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CHAPTER 9 Cardiac Arrhythmias
113
When the rate is 250 beats per minute or greater, the arrhythmia is
arbitrarily classified as atypical atrial flutter, and when the rate is less
than 250 beats per minute, it is arbitrarily classified as atrial tachycardia.
Like typical atrial flutter, these arrhythmias are paroxysmal sustained
or persistent arrhythmias, and when manifesting with 2:1 conduction,
they may be misdiagnosed as sinus tachycardia if the abnormal P-wave
vector and fixed heart rate over time are not recognized. Therapy and
prognosis are otherwise similar to those for typical atrial flutter.
Atrial Fibrillation
Overview and Classification
AF is a chaotic atrial rhythm related to continuous and variable activation of the atria. There are no distinct P waves or periods of atrial
quiescence. It is characterized electrocardiographically by a wavering
baseline associated with an irregular ventricular response (see Fig.
9.6D).
AF is the most common clinically significant arrhythmia. It affects
2.2 million people in the United States. Its prevalence is between 0.4%
and 1% in the general population, and it increases with age, reaching
8% in those older than 80 years. Patients with AF have a higher risk of
stroke, heart failure, and mortality. However, the role of AF as an independent determinant of mortality is uncertain because it commonly
coexists with other important conditions. Patients with lone AF do not
have an increased mortality rate, and carefully designed trials exploring
the benefit of maintenance of sinus rhythm over rate control show, in
most populations, no survival benefit for sinus rhythm. One exception
may be in patients with systolic heart failure in addition to AF where
ablation of AF may have a survival advantage. The recently completed
CASTLE-AF (Catheter Ablation vs. Standard Conventional Treatment
in Patients with LV Dysfunction and AF) showed a significant reduction in mortality with catheter ablation of AF in this select population.
AF is often classified by its clinical presentation and pattern. When AF
is first detected, it is called new onset, and its ultimate pattern is initially undetermined. When AF relapses during follow-up, it is called
recurrent and classified by its clinical pattern. If AF terminates spontaneously, it is called paroxysmal AF. Although episodes lasting up to 7
days are defined as paroxysmal, most episodes of paroxysmal AF terminate within the first 24 hours and many terminate within minutes
or hours of onset. When AF lasts longer than 7 days, it is designated
as persistent. AF that persists for a long interval, typically more than
a year, without return of an interim period of sinus rhythm (spontaneously or as a result of medical intervention such as cardioversion) is
termed long-standing persistent AF. Finally, when a clinical decision is
made to no longer try to maintain sinus rhythm, the term permanent
AF is used.
Mechanisms of Atrial Fibrillation
Because of its chaotic nature, it has been difficult to study AF, and its
mechanisms remain incompletely understood. The initiation of spontaneous AF is a consequence of rapid electrical firing from preferential
focal sites of origin. The most common site of focal origin is from left
atrial muscle sleeves extending along the outer surface of the pulmonary veins. When firing does not originate from a pulmonary vein, it
is commonly from the left atrial tissue immediately adjacent to one
of the veins or occasionally from one of the other thoracic veins such
as the ostium of the superior vena cava or the ostium of the coronary
sinus. Atrial rates recorded in and around the pulmonary veins are significantly higher than at other atrial sites, suggesting that activity in
the region of the veins is important in perpetuating AF after initiation.
These insights have produced highly effective techniques for the
cure of AF. Ablation techniques designed to isolate these trigger sites
from the atrium have success rates of 70% to 80% for the cure of
paroxysmal AF and somewhat lower rates for the cure of persistent AF.
Ablation restricted to the region of the pulmonary veins and adjacent
left atrium is curative in most patients with AF, implying that most
cases of AF are arrhythmias entirely contained within and maintained
by the left atrium and connecting veins. In the same way that typical
atrial flutter is the characteristic arrhythmia of the right atrium, AF is
the characteristic arrhythmia of the left atrium.
Anticoagulation and Atrial Fibrillation
During AF (and to some extent, atrial flutter), the atria have incomplete and ineffective contractions. Blood stasis occurs and may result
in the formation of intracardiac thrombus, which may lead to thromboembolism and stroke. The overall risk of stroke in patients with
AF is 5% per year. Certain risk factors may adjust this risk, including age, gender, rheumatic heart disease, prior stroke, left ventricular
dysfunction, vascular disease, hypertrophic cardiomyopathy, left atrial
enlargement, hypertension, and diabetes.
Scoring systems have been developed to estimate a patient’s
AF-related stroke risk based on his or her constellation of risk factors.
Formerly, the most used system was the CHADS2 score (cardiac failure,
hypertension, age ≥75 years, diabetes mellitus, and prior stroke). This
system has been well validated in assessing the stroke risk of patients
with AF. It assigns a single point for age of 75 years or older, diabetes,
history of heart failure, and hypertension. It assigns two points for a
history of stroke or transient ischemic attack. A score of 0 correlates
with a relatively low risk of stroke at 1.9% per year, a score of 1 has a
stroke risk of 2.8% per year, a score of 2 has a risk of 4.0% per year,
and a score of 3 or higher has a stroke risk of more than 5.9% per year.
The CHADS2 underwent further refinement to increase the granularity of stroke risk stratification with the creation of the CHA2DS2VASc (vascular disease, age, and sex) scoring system, currently the
primary score for thromboembolic risk stratification. In this system,
congestive heart failure, hypertension, diabetes mellitus, vascular disease, age between 65 and 74 years, and female gender are assigned
1 point, and age of 75 years or older and prior stroke are assigned 2
points. A CHA2DS2-VASc score of 0 was associated with a 0% stroke
rate, a score of 1 with a 0.6% per year risk, a score of 2 with a 1.6% risk,
and a score of 3 with a risk of 3.9%. This system may be most useful for
identifying truly low-risk patients.
After a patient’s individualized stroke risk is determined, it can be
balanced against the risk of anticoagulation to determine what would
be appropriate for stroke prevention. A useful tool for estimating
bleeding risk due to oral anticoagulation is the HAS-BLED (hyperten-
sion, abnormal renal/liver function, stroke, bleeding history or predisposition, labile international normalized ratio, elderly, drugs/alcohol)
score. Patients with a HAS-BLED score of 0 had a risk of 0.59 severe
bleeds per 100 patient-years, those with a score of 1 had a risk of 1.51,
those with a score of 2 had a risk of 3.20, and those with a score of 3
had a risk of 19.51.
In patients with an acceptable bleeding risk, and with a CHA2DS2VASc score of 2 or greater in men or 3 or greater in women, the 2019
AHA guidelines recommend oral anticoagulation to help prevent
embolic stroke. Recommended agents include warfarin, dabigatran,
rivaroxaban, apixaban or edoxaban. For patients with low CHA2DS2VASc scores, aspirin is no longer recommended. Oral anticoagulants
might be reasonable for intermediate CHA2DS2-VASc scores (1 in men
and 2 for women), but this has less evidence.
