Cardiovascular responses during orthostasis: effect of an increase in VO2max.

Cardiovascular responses to orthostasis were assessed in eight men (18-29 years old) before and after an 8-d cycle ergometer exercise training (E) regimen for 2 h . d-1 at 65% maximal O2 uptake (VO2 max). Each subject underwent 60 degrees head-up tilt (60 min max) before (T1) and after (T2) E. Heart rate (HR), systolic (SBP), and diastolic (DBP) pressures were measured each min before, during, and after tilt; pulse pressure (PP), mean arterial pressure (MAP), and rate-pressure-product (RPP) were calculated. Changes in plasma renin activity (PRA), vasopressin (pVP), hematocrit, hemoglobin, and plasma volume (PV, T-1824) were measured from venous blood samples taken pre- and immediately post-tilt. Following E, VO2 max increased by 8.3% (p less than 0.05), resting HR decreased by 8.1% (p less than 0.05), and PV increased by 430 ml (12.2%, p less than 0.05). Mean (+/- S.E.) tilt duration went from 40.0 +/- 5.1 min during T1 to 46.7 +/- 3.4 min during T2 (NS); mean tilt HR decreased from 86 +/- 4 bpm to 77 +/- 3 bpm (p less than 0.05), RPP decreased from 10,320 +/- 390 to 9,317 +/- 310 mm Hg . bpm (p less than 0.05), while mean SBP, DBP, PP, and MAP were unchanged. Plasma volume decreased during tilt by 479 ml in T1 compared to 544 ml in T2 (p less than 0.05), while % delta PV were similar: -13.6% and -13.9%, respectively. Peak fluid-accumulation in the calf and total leg during tilt increased (p less than 0.05) following E.(ABSTRACT TRUNCATED AT 250 WORDS)