Pressure-velocity relation in the isolated rabbit left ventricle

The relation between pressure (P) and ejection velocity (V) was determined in the coronary-perfused rabbit left ventricle. The heart was isolated and a thin latex balloon was inserted into the left ventricle. The volume of the ventricle was changed at a constant velocity with a magnetic shaker connected to the balloon at various phases of contraction and the corresponding pressure change was measured. The pressure-velocity relation determined by this isovelocity method was well fitted by a hyperbolic equation, (P + A) (V + B) = B(PO + A), where A and B are constants and Po is the maximum systolic pressure. Using the thick-walled spherical ventricle model, the Hill equation of the ventricular wall muscle with constants a and b was obtained, and Hill constants (a and b) were calculated from ventricular constants (A and B). The constants A and B/V (V is the volume of ventricle) increased with decreases in ventricular volume, but the ratio Vmax/TO (= b/aL, L is the length of the muscle) was not affected. The value of Vmax/TO was 1.14 +/- 0.22 X 10(-5) cm2/(dyn X sec) (mean +/- S. D. in 9 rabbits). The constants A and B/V, especially A, increased and the ratio Vmax/TO decreased under the negative inotropic effect, such as decreased heart rate or decreased coronary perfusion pressure. This ratio would therefore be a good indicator for assessment of the ventricular performance as well as the contractility of the wall muscle.