The reduced oxygen breathing paradigm for hypoxia training: physiological, cognitive, and subjective effects.

UNLABELLED: The current training program for hypoxia familiarization requires a low-pressure chamber that places aviator trainees at risk for decompression sickness. A cost-effective reduced oxygen-breathing (ROB) paradigm that decreases oxygen (O2) concentration leading to normobaric hypoxia was assessed as an alternative to the hypobaric chamber. PURPOSE: To help establish the validity of the ROB paradigm, this report documents cognitive performance, cardiopulmonary and subjective changes during ROB exposure. METHODS: Performance on a two-dimensional tracking task, as well as BP, heart rate, end-tidal carbon dioxide (ETCO2), O2 saturation, and subjective reports of hypoxia symptoms were observed in 12 U.S. Navy divers during exposure to normoxic air followed by one of four experimental gas mixtures per session. All participants received all gas conditions that differed in their relative concentrations of O2 and nitrogen (6.20/93.80, 7.00/93.00, 7.85/92.15, and 20.85/79.15% O2/N2). RESULTS: ROB caused increases in tracking task error (p < 0.0001). ROB also increased heart rate (p < 0.001) and systolic BP (p = 0.004), and decreased ETCO2 and O2 saturation (p < 0.0001). Finally, subjects responded to ROB-induced hypoxia with higher subjective symptom ratings (p < 0.0001). CONCLUSIONS: These data are consistent with those expected from hypoxic states and support the validity of the ROB paradigm for hypoxia training. Future validation studies comparing a ROB device with hypobaric chambers are needed.

Hemodynamic characteristics of young men at risk for hypertension at rest and during laboratory stressors.

Examined hemodynamic activity--at rest, during mental arithmetic, and during hand cold pressor--in young men varying in risk for hypertension. Classification into low-risk (n = 72), moderate-risk (n = 20), and high-risk (n = 13) groups was based on resting systolic blood pressure (SBP) and parental history of essential hypertension (PH). Dependent variables were SBP, diastolic BP (DBP), heart rate, and rate-pressure product (RPP). Progressively greater hemodynamic activity was seen across risk groups at rest and during the tasks. Risk groups differed significantly in SBP, DBP, and RPP at baseline (ps less than .003) and in size of response to mental arithmetic (ps less than .05) but not to cold pressor. These relationships were either absent or weaker when using either risk factor alone to form risk groups. These findings suggest that hemodynamic reactivity to mental stress is predicted better by a combination of resting SBP and PH than by either risk factor alone and that physiological reactivity may be an important accompaniment of increased risk for hypertension.

Cardiovascular responses to occupational stress in male medical students: a paradigm for ambulatory monitoring studies.

Hemodynamic responses to systematic variations in occupational stress were examined using ambulatory blood pressure monitors (ABPMs). The goals of this study were (a) to illustrate the effectiveness of employing naturally occurring, controlled variations in psychological stress levels in studies incorporating ABPMs and (b) to document the patterns of cardiovascular responses to acute examination stress in medical students. Heart rate, systolic blood pressure, diastolic blood pressure, rate-pressure product, and mood states were measured before, during, and after low-stress (lecture) and high-stress (examination) work in 44 healthy male medical students. The lecture day was characterized by stable patterns of cardiovascular activity across all three periods. Hemodynamic activity and reports of activation and distress were greater on the examination day than on the lecture day. Cardiovascular activity during the preexamination period was as high as that seen during the examination period itself, indicating an anticipatory stress effect. Pressor activity decreased after the examination, although some residual chronotropic activity was seen. Systematic changes in physiological responses to controlled variations in the level of this naturalistic stressor support the use of this paradigm for other ABPM studies. The elevations in preexamination responses seen here suggest the need to consider prestressor experiences in such studies.

Heart rate reactivity, behavior pattern, and parental hypertension as predictors of cardiovascular activity during cognitive challenge.

Hemodynamic responses were studied during work on serial subtraction and digits backwards tasks in 99 healthy male undergraduates jointly classified as high or low heart rate reactors, Type A or Type B behavior pattern, and as having positive or negative parental history of hypertension. Heart rate, systolic blood pressure, and diastolic blood pressure were recorded, and rate pressure product was calculated at rest and during the tasks. High heart rate reactors to a cold pressor task responded with relatively higher heart rate and rate pressure product during both cognitive tasks than low heart rate reactors. Type A subjects defined by Jenkins Activity Survey did not differ from Type Bs on any of these physiological variables, although behavioral evidence demonstrated greater effort and superior performance by the Type As. Subjects with a positive parental history of hypertension manifested greater systolic and diastolic pressures and rate pressure product responses to the cognitive challenge than those lacking such parental history. These results indicate that the individual difference variables of heart rate reactivity and parental history of hypertension predicted different hemodynamic response patterns to behavioral challenge.

Caffeine may potentiate adrenocortical stress responses in hypertension-prone men.

The effect of caffeine on blood cortisol levels and blood pressures was examined during rest and in response to a challenging psychomotor task in men with a low versus high risk of essential hypertension. Thirty-four healthy men ages 21-35 years were selected such that 17 were at high risk for hypertension (positive parental history and screening blood pressures of 135/85-155/95 mm Hg) and 17 were at low risk (negative parental history and no pressures above 132/84 mm Hg). Testing consisted of quiet rest (20 minutes); oral placebo (grapefruit juice) or caffeine administration (3.3 mg/kg in grapefruit juice); rest during a postdrug absorption period (40 minutes); work on an unsignalled simple reaction time task (15 minutes); and quiet rest (20 minutes). Blood pressures were recorded at 2-minute intervals, and blood samples were withdrawn via an indwelling catheter at the end of the baseline, drug absorption, task, and recovery periods. The combination of task plus caffeine produced the highest blood pressures in men at risk for hypertension. Cortisol levels were found to be sustained during rest in members of the high risk group after they had consumed caffeine, whereas members of the low risk group showed a modest decline. The high risk subjects also showed a significant rise in cortisol during (+3.7 micrograms/dl) and after (+4.0 micrograms/dl) work on the reaction time task after caffeine consumption.(ABSTRACT TRUNCATED AT 250 WORDS)