When does relaxation actually begin? The early phases of relaxation.

Exact timing of relaxation within the cardiac cycle was carried out by aid of polygrams consisting of epicardial segmental length curves, contractile force, left ventricular, left atrial and aortic pressures, indirect carotid curves, apex cardiogram, their derivatives and ECG reference tracings in the open-chest dog heart. The relaxation starts in late systole before aortic valve closure, at the nadir of the epicardial segmental length curve and at the climax of the contractile force curve. Onset of relaxation is indicated by the late systolic knee of the left ventricular pressure curve and, by the apex cardiogram as well as by the start of the downstroke of the negative phase on their first derivatives. The earliest phase of relaxation, from its beginning to the isovolumic relaxation period (IVRP), is proposed to be named "auxobaric relaxation period" (ABRP). Considerable lengthening was found during both ABRP and IVRP. This lengthening may provide an explanation for pre-inflow volume increments. The higher the frequency the longer the ABRP and the shorter the IVRP. In the more severe form of ischemia there is a sudden lengthening during ABRP and IVRP (late systolic bulging) whereas in the less severe form only onset of the relaxation is postponed within the cardiac cycle. The data suggest that ischemia leads to alterations in the early phase of relaxation even before involvement of the contraction phase (hypokinesis, akinesis, dyskinesis).