Warfarin is the longest-studied antithrombotic used for reducing
the rate of AF-related stroke and reduces the risk by 50%. Warfarin can
be difficult to administer; the level of blood-thinning effect must be
constantly monitored with international normalized ratio (INR) blood
testing. An INR less than 2.0 is associated with higher rates of ischemic
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stroke; a level greater than 3.0 is associated with increased intracranial
bleeding. On average, a therapeutic INR (between 2.0 and 3.0) is maintained in only two thirds of cases, and there are many drug and dietary
interactions with warfarin.
Several newer oral anticoagulants (NOACs) have effectiveness and
bleeding risk rates similar to warfarin, but they do not require drug
level monitoring. They include dabigatran, rivaroxaban, apixaban,
and edoxaban. These drugs have been studied in large patient groups
and found to be noninferior to warfarin, and some may be superior in
certain aspects. NOACs are preferred in eligible patients over warfarin
except in cases of moderate-to-severe mitral stenosis or the presence of
a mechanical heart valve.
Percutaneous occlusion of the left atrial appendage with the
Watchman device has been compared to Coumadin in patients with
nonvalvular atrial fibrillation and found generally to offer similar protection against stroke. Oral anticoagulation remains the preferred therapy for stroke prevention in most patients; however, in those who are
poor candidates for long-term anticoagulation (because of the propensity for bleeding or poor drug tolerance or adherence), the Watchman
device provides an alternative.
The highest risk of stroke related to AF occurs at time of conversion
to sinus rhythm achieved spontaneously or by chemical or electrical
cardioversion. If thrombus has formed within the left atrium or left
atrial appendage, it may not leave the atria during AF due to ineffective atrial mechanics. However, after sinus rhythm is restored, the
improved atrial function may eject the thrombus and cause embolic
stroke or other systemic embolic sequelae. Even with restoration of
electrical atrial systole, the recovery of normal atrial mechanics may
be delayed several days to weeks (i.e., atrial stunning). To reduce the
risk of pericardioversion stroke, it is important to reduce the risk of
preexisting thrombus and to prevent formation in the time period
immediately after cardioversion.
The risk of preexisting thrombus can be reduced by 3 weeks of oral
anticoagulation or Doppler transesophageal echocardiography (TEE)
before cardioversion. These steps are recommended for any patient
who has been in AF for an unknown period or has been documented
to be in AF more than 48 hours. Although thrombi have been identified in patients with AF for shorter periods, current clinical practice
presumes that most thrombus formation requires at least 48 hours.
Thrombus related to AF occurs most commonly in the left atrial
appendage, which cannot be well visualized by transthoracic echocardiography; TEE is often recommended before cardioversion for optimal imaging of the left atrial appendage. After cardioversion, at least
4 weeks of oral anticoagulation is recommended for everyone, with
the exception of low CHA2DS2-VASc score patients (0 in men or 1 in
women) who had AF less than 48 hours prior to the cardioversion, in
whom postconversion anticoagulation may be omitted.
Acute Management of Atrial Fibrillation: Rate Control
The acute management of AF centers on the control of the ventricular response, timely restoration of sinus rhythm, and identification of
potentially reversible factors that might have precipitated the arrhythmia. AF with rapid ventricular response results in acute deterioration
in stroke volume and cardiac output and an increase in myocardial
oxygen demand with the potential for coronary ischemia. Patients who
are symptomatic must be controlled promptly. When pursuing rate
control for acute AF of recent onset, the fastest way to achieve rate
control is the restoration of sinus rhythm. If rate control in ongoing
rapidly conducted AF proves difficult or is not well tolerated, cardioversion should be undertaken early.
For the acute control of rapidly conducted AF, intravenous administration of a β-blocker (i.e., esmolol, metoprolol, or propranolol)
or a nondihydropyridine calcium-channel blocker (i.e., diltiazem or
verapamil) is preferred. In the setting of decompensated heart failure,
the use of a calcium-channel blocker may exacerbate heart failure and
should be avoided. In this setting, digoxin is a useful agent for resting
rate control. Digoxin is also a useful second-line drug in addition to a
calcium-channel or β-blocker for resting rate control. If this therapy
is ineffective or not tolerated, intravenous amiodarone is a useful rate
control agent, especially in the setting of congestive heart failure, and it
may facilitate restoration of sinus rhythm.
Long-term targets for rate control of permanent AF have been
a matter of debate. The Rate Control Efficacy in Permanent Atrial
Fibrillation II (RACE II) study showed no advantage to strict rate control. Targeting a resting rate of less than 80 beats per minute showed
no advantage over a target of less than 110 and was much harder to
achieve. For long-term management, the results suggest that achieving
a resting heart rate of less than 110 beats per minute may be sufficient
and safe.
Acute Management of Atrial Fibrillation: Restoration of Sinus
Rhythm
When sinus rhythm is restored in the first 48 hours of acute AF, the
thromboembolic risk is low, and anticoagulation is not required.
New-onset AF should be managed with a plan to restore sinus rhythm
during this period if possible. At least one half of new-onset AF episodes terminate spontaneously in the first 24 to 48 hours.
Pharmacologic conversion of atrial fibrillation. Pharmacologic
conversion of AF can be undertaken when restoration of sinus rhythm
is not urgent. Several antiarrhythmic drugs have been effective
in increasing the rate of early conversion of AF. Pharmacologic
conversion usually is more successful with AF of recent onset than
with chronic AF.
Oral agents with efficacy in the early conversion of AF include
flecainide, propafenone, and dofetilide. Oral amiodarone and sotalol
have been associated with a 27% and 24% conversion rate, respectively,
occurring after 28 days of therapy. However, due to low early conversion rates, these oral drugs are not recommended for conversion.
Intravenous agents with efficacy for early conversion include ibutilide
and amiodarone. Ibutilide is limited by a relatively high 4% rate of
drug-induced QT prolongation and TdP VT. This risk is even higher in
the setting of LV dysfunction, electrolyte disturbances, or heart failure.
Ibutilide should be reserved for the pharmacologic conversion of stable
patients with a baseline normal QT interval. In contrast, intravenous
amiodarone is well tolerated by unstable patients and is the preferred
pharmacologic agent for conversion in the critically ill.
Electrical cardioversion of atrial fibrillation. Electrical cardio-
version should be performed urgently in the case of severe compromise
related to acute AF, including angina, heart failure, hypotension, and
shock. Cardioversion should also be attempted at least once electively in
most cases of new-onset AF regardless of tolerance. When performing
electrical cardioversion, an anterior-posterior patch or paddle position is
more effective than the conventional anterior-to-lateral patch or paddle
position used for ventricular defibrillation. Although low-output discharges may be effective in some patients, a strategy of starting at higher
outputs decreases the number of shocks required and the average cumulative energy delivered. An initial shock energy of 200 J is recommended.
After a failed initial shock, full output should be used for the next attempt.
Long-Term Maintenance of Sinus Rhythm
Antiarrhythmic therapy. Despite the association of AF with an
increase in stroke-related and all-cause mortality, no study has
established a benefit for pharmacologic maintenance of sinus
rhythm in terms of stroke risk or survival. This may be because AF is
CHAPTER 9 Cardiac Arrhythmias
115
merely a marker and not a mechanism of mortality. It may also be a
consequence of the relative inefficacy of pharmacologic therapy in the
maintenance of sinus rhythm and the difficulty of establishing whether
patients thought to be in sinus rhythm are consistently in sinus rhythm
at follow-up.
