The Effects of 12 Days Exposure to 4,300M Altitude on the Sympathetic Neural and Cardiovascular Responses to Head‐Up Tilt

Abstract

Musclesympathetic nerve activity (MSNA) increases with acute and chronic altitude exposure. However, the effects of altitude exposure on the neural and hemodynamic responses to orthostasis are not clear. The purpose of the current study was to evaluate the effects of altitude exposure (12 days; 4,300 m, Pikes Peak, CO) on MSNA, heart rate (HR) and systolic (SBP) and diastolic (DBP) blood pressure during supine rest and 30° and 45°head up tilt (HUT). We hypothesized that: (1) MSNA would be elevated upon return to sea level (SL, 50 m) from Pikes Peak and (2) increased resting MSNA would be associated with decreased responses to HUT due to a potential ‘ceiling effect’. Supine MSNA, HR, SBP and DBP were measured in 9 healthy subjects (7 M, 2 F; 24 ± 8 years)prior to‐, and within 24 hr of return from altitude. MSNA was measured via peroneal microneurography. Subjects rested supine for 10–15 min to collect baseline data, then were tilted to 30° HUT for 2–3min, followed by 45° HUT for 2–3 min. Supine MSNA was increased within 24 hr return from altitude (27 ± 5 vs 17 ±6 bursts/min [SL], p < 0.05, ANOVA). MSNA at 30° HUT was higher within 24 hr return from altitude (36 ± 8bursts/min vs 25 ± 7 [SL], p< 0.05). MSNA tended to be higher at 45° HUT (39 ± 8 bursts/min vs 30 ± 9 [SL], p = 0.07). There was no difference in the extent of change in MSNA from supine to 30° HUT orsupine to 45° HUT between SL and post‐altitude (p > 0.05). There were no changes in HR, SBP or DBP supine, 30° HUT, and 45° HUT post‐Pikes Peak compared to sea level(p > 0.05 for all). Our data suggest that 12 days of altitude exposure increases resting (supine) MSNA but does not alter the ability of sympathetic vasoconstrictor nerves to increase activity in response to a baroreflex stimulus (HUT).Support or Funding InformationFunded by USAMRMC; author views not official US Army or DOD policy.