Orthostatic responses at 4860 m in low, moderate, and high altitude residents

Could Orthostatic Stress Responses Predict Acute Mountain Sickness Susceptibility Prior to High Altitude Travel? A Pilot Study

Bellovary, Bryanne N., Andrew D. Wells, Zachary J. Fennel, Jeremy B. Ducharme, Jonathan M. Houck, Trevor J. Mayschak, Ann L. Gibson, Scott N. Drum, and Christine M. Mermier. Could orthostatic stress responses predict acute mountain sickness susceptibility before high altitude travel? A pilot study. High Alt Med Biol. 24:19-26, 2023. Purpose: This study assessed head-up tilt (HUT) responses in relation to acute mountain sickness (AMS)-susceptibility during hypoxic exposure. Materials and Methods: Fifteen participants completed three lab visits: (1) protocol familiarization and cycle maximal oxygen consumption (VO₂max) test; (2) HUT test consisting of supine rest for 20 minutes followed by 70° tilting for ≤40 minutes; and (3) 6 hours of hypobaric hypoxic exposure (4,572 m) where participants performed two 30-minute cycling bouts separated by 1 hour at a 50% VO₂max workload within the first 3 hours and rested when not exercising. During HUT, systolic blood pressure (SBP), diastolic blood pressure, heart rate (HR), and variability (blood pressure variability [BPV] and HR variability [HRV]) were measured continuously. The AMS scores were determined after 6 hours of exposure. Correlations determined relationships between HUT cardiovascular responses and AMS scores. Repeated-measures analysis of variance (ANOVA) assessed differences between those with and without AMS symptoms during HUT. Results: Higher AMS scores correlated with greater change in SBP variability (r = 0.52, p = 0.048) and blunted changes in HRV (root mean square of successive differences between normal heartbeats r = 0.81, p = 0.001, percentage of adjacent normal sinus intervals that differ by more than 50 milliseconds [pNN50] r = 0.87, p < 0.001) during HUT. A pNN50 interaction (p = 0.02) suggested elevated cardiac sympathetic activity at baseline and a blunted increase in cardiac sympathetic influence throughout HUT in those with AMS (pNN50 baseline: AMS = 26.2% ± 15.3%, no AMS = 51.0% ± 13.5%; first 3 minutes into HUT: AMS = 17.2% ± 19.1%, no AMS = 17.1% ± 10.9%; end of HUT: AMS = 6.2% ± 9.1%, no AMS 11.0% ± 10.0%). Conclusions: The results suggest autonomic responses via HUT differ in AMS-susceptible individuals. Changes in HRV and BPV during HUT may be a promising predictive measurement for AMS-susceptibility, but further research is needed for confirmation.

Orthostatic Resiliency During Successive Hypoxic, Hypoxic Orthostatic Challenge: Successful vs. Unsuccessful Cardiovascular and Oxygenation Strategies

Introduction: Rapid environmental changes, such as successive hypoxic-hypoxic orthostatic challenges (SHHOC) occur in the aerospace environment, and the ability to remain orthostatically resilient (OR) relies upon orchestration of physiological counter-responses. Counter-responses adjusting for hypoxia may conflict with orthostatic responses, and a misorchestration can lead to orthostatic intolerance (OI). The goal of this study was to pinpoint specific cardiovascular and oxygenation factors associated with OR during a simulated SHHOC.

Methods: Thirty one men underwent a simulated SHHOC consisting of baseline (P0), normobaric hypoxia (Fi02 = 12%, P1), and max 60 s of hypoxic lower body negative pressure (LBNP, P2). Alongside anthropometric variables, non-invasive cardiovascular, central and peripheral tissue oxygenation parameters, were recorded. OI was defined as hemodynamic collapse during SHHOC. Comparison of anthropometric, cardiovascular, and oxygenation parameters between OR and OI was performed via Student’s t-test. Within groups, a repeated measures ANOVA test with Holm-Sidak post hoc test was performed. Performance diagnostics were performed to assess factors associated with OR/OI (sensitivity, specificity, positive predictive value PPV, and odd’s ratio OR).

Results: Only 9/31 were OR, and 22/31 were OI. OR had significantly greater body mass index (BMI), weight, peripheral Sp02, longer R-R Interval (RRI) and lower heart rate (HR) at P0. During P1 OR exhibited significantly higher cardiac index (CI), stroke volume index (SVI), and lower systemic vascular resistance index (SVRI) than OI. Both groups exhibited a significant decrease in cerebral oxygenation (TOIc) with an increase in cerebral deoxygenated hemoglobin (dHbc), while the OI group showed a significant decrease in cerebral oxygenated hemoglobin (02Hbc) and peripheral oxygenation (TOIp) with an increase in peripheral deoxygenated hemoglobin (dHbp). During P2, OR maintained significantly greater CI, systolic, mean, and diastolic pressure (SAP, MAP, DAP), with a shortened RRI compared to the OI group, while central and peripheral oxygenation were not different. Body weight and BMI both showed high sensitivity (0.95), low specificity (0.33), a PPV of 0.78, with an OR of 0.92, and 0.61. P0 RRI showed a sensitivity of 0.95, specificity of 0.22, PPV 0.75, and OR of 0.99. Delta SVI had the highest performance diagnostics during P1 (sensitivity 0.91, specificity 0.44, PPV 0.79, and OR 0.8). Delta SAP had the highest overall performance diagnostics for P2 (sensitivity 0.95, specificity 0.67, PPV 0.87, and OR 0.9).

Discussion: Maintaining OR during SHHOC is reliant upon greater BMI, body weight, longer RRI, and lower HR at baseline, while increasing CI and SVI, minimizing peripheral 02 utilization and decreasing SVRI during hypoxia. During hypoxic LBNP, the ability to remain OR is dependent upon maintaining SAP, via CI increases rather than SVRI. Cerebral oxygenation parameters, beyond 02Hbc during P1 did not differ between groups, suggesting that the during acute hypoxia, an increase in cerebral 02 consumption, coupled with increased peripheral 02 utilization does seem to play a role in OI risk during SHHOC. However, cardiovascular factors such as SVI are of more value in assessing OR/OI risk. The results can be used to implement effective aerospace crew physiological monitoring strategies.