The largest and best designed trial addressing this issue was the
Atrial Fibrillation Follow-up Investigation of Rhythm Management
(AFFIRM) trial. The study included 4060 patients randomly assigned
to rhythm control with antiarrhythmic drugs, most commonly
amiodarone, or to rate control without attempts to maintain sinus
rhythm. AFFIRM demonstrated no advantage in stroke or mortality
rates using a strategy of sinus rhythm maintenance compared with rate
control. Either strategy can be offered to patients with an expectation
of similar outcomes with regard to hard end points. The decision to
pursue sinus rhythm usually is determined by the management of
symptoms that may be better addressed by maintaining sinus rhythm
in selected patients.
In the absence of antiarrhythmic drugs, more than 80% of patients
relapse during the first year after cardioversion of AF. Antiarrhythmic
drugs remain the primary strategy for maintaining sinus rhythm after
cardioversion and for preventing symptomatic episodes in patients
with paroxysmal AF. However, antiarrhythmic therapy has many limitations, and alternative ablative therapies may over time overtake antiarrhythmic therapy in the management of AF.
All antiarrhythmic drugs have the potential for proarrhythmia, the
unintended precipitation of a new arrhythmic problem caused by the
drug. Adverse rhythm effects of drugs may include sinus node dysfunction, heart block, promotion of drug-slowed atrial flutter permitting
rapid 1:1 conduction, and promotion of potentially lethal ventricular
arrhythmias. Class I drugs such as flecainide, propafenone, and disopyramide may result in significant direct myocardial depression and
consequent exacerbation of heart failure. The array of potential adverse
effects of antiarrhythmic drugs is beyond the scope of this chapter, but
certain essential concepts are important to recognize.
Class I drugs such as flecainide and propafenone, which work by
slowing conduction, have a high risk of ventricular proarrhythmia and
potential for sudden death in the setting of heart failure, LV dysfunction, and coronary artery disease. Use of these drugs is restricted to
patients with preserved cardiac function and no evidence of obstructive coronary artery disease. However, in this selected group of patients
with normal hearts, these drugs are exceedingly safe, well tolerated,
and often effective.
Class III drugs, which prolong repolarization and refractoriness,
include sotalol, dofetilide, dronedarone, and amiodarone. They are
safe for patients with coronary artery disease, and in the case of dofetilide and amiodarone, they are safe for those with congestive heart failure. However, sotalol and dofetilide may provoke TdP, even in patients
with normal cardiac function, and they must be used with caution.
Amiodarone has greater long-term efficacy than other drugs and a
lower risk of proarrhythmia, but long-term somatic toxicity consisting of thyroid dysfunction, pulmonary, and occasional hepatotoxicity limits the use of this drug in older patients or those with limited
expected longevity or an inability to safely tolerate alternative agents
due to advanced cardiac disease or proarrhythmia. Amiodarone is
highly effective for the short-term, acute management of arrhythmias
in critically ill patients when the potential risk of long-term toxicity is
not an issue.
Dronedarone was derived by modification of the amiodarone molecule. Like amiodarone, the drug has a low risk of proarrhythmia and
TdP VT. Unlike amiodarone, the drug does not cause thyroid toxicity. In common use, hepatotoxicity is also uncommon with dronedarone. However, rare cases of hepatic failure have been associated
Heart disease
CAD
Sotalol
Ye s
Hypertension
LVH ≥1.4 cm
Ye s
Propafenone
Amiodarone
No
Flecainide
Sotalol
Catheter
ablation
No (or minimal)
Heart failure
Flecainide
Propafenone
Sotalol
Amiodarone
Dofetilide
Fig. 9.7 A strategy for the selection of therapy to maintain sinus rhythm
in patients with recurrent atrial fibrillation. Patients are stratified by the
presence or absence of structural heart disease, and drugs expected to
have the greatest efficacy and lowest therapeutic risk in each group are
selected. Catheter ablation becomes a therapeutic option after failure
of at least one antiarrhythmic drug. The class IC drugs flecainide and
propafenone are not advised for patients with heart failure or coronary
artery disease (CAD). Amiodarone is an acceptable first-line drug for
those with heart failure and severe left ventricular hypertrophy. Because
of its potential for somatic toxicity, amiodarone is otherwise reserved as
a second-line agent that is used as an alternative to catheter ablation.
Catheter
ablation
Amiodarone
Dofetilide
Catheter
ablation
Dofetilide
Amiodarone
with dronedarone use. Dronedarone has increased mortality rates for
patients with recently decompensated heart failure and when used as a
simple rate control agent in patients with permanent AF. It is contraindicated in these settings.
In addition to being useful agents for the prevention of AF, sotalol,
dronedarone, and amiodarone provide substantial rate control during
relapses of AF. However, rate control with other antiarrhythmic agents
may not be adequate to prevent rapid conduction with relapse, and
class I drugs such as flecainide may accelerate response at the time
of relapse. Antiarrhythmic drugs other than sotalol, dronedarone, or
amiodarone should therefore be combined with a rate control agent
such as a β-blocker or nondihydropyridine calcium-channel blocker
during long-term therapy. Fig. 9.7 is a proposed strategy for antiarrhythmic drug selection for the long-term maintenance of sinus
rhythm in patients with AF.
Surgical ablation of atrial fibrillation. The surgical treatment
of AF was pioneered by Cox with the development of the atrial maze
procedure. The procedure was predicated on the concept that AF was
maintained by multiple interacting wave fronts of activity. By surgically
dividing the atria into narrow channels, most with connection back
to the sinus node, it was thought that AF could be abolished while
preserving physiologic activation and contraction of the atrium. The
circuitous path left for atrial activation and the multiple barriers
created in the atrium intended to prevent AF gave rise to the term maze
procedure to describe the technique. The initial procedure was thought
to be highly successful but was associated with significant surgical risks
and problems with sinus node dysfunction. Because of the surgical
complexity of making and then closing multiple incisions in the atria
and the complications associated with the procedure, the initial cutand-sew maze procedure has fallen out of clinical use.
116 SECTION II Cardiovascular Disease
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Although the original maze procedure is no longer used, many
techniques have been developed to simplify the operation by substituting linear thermal ablation (by heating or cooling tissue) to create
lines of conduction block in the atria without the need for extensive
atrial dissection and reconstruction. Surgical ablation is commonly
applied in patients with a history of AF who are undergoing concomitant heart operations for other indications such as valvular or coronary
disease. Less frequently, surgical ablation has been applied as a standalone procedure for the sole management of AF. In that setting, various minimally invasive techniques have been developed. However, the
techniques used vary widely from one center to another and long-term
reporting of outcomes is inconsistent. In a large series that included
282 patients undergoing an open bi-atrial ablation procedure, 78%
were in sinus rhythm without antiarrhythmic therapy at the 1-year follow-up evaluation.
Another important potential benefit of surgical ablation for AF
is that it provides an opportunity to eliminate the left atrial appendage as a potential site of thrombus formation and source of thromboembolism. This can be accomplished by complete amputation of
the appendage with oversewing of the appendage or clamping off the
opening to the appendage with special devices designed for this purpose. This may be especially important in patients with absolute or
relative contraindications to anticoagulation.
