Aerobic fitness and the cardiovascular response to stress

The observation that aerobically fit persons react to physical load with a smaller sympathetic response than do less fit subjects suggests that their response to psychological stress might also be reduced. The evidence for this, however, is far from consistent. It was argued that this inconsistency might be due to the incomplete measurement of the response system involved. In the present experiment two groups, which differed strongly in maximal aerobic power, were compared with respect to their cardiovascular response to a laboratory stressor. In addition to the traditional heart rate and blood pressure measurements, pre-ejection period, cardiac output, and peripheral resistance were assessed. Since only the part of the stress response that is not accounted for by metabolic needs might have pathological significance, the so-called "additional" responses were also measured. Fitness was shown to be associated with smaller sympathetic effects on both heart and vessels. The decrement in pre-ejection period and the heart rate response were smaller in the high fit group. The total peripheral resistance and diastolic blood pressure responses pointed to a much stronger vascular reactivity in the low fit group. Unexpectedly cardiac output did not increase during stress. The strong heart rate response in the low fit group was compensated by a reduction in stroke volume, which probably originated in an increased peripheral resistance. The largest discrepancy between the effects of sympathetic activation during stress and exercise occurred in the vessels, as demonstrated by the large "additional" response in total peripheral resistance during stress. The data point to the often neglected role of vascular processes in experiments of this kind. Furthermore, it seems necessary to take the effect of fitness into account when comparing subjects with respect to their cardiovascular stress response.