Location of Conduction Pathways

The reentrant circuit of “typical” atrial flutter revolves around the tricuspid valve in the right atrium. Like reentrant supraventricular arrhythmias in general, it is initiated by a premature atrial complex (PAC) that blocks in one direction, while propagating in another. The signal then keeps circling in the same trajectory over and over again. In atrial flutter the typical frequency is about 300 cycles per minute (which is determined by the atrial size and conduction velocity) producing identical F waves.

The “bottleneck” where the signal usually blocks is the relatively narrow area between the tricuspid valve and inferior vena cava at the bottom of the right atrium (cavo-tricuspid isthmus). Conduction of the initiating PAC is blocked from propagating from the middle area of the right atrium to its lateral wall through this narrow isthmus. However, the electrical signal can propagate through the rest of the atrium and reach the isthmus from the other side. By this time the isthmus tissue has already recovered electrically and is ready to conduct. The signal goes through the isthmus in a counterclockwise direction, and the large (macro-) reentrant cycle starts again.

The classic “sawtooth” pattern of F waves that are predominantly negative in leads II, III, and aVF and positive in V₁ with a very regular ventricular (QRS) rate of about 150/min (functional 2:1 AV block) is suggestive of the counterclockwise (common) type of typical right atrial flutter (Fig. 15-2A). Less frequently the same circuit gets initiated in the opposite direction, producing “clockwise” flutter. The polarity of the F waves will then be reversed: positive in leads II, III, and aVF, and negative in lead V₁ (Fig. 15-2B). Clockwise and counterclockwise flutter can occur in the same patient and both are usually isthmus-dependent.^‡ Clinically, the development of atrial flutter most often indicates the presence of underlying structural/electrical atrial disease.

Figure 15-2 A, Typical atrial flutter most commonly involves a reentrant (“merry-go-round”-like) circuit in the right atrium, proceeding in a highly consistent, counterclockwise pathway. The cycle length (rotation time) is about 200 msec, corresponding to an atrial rate of 300/min. Note that the “sawtooth” flutter (F) waves (arrows) are negative in the inferior leads (II, III, and aVF) and V₆, but positive in V₁. In the absence of drugs or atrioventricular node disease, the ventricular response is often exactly half the atrial rate (i.e., 150 beats/min). B, With the “clockwise” variant, the flutter (F) waves are positive in the inferior leads and V₆, and negative in V₁. These variants have the same clinical implications.

Conduction to the Ventricles

The atrial rate during typical atrial flutter, as noted, is around 300 cycles/min (range usually between about 240 to 330 cycles/min). Slower rates can be due to drugs that slow atrial conduction. Fortunately, the AV node cannot conduct electrical signals at that rate to the ventricles—although a bypass tract in the Wolff-Parkinson-White (WPW) syndrome (see Chapter 12) can! Thus, with atrial flutter, physiologic AV block develops (usually with a 2:1 A/V ratio) (Figs. 15-2 and 15-3). In the presence of high vagal tone, AV nodal disease, or AV nodal blocking drugs (e.g., beta blockers, digoxin, and certain calcium channel antagonists) higher degrees of AV block can be seen, for example with a 4:1 conduction ratio (Figs. 15-3 and 15-4).

Figure 15-3 A, Notice the variable appearance of flutter waves in different leads. In lead I, the waves are barely apparent, whereas in leads II and III, the classic “sawtooth” waves appear. The ventricular rate is about 160 beats/min, and the flutter rate is about 320 beats/min; thus 2:1 AV conduction is present. B, Carotid sinus massage produces marked slowing of the ventricular rate by increasing vagal tone.

Often the AV nodal conduction shows more complex patterns and the degree of AV block varies in a periodic way, producing flutter/QRS ratios with repeating patterns (Fig. 15-4) of RR intervals (group beating). This phenomenon is believed to be due to multiple levels of block within the conduction system. Variable AV block may be due to other mechanisms (e.g., AV Wenckebach) and produce noninteger ratios of F waves to QRS complexes (Fig. 15-5).

Figure 15-4 Atrial flutter from different patients (A through E) showing variable patterns of conduction (block). As shown, the block may alternate between two values. In other cases it is more variable.

Figure 15-5 With atrial flutter, the ventricular response may be variable, but not always a simple fraction (½, ⅓, ¼) of the atrial rate. Even in these cases, the response usually shows some underlying patterns, in contrast to the random-appearing ventricular rate in atrial fibrillation.

On the other hand, atrial flutter with 1:1 AV conduction (Fig. 15-6), although uncommon, can occur in three major settings:

Figure 15-6 Atrial flutter with 2:1 AV conduction (A) compared with 1:1 (one-to-one) AV conduction (B) in the same patient. In the latter case, the flutter waves are hard to locate. Owing to the very rapid ventricular response (about 300 beats/min), atrial flutter is a medical emergency, often necessitating direct current (DC) cardioversion (see later discussion).

Atrial flutter with sustained 1:1 AV conduction represents an emergency situation, requiring consideration of immediate synchronized electrical cardioversion because of the dangerously rapid ventricular rate.