Don't always trust that computer interpretation at the top of the ECG tracing! In this case, the ominous reading of second degree AV block, Mobitz II, is incorrect. A much more benign rhythm disturbance is evident: atrial bigeminy. Using a differential diagnostic approach to this rhythm, one can exclude other rhythms if the diagnosis of atrial bigeminy is not recognized immediately.
There are a number of irregular cardiac rhythms that characteristically occur at a normal rate; some are benign while others are not (see table). There are a variety of ways to categorize these rhythms: serious (e.g., second degree atrioventricular (AV( block) vs. harmless (e.g., normal sinus rhythm with premature complexes); regularly irregular vs. irregularly irregular; those with an underlying sinus rhythm vs. those rhythms that are not originating from the sinus node. It may be advisable to use a combination of these strategies when deciphering a rhythm after first eyeballing the tracing and focusing on its most distinguishing characteristic.
The most striking thing about this tracing is the periodicity of the QRS complexes (see figure 2). With the exception of the 10th QRS complex (a premature ventricular complex), the QRS complexes exhibit grouped beating, a sort of regularly irregular rhythm. This immediately calls to mind second degree AV block, a rhythm where some of the P waves are not followed by QRS complexes, resulting in pauses in the regularity of the tracing.
First and third degree AV block are generally regular rhythms, and do not feature grouped beating. Mobitz type I second degree AV block usually exhibits the Wenckebach phenomenon of progressively lengthening PR intervals until the dropped QRS complex results in a pause. The rarer and more ominous Mobitz II second degree AV block (which frequently progresses to complete heart block) features normal or slightly prolonged PR intervals until a ventricular complex is dropped.1
Close inspection of the P-QRS relationship on this tracing reveals two key things: All P waves have associated QRS complexes (therefore there is no AV block), and the P waves are not all of the same morphology. This is not second degree AV block, and the regular occurrence of sinus P waves (P wave #s 1, 3, 5, etc.) excludes other non-sinus rhythms (e.g., atrial flutter, atrial fibrillation). The relatively slow atrial rate excludes atrial tachycardia with variable block as well because this rhythm usually features atrial rates in the range of 150-250 bpm; indeed, atrial tachycardia bears a closer resemblance to atrial flutter.2
Continuing to focus on the regular irregularity of the rhythm, other rhythms in the differential diagnosis can be discarded. Sinus arrhythmia has a periodicity to it, yet the P waves do not vary. It is the P-P interval that does, by more than 0.16 sec.2 This is most commonly due to respiratory variation, especially in a younger person, so the periodicity seen in this figure would be too rapid to be tied to the respiratory cycle.
Sinoatrial (SA) block (also called sinus exit block) can be regularly or irregularly irregular. Like AV block, it has three degrees, but unlike its AV counterpart, only second degree SA block can be detected on the 12-lead ECG (i.e., without a direct recording of SA nodal activity). SA block differs from AV block in that SA block features a dropped P-QRS-T, whereas AV block features a dropped QRS-T. Our tracing seems to be regularly missing a P-QRS-T, so SA block is a possibility. However, there is more than one P wave morphology in the case ECG, and thus this is not SA block.
Finally, normal sinus rhythm with blocked, or nonconducted, premature atrial beats deserves mention because this dysrhythmia is the most common cause of an unexpected atrial pause.3 The pause results from the relatively early occurrence of the premature atrial contraction (PAC) during a time when the AV node is still refractory, thus preventing conduction of the impulse to the ventricles, resulting in a pause in QRS complexes. This rhythm is usually not so regularly irregular, however.
In this tracing (see figure), every other beat (P wave #s 2,4,6, etc.) features a P wave that is slightly different from the preceding one. Thus there seems to be the sinus focus, followed by premature atrial beats originating from some other focus or foci. When a PAC follows every sinus beat, the rhythm is termed atrial bigeminy; if every third beat is a PAC, the term is atrial trigeminy, and if every fourth beat is a PAC, the rhythm is atrial quadrigeminy. These are all regularly irregular rhythms.
Most emergency physicians are more familiar with the ventricular forms of bigeminy, trigeminy, and quadrigeminy, in part because the wide-QRS-complex premature ventricular contractions are easy to spot.
Atrial bigeminy, as a manifestation of PACs, is a harmless rhythm in the proper clinical context. Most people with PACs do not have organic heart disease, although PACs are more common in patients with heart disease than in those without. Patients should be questioned regarding caffeine intake, fatigue, smoking, alcohol use, and emotional stress, as these factors have been linked to the occurrence of PACs.2