Lesson topic №25. Нарушения ритма (Cardiac arrhythmia in the outpatient setting)
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Supraventricular tachycardia: atrioventricular nodal re-entrant tachycardia
AVNRT that results due to the presence of a re-entry circuit within or adjacent to the AV node.
Prototype of dual AV nodal pathways physiology in sinus rhythm, with a premature atrial complex (PAC) which trigger a typical “slow–fast”
AVNRT.
Supraventricular tachycardia: atrioventricular nodal re-entrant tachycardia
Typically, AVNRT is a narrow complex tachycardia, i.e. QRS duration <120 ms, unless there is aberrant conduction, which is usually of the RBBB type, or a previous conduction defect exists.
In the typical form of AVNRT (also called slow–fast AVNRT), retrograde P waves are constantly related to the QRS and, in the majority of cases, are indiscernible or very close to the QRS complex. Thus, P waves are either masked by the QRS complex or seen as a small terminal P' wave that is not present during sinus rhythm.
In the atypical form of AVNRT, P waves are clearly visible before the QRS, i.e. RP>PR, denoting a long RP tachycardia, and are negative or shallow in leads II, III, aVF, and V6, but positive in V1.
Supraventricular tachycardia: atrioventricular nodal re-entrant tachycardia
Atrioventricular nodal re-entrant tachycardia. (A) Typical atrioventricular nodal re-entrant
tachycardia. |
(B) |
Atypical |
atrioventricular nodal re-entrant |
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tachycardia. (C) Atypical AVNRT |
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with (unusual) left bundle branch |
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block aberration. Retrograde P |
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waves are indicated by arrows. |
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Supraventricular tachycardia: atrioventricular nodal re-entrant tachycardia
Synchronize cardioversion is recommended for haemodynamically unstable patients
Vagal manoeuvres, preferably in the supine position with leg elevation, are recommended.
Adenosine is recommended if vagal manoeuvres fail.
Verapamil or diltiazem i.v. should be considered if vagal manoeuvres and adenosine fail.
Beta-blockers (i.v. esmolol or metoprolol) should be considered if vagal manoeuvres and adenosine fail.
Synchronized cardioversion is recommended when drug therapy fails.
Supraventricular tachycardia: atrioventricular nodal re-entrant tachycardia
Сatheter ablation for AVNRT is the current treatment of choice for symptomatic patients because it substantially improves quality of lif and reduces costs.
Patients with minimal symptoms and short-lived, infrequent episodes of tachycardia can be followed-up without the need for ablation or long-term pharmacological therapy.
Chronic administration of antiarrhythmic drugs decreases the frequency and duration of AVNRT (diltiazem or verapamil or beta-blocker or diltiazem plus beta-blocker).
In view of the excellent success rate and minimal risk of catheter ablation in symptomatic cases, the value of long-term antiarrhythmic drug therapy seems very limited.
Supraventricular tachycardia: atrioventricular re-entrant tachycardia
AVRTs use an anatomically defined re-entrant circuit that consists of two limbs: first, the AVN–HPS (Atrioventricular node - His–Purkinje system), and second, an AP (accessory pathway) also called the bypass tract. The two limbs are characterized by differences in refractoriness and conduction times, with critically timed premature atrial or ventricular beats initiating re-entrant tachycardia.
APs are single or multiple strands of myocardial cells that bypass the physiological conduction system, and directly connect atrial and ventricular myocardium. These AV connections are due to incomplete embryological development of the AV annuli, without complete separation of the atria and ventricles.
AVRT is the most common tachycardia associated with APs. Two mechanisms of re-entry are possible according to the antegrade or retrograde conduction over the AVN–HPS, and are classified as orthodromic and antidromic AVRT.
Supraventricular tachycardia: Wolff– Parkinson–White syndrome
WPW syndrome refers to the presence of an overt (manifest) accessory pathway, thus resulting in the so-called pre-excitation, in combination with usually recurrent tachyarrhythmias.
During sinus rhythm, a typical pattern in the resting ECG with the following characteristics is present:
1.a short PR interval (≤120 ms);
2.slurred upstroke (or downstroke) of the QRS complex (‘delta wave’);
3.a wide QRS complex (>120 ms).
Supraventricular tachycardia: orthodromic atrioventricular re-entrant tachycardia
Orthodromic AVRT accounts for >90% of AVRTs and for 20 − 30% of all sustained SVTs.
The re-entrant impulse conducts from the atrium to the ventricle through the AVN– HPS, which is the anterograde limb of the re-entrant circuit, whereas the accessory pathway conducts from the ventricle to the atrium, and serves as the retrograde limb of the re-entrant circuit.
Supraventricular tachycardia: orthodromic atrioventricular re-entrant tachycardia
During tachycardia, the following ECG features can be present:
1. RP interval constant and, usually but not invariably, up to one-half of the tachycardia сycle length;
2.
3.
narrow QRS;
functional BBB usually associated with an accessory pathway ipsilateral to the blocked bundle, especially in young patients (aged <40 years); and
4. ST-segment depression.
Supraventricular tachycardia: antidromic atrioventricular re-entrant tachycardia
The re-entrant impulse travels from the atrium to the ventricle through the accessory pathway with anterograde conduction; meanwhile, retrograde conduction occurs over the AVN or another accessory pathway.