Supraventricular rhythms appear on an electrocardiogram as narrow complex rhythms, which may be regular or irregular. They may have a normal rate, tachycardia, or bradycardia depending on the underlying arrhythmia mechanism and presence of atrioventricular nodal block. Bundle branch blocks may be present, either at baseline or due to rate-related aberrancy, which can make the QRS complex wide, though they are typically narrow complex. When evaluating the rhythm, the most important step is to evaluate for the presence of P waves, and determine if the morphology, axis, and duration matches the normal sinus rhythm P wave.
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 (waveform 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 (since it is an escape rhythm) and the P wave morphology differs from that of the sinus P wave, depending 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 (waveform 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) or 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 morphology, axis, and duration vary based on the location the signal originates within the atrium. The QRS complexes of an ectopic atrial rhythm resemble those seen during sinus rhythm since myocardial activation is via the His Purkinje system. However, since atrial activation is abnormal and no longer via the normal intra-atrial pathways, right and left atrial abnormalities (hypertrophy or conduction abnormality) cannot be reliably diagnosed.