Load dependence of the single beat maximal pressure (stress)/volume ratios in humans
MR Starling,
DG Montgomery,
and
RA Walsh
University of Michigan, Department of Internal Medicine, Ann Arbor.
To determine whether the slopes of the single beat maximal pressure (stress)/volume ratios are sensitive to changes in loading conditions in humans, 16 patients without cardiac disease were studied with simultaneous micromanometer-determined left ventricular pressures and biplane contrast cineangiograms under control conditions and during methoxamine and nitroprusside infusions. Left ventricular volumes were calculated with use of a Simpson's rule algorithm, wall thickness was obtained iteratively, and both midwall circumferential and meridional stresses were computed frame by frame. The maximal pressure/volume and both circumferential and meridional maximal stress/volume ratios were calculated using a single beat from each loading condition assuming a zero volume-axis intercept. Mean left ventricular systolic pressure increased 47% during the methoxamine infusion and decreased 22% during the nitroprusside infusion compared with control (p less than 0.001 for both). Despite these changes in left ventricular systolic pressure, heart rate was eliminated as a confounding variable by right atrial pacing; and mean maximal rate of change of left ventricular pressure [(+)dP/dtmax] and rate of change at developed pressure 40 mm Hg [(+)(dP/dt) per DP40] values did not differ significantly. Mean single beat maximal pressure/volume ratios also did not differ significantly among the three loading conditions. In contrast, mean single beat circumferential and meridional maximal stress/volume ratios were 3.15 +/- 1.83 and 1.40 +/- 0.82 g/cm2 per ml at control; they increased to 4.47 +/- 2.44 and 2.21 +/- 1.25 g/cm2 per ml during the methoxamine infusion (p less than 0.001 for both), and they decreased during the nitroprusside infusion to 2.58 +/- 1.47 and 1.14 +/- 0.57 g/cm2 per ml (p less than 0.05 and p less than 0.01, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)
