COMPARATIVE-ASSESSMENT OF RIGHT-VENTRICULAR PERFORMANCE FROM THE PRESSURE-VOLUME RELATIONSHIP IN DOUBLE-MUSCLED AND CONVENTIONAL CALVES

Forty-one and 55 records of right-sided and systemic arterial pressure s, cardiac output, and end-diastolic and end-systolic right ventricula r volumes were collected from a group of 6 conventional and 6 double-m uscled calves, respectively. In each group, the mean right ventricular pressure-volume loop was constructed. Global cardiac performance was significantly lower in the double-muscled than in the conventional cal ves. The right ventricular end-diastolic and end-systolic volumes, as well as the diastolic portion of the mean pressure-volume loop, were s imilar in the 2 groups. Those results suggest that the reduced cardiac performance of double-muscled calves is not due to a lowered ventricu lar preload and that diastolic properties of their myocardium are simi lar to those of conventional calves. When expressed on a body weight b asis, however, the right ventricular end-diastolic and end-systolic vo lumes were lower in the double-muscled than in conventional calves. Wh en expressed as a function of probable metabolic demand, therefore, th e volumetric capacity of the cardiac pump appears to be reduced in dou ble-muscled calves. The significantly lower right ventricular ejection fraction, maximal rate of ventricular pressure rise and right ventric ular peak-systolic pressure to end-systolic volume ratio measured in d ouble-muscled as compared with conventional calves suggest that reduce d myocardial contractility may also be partly responsible for the sign ificantly lower stroke index of the former calves. The cardiac pump of double-muscled cattle thus seems to be less effective than that of co nventional cattle because of reduced volumetric capacity and lowered s trength of contraction.