Catheter ablation of atrial fibrillation. Catheter ablation has
become a common procedure for the management of AF after failure
of initial attempts at medical therapy. Initial attempts to cure AF
using catheter techniques were based on attempts in the early 1990s to
emulate the linear lesion set of the Cox maze procedure with multiple
endocardial lesions. High complication rates and limited efficacy led to
abandonment of this approach.
In 1998, Haissaguerre reported the important role of rapid activity
originating in the musculature of the pulmonary veins in initiation of
paroxysmal AF. This led to the development of procedures designed
to target the pulmonary veins and eventuated in the technique of electrical pulmonary vein isolation (PVI), which is currently the primary
ablative approach to treatment of paroxysmal AF by catheter techniques. This technique has had acceptably high success rates (≈70%)
at multiple centers for the treatment of paroxysmal AF without antiarrhythmic therapy.
Despite the high success rate of catheter PVI ablation for the treatment of paroxysmal AF, this technique has not proved reliably effective in the management of more persistent forms of AF, especially
long-standing persistent AF. This likely reflects the importance of factors other than pulmonary vein activity in the initiation and maintenance of persistent AF that are not addressed by PVI ablation. Multiple
ablative techniques are currently used in an attempt to increase the
success rates for patients with persistent AF. They have included addition of linear lesions to block reentrant wave fronts, ablation of regions
of unusually rapid atrial activity during ongoing AF, and interruption
of stable rotors of atrial activity identified during multisite mapping of
AF. Although these techniques have improved success rates in limited
series, it is uncertain which, if any, of these methods represents the
optimal approach to the ablation of long-standing persistent AF.
In summary, catheter ablation is the preferred secondary strategy
for treatment of symptomatic AF after initial attempts at medical therapy have failed. Simple pulmonary vein isolation has a high success
rate for the management of patients with paroxysmal AF. Success rates
for all ablative techniques are lower for persistent AF, especially for
long-term AF. As in the case of surgical ablation, multiple techniques
are used at various centers, and the different strategies for follow-up
and definitions of response have made it difficult to ascertain the relative efficacy of the various approaches in common use.
Catheter ablation of the atrioventricular node. Although less
commonly used today than in the past, the older technique of catheter
ablation of the AV node resulting in complete heart block followed
by placement of a ventricular pacemaker to maintain physiologic
heart rates remains an option for patients when rate control cannot
be achieved medically. This technique continues to have an important
role in the management of patients who are too infirm to safely
undergo AF ablation or in patients for whom ablative techniques have
failed to control the arrhythmia.
For a deeper discussion on this topic, please see Chapter 58,
“Supraventricular Cardiac Arrhythmias,” in Goldman-Cecil Medicine,
26th Edition.
SYNCOPE
Syncope is a sudden loss of consciousness that is transient. Syncope
has cardiac causes (e.g., low cerebral blood pressure) and noncardiac
causes. Common causes and categories of syncope are outlined in
Table 9.4. Cerebrovascular disease or stroke uncommonly manifests
as syncope unless a large cerebral territory is involved. Syncope is a
common reason for emergency room or hospital admission.
The diagnostic approach to a patient with syncope is given in Fig.
9.8. Most causes can be identified by the medical history and physical
examination alone. Conditions surrounding the syncopal episode often
suggest a cause. For example, vasovagal episodes often occur during
stress, pain, straining, coughing, or urination. Exercise-induced syncope may indicate obstructive coronary disease, channelopathies such
as long QT or CPVT, obstructive cardiomyopathy, aortic stenosis, or
arrhythmia. A history of palpitations or syncope with no warning may
be related to cardiac arrhythmias. Very long episodes of syncope (>5
minutes) suggest noncardiac causes. A recent change in medications
or dizziness with position changes suggests orthostatic hypotension.
Witnessed limb movements or posturing is not specific for neurologic
causes and can result from any type of cerebral hypoperfusion, even
from cardiac causes.
Beyond the history, physical examination, and routine ECG, further testing has little diagnostic utility. Holter or loop recorders may be
useful. Implantable loop recorders may have utility in cases of recurrent, infrequent syncope. Electrophysiologic testing may be useful
in some patients with other abnormalities suggesting an arrhythmic
cause.
Despite thorough evaluations, more than 30% of patients with syncope have no identifiable cause. Cardiac causes of syncope have the
highest morbidity and mortality rates. Because patients with unknown
causes of syncope have long-term outcomes similar to those with noncardiac syncope, the major goal of an evaluation is to identify cardiac
causes of syncope.
VENTRICULAR ARRHYTHMIAS AND SUDDEN
CARDIAC DEATH
Ventricular ectopy is defined as cardiac beats that originate from
within the right or left ventricular muscle or conduction system.
Premature ventricular contractions (PVCs) can occur singly or as ventricular couplets or triplets. VT is four or more consecutive beats that
originate from the ventricle at a rate of at least 100 beats per minute.
VT is classified as sustained if it lasts longer than 30 seconds or requires
termination due to hemodynamic instability; otherwise, it is classified
as nonsustained VT (NSVT).
Ventricular ectopy also may be classified based on maintenance of
a similar electrocardiographic morphology. The beats of monomorphic VT (MMVT) appear to be identical and usually originate from
CHAPTER 9 Cardiac Arrhythmias
TABLE 9.4 Causes of Syncope
Cause Features
Peripheral Vascular or Circulatory
Vasovagal syncope (neurally mediated) Prodrome of pallor, yawning, nausea, diaphoresis; precipitated by stress or pain; occurs when patient is upright,
aborted by recumbency; fall in blood pressure with or without a decrease in heart rate
Micturition syncope Syncope with urination (probably vagal)
Post-tussive syncope Syncope after paroxysm of coughing
Hypersensitive carotid sinus syndrome Vasodepressor and/or cardioinhibitory responses with light carotid sinus massage
Drugs Orthostasis; occurs with antihypertensive drugs, tricyclic antidepressants, phenothiazines
Volume depletion Orthostasis; occurs with hemorrhage, excessive vomiting or diarrhea, Addison’s disease
Autonomic dysfunction Orthostasis; occurs in diabetes, alcoholism, Parkinson’s disease, deconditioning after a prolonged illness
Central Nervous System
Cerebrovascular Transient ischemic attacks and strokes are unusual causes of syncope; associated neurologic abnormalities are
usually identified
Seizures Warning aura sometimes present, jerking of extremities, tongue biting, urinary incontinence, postictal confusion
Metabolic
Hypoglycemia Confusion, tachycardia, jitteriness before syncope; patient may be taking insulin
Cardiac
Obstructive Syncope is often exertional; physical findings consistent with aortic stenosis, hypertrophic obstructive cardiomyopa-
thy, cardiac tamponade, atrial myxoma, prosthetic valve malfunction, Eisenmenger’s syndrome, tetralogy of Fallot,
primary pulmonary hypertension, pulmonic stenosis, massive pulmonary embolism
Arrhythmias Syncope may be sudden and occurs in any position; episodes of dizziness or palpitations; may be history of heart
disease; bradyarrhythmias or tachyarrhythmias may be responsible—check for hypersensitive carotid sinus
117
the same area of the heart. Ventricular flutter is a term that may be
used to describe MMVT with rates of more than 300 beats per minute.
