The demonstration of alternating contractile state in pulsus alternans

Abstract

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. The contractile state of the strong beats was consistently greater than that of the weak beats when contractility was defined in terms of: (a) V_max (3.13 > 2.53 circ/sec, P < 0.01); and (b) the velocity of circumferential fiber shortening (0.84 > 0.39 circ/sec, P < 0.001) and developed tension (82.5 > 74 g/cm, P < 0.01) at isolength. The length-tension-velocity relationship of the left ventricle also varied between strong and weak beats when: (a) the maximum velocity of contractile element shortening at least common tension (1.68 > 1.28 circ/sec, P < 0.05); and (b) the velocity of circumferential fiber shortening (0.81 > 0.39 circ/sec, P < 0.001) at maximum developed tension were examined. Analysis of the length-tension-velocity characteristics of sequential beats at the onset of alternans in three dogs suggests that an alternation in contractility initiates alternans, with secondary alternations in ventricular filling. Cross-clamping of the aorta in three other dogs essentially eliminated the alternating changes in end-diastolic length and pressure, while the resultant isovolumic contractions continued to demonstrate clear evidence of pulsus alternans in the ventricular systolic pressure, suggesting the persistance of an alternating contractile state. The evidence suggests that an important mechanism in the production and propagation of pulsus alternans, as produced in the intact canine ventricle by rapid pacing, is a beat-to-beat alternation in contractile state with secondary alternations in ventricular filling.