Sinus and Escape Rhythms


Part II of this book deals with physiologic and abnormal cardiac rhythms. Systematically analyzing the cardiac rhythm from the electrocardiogram (ECG) allows you to address two key and interrelated sets of questions:

  • 1.

    What pacemaker is controlling the heartbeat? There are three major possibilities:

    • a.

      The sole pacemaker is the sinus (sinoatrial [SA]) node.

    • b.

      Sinus beats are present but interrupted by extra (ectopic) heartbeats. Ectopic beats, in turn, come in two general classes: (1) premature, occurring before the next sinus beat is due, or (2) escape, occurring after a relatively short or long pause.

    • c.

      The atria are under the control of one or more stimuli originating outside the side node. For example, loss of sinus control is the case in atrial fibrillation, atrial tachycardia (unifocal or multifocal), atrioventricular (AV) nodal reentrant tachycardia (AVNRT), atrioventricular reentrant (bypass-tract mediated) tachycardia (AVRT), ventricular tachycardia (with ventriculo-atrial conduction), or with an electronically paced rhythm ( Chapter 22 ).

  • 2.

    Next, you should ask: What, if any, is the signaling (communication link) between the sinus (or other supraventricular) pacemaker(s) and the ventricles? The physiologic situation (“normal sinus rhythm”) occurs when every sinus depolarization results in a ventricular beat, which requires timely conduction of the impulse through the atria, AV junction (AV node and His–bundle) and the bundle branches, into the ventricular myocardium.

This chapter focuses on sinus rhythm and its variants, as well as on escape or subsidiary pacemakers, those that act as “backup electrical generators” when the sinus node fails to fire in a timely way or when the sinus impulse is blocked from stimulating the surrounding atrial tissue. Subsequent chapters deal with premature beats and the major sustained ectopic rhythms, both supraventricular and ventricular, as well as with AV heart block and AV dissociation, and with preexcitation.

Projecting ahead to Chapter 17 , we will discuss one of the most extreme forms of disrupted AV signaling called complete (third-degree) heart block in which sinus rhythm may still control the atria but none of these sinus impulses traverses the AV junction to the ventricles. Instead, a subsidiary pacemaker, located in the AV junction or in the His–Purkinje–ventricular system, controls the ventricles.

After that, we discuss another type of AV conduction anomaly, one associated not with delays but with early or preexcitation of the ventricles, the substrate of the Wolff–Parkinson–White (WPW) patterns and related syndromes ( Chapter 18 ).

Keep in mind the underlying principle: all normal and abnormal cardiac electrical function is based on the key properties of automaticity (impulse formation) and conductivity (impulse propagation and recovery).

Sinus Rhythms

“Normal” Sinus Rhythm

Sinus rhythm is the primary physiologic mechanism of the heartbeat. You diagnose it by finding P waves with a polarity predictable from simple vector principles (see Chapter 5 ). When the sinus node paces the heart, atrial depolarization spreads from right to left and downward toward the AV junction. A single arrow (vector) representing the overall trajectory of this depolarization wavefront is directed downward and toward the (patient’s) left. Therefore with sinus rhythm, the P wave is always positive in lead II and negative in lead aVR ( Fig. 13.1 and see Fig. 5.3 ). The sinus P wave is usually also positive in leads I, aVF, and III. In addition, with sinus rhythm, the P wave is usually biphasic in V 1 , typically with small positive and negative components (<40 msec) and is positive in the lateral chest leads.

Fig. 13.1, Sinus rhythm with 1:1 AV conduction (“normal sinus rhythm”). The heart rate is about 80 beats/min. Each QRS complex is preceded by a P wave that is negative in lead aVR and positive in lead II. The sinus P wave in lead V 1 is usually biphasic with an initial positive component (right atrial activation) followed by a small negative component (left atrial activation).

Reminders

  • If you state that the rhythm is “normal sinus” and do not mention any AV node conduction abnormalities, listeners will reasonably assume that each P wave is followed by a QRS complex and vice versa. A more rigorous but physiologically unambiguous way of stating this finding would be “Sinus rhythm with 1:1 AV conduction.”

  • Strictly speaking, when you diagnose “sinus rhythm,” you are only describing the physiologic situation in which the sinus node is generating P waves (upright in lead II, negative in aVR). But the term sinus rhythm by itself says nothing about AV conduction. Sinus rhythm (i.e., activation of the atria from the SA node) can occur not only with normal (1:1) AV conduction but with any degree of AV heart block (including second- or third-degree), or even with ventricular tachycardia (when AV dissociation is present). In the most extreme case, a patient can have an intact sinus node consistently firing off impulses in the absence of any ventricular activation, leading to ventricular asystole and cardiac arrest (see discussion in the next section about vasovagal syncope; see also Chapter 21 ).

By convention, normal sinus rhythm in a resting or somewhat active subject is usually defined as sinus rhythm with normal (1:1) AV conduction and a normal PR interval at a heart rate between 50 to 60 and 100 beats/min. Sinus rhythm with a heart rate greater than 100 beats/min is termed sinus tachycardia ( Fig. 13.2 ). Sinus rhythm with a heart rate of less than 50 to 60 beats/min is termed sinus bradycardia ( Fig. 13.3 ). However, be aware that the “normal” adult heart rate is context dependent. For an endurance athlete at rest or during deep sleep, the physiologic sinus rate may be as slow as 30 beats/min, transiently. In contrast, a young adult’s heart rate during near maximal exercise may approach 200/min. Indeed, as discussed later in this chapter, if your sinus rate during vigorous exertion were within the usually quoted “normal” range of 60 to 100 beats/min, that finding would be distinctly abnormal.

Fig. 13.2, Sinus tachycardia. The heart rate is close to 150 beats/min. Note the positive (upright) P waves in lead II. There is nonspecific T wave flattening (see Chapter 11 ).

Fig. 13.3, Sinus bradycardia. Sinus rhythm is present but at a very slow rate of about 38 beats/min.

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here