Polymorphic VT (PMVT) has a more variable appearance on the ECG
than MMVT. TdP is a special form of PMVT that has a repetitive,
undulating periodicity and usually implies a long-QT triggered mechanism. VF is the most chaotic form of ventricular ectopy. It is associated with no meaningful cardiac output and usually leads to death
unless rapidly treated. The other forms of VT may eventually degrade
into VF.
Determining whether a patient has a rhythm of ventricular origin
usually is done by 12-lead surface ECG. Ventricular ectopy typically
has a wide QRS morphology (Fig. 9.9). Not all wide QRS morphologies are ventricular in origin, and there are criteria for determining
whether a wide-complex tachycardia is supraventricular or ventricular. SVT may appear as a wide-complex tachycardia if it conducts to
the ventricle with aberrancy (e.g., bundle branch block) or through an
accessory pathway (e.g., WPW syndrome). Features that may help distinguish between SVT and VT include AV dissociation with capture
beats and fusion beats and the QRS morphology and duration (Table
9.5). The Brugada algorithm is commonly used for determining the
site of origin of wide-complex tachycardia. The tachycardia has a ventricular origin in more than 90% of patients with a history of ischemic
heart disease.
VT may occur by the same mechanisms as other tachycardias,
such as reentry, enhanced automaticity, or triggered activity. VT often
occurs as a reentrant tachycardia around an area of prior MI scar in
the left ventricle. VT in the chronic phase of ischemic heart disease
is mediated by reentry through channels or sheets of surviving myocardium, especially in the partially spared border zone of a region of
scar resulting from a prior MI. In these channels, conduction is abnormally slow due to poor coupling between sparse surviving myocytes.
Susceptibility to sustained VT increases with worsening left ventricular
dysfunction, likely due to the greater extent of ventricular scar.
VT can occur in the absence of ischemic heart disease in the form
of idiopathic VT, nonischemic cardiomyopathies, hypertrophic cardiomyopathies, arrhythmogenic RV dysplasia, bundle branch reentry,
cardiac ion channel disorders, or electrolyte disturbances. The right
ventricular outflow tract (RVOT) is the most common origin of idiopathic VT, which is likely caused by triggered activity. This form of VT
(or PVCs) is usually sensitive to catecholamines and may terminate
with adenosine (i.e., adenosine-sensitive VT). Another common form
of idiopathic VT originates from the left ventricular conduction system
(i.e., fascicular VT) and may be verapamil sensitive. Idiopathic VTs are
common targets for successful catheter ablation.
Nonsustained VT usually does not require specific therapy unless
the patient is symptomatic. The Cardiac Arrhythmia Suppression Trial
treated PVCs and NSVT after the acute phase of MI with class I antiarrhythmic drugs, and the trial demonstrated increased mortality rates
when the arrhythmias were treated. If VT is attributed to reversible
causes such as electrolyte disturbances or acute ischemia, the underlying mechanism should be treated. VT not due to reversible causes
may be treated with β-blockers, antiarrhythmic drug therapy (e.g.,
amiodarone), or catheter ablation. If urgent treatment is required due
to hemodynamic instability, direct current cardioversion is performed.
It should be synchronized to the QRS complex if a regular morphology exists; otherwise, it should be nonsynchronized. Performing direct
current cardioversion during the refractory period (T wave) of MMVT
may degrade the rhythm to VF. An ICD often is used in patients who
survive VT or VF to quickly treat recurrent episodes. Endocardial and
epicardial catheter ablation has become an effective treatment for VT.
Prevention of Sudden Cardiac Death
SCD is defined as death within 1 hour of the onset of symptoms. It may
result from a variety of cardiac or noncardiac conditions (Table 9.6). SCD
is one of the most common causes of death, with 400,000 events occurring annually in the United States. The most common cause of SCD is VT
118 SECTION II Cardiovascular Disease
History
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Physical examination including orthostatic
vital signs and carotid sinus massage
ECG
Documented
arrhythmia
Specific therapy including
AA drugs, EPS, AICD, and
pacemakers
Normal and no
structural
heart disease
Consider tilt-table
testing
Fig. 9.8 Approach to the evaluation of syncope. AA, Antiarrhythmic; AICD, automatic implantable cardiovert-
er-defibrillator; AS, aortic stenosis; CMP, cardiomyopathy; ECG, electrocardiogram; EPS, electrophysiologic
study; MS, mitral stenosis; SAECG, signal-averaged ECG.
Suggests cardiac cause
Suspected
arrhythmia
Holter monitor
Event monitor
SAECG
Abnormal, or normal
with underlying
ischemic heart disease
or cardiomyopathy
Electrophysiologic
Does not suggest cardiac cause
Further work-up as indicated
by diagnosis
Suggests structural
heart disease
(AS, MS, CMP)
Echocardiogram Tilt-table testing
study
Suggests
ischemic
event
Exercise test
Cardiac catheterization
Normal ECG
No structural
heart disease
A
B
Fig. 9.9 Ventricular arrhythmias. (A) Monomorphic ventricular tachycar-
dia (VT). Notice the wide QRS with a stable appearance with each beat.
Detecting P waves during VT is difficult due to the overlying ventricular activity, but it is visible at several points on this tracing, some of
which are marked by arrows. The AV dissociation is diagnostic of VT and
excludes supraventricular tachycardia. (B) An initially organized agonal
(preterminal) rhythm (arrows) degenerates into coarse ventricular fibrillation. Notice the irregular baseline and the absence of organized QRS
complexes. During ventricular fibrillation, there is no forward cardiac
output, and cardiac arrest immediately ensues.
or VF. Cardiac conditions that increase the risk of SCD include LQTS,
hypertrophic cardiomyopathy, Brugada syndrome, arrhythmogenic RV
dysplasia, and nonischemic or ischemic cardiomyopathy. The most common cardiac condition that may lead to SCD is acute or distant MI.
The successful treatment of SCD due to VF usually requires rapid
access to cardioversion; if treatment is delayed by more than 5 to 10
minutes, permanent brain injury is common. AEDs can reduce the
TABLE 9.5 Differentiation of Ventricular
Tachycardia From Supraventricular
Tachycardia With Aberrancy
Helpful Features Implications
Positive QRS concordance Diagnostic of VT
AV dissociation, capture beats, or fusion beats Diagnostic of VT
Atypical RBBB (monophasic R, QR, RS, or triphasic QRS in
V1; R:S ratio <1, QS or QR, monophasic R in V6)
Atypical LBBB (R >30 min or R to S [nadir or notch] >60 min
in V1 or V2; R:S ratio <1, QS or QR in V6)
Shift of axis from baseline Suggests VT
History of CAD Suggests VT
QRS during tachycardia identical to QRS during sinus
rhythm
Termination with adenosine Suggests SVT
AV, Atrioventricular; CAD, coronary artery disease; LBBB, left bundle
branch block; RBBB, right bundle branch block; SVT, supraventricular
tachycardia; VT, ventricular tachycardia.
time to defibrillation and improve survival when placed in public
areas, although they have been less effective when installed in private
residences, even for patients at risk for SCD.
