RESUMO
The influence of acute volume changes during diastole on the contractile state of the left ventricle has been studied in the closed-chest dog. Volume changes were introduced by means of a servo-controlled pump system connected to the left ventricular cavity by an apical cannula. Pressure measurements were made in the left ventricle and aorta. Flow sensors in the mitral valve and around the ascending aorta monitored ventricular inflow and outflow patterns. The ventricular performance was evaluated in terms of the ratio between end-systolic pressure and end-systolic volume (P/Ves). By changing the time of occurrence of the volume interventions from the rapid filling phase of diastole to the atrial contraction phase, the relative contributions of rapid filling and atrial contraction to the mitral flow were changed. When the rapid filling was changed by the volume intervention, the effect on the contractile status of the heart, expressed as the P/Ves value, was small. In contrast, when the volume intervention took place during the atrial contraction phase, the effect on the P/Ves value was much larger. Comparison with muscle fiber experiments suggests that length-dependent calcium sensitivity of troponin and length-dependent conductivity of the sarcolemma are the underlying fundamental mechanisms. Therefore, we conclude that the influence of an intervention in ventricular filling on the inotropic state of the left ventricle is dependent on the timing of the intervention.
