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

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Abstract
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 (ΔL). The effect of filling pressure on dL/dtmax appeared by comparing moderate volume loading and maximal volume loading where ΔL 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 than the effect of ΔL since, for matched ΔL during control and during volume loading, dL/dtmax increased by 37.9% while a 25% increase of ΔL increased dL/dtmax by 55.6%. Heart rate and afterload changes did not appear as influencing dL/dtmax for matched ΔL. In conclusion, although an increased filling pressure does increase 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.