Similar orthostatic defense in active, healthy young adult and late middle-aged men

Orthostatic defense is commonly validated with a 60 ° to 80 ° head-up tilt test, addressing the step response rather than the response to permanent orthostasis. During the initial phase of tilt, neural factors predominate, while later, the slower humoral factors fade in. It has been demonstrated that, during adaptation of the circulatory system to the standing conditions, overshoot and undershoot occur. These oscillations hamper straight-forward interpretation of a tilt test, and may contribute to the inconclusiveness of current studies regarding the aging of orthostatic defense. Gradual, progressive, orthostatic load testing seems a valuable alternative. We used a novel, incremental, head-up tilt protocol (0 ° to 80 °, 13 increments) to impose graded orthostatic stress on 46 healthy young adult men (mean age ±SD 25 ± 3 years), ana on 16 healthy late middle-aged men (60 ± 4 years), while recording the electrocardiogram and the blood pressure. A first-order estimate of the heart rate range associated with the sympathovagal transition was made by combined analysis of heart rate and heart rate variability trends. We observed similar responses in heart rate, heart rate variability, and blood pressure. Supine heart rate (61 ± 8 vs 61 ± 7 beats/min), heart rate at the maximal tilt angle (86 ± 13 vs 84 ± 12 beats/min), sympathovagal transition (112 ± 82 vs 111 ± 76 beats/min), percent increase of the rate-pressure product (49 ± 23% vs 43 ± 20%), and the slope of the linear regression of the mean blood pressure on the sine of the tilt angle (8.7 ± 8.3 vs 9.2 ± 7.1 mm Hg) did not differ significantly. We conclude that aging, per se, does not impair orthostatic defense under gradually increasing orthostatic stress.