Ventricular Response to Increased Outflow Resistance in Absence of Elevated Intraventricular End-Diastolic Pressure

In the isolated, supported, canine heart preparation, the response of the left ventricle to augmented outflow resistance was compared under conditions in which ventricular end-diastolic pressure and volume were increased and in which the pressure and volume remained virtually constant. The compensatory changes were much greater when end-diastolic pressure and volume were increased, even if these alterations in pressure and volume subsequently returned to or toward control levels. Immediately after resistance was augmented, peak ventricular power and stroke work diminished to less than control values. Within a very few beats, however, both of these parameters rose to exceed control levels. However, definite adaptive changes were also observed under conditions in which ventricular dimensions and pressures did not change appreciably, even temporarily. After a similar initial reduction in peak power and work after resistance was increased, peak power rose progressively to a value still significantly below control, while stroke work attained control levels by the end of a mean interval of 32 seconds of increased resistance. The intrinsic adaptation of the ventricle to increased resistance per se undoubtedly contributes to the total compensatory response, even under conditions in which diastolic stretch of the ventricle plays a major role.