Chapter 13 Sinus and Escape Rhythms
Part II of this book deals with cardiac rhythms in health and disease. A key set of questions asks the following: What pacemaker(s) is (are) controlling the heartbeat? Is the controller entirely in the sinus node (the normal pacemaker)? Or is sinus rhythm present, accompanied by extra (ectopic) heartbeats? Ectopic beats come in two general classes: (1) they may be premature, occurring before the next sinus beat is due, or (2) they may come after a pause or delay, so-called escape beats.
This chapter discusses sinus rhythms and on certain escape or subsidiary pacemakers that act as “backup electrical generators.” Subsequent chapters deal with premature beats and major ectopic rhythms, both supraventricular and ventricular, as well as with the major forms of atrioventricular (AV) heart block and AV dissociation. In complete AV block, sinus rhythm may control the atria while the ventricles are controlled by a lower pacemaker, in the AV junction or in the His-Purkinje-ventricular system.
Sinus rhythm is the primary physiologic mechanism of the heartbeat. You diagnose it by finding P waves with a predictable polarity (see Chapter 4). When the sinus (also called the sinoatrial or SA) node is pacing the heart, atrial depolarization spreads from right to left and downward toward the AV junction. An arrow representing this depolarization wave points downward and toward the (patient’s) left. Therefore, with normal sinus rhythm, the P wave is always positive in lead II and negative in lead aVR (see Figs. 4-3 and 13-1).
Figure 13-1 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 P wave in lead V1 is usually biphasic with an initial positive component (right atrial activation) followed by a small negative component (left atrial activation).
If you state that the rhythm is “normal sinus” and do not mention any AV node conduction abnormalities, listeners will assume that each P wave is followed by a QRS complex and vice versa. The more technical and physiologically unambiguous way of stating this finding is to say: “Sinus rhythm with 1:1 AV conduction.”
However, do not forget that sinus rhythm (i.e., the sinus node is the dominant or sole pacemaker of the atria) can exist not only with normal (1:1) AV conduction but with any degree of AV heart block (including complete or third degree) or even with ventricular tachycardia or asystole.
By convention, normal sinus rhythm in a resting subject is usually defined as sinus rhythm with a heart rate between 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 60 beats/min is called sinus bradycardia (Fig. 13-3). Some authors define sinus bradycardia based on a heart rate of less than 50 beats/min.
The heart, like other organs, has a special nerve supply from the autonomic nervous system, which controls involuntary muscle action. The autonomic nerve supply to the heart (in particular, the SA and AV nodes) consists of fibers with opposing effects: the sympathetic nerves and the parasympathetic nerves. Sympathetic stimulation increases the heart rate and the strength of myocardial contraction. Sympathetic stimulation also occurs by secretion of circulating hormones called catecholamines (especially, norepinephrine and epinephrine), produced by the adrenal glands.
Parasympathetic stimulation (from the vagus nerve) produces slowing of the sinus rate as well as increased conduction time through the AV nodal area. It can also cause a pacemaker “shift” from the SA node to the low right atrial area producing so-called low atrial rhythm with negative P waves in leads II, III, and aVF (Fig. 13-4).
Figure 13-4 Notice the change in P wave polarity from positive to negative in lead II. This shift from sinus bradycardia here to an ectopic (low) atrial escape rhythm may occur as a normal (physiologic) variant, especially with high vagal tone, or in a variety of pathologic settings.
In this way the autonomic nervous system exerts a counterbalancing control of the heart rate. The sympathetic nervous system acts as a cardiac accelerator, whereas the parasympathetic (vagal) stimulation produces a braking effect. For example, when you become excited or upset, or are exercising, increased sympathetic stimuli (and diminished parasympathetic tone) result in an increased heart rate and increased contractility, producing the familiar sensation of a pounding sensation in the chest (palpitations).
Note that the sensation of “palpitations” may be associated with an entirely normal heartbeat, with isolated premature beats (atrial or ventricular), or, more seriously, with an actual run of ectopic (nonsinus) heartbeats (e.g., from atrial fibrillation, paroxysmal supraventricular tachycardia, or ventricular tachycardia).
Sinus tachycardia is sinus rhythm with a heart rate exceeding 100 beats/min. In adults the heart rate with sinus tachycardia is generally between 100 and 180 beats/min. Even faster rates, transiently up to 200 beats/min or so, can be observed in healthy young adults during maximal exercise.
Aging decreases the capacity to generate very rapid sinus rates. Elderly individuals (especially those older than 70 years) rarely show sinus tachycardia at rates above 140 to 150 beats/min even during maximal exertion. Indeed, heart rates above this range in the elderly, especially at rest, usually indicate the presence of a nonsinus tachycardia (e.g., atrial fibrillation or flutter, or a paroxysmal supraventricular tachycardia).
Figure 13-2 shows an example of sinus tachycardia. Each sinus P wave is followed by a QRS complex, indicating sinus rhythm with 1:1 AV conduction. Sinus tachycardia (or bradycardia), however, can occur with any degree of AV block. Notice that the P waves are positive in lead II. With sinus tachycardia at very fast rates, the P wave may merge with the preceding T wave and become difficult to distinguish.