ICDs used in the treatment of SCD have improved mortality rates.
Patients who are at high risk for SCD are often offered an ICD to enable
rapid defibrillation before the onset of anoxic brain injury. If a patient
survives the first episode of SCD due to documented or presumed VT
Suggests VT
Suggests VT
Suggests SVT
CHAPTER 9 Cardiac Arrhythmias
119
TABLE 9.6 Causes of Sudden Cardiac Death
Noncardiac Causes
Central nervous system hemorrhage
Massive pulmonary embolus
Drug overdose
Hypoxia secondary to lung disease
Aortic dissection or rupture
Cardiac Causes
Ventricular fibrillation
Myocardial ischemia or injury
Long QT syndrome
Short QT syndrome
Brugada syndrome
Arrhythmogenic right ventricular dysplasia
Ventricular tachycardia
Bradyarrhythmias, sick sinus syndrome
Aortic stenosis
Tetralogy of Fallot
Pericardial tamponade
Cardiac tumors
Complications of infective endocarditis
Hypertrophic cardiomyopathy (arrhythmia or obstruction)
Myocardial ischemia
Atherosclerosis
Prinzmetal angina
Kawasaki arteritis
or VF from nonreversible or unknown causes, he or she is offered an
ICD. ICDs are extremely successful in the detection and treatment of
VT or VF. They do not always prevent loss of consciousness because it
takes 15 to 20 seconds to treat the arrhythmia, and low cardiac output
may cause syncope before restoration of normal rhythm, especially if
several cardioversions are required.
The earliest ICD trials examined their use in the secondary prevention of SCD (i.e., treating patients who had already survived an episode
of cardiac arrest). The largest study was the Antiarrhythmics Versus
Implantable Defibrillators (AVID) trial, which randomized patients
with a history of poorly tolerated sustained VT or cardiac arrest to
empirical amiodarone or ICD implantation. In this trial and several
others, ICD therapy was associated with a lower risk of arrhythmic and
all-cause death compared with antiarrhythmic therapy.
Several trials have examined the use of ICDs for the primary prevention
of SCDs (i.e., treating patients who are at risk for SCD). The first was the
Multicenter Automatic Defibrillator Implantation Trial (MADIT), which
enrolled patients with a prior MI and an ejection fraction of 35% or less
who had frequent ventricular ectopy and inducible VT at electrophysiologic testing. The study demonstrated a substantial mortality reduction
with ICD therapy. MADIT-II enrolled patients with a prior MI and an ejection fraction of 30% or less in the chronic phase, without requiring invasive
testing. A significant mortality benefit was associated with ICD therapy.
The Sudden Cardiac Death in Heart Failure trial enrolled a broader
population consisting of patients with ischemic and nonischemic cardiomyopathy, symptomatic heart failure, and an ejection fraction of
35% or less. A survival benefit was found for patients treated with an
ICD compared with conventional therapy or empirical amiodarone
therapy. The degree of benefit was similar for patients with ischemic
or nonischemic cardiomyopathy, suggesting that primary prevention
with ICDs for patients with prior MI or nonischemic cardiomyopathy
and heart failure was appropriate.
TABLE 9.7 Predictors of Sudden Cardiac
Death After Myocardial Infarction
Decreased left ventricular ejection fraction
Residual ischemia
Delayed enhancement on cardiac MRI
Late potentials on signal-averaged electrocardiography
Decreased heart rate variability
Prolonged QT on ECG
Induction of sustained MMVT with programmed electrical stimulation
Complex ventricular ectopy (e.g., NSVT) on ambulatory monitoring
ECG, Electrocardiogram; MMVT, monomorphic ventricular tachycardia;
MRI, magnetic resonance imaging; NSVT, nonsustained ventricular
tachycardia.
The risk of SCD after MI is highest in the few months after the index
event. However, ICDs have not been effective when implanted immediately after MI or revascularization procedures. The reason for this is
unclear; it may reflect the large percentage of patients who have improved
cardiac function early on, which decreases the risk of SCD and therefore the benefit of an ICD. Alternatively, the mechanism for SCD in the
early period after an MI or revascularization procedure may be recurrent
ischemia rather than reentrant tachycardia and therefore less amenable
to ICD therapy. The Defibrillator in Acute Myocardial Infarction Trial
(DINAMIT) randomized 675 patients with low ejection fractions immediately after MI to ICD or medical therapy; no difference in mortality rates
was seen. The current recommendations are to avoid primary prevention
with ICDs within 40 days of an MI or 3 months of revascularization.
A significant challenge in modern medicine is identifying patients
who have an elevated risk of SCD to allow effective use of primary prevention interventions such as ICDs. Some known predictors of SCD
after MI are shown in Table 9.7, but many are not specific or sensitive
enough for practical use. Reduced ejection fraction has been the most
successful noninvasive measure that can predict increased risk of SCD.
An electrophysiologic study is a minimally invasive catheter procedure
that with electrical stimulation can help to identify patients who are
prone to VT. Electrophysiologic studies are most sensitive in patients
with prior MI, but they may be less useful in other cardiac conditions.
Cardiac magnetic resonance imaging (MRI), which can directly image
cardiac function and cardiac scar or fibrosis, is showing great promise
as a more sensitive and specific, noninvasive risk predictor of SCD.
Ventricular Tachycardia and Ventricular Fibrillation
Without Evident Heart Disease
Ventricular arrhythmias occurring in the absence of structural heart
disease usually carry a benign prognosis but can be associated with
SCD in patients with genetic arrhythmic syndromes predisposing to
life-threatening polymorphic VT. Genetic screening for these syndromes is important to identify at-risk family members.
Idiopathic Ventricular Tachycardia
Idiopathic VT most commonly originates from the outflow tracts, with
approximately 80% localized to the RVOT and the remainder originating in the left ventricular outflow tract (LVOT), the aortic sinuses
of Valsalva, and the region of the aortomitral continuity. Idiopathic
RVOT VT manifests with the characteristic electrocardiographic
findings of left bundle branch block and inferior axis VT QRS morphology. Triggered activity is the mechanism underlying outflow tract
tachycardias. This calcium-dependent mechanism explains why an
outflow tract VT often terminates with adenosine, β-blockers, and calcium-channel blockers.
120 SECTION II Cardiovascular Disease
A
C D
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I aVR V1
II
III
V1
II
V5
I aVR V1
II
III
V1
II
V5
aVL
aVF
aVL
aVF
V4
V2
V3
V5
V6
I aVR V1
II
III
V1
II
V5
aVL
aVF
V2
V3
V4
V5
V6
B
V4
V2
V3
V5
V6
I aVR V1
II
III
V1
II
V5
aVL
aVF
V2
V3
V4
V5
V6
Fig. 9.10 Characteristic electrocardiograms associated with genetic disorders predisposing to SCD. (A) ARVC
ECG demonstrating inverted T waves V1-V3 during sinus rhythm. (B) Monomorphic ventricular tachycardia
with left bundle branch block morphology characteristic of ARVC. (C) Type I Brugada ECG pattern with coving
ST elevation and T inversion in V1-V2. (D) ECG from patient with hereditary LQT1, with mutation KCNQ1.
