Physical fitness and cardiovascular regulation: mechanisms of orthostatic intolerance

Abstract

We studied three groups of eight men each--high, mid, and low fit (peak O2 consumption 60.0 +/- 0.8, 48.9 +/- 1.0, and 35.7 +/- 0.9 ml.min-1.kg-1)--to determine the mechanism of orthostatic intolerance in endurance athletes. Tolerance was defined by progressive lower body negative pressure (LBNP) to presyncope. Maximal calf vascular conductance (Gmax) was measured. The carotid baroreflex was characterized using both stepwise R-wave-triggered and sustained (2 min) changes in neck chamber pressure. High-fit subjects tended to have lower LBNP tolerance than mid- and low-fit subjects but similar baroreflex responses. Subjects with poor LBNP tolerance had larger stroke volumes (SV) (120 +/- 6 vs. 103 +/- 3 ml) and greater decline in SV with LBNP to -40 mmHg (40 +/- 2 vs. 26 +/- 4%). Stepwise multiple linear regression analysis revealed that Gmax and steady-state gain of the carotid baroreflex contributed significantly toward explaining interindividual variations in LBNP tolerance. Thus endurance athletes may have decreased LBNP tolerance, but apparently not as a simple linear function of aerobic fitness. Orthostatic tolerance depends on complex interactions among functional characteristics that appear both related (Gmax and SV) and unrelated (baroreflex function) to fitness or exercise training.

Short-term Aerobic Exercise Reduces Nitroglycerin-induced Orthostatic Intolerance in Older Adults With Type 2 Diabetes

“…All investigations demonstrating either negative or neutral effects of aerobic exercise involved the examination of very intense training in young athletes (VO 2 max > 60 mL·kg −1 ·min −1 ) 7-10 and most involved a cross-sectional design. [8][9][10] The only study that involved both a prospective intervention and a subject population of nonathletes demonstrated a large improvement in orthostatic tolerance to upright tilting in young adult military recruits, congruent with the results of this study. 6 To our knowledge, our study is the first to show that even older adults at very high cardiovascular risk due to type 2 diabetes, and age can improve orthostatic tolerance with regular aerobic exercise.…”

“…Vigorous aerobic training has been shown to improve orthostatic tolerance in young normal subjects, 6 but some investigations have demonstrated a worsening [7][8][9] or no 10 effect of aerobic training on orthostatic tolerance. The impact of aerobic exercise on GTN-induced orthostatic intolerance in older adults with type 2 diabetes has not been previously examined.…”

Vasoconstrictor Reserve and Sympathetic Neural Control of Orthostasis

“…None was an endurance-trained athlete. 1 No woman was pregnant during the experiment. All subjects gave their written informed consent, approved by the Institutional Review Boards of the University of Texas Southwestern Medical Center at Dallas and Presbyterian Hospital of Dallas.…”

“…This method has been validated against standard invasive techniques, including thermodilution and direct Fick at rest, during exercise and changes in orthostatic stress, with a typical error (expressed as coefficient of variation) of 4% to 5%. 16 This method has been used extensively in our laboratory and others, 1,2,5,12,15,17 and it has been reviewed recently in detail. 18 Stroke volume (SV) was calculated from cardiac output and the HR measured during rebreathing.…”

Arterial blood pressure response to head-up tilt test and orthostatic tolerance in nurses

“…First, we did not evaluate the physical fitness of the subjects, which can affect orthostatic tolerance [45]; thus, we did not measure Vo 2max . Second, we did not measure blood volume, which also affects orthostatic tolerance [11,12].…”