3

Interpreting a rhythm strip

Just the facts

In this chapter, you’ll learn:

the components of an ECG complex and their significance and variations

techniques for calculating the rate and rhythm of an ECG recording

the step-by-step approach to ECG interpretation

properties of normal sinus rhythm.

A look at an ECG complex

An ECG complex represents the electrical events occurring in one cardiac cycle. A complex consists of five waveforms labeled with the letters P, Q, R, S, and T. The middle three letters —Q, R, and S—are referred to as a unit, the QRS complex. ECG tracings represent the conduction of electrical impulses from the atria to the ventricles. (See Normal ECG, page 44.)

The P wave

The P wave is the first component of a normal ECG waveform. It represents atrial depolarization—conduction of an electrical

impulse through the atria. When evaluating a P wave, look closely at its characteristics, especially its location, configuration, and deflection. A normal P wave has the following characteristics:

•location—precedes the QRS complex

•amplitude—2 to 3 mm high

•duration—0.06 to 0.12 second

•configuration—usually rounded and upright

INTERPRETING A RHYTHM STRIP

44

• deflection—positive or upright in leads I, II, aVF, and V2 to V6; usually positive but variable in leads III and aVL; negative or inverted in lead aVR; biphasic or variable in lead V1.

If the deflection and configuration of a P wave are normal— for example, if the P wave is upright in lead II and is rounded and smooth—and if the P wave precedes each QRS complex, you can assume that this electrical impulse originated in the sinoatrial (SA) node. The atria start to contract partway through the P wave, but you won’t see this on the ECG. Remember, the ECG records electrical activity only, not mechanical activity or contraction.

Normal ECG

This strip shows the components of a normal ECG waveform.

ECG tracings represent the conduction of electrical impulses from the atria to the ventricles.

The odd Ps

Peaked, notched, or enlarged P waves may represent atrial hypertrophy or enlargement associated with chronic obstructive pulmonary disease, pulmonary emboli, valvular disease, or heart failure. Inverted P waves may signify retrograde or reverse conduction from the atrioventricular (AV) junction toward the atria. When an upright sinus P wave becomes inverted, consider retrograde or reverse conduction as possible conditions.

Varying P waves indicate that the impulse may be coming from different sites, as with a wandering pacemaker rhythm, irritable atrial tissue, or damage near the SA node. Absent P waves may signify conduction by a route other than the SA node, as with a junctional or atrial fibrillation rhythm.

The PR interval

The PR interval tracks the atrial impulse from the atria through the AV node, bundle of His, and right and left bundle branches. When evaluating a PR interval, look especially at its duration.

Changes in the PR interval indicate an altered impulse formation or a conduction delay, as seen in AV block. A normal PR interval has the following characteristics (amplitude, configuration, and deflection aren’t measured):

•location—from the beginning of the P wave to the beginning of the QRS complex

•duration—0.12 to 0.20 second.

These characteristics are different for pediatric patients. (See

Pediatric rates and intervals, page 46.)

The short and long of it

Short PR intervals (less than 0.12 second) indicate that the impulse originated somewhere other than the SA node. This variation is associated with junctional arrhythmias and preexcitation syndromes. Prolonged PR intervals (greater than 0.20 second) may represent a conduction delay through the atria or AV junction due to digoxin toxicity or heart block—slowing related to ischemia or conduction tissue disease.

The QRS complex

The QRS complex follows the P wave and represents depolarization of the ventricles. Immediately after the ventricles depolarize, as represented by the QRS complex, they contract. That contraction ejects blood from the ventricles and pumps it through the arteries, creating a pulse.

INTERPRETING A RHYTHM STRIP

46

Ages and stages

Pediatric rates and intervals

The hearts of infants and children beat faster than those of adults because children have smaller ventricular size and higher metabolic needs. The fast heart rate and small size produces short PR intervals and QRS intervals.

Not necessarily mechanical

Whenever you’re monitoring cardiac rhythm, remember that the waveform you see represents the heart’s electrical activity only. It doesn’t guarantee a mechanical contraction of the heart and a subsequent pulse. The contraction could be weak, as happens with premature ventricular contractions, or absent, as happens with pulseless electrical activity. So before you treat the strip, check the patient.

It’s all normal

Pay special attention to the duration and configuration when evaluating a QRS complex. A normal complex has the following characteristics:

•location—follows the PR interval

•amplitude—5 to 30 mm high but differs for each lead used

•duration—0.06 to 0.10 second, or half of the PR interval. Duration is measured from the beginning of the Q wave to the end of