Influence of cardiac output distribution on cardiac filling pressure during rest and dynamic exercise in dogs
Sheriff, D.D.; Zhou, X.
American Journal of Physiology 267(6 Pt 2): H2378-H2382
1994
ISSN/ISBN: 0002-9513 PMID: 7810738 Document Number: 432232
The distribution of cardiac output (CO) between compliant and noncompliant organs is an important determinant of the slope of the relationship predicted between CO and right atrial pressure (RAP). However, curves relating CO to RAP at rest are shifted rightward (higher CO) and upward (higher RAP) by exercise with no change in slope, despite a large rise in the fraction of CO directed to noncompliant muscle vasculature, 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 transducers implanted on the ascending aorta and the terminal aorta. Total muscle blood flow (MBF) was estimated from terminal aortic flow by assuming that all of the increase in CO in mild dynamic exercise is directed to muscle. CO was reduced by lowering ventricular pacing rate at rest and during graded treadmill exercise (2 and 4 miles/h at 0% grade). Exercise increased the fraction of CO directed to muscle (MBF/CO) (P lt 0.001). The effect of changes in CO on MBF/CO depended on exercise intensity (P lt 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 relationship between CO and RAP. During exercise at 2 miles/h, MBF/CO changed 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 increase the slope of the measured relationship between CO and RAP. Thus, when CO is reduced by ventricular pacing, directionally opposite changes in the distribution of CO between compliant and noncompliant vasculatures 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.