Effects of selective myocardial inactivation on left ventricular function during the isovolumetric phase of systole.

Abstract

The pressure changes in the left ventricle during the isovolumetric phase of systole have been studied in the dog before and three months after selective inactivation of between 5 per cent and 8 per cent of left ventricular myocardium. In fourteen animals the anterior papillary muscle was inactivated and in thirteen animals an equivalent mass of myocardium in the antero‐lateral wall of the left ventricle was inactivated. In both series left ventricular systolic pressure and mean cycle length were not significantly changed by the procedures. In the animals with lateral myocardial inactivation the mean left ventricular end diastolic pressure (LVEDP) was raised from 4·4 mm. Hg to 6·3 mm. Hg. The maximum rate of isovolumetric pressure rise was not significantly changed; it was 2530 mm. Hg/sec. initially and 2580 mm. Hg/sec. 3 months after inactivation. The mean time to reach the maximum rate of pressure rise was also unaffected, being 31 m/sec. initially and 30 m/sec. 3 months after inactivation. The first derivative of left ventricular pressure with respect to time was plotted; it was unchanged up to the peak rate of pressure rise (dP/dt max), apart from the higher LVEDP, but thereafter the curve showed a more rapid falling off. In the animals with papillary muscle damage the mean LVEDP was increased from 4·6 mm. Hg to 5·9 mm. Hg, (dP/dt) max fell from 2630 mm. Hg/sec. to 2020 mm. Hg/sec., and t(dP/dt)max lengthened from 29 m/sec. to 38 m/sec. All these changes were statistically significant. In the first derivative of pressure with respect to time the changes were seen as a fall in the peak value and a shift of the peak to the right. In six of the fourteen animals with papillary muscle damage mitral valve incompetence was produced. The changes in the isovolumetric portion of the pressure curve occurred whether or not mitral valve incompetence was present, but were accentuated when it was.The contraction of the ventricle is a sequence of the contractions of its parts; the present results demonstrate that papillary muscle contraction is important early in thie seqcuence, at the start of the isovolumetric phase of systole.