By Ken Grauer, MD
Figure. Telemetry tracing interpreted as showing a bigeminal rhythm.
Clinical Scenario: The telemetry tracing shown in the Figure was interpreted as showing "bigeminy," in that every other beat was thought to be a PVC. Do you agree with this interpretation?
Interpretation: A bigeminal rhythm is one in which every other beat is ectopic, with a fixed relation to each preceding beat. This definition is satisfied in the Figure. However, it is unlikely that the bigeminal rhythm seen in this Figure reflects the presence of premature ventricular contractions (PVCs).
Although reference to the term "bigeminy" is most often associated with a sinus rhythm in which every other beat is a PVC, it is important to appreciate that there are other causes of this type of patterned beating. Thus, instead of ventricular bigeminy there may be atrial or junctional bigeminy if every other beat on the tracing is early and arises from a premature atrial contraction (PAC) or premature junctional contraction (PJC).
Rather than ventricular bigeminy, one should suspect that the rhythm in the Figure represents atrial bigeminy. The reason the QRS complex of early occurring beats looks different than the QRS of sinus-conducted beats is that each PAC in this tracing (i.e., beats 3, 5, 7, 9, 11, 13, and 15) is conducted with aberration, a consequence of PAC occurrence during the relative refractory period.
The most helpful criterion for distinguishing between PVCs and PACs that conduct with aberration is detecting of a premature P wave preceding the widened and abnormal appearing early QRS complex. Sometimes identification of this premature P wave is easy, as would be the case when there is an obvious spiked deflection deforming the T wave that precedes the anomalous beat. At other times, evidence of the "telltale" premature P wave is much more subtle. Such is the case in the Figure, in which the key clue to the etiology of this bigeminal rhythm lies with inspection of the T wave of beat No. 1 on the tracing. Note how the amplitude of this T wave is smaller than that of each of the sinus-conducted beats (i.e., beats 2, 4, 6, 8, 10, 12, and 14). We suspect this may be due to the hidden presence of a premature P wave in the T wave of each of the sinus beats. Further support for a supraventricular etiology for each bigeminal beat comes from the fact that although different in appearance, the QRS complex of bigeminal beats is only minimally widened.
As is often the case, certainty of diagnosis is lacking on the basis of a single rhythm strip. Additional rhythm strips and ideally a 12-lead ECG obtained during the bigeminal rhythm would be needed to confirm QRS width and morphology of the bigeminal beats. The slightly different appearance of the QRS complex of beat No. 1 (which is taller and lacking the small initial q wave of the sinus beats) raises other questions (junctional etiology for this beat?). Nevertheless, the key point to emphasize is that not all bigeminy is ventricular, and there are several indications on this tracing that the bigeminal beats seen here are most likely to be PACs with aberrant conduction rather than PVCs.
Dr. Grauer, Professor and Associate Director, Family Practice Residency Program, Department of Community Health and Family Medicine, College of Medicine, University of Florida, Gainesville, is on the Editorial Board of Emergency Medicine Alert.