Patients in their third or fourth decade typically have palpitations,
shortness of breath, and lightheadedness at presentation. Reports of
cardiac arrest are rare, and treatment is directed at controlling symptoms. β-Blockers and calcium-channel blockers are often used initially,
although some patients require catheter ablation or antiarrhythmic
drug therapy. A subset of asymptomatic patients may develop tachycardia-mediated cardiomyopathy due to frequent ventricular ectopy. The
PVC burden posing the greatest risk for producing left ventricular dysfunction is likely more than 10,000 PVCs daily. Fortunately, PVC suppression with catheter ablation usually improves ventricular function.
ARVC patients develop ventricular arrhythmias with associated
symptoms, including palpitations, lightheadedness, syncope, and SCD.
Given the typical RV origin of arrhythmias in ARVC, the ventricular
arrhythmias have a left bundle branch morphology (Fig. 9.10B). The
surface ECG during sinus rhythm may demonstrate inverted T waves
in the V1 to V3 leads or epsilon waves, which are low-amplitude deflections at the end of the QRS complex in the right precordial leads resulting from slowed RV conduction (Fig. 9.10A).
Distinguishing ARVC from idiopathic RVOT VT is essential
because of the different prognostic and therapeutic implications of
the two diagnoses. The diagnosis of ARVC is established by the ARVC
Arrhythmogenic Right Ventricular Cardiomyopathy or Dysplasia
Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an
inherited cardiomyopathy with typically autosomal dominant transmission. It is associated with mutations affecting desmosomes, which
are molecular complexes of cell adhesion proteins that bind cardiac
myocytes. Although morphologic changes in the RV free wall predominate, biventricular or primary left ventricular variants occur. Due to
Task Force Criteria. Risk factors for SCD of ARVC patients include
prior aborted episodes of SCD, syncope, young age, LV dysfunction,
and markedly diminished RV function.
Patients with documented ARVC typically receive ICDs. Adjunctive
therapy with antiarrhythmic drugs or ablation, particularly strategies
incorporating combined epicardial and endocardial ablation, may be
useful in treating symptomatic VT.
myocyte death, large portions of the right ventricle are replaced with
adipose tissue, leading to wall motion abnormalities, cardiac dysfunction, and aneurysm formation. Structural changes spread from
the epicardium to the endocardium. RV imaging classically demonstrates RV enlargement with focal wall motion abnormalities and RV
hypokinesis. The RV free wall is not well imaged by routine cardiac
echocardiography, and MRI has become the gold standard for the
diagnosis of ARVC.
Congenital Long QT Syndrome
Congenital LQTS is a genetic disorder characterized by abnormal
cardiac repolarization producing QT prolongation on the ECG (corrected QT [QTc] >440 milliseconds in men and >460 milliseconds in
women) (Fig. 9.10D). It is a leading cause of SCD in the young.
Mutations in 16 genes that participate in cardiac repolarization have
been identified in patients with LQTS. Mutations of KCNQ1 (encodes
CHAPTER 9 Cardiac Arrhythmias
121
the α-subunit of the IKs potassium channel) produce LQT1; mutations
of KCNH2 (encodes the α-subunit of the IKr potassium channel) produce LQT2; and mutations of SCN5A (encodes the α-subunit of the
cardiac sodium channel) cause LQT3. Together, they account for 75%
of cases of congenital LQTS.
Decreased outward potassium currents or increased inward
sodium currents prolong action potential duration, predisposing to
early afterdepolarizations and TdP, a specific type of polymorphic VT.
Symptoms typically begin during adolescence and include syncope,
seizures, and SCD. The arrhythmia triggers in LQTS are gene specific.
Patients with LQT1 are at risk during high adrenergic states, such as
exercise; arrhythmias in LQT2 are triggered by sudden noises such as
alarms; and LQT3 patients are more likely to experience arrhythmias
during sleep. The autosomal dominant Romano-Ward variant has a
prevalence of 1 case in 2000 live births.
Chronic treatment is directed at prevention of SCD. Initial therapy
includes avoidance of QT-prolonging agents and initiation of β-block-
ers in symptomatic patients and asymptomatic patients with significant QT prolongation. ICDs are recommended after resuscitation
from a cardiac arrest and for recurrent syncope despite β-blockade.
The acute treatment of TdP is different from that of other forms of
VT because many antiarrhythmic agents prolong the QT interval and
should therefore be avoided.
Brugada Syndrome
The Brugada syndrome is a genetic disorder predisposing to polymorphic VT and SCD. The ECG characteristically displays coving ST elevation in the right precordial leads, V1 to V3, and a right bundle branch
block pattern (Fig. 9.10C). These electrocardiographic abnormalities
may be dynamic, and they are characteristically exacerbated by fever
and therapy that blocks sodium channels.
The syndrome is linked to mutations in SCN5A, which encodes the
cardiac sodium channel. Mutations result in a reduction in the sodium
current. The mode of transmission is autosomal dominant. Patients
typically have syncope or cardiac arrest, often occurring during sleep.
Although quinidine, by virtue of its ability to block transient outward potassium current (Ito), may have a therapeutic role, there are
no established medical therapies to prevent VT in Brugada syndrome.
Intravenous β-adrenergic stimulation with isoproterenol or a similar agent, by virtue of its ability to augment the sodium current, is
potentially useful in the acute management of recurrent VT or VF in
Brugada syndrome. Paradoxically, because of a protective effect of catecholamine stimulation, β-blockers are potentially harmful in patients
with Brugada syndrome and should be avoided.
ICDs represent the only proven therapy for prevention of cardiac
arrest. ICD therapy is recommended for secondary prevention of
SCD. For high-risk patients with a spontaneous Brugada electrocardiographic pattern and syncope, primary prevention with an ICD is
indicated.
Catecholaminergic Polymorphic Ventricular Tachycardia
CPVT is a genetic disorder that alters myocardial calcium handling,
resulting in exercise-induced polymorphic or bidirectional VT.
Exercise-triggered syncope or SCD during childhood is the common
presenting symptom. About 50% to 60% of patients have an inherited
or sporadic autosomal dominant mutation affecting the cardiac ryanodine receptor gene (RYR2), producing abnormal calcium-induced
calcium release from the sarcoplasmic reticulum and intracellular calcium overload.
β-Blockers along with exercise restriction represent the primary therapy, although arrhythmia breakthrough is common. ICD therapy may
be used for secondary prevention, although ICD shocks can produce
catecholamine surges that may exacerbate the underlying arrhythmia.
Left cardiac sympathetic denervation is useful in selected cases.
Acquired Long QT Syndrome
Environmental factors may prolong cardiac repolarization and produce
QTc prolongation, leading to the development of early afterdepolarizations and TdP. Patients with acquired LQTS may have background
genetics predisposing them to develop excessive QTc prolongation and
polymorphic VT in response to electrolyte abnormalities (i.e., hypokalemia, hypomagnesemia, and hypocalcemia), bradycardia, and the
use of QT-prolonging medications. Most QTc-prolonging drugs block
the rapid component of the delayed rectifier potassium channel (IKr)
encoded by the KCNE2 gene. Drugs known to prolong the QTc interval are updated on an Internet registry. Therapy for acquired LQTS
requires reversal of inciting physiologic factors and discontinuation of
offending medications.
