Mechanism of sustained mechanical alternans. Effect of variations in ventricular billing volume.

Abstract

This study investigations the phenomenon of sustained mechanical alternans (SMA) through the use of quantitative criteria. Discrete analysis is used to demonstrate that the hemodynamic variables during SMA are governed by a simple mathematical relation. The analysis shows that the value of the slope created by the two alternating beats on the stroke volume (SV)-end-diastolic volume (EDV) plane is gamma = (mu - 1) (1 + beta)/(mu - beta), where mu = SVs/SVw, and SVs and SVw denote the strong and weak beats, respectively, in the presence of one contractile state, and the beats associated with the higher and lower contractile states, respectively, in the presence of two alternating contractile states; beta = FVs/FVw, where FVs and FVw are the filling volumes after SVs and SVw, respectively. This equation is valid, whether SMA is exhibited in the presence of one or two contractile states and irrespective of the SV-EDV functional relation. Assuming constant afterload, a criterion based on this slope (gamma) is described to determine if SMA is caused by variations in EDV and FV. The slope of the SV-EDV curve in the presence of one contractile state (denoted as gamma) was determined directly (34 runs in eight dogs) by preventing FV in a beat, after a steady state, using a remote-controlled mitral valve. The slope gamma = 0.892 +/- 0.078 was found to agree with data in the literature. In 10 other dogs, mitral flow and aortic flow were measured in 55 series of SMA. In 51 series, gamma was greater than 1. Because the experimental slope in the presence of one contractile state, gamma, is smaller than or equal to 1, the possibility that one contractile state is involved in this series is rejected. On the other hand, when two contractile states are involved, the slope that the two successive beats create on the SV-EDV plane, gamma, is determined by connecting the two SV-EDV relations. This slope tends to be greater than 1. Thus, in these 51 series, SMA cannot be explained as a result of the Frank-Starling mechanism and variations in FV but as a result of two alternating contractile states. In the other four series, the value of gamma can be compatible either with the presence of one contractile state or with two alternating contractile states. This quantitative analysis enables the classification of the various types of SMA into subcategories with well-defined features. The quantitative analysis presented here shows that the common genesis of SMA is an alternating contractile state.