The Effect of Changes in Posture and of Graded Exercise on Stroke Volume in Man *

Abstract

Interest has been revived recently in the relative importance of changes in heart rate and stroke volume during exercise in man. Rushmer and Smith have examined the literature and concluded that an increase in stroke volume is not an essential feature of the ventricular response in normal human subjects (1, 2). Contrary views, particularly in the older literature, were attributed to differences in the technics used to measure cardiac output and in the degree of training of the subjects (1). However, in the studies reviewed by Rushmer, the subjects had exercised in various postures. This could have accounted for much of the apparent discrepancy (3), a point which Rushmer has now emphasized (4).The present investigation demonstrates the importance of the posture of the subject in the interpretation of changes in stroke volume from rest to exercise. The stroke volume increases only slightly once exercise has begun, regardless of the severity of the exercise or the position assumed by the subject. METHODSSeven healthy "untrained" men, aged 25 to 35 years, were studied. Cardiac output was measured by the indicator-dilution method. A cardiac catheter was placed with its tip in the superior vena cava. The indicator, cardio-green dye,1 was given in a dose of 7.5 mg by sudden injection from a specially calibrated syringe. The syringe was attached to the catheter throughout the experiment and was filled before each injection from a supply syringe connected to it by a three-way tap. Blood was sampled continuously for each dilution curve from the radial artery through a Wood oximeter at * This investigation was supported in part by a grant from the Minnesota Heart Association. rates of 28 to 50 ml per minute. The dilution curve was recorded on a photokymograph. Not more than 40 ml was withdrawn for any curve. On completion of each curve the blood was reinfused.Samples of blood were withdrawn from the radial artery before and after each experiment. Known amounts of dye were added to 10 ml aliquots of blood and the resulting mixtures were drawn through the cuvet oximeter to permit calibration of tIle dilution curves. The calibrations for the two samples invariably were close (Figure 1, upper panel) and were reproducible from day to day. Calibrations for blood from different subjects were similar. Figure 1

Cardiovascular Adjustments to Gravitational Stress

“…Many other investigators (21,24,119,120,287,309,345) have reported similar results. Stroke volume decreases in the upright position by about 33%.…”

“…Guyton and Greganti (135) postulated that the venous HIP was located within the heart to maintain stable ventricular filling pressure and cardiac output during changes in body position. Stroke volume, however, decreases by about 30% in humans tilted from the horizontal to the head-up position (224,260,345) mainly because of changes in ventricular filling pressures. Thus a species difference in the response to postural changes exists.…”

Left ventricular performance in normal subjects: a comparison of the responses to exercise in the upright and supine positions.

“…Hemodynamic studies have generally, but not unanimously, shown that stroke volume increases with transition from rest to exercise in both the upright and supine positions. ", 4,[8][9][10][40][41][42] LV end-diastolic volume during supine exercise has been shown to decrease in studies in which metallic epicardial markers" and angiography were used.14 Using angiography, Gorlin et al12 found no substantial change in LV enddiastolic volume. Crawford et al6 demonstrated echocardiographically that diastolic diameter does not change during moderate supine and upright bicycle exercise.…”

Effect of posture on ampicillin pharmacokinetics, glomerular filtration rate and renal plasma flow in resting subjects.

“…The experiments were started when the subjects had been either sitting or lying for 30 min, a period intermediate between the previously used 15 min (Chapman et al, 1960;Wang et al, 1960) and 40 min (Waterfield, 1931;Chapman et al, 1948) and identical to the equilibration period of Radigan & Robinson (1949) and Swartz et al (1973). Changes in posture were not permitted until the end of the experiment.…”

“…Differences in pharmacokinetics between supine rest and ambulation could be due both to the changes in posture and the change of physical activity. The relative contribution of these factors has been evaluated with respect to cardiac output and stroke volume (Reeves et al, 1961;Chapman et al, 1960;Wang et al, 1960), but no data are available on the relative importance of posture and level of exercise for liver and renal blood flow. Furthermore, in resting subjects, the relation between renal blood flow and pharmacokinetics of drugs eliminated via the renal route in the lying, sitting, and standing position is unknown.…”