Genetic Testing for Channelopathies
Commercial laboratories offer genetic testing for congenital LQTS,
Brugada syndrome, and CPVT. The yields of genetic testing vary from
25% for Brugada syndrome up to 80% for congenital LQTS. The limited sensitivity of current assays and the common finding of genetic
variants of unknown significance represent ongoing challenges.
Despite these considerations, cascade screening or screening of family
members for a disease-causing mutation once characterized in a proband has been effectively used to identify mutation carriers.
Mutation-positive family members may benefit from prophylactic
therapy. Reassurance for mutation-negative individuals is also valuable. Before ordering genetic testing, patients should be thoroughly
informed of the risks, benefits, and limitations of testing. Genetic
counselors ideally play an important advisory role.
For a deeper discussion on this topic, please see Chapter 59,
“Ventricular Arrhythmias,” in Goldman-Cecil Medicine, 26th Edition.
SUMMARY
Cardiac arrhythmias are caused by disorders of action potential formation or propagation and are broadly categorized as abnormally
slow rhythms (i.e., bradycardias) or abnormally rapid rhythms (i.e.,
tachycardias). The cardiac cellular action potential is composed of five
phases determined by the activity of multiple ion channels, including
the rapid sodium channel, several potassium channels, and a calcium
current. Disruptions of these currents may lead to abnormal automaticity and triggered activity, which may mediate pathologic tachyarrhythmias. Reentry is the dominant mechanism of clinically significant
tachyarrhythmias and requires a functional or fixed obstacle to propagation, an area of slowed conduction, and differential refractoriness for
initiation and perpetuation of the arrhythmia.
Antiarrhythmic drugs are commonly divided into four broad
groups using the Singh–Vaughan Williams classification. Despite its
clinical utility, many antiarrhythmic drugs have multiple effects and
do not fit neatly into this framework. Some, such as adenosine and
digoxin, fall completely outside of it. Class I drugs slow membrane
conduction by blockade of the sodium channel. Class II drugs, or
β-blockers, function by blockade of the cardiac β-receptor. Class III
drugs prolong repolarization and the QT interval. Class IV drugs block
the slow calcium channel and are primarily active in slow-response
myocytes such as the sinus and AV node.
All bradycardia is a consequence of impairment of sinus node
function or AV conduction, or both. Sinus and AV nodal function
is strongly influenced by autonomic tone. Parasympathetic tone
dominates at rest, and significant bradycardia and second-degree
122 SECTION II Cardiovascular Disease
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AV block may be observed in normal patients due to increased
parasympathetic tone, especially during sleep or athletic training.
Clinical sinus node dysfunction manifests as one of several syndromes, including sinus bradycardia, chronotropic incompetence,
exit block, and bradycardia-tachycardia syndrome due to sinus
pauses and bradycardia when concomitant atrial arrhythmias terminate to sinus rhythm.
AV conduction disturbances may occur at the AV nodal level or
infranodal level. A block at the level of the AV node tends to be indolent, characterized by gradual progression and competent subsidiary
escapes that usually protect the patient from catastrophic bradycardia.
This permits asymptomatic patients to be followed clinically for the
development of symptoms before intervention. In contrast, second- or
third-degree infranodal block at the His bundle, or more commonly
at the level of the bundle branches, is potentially malignant and is
often not accompanied by stable escape mechanisms. If not managed
appropriately, it can cause sudden death. Clues to an infranodal level
of block are Mobitz II periodicity, associated bundle branch block,
worsening heart block with tachycardia or exercise, and a wide QRS
escape rhythm different from the conducted QRS in the setting of a
high-degree or third-degree AV block.
Tachycardias are broadly categorized as SVTs, which depend on
the atrium and AV conduction system, and ventricular arrhythmias,
which depend on the ventricular myocardium. Supraventricular
arrhythmias are further categorized as PSVTs, which depend on AV
nodal conduction, and intra-atrial arrhythmias, which depend only on
atrial tissue and not on AV conduction. The PSVTs include AVNRT
and AV reciprocating tachycardia related to WPW syndrome. Intraatrial arrhythmias include organized atrial arrhythmias, such as focal
atrial tachycardia, atrial flutter, macro-reentrant atrial tachycardia, and
AF, a common disorganized atrial arrhythmia. Recurrent atrial flutter
and AF carry a risk of thromboembolism and, based on risk stratification, should be treated with antithrombotic therapy when appropriate. Catheter ablation has an important role in the management of all
supraventricular arrhythmias but remains a second-line strategy for
AF, for which success rates are lower and complication rates are higher
than for other supraventricular arrhythmias.
Ventricular arrhythmias include isolated ventricular premature beats; short, nonsustained runs of tachycardia; and sustained
ventricular arrhythmias. Sustained VT lasts more than 30 seconds or
requires intervention before then. It is classified as monomorphic if
beats all share a single electrocardiographic morphology, polymorphic
if the electrocardiographic morphology is variable, TdP when the morphology is variable and the arrhythmia is associated with pathologic
QT prolongation, and VF when the surface ECG continuously varies
without distinct QRS complexes. VT is poorly tolerated and is the
major cause of cardiac arrest. Although commonly seen in the setting
of ischemic heart disease, idiopathic VT may be seen in the absence of
structural heart disease.
Antiarrhythmic drugs have not been effective in reducing the risk
of SCD after MI. In contrast, ICDs have been shown to improve mortality rates for patients with impaired LV function after an MI and
patients with heart failure and impaired LV function with or without
coronary disease.
In addition to advanced structural heart disease as a cause for VT,
several syndromes may result in VT in the absence of evident structural
heart disease. They include the syndrome of idiopathic VT, ARVC,
arrhythmogenic RV dysplasia, congenital LQTS, Brugada syndrome,
and CPVT. Several of these conditions are familial, and genetic testing
and family screening have important roles in their management.
SUGGESTED READINGS
Al-Khatib SM, Stevenson WG, Ackerman MJ, et al.: 2017 AHA/ACC/HRS
guideline for management of patients with ventricular arrhythmias and the
prevention of sudden cardiac death, Circulation 138:e272–e391, 2018.
Calkins H, Hindricks G, Cappato R, et al.: 2017 HRS/EHRA/ECAS/APHRS/
SOLAECE expert consensus statement on catheter and surgical ablation of
atrial fibrillation, Heart Rhythm 14:e275–e444, 2017.
January CT, Wann LS, Calkins H, et al. AHA/ACC/HRS Focused Update of
the 2014 AHA/ACC/HRS Guideline for the Management of Patients With
Atrial Fibrillation. A Report of the American College of Cardiology/Amer-
ican Heart Association Task Force on Clinical Practice Guidelines and the
Heart Rhythm Society 2019:25873.
Priori SG, Wilde AA, Horie M, et al.: Executive summary: HRS/EHRA/APHRS
expert consensus statement on the diagnosis and management of patients
with inherited primary arrhythmia syndromes, Heart Rhythm 10:e85–e108,
2013.
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