Pulsus alternans was induced in 11 anesthetized, open-chest dogs by rapid atrial pacing, and the left ventricular filling characteristics and length-tension-velocity relationship of alternating beats were compared. The end-diastolic circumferences (cire) of the strong beats were slightly, but significantly, increased over the weak beats (7.3 > 6.9 cm, P < 0.01), confirming that diastolic filling does alternate in pulsus alternans. This alternation in initial fiber length seemed to result from an alternation in the prior end-systolic length, rather than from an alternation in diastolic filling time or compliance. There was also no difference in end-diastolic tension as measured by an isometric strain gauge suggesting no difference in contractile element relaxation before weak and strong beats.
R. Joe Noble, Donald O. Nutter