DIASTOLIC VISCOUS PROPERTIES OF THE INTACT CANINE LEFT-VENTRICLE

The diastolic viscous properties of the intact heart were studied in seven conscious dogs and in four right heart bypass preparations. The passive elastic properties were first determined by fitting a linear relation between the natural logarithm (ln) of the left ventricular pressure and the diameter or between ln of the diastolic wall stress and the strain (r=0.93-0.99). When the velocity of lengthening was less than 1 diam/sec, the pressure and stress data fell within the 95% confidence interval of the static elastic curve, both at low and high strain. At velocities of lengthening greater than 1 diam/sec, the pressure and stress values always deviated from the passive elastic relationship, but the magnitude of this deviation was not linearly related to the velocity of lengthening. When pressure and stress data at high velocities of lengthening were compared, the pressure or stress deviation from the passive elastic curve was always greater at high diameter than at low diameter. Moreover, the pressure and stress data at the time of minimum left ventricular pressure after systole showed greater deviation from the passive curve than expected from the diameter and the velocity of lengthening alone (+8 mm Hg in conscious dogs, P<0.002; +2.4 mm Hg in open-chest dogs, P<0.05), suggesting incomplete ventricular relaxation at that time. We conclude that the viscous properties of the left ventricle, like those of isolated cardiac muscle, are not linear but increase with length. These observations, together with the influence of incomplete ventricular relaxation in early diastole, preclude the determination of diastolic properties from only one diastolic cycle.