ECG tutorial: Atrial and atrioventricular nodal (supraventricular) arrhythmias

ESCAPE ATRIAL BEATS

Escape atrial beats or rhythm may occur after a long sinus pause, usually resulting from sinus node exit block or sinus node arrest (figure 1). (See "Sinoatrial nodal pause; arrest; and exit block".) If the pause is long enough, there will be an escape atrial rhythm at a rate correlating with the intrinsic automaticity of the atrial focus. This may be a single atrial beat, multiple atrial complexes, or a sustained atrial rhythm due to an accelerated or ectopic pacemaker.

The rate of the atrial rhythm is slower than that of the sinus node and the P wave morphology differs from that of the sinus P wave. Since the atrial focus is often within the atrium, close to the atrioventricular (AV) node, the time for atrial conduction to the AV node is decreased and the PR interval may be shorter than that seen during sinus rhythm. The P wave morphology depends upon the location of the ectopic atrial focus.

ECTOPIC ATRIAL RHYTHM

Ectopic atrial rhythm occurs when the dominant pacemaker is an ectopic focus in the atrium (figure 2). This may result from sinus node failure and the development of an escape atrial rhythm (generally at a rate of 30 to 60 beats per minute). Another cause is the acceleration of an ectopic atrial focus, as with sympathetic nervous system activation. If the rate of this focus exceeds that of the sinus node, an atrial rhythm will be present at a rate faster than the intrinsic sinus rate. In such cases, sinus node impulse generation is suppressed.

The direction of atrial activation may be altered when an atrial rhythm is present since the pacemaker focus is within the atrial myocardium. The P wave is often short in duration and small in amplitude. Its axis depends upon the location of the ectopic pacemaker and the vector resulting from the direction of atrial activation. The P wave is inverted due to retrograde atrial activation when the pacemaker focus is in the low atrium, the area of the coronary sinus or the left atrium (left atrial rhythm). This is often referred to as a "coronary sinus rhythm." The QRS complexes of an atrial tachycardia resemble those seen during sinus rhythm since myocardial activation is via the His Purkinje system.

ATRIAL TACHYCARDIA

Atrial tachycardia with 1:1 conduction is a supraventricular tachyarrhythmia that has a rate of 140 to 220 beats per minute (figure 3). The QRS complexes occur at regular intervals (there is constant RR cycle length), and there is a P wave with a uniform morphology and the same PR interval. The baseline between successive P waves is flat and isoelectric on the electrocardiogram. The QRS complexes are similar to those seen during sinus rhythm since activation of the ventricular myocardium is unaltered and is via the His Purkinje system. Commonly, there is a warm up phase at the onset of the tachycardia, during which gradual rate acceleration or progressive shortening of the RR cycle length between the first several beats occurs.