In sinus tachycardia, the rhythm is regular. As already mentioned, there can be some minor variations in the RR interval with any sinus rhythm, but in general the RR interval is quite constant in sinus tachycardia. Each QRS is preceded by a P wave, which is usually but not always obvious in each lead (Fig. 7.3). The atrial rate with sinus tachycardia is 101 to about 190. It is impossible to make a normal heart go as fast as 250 beats per minute with sinus tachycardia. The ventricular rate in sinus tachycardia is the same as the atrial rate. The atrial rate limitation can be exceeded by atrial pacing, but at rates greater than about 200 beats per minute the normal atrioventricular (AV) node fails to conduct all of the supraventricular beats, and a pattern of AV block appears, usually Mobitz I (see Chap. 4). Thus, the AV node imposes a limit to the ventricular rate even with supraphysiological atrial rates.

Atrial fibrillation is characterized by an irregularly irregular ventricular rhythm and a “wavy” baseline as a result of the chaotic, disorganized atrial activity. There is generally no pattern whatever to the sequence of ventricular (QRS) complexes in atrial fibrillation (Fig. 7.4). Sometimes there can be brief periods of regular or nearly regular ventricular depolarizations, but such regularity is short lived and may not be quite as regular as it may initially appear on casual inspection after when one takes a moment to carefully measure the RR intervals (Fig. 7.5).

Rarely, in patients with a history of atrial fibrillation who have received excessive digitalis (“digitalis intoxication”), one can observe a regular rhythm. That regularization of rhythm as a manifestation of digitalis toxicity will be further described later in this chapter since it actually represents a different arrhythmia. However, these conditions of regularity associated with atrial fibrillation are rare, and atrial fibrillation is almost always irregularly irregular. In fact, any irregularly irregular rhythm should raise the likely possibility of atrial fibrillation until further evaluation proves otherwise. The presence of the small fibrillatory waves (“f” waves) serves to confirm the diagnosis. When the fibrillatory waves are large, the term “coarse” atrial fibrillation is sometimes used, as opposed to “fine” atrial fibrillation where the fibrillatory waves are small and nearly inapparent.

The atrial rate with atrial fibrillation is very fast, reaching 400–800 beats per minute. The ventricular rate is limited to about 200, just as with atrial pacing, since the normal AV node cannot conduct more rapidly than that. In the uncommon circumstance of an accessory pathway that bypasses the AV node, as with Wolff–Parkinson–White (WPW) or other preexcitation syndromes, the conduction can be much faster and may greatly exceed 200 beats per minute (Fig. 7.6). Therefore, in patients with preexcitation syndromes, the development of atrial fibrillation can be a catastrophic event since cardiac output falls greatly with rapid heart rates because ventricular filling time is severely shortened. Occasionally atrial fibrillation is present with a normal or slower-than-normal ventricular heart rate (Fig. 7.7). This usually occurs in patients on a medication that reduces AV conduction (e.g., verapamil) or with intrinsic AV node disease.

It is interesting that the atrial rate with atrial flutter is usually within the fairly narrow range of about 300 ± 30 beats per minute, even though the rate can range between 250 and 400 beats per minute. The ventricular rate is usually about 150, since there is generally a fixed 2 to 1 block of conduction at the AV node when the atrial rate is so fast. The distinguishing flutter (F) waves in the classical “saw-tooth pattern” are often but not always recognizable on a rhythm strip, but generally are demonstrable on a 12-lead tracing (Fig. 7.8). Sometimes there is a greater degree of AV block, but this is not common and usually occurs, as with low rates in atrial fibrillation, if the patient is on a medication that increases AV block (e.g., verapamil) or the patient has intrinsic AV node disease (Fig. 7.9). Occasionally, the AV block with atrial flutter is variable, and this leads to an irregular ventricular rate. Any supraventricular tachycardia with a regular ventricular rate of 150 beats per minute should raise the possibility of atrial flutter and this should be the working diagnosis until further examination of the tracing confirms some other arrhythmia [1]. At least some cases of atrial flutter involve a large reentrant circuit involving the right atrium [2].

The atrial rate for PAT is usually 115–240 with an equal ventricular rate (1:1 AV conduction). If both the sino-atrial (SA) node and the AV node have rate limits of about 200 beats per minute, how is it possible that PAT achieves a rate over 200 beats per minute? First, PAT usually does not involve the SA node, and so the rate limitation imposed by the SA node is avoided. That also explains why the atrial depolarization with PAT usually causes abnormal P waves or no apparent P waves at all (hidden by QRS and/or T waves). Second, PAT is usually due to a reentry mechanism that involves the AV node. When electrophysiological testing is done and confirms that the reentry involves the AV node, it is properly called AV nodal reentry tachycardia, or AVNRT. When electrophysiologic studies show that the reentry does not include the AV node, it is called AV reciprocating tachycardia, or AVRT. Because at least part of the AV node (the usual bottleneck of conduction into the ventricle) is bypassed, the ventricular rate can reach 240 beats per minute. Whenever there is a supraventricular tachycardia with a ventricular rate greater than 200, the first thing that should come to mind is PAT. In fact, it is not likely to be anything else. Rarely, one might see a brief burst of atrial fibrillation that will go 200 beats per minute or so, but with rates around 240 the rhythm is usually PAT. The only exception to this dictum is the situation where there is an accessory pathway bypassing the AV node, in which case atrial flutter or fibrillation may achieve rates as high or higher than PAT (see Fig. 7.6).

At this point it is appropriate to elaborate on WPW syndrome, the classic preexcitation syndrome. Accessory pathways between the atria and the ventricles are typical for preexcitation syndromes, and in WPW an accessory pathway is responsible for the short PR interval (because the impulse gets into the ventricle faster than going through the AV node) and the pathognomonic “delta wave” (from slightly aberrant initial depolarization of the ventricle) (Fig. 7.10). WPW syndrome is not an arrhythmia—it is a syndrome that predisposes to PAT because the accessory pathway can become part of a conduction circuit. WPW can be Type A (tall R wave in V₁) or Type B (negative QRS in V₁).

In the presence of digitalis toxicity, PAT may develop with a 2:1 or greater block, and the rhythm may still be regular (Fig. 7.11). When digitalis toxicity occurs in a patient previously in atrial fibrillation, the irregular rhythm of atrial fibrillation may be replaced by a regular PAT in fixed block. Without an EKG, this situation could be mistaken on physical examination for normal sinus rhythm. Therefore, when a patient previously in atrial fibrillation treated with digitalis presents with a regular rhythm, one must obtain a tracing to determine if the rhythm is really sinus rhythm or is PAT or atrial flutter with fixed block. On the other hand, PAT may rarely occur with variable block, and without an EKG it would be indistinguishable from atrial fibrillation. While PAT with block has classically been observed in digitalis intoxication, it may also occur in other settings (Fig. 7.12).

MAT is an uncommon supraventricular tachycardia that is usually irregular. The diagnosis of MAT requires that the patient has a supraventricular tachycardia with at least three different configurations of P waves and varying PR intervals (Fig. 7.13). This means that there are at least three separate foci of supraventricular activation. It is called MAT when the rate is over 100 beats per minute. When the rate is normal and there are three or more supraventricular foci of activation, the rhythm is termed “wandering atrial pacemaker.” Wandering atrial pacemaker, in a sense, is slow MAT, or MAT is wandering atrial pacemaker with a rapid rate.