Diastolic left ventricular function Experimental study of the early filling period using the Voigt model

Abstract

The left ventricular (LV) diastolic function is usually described by the diastolic part of the LV pressure-volume relationship (PVR). The mathematical analysis of the PVR provides parameters of diastolic LV as well as myocardial function. Up to now, all approaches assume the myocardium as a passive elastic medium. This implies that each increase in LV volume is associated with a corresponding increase in LV pressure. Obviously this does not agree with actual PVR's during the LV early filling phase, where a decrease of pressure (due to relaxation) can be observed. Therefore, we analysed the LV diastolic PVR using the Voigt model to provide a better understanding of the concomitant processes of relaxation and filling especially during the early diastole. LV volumes and pressures were measured in an experimental study (6 pigs) using biplane cineangiography (100 frames/s) and simultaneous pressure measurements (catheter tip manometer). LV volumes were determined from all cine frames of total cardiac cycles. Instantaneous (beat to beat) changes in duration (Deltat(f)) of the LV filling period were achieved by atrial stimulation and by releasing post-extrasystolic beats. A total of 26 PVR's with different Deltat(f) were evaluated. According to the Voigt model the compliance (product of the resting volume V(0) and the elastic coefficients) of the total muscle fibre (V(0).K(tot)) and of the serial elastic element (V(0).K(se)) were calculated (average: V(0).K(tot) = 3.43 ml/ mmHg; V(0).K(se) = 5.07 ml/mmHg; n = 26). The interindividual differences between the compliances were in some individuals significant (V(0).K(tot): p < 5% in 6 of 15 Wilcoxon tests ; V(0).K(se): p < 5% in 3 of 15 Wilcoxon tests) but the intraindividual range of these parameters (due to different Deltat(f)) were in some cases as pronounced as the interindividual differences. As in 22 of 26 measurements V(0).K(tot) is less than V(0).K(se) it must be concluded that the early diastolic PVR cannot be described by passive elasticity and the laws of elastomechanics only. These inconsistencies can be explained by the early diastolic LV suction (untwisting) as well as by recent findings regarding the titin filament. Both mechanisms augment LV diastolic filling even in unfavourable working conditions (shortened filling) such as exercise.