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The contractile state of the intact canine left ventricle was investigated by determining the instantaneous relations between tension and contractile element velocity (V CE ) during the course of single, isovolumic beats produced by balloon occlusion of the aorta. The relative sensitivity of this relation was compared with that of the ventricular function curve (VFC) by exerting small inotropic influences. In seven experiments, low doses of norepinephrine always shifted the isovolumic force-velocity (FV) relation, increases occurring in maximum V CE (32.7%) and maximum tension (P 0 ) (16.9%). The VFC was unchanged in four of the seven dogs. Moderate hypothermia (average 30.8°C) in four dogs increased P 0 (average 28%), with no change, or a fall, in maximum VCE; the VFC during hypothermia was shifted upward and to the left in two experiments and unchanged in two. Moderate increases in heart rate always produced increases in maximum V CE (average 13.5%) but no change in the VFC was observed. Thus, the isovolumic FV relation proved more sensitive than the VFC in detecting changes in myocardial contractile state and allowed more complete definition of these alterations by providing separation of changes in shortening velocity from alterations in the strength of myocardial contraction.
Covell et al. (Mon,) studied this question.