Effect of aerobic fitness on hemodynamic responses to upright tilting.

The purpose of this study was to examine the influence of aerobic fitness on heart rate, systolic and diastolic blood pressure, mean arterial pressure, and pulse pressure responses to upright tilting in 10 below-average (BA Group) and 10 above-average (AA Group) aerobically fit adults. Aerobic fitness levels were predicted using the Astrand and Ryhming submaximal exercise test. Mean predicted maximal aerobic capacities were significantly higher (p less than .05) for the AA Group (46.0 +/- 6.0 mL.kg-1.min-1) compared with the BA Group (31.0 +/- 4.8 mL.kg-1.min-1) subjects. Heart rate and blood pressure responses were obtained at rest (supine position for 30 minutes) and during 5 minutes of upright tilting to 70 degrees (electric tilt table). A two-factor analysis of variance indicated that the increase in heart rate to upright tilting was significant (p less than .05) (significant treatment effect). More importantly, the study revealed a significant treatment x group interaction (p less than .05), indicating that the heart rate response for the BA Group was significantly greater than for the AA Group. Similar results occurred for blood pressure responses. A separate analysis of variance demonstrated significant changes in hemodynamic responses to upright tilting and significant treatment x group interactions. These results demonstrated significantly smaller (p less than .05) changes in blood pressure and heart rate to upright tilting for the AA Group compared with the BA Group. The results, therefore, indicate that reflex responses to central hypovolemia are different between above-average and below-average aerobically fit adults.