Sex Difference in the Effect of Whole Body Heating on Cardiovascular Functions.

Cold pressor test (CPT) is a well-known method for evaluating non-baroreflex mediated autonomic cardiovascular functions in humans. It has been reported that autonomic cardiovascular response to CPT differs in males and females and that heat stress attenuates the increase in arterial blood pressure during CPT. Study has also indicated that heat stress attenuates the increase in arterial blood pressure during CPT. The present study assessed the autonomic cardiovascular reactivity in males and females during cold pressor test before and after whole body heating for 40 min. 20 healthy Indian males and 18 females participated in the study. The participants were exposed to 40°C dry bulb temperature and 40% relative humidity in a simulated thermal chamber. They performed CPT before and after heat stress and their beat to beat heart rate and blood pressure were recorded. It was observed that baroreflex sensitivity (BRS) was significantly lower in females during CPT before heat stress (p<0.01) as well as after heat stress (p<0.01). Moreover, following whole body heating, the BRS during CPT increased slightly in females but not in males. Stroke Volume (SV) increased significantly during CPT before heat stress in females from pre-heat baseline (82 ml/beat vs. 101 ml/beat) (p<0.001) as well as during CPT after heat stress from post-heat baseline (75 ml/beat vs. 95 ml/ beat) (p<0.01). SV increased significantly in males during CPT after heat stress only (p<0.01) as compared to post-heat baseline (82.5 ml/beat vs. 94.5 ml/beat). Frequency domain analysis of heart rate variability indicated that during CPT, low frequency power in males was higher than females (p<0.05) and high frequency power was higher in females than males (p<0.05). This suggests that autonomic modulation of cardiovascular function during CPT in males is mediated mainly via sympathetic neural system and in females it is mediated via vagal system. LF/HF ratio during CPT was found to be significantly higher in males (2.54) than females (1.27) before heat stress (p<0.01).

Subjective and objective convergence of the eyes at simulated altitude of 18,000 feet Preceded by short-term exposure to heat stress.

Armed forces personnel including military aviators are quite often exposed concurrently to various environmental stressors like high environmental temperature and hypoxia. Literatures have suggested that exposure to one environmental stressor may modify the physiological response on subsequent exposure to same or different stressor. The present study was undertaken to investigate the impact of cross tolerance between two environmental stressors of aviation (heat and hypoxia) in ten healthy adult males in a simulated altitude chamber in a within subject experimental study. They were assessed for their convergence ability of the eyes at ground and at simulated altitude of 18,000 ft with or without pre-exposure to heat stress. Subjective convergence at simulated altitude of 18,000 ft did not show any improvement following pre-exposure to heat stress. Objective convergence was improved following pre-exposure to heat stress and was found to be 10.76 cm and 9.10 cm without and with heat stress respectively at simulated altitude of 18,000 ft. Improved objective convergence at high altitude as a result of pre-exposure to heat stress is indicative of better ocular functions. This might benefit aviators while flying at hypoxic condition.

Comparison of Cardiorespiratory Responses Between Surya Namaskar and Bicycle Exercise at Similar Energy Expenditure Level.

Surya Namaskar (SN), a popular traditional Indian yogic practice called “Sun Salutations”, includes practice of twelve physical postures involving alternate backward bending and forward bending postures. The practice of twelve postures in succession makes one round of its practice. Many people practise it as part of their daily physical fitness regimen. No study is available to compare cardiorespiratory responses of SN with bicycle exercise (BE). 20 healthy Yoga instructors practicing various Yogic practices including SN since last 7-8 years participated in the study. They performed SN in the laboratory according to their customary daily practice routine. The subject also performed incremental load bicycle exercise test till exhaustion on their second visit for measuring their 􀀅 V O2 max. SN and BE were compared at three similar exercise intensity levels in terms of % of 􀀅 V O2 max. The exercise intensities were light (10-20% 􀀅 V O2 max), moderate (21-40% 􀀅 V O2 max) and high intensities (41-50% 􀀅 V O2 max). Heart rate at high work intensity was significantly higher in BE than SN (P<.001). Ventilation and carbon dioxide output were significantly higher in BE than SN at high exercise intensity (P<0.001). Overall, cardiorespiratory stress is less in SN than BE at similar work intensities