INFLUENCE OF CARDIAC-OUTPUT DISTRIBUTION ON CARDIAC FILLING PRESSURE DURING REST AND DYNAMIC EXERCISE IN DOGS

The distribution of cardiac output (CO) between compliant and noncompl iant organs is an important determinant of the slope of the relationsh ip predicted between CO and right atrial pressure (RAP). However, curv es relating CO to RAP at rest are shifted rightward (higher CO) and up ward (higher RAP) by exercise with no change in slope, despite a large rise in the fraction of CO directed to noncompliant muscle vasculatur e, which is predicted to decrease the slope. We sought to test whether reductions in CO imposed during rest and exercise are accompanied by changes in its distribution that would favor constant slopes. Six dogs had atrioventricular block produced surgically and had blood flow tra nsducers implanted on the ascending aorta and the terminal aorta. Tota l muscle blood flow (MBF) was estimated from terminal aortic flow by a ssuming that all of the increase in CO in mild dynamic exercise is dir ected to muscle. CO was reduced by lowering ventricular pacing rate at rest and during graded treadmill exercise (2 and 4 miles/h at 0% grad e). Exercise increased the fraction of CO directed to muscle (MBF/CO) (P < 0.001). The effect of changes in CO on MBF/CO depended on exercis e intensity (P < 0.01). At rest, MBF/CO fell from 0.53 to 0.45 when CO was reduced; this is expected to reduce the slope of the measured rel ationship between CO and RAP. During exercise at 2 miles/h, MBF/CO cha nged little when CO was reduced. During exercise at 4 miles/h, MBF/CO rose from 0.69 to 0.76 when CO was reduced; this is expected to increa se the slope of the measured relationship between CO and RAP. Thus, wh en CO is reduced by ventricular pacing, directionally opposite changes in the distribution of CO between compliant and noncompliant vasculat ures in rest and mild dynamic exercise tends to make the slope of the relationship between CO and RAP in rest equal to the slope in exercise .