Role of myocardial restoring forces in the determination of early diastolic peak velocity of fibre lengthening in the conscious dog

Factors determining peak velocity of fibre lengthening (dL/dtmax) were examined in 12 conscious dogs previously instrumented with a left ventricular pressure micromanometer and ultrasonic crystals measuring internal left ventricular diameter and myocardial segment length. dL/dtmax was studied during control state, atrial pacing, volume loading and methoxamine injection. The behaviour of dL/dtmax was similar in diameters and in segments. A significant correlation was found during control in each dog between dL/dtmax and the extent of systolic shortening (delta L). The effect of filling pressure on dL/dtmax appeared by comparing moderate volume loading and maximal volume loading where deltaL were similar while dL/dtmax was increased by 7.1% in segments and 20.2% in diameters. However, the effect of filling pressure on dL/dtmax was less pronounced that the effect of delta L since, for matched delta L during control and during volume loading, dL/dtmax increased by 37.9% while a 25% increase of delta L increased dL/dtmax by 55.6%. Heart rate and afterload changes did not appear as influencing dL/dtmax for matched deltaL. In conclusion, although and increased filling pressure does increased significantly dL/dtmax, the extent of systolic shortening, and thus restoring forces, appear as the primary determinant of peak velocity of fibre lengthening in basal conditions.