Rate dependent variation in concealed bigeminy

A 63 year old man with angina pectoris was found to have frequent extrasystoles separated by odd as well as even numbers of conducted sinus beats. Analysis of 935 conducted sinus beats showed that premature ventricular beats without interpolation were separated by odd numbers of conducted sinus beats in 117 sequences and even numbers (exceptions) in 48 sequences (p less than 2 X 10(-4)). Tabulation of cycle lengths revealed that cycles with even numbers of conducted sinus beats were characterized by a significant reduction in the preceding postextrasystolic pause (1441+/-76 msec vs. 1487 +/-59 msec; p less than .001) and second sinus cycle (720 +/- 44 msec vs. 750 +/- 38 msec; p less than .001). Premature ventricular beats were interpolated in 41 additional sequences. Interpolated extrasystoles were separated by the expected even numbers of conducted sinus beats in 39 cases and odd numbers (exceptions) in only two cases. The frequency of exceptions to the usual rules for concealed bigeminy was therefore 2/39 during interpolation and 48/117 without interpolation (p less than .01). This case demonstrates that: 1) a reduction in cycle length may be associated with exceptions to the usual rules for concealed bigeminy, and 2) the frequency of exceptions to concealed bigeminy may be altered by the presence of interpolation. Only one previous case has contained statistical documentation of these circumstances. The diagnosis of a concealed ventricular rhythm may be facilitated by careful analysis at multiple cycle lengths.