Components of aircraft life support systems interact with each other and the user.

The life support system in a tactical aircraft provides necessary supplemental oxygen to the aircrew. However, interactions among its various components may generate unexpected breathing loads. We focus here on the interactions between a regulator and breathing mask commonly used together in the U.

Transcutaneous and End-Tidal CO₂ Measurements in Hypoxia and Hyperoxia.

Transcutaneous measurement of carbon dioxide (CO₂) has been proposed for physiological monitoring of tactical jet aircrew because in some clinical settings it mirrors arterial CO₂ partial pressure (Pco₂). End-tidal monitoring in laboratory settings is known to give high-fidelity estimates of Pco₂. The correspondence between end-tidal (Pco₂) and transcutaneous Pco₂ (tcPco₂) was examined in healthy volunteers under laboratory conditions of hyperoxia and hypoxia.

Risks from Breathing Elevated Oxygen.

Effects of breathing gas with elevated oxygen partial pressure (Po₂) and/or elevated inspired oxygen fraction (Fo₂) at sea level or higher is discussed. High Fo₂ is associated with absorption problems in the lungs, middle ear, and paranasal sinuses, particularly if Fo₂ > 80% and small airways, Eustachian tubes, or sinus passages are blocked. Absorption becomes faster as cabin altitude increases.

Pulmonary effects of repeated six-hour normoxic and hyperoxic dives.

This study examines differential effects of immersion, elevated oxygen partial pressure, and exercise on pulmonary function after series of five daily six-hour dives at 130 kPa (1.3 ATA), with 18 hours between dives. Five cohorts of 10 to 14 divers participated.

Effects of prolonged and repeated immersions on heart rate variability and complexity in military divers.

Background: The influence of prolonged and repeated water immersions on heart rate variability (HRV) and complexity was examined in 10 U.S. Navy divers who completed six-hour resting dives on five consecutive days.

Breathing 100% oxygen during water immersion improves postimmersion cardiovascular responses to orthostatic stress.

Physiological compensation to postural stress is weakened after long-duration water immersion (WI), thus predisposing individuals to orthostatic intolerance. This study was conducted to compare hemodynamic responses to postural stress following exposure to WI alone (Air WI), hyperbaric oxygen alone in a hyperbaric chamber (O HC), and WI combined with hyperbaric oxygen (O WI), all at a depth of 1.35 ATA, and to determine whether hyperbaric oxygen is protective of orthostatic tolerance.

Residual oxygen time model for oxygen partial pressure near 130 kPa (1.3 atm).

A two-part residual oxygen time model predicts the probability of detectible pulmonary oxygen toxicity P(P[O2tox]) after dives with oxygen partial pressure (PO2) approximately 130 kPa, and provides a tool to plan dive series with selected risk of P[O2tox]. Data suggest that pulmonary oxygen injury at this PO2 is additive between dives. Recovery begins after a delay and continues during any following dive.

Thermoneutral water immersion and hyperbaric oxygen do not alter cortisol regulation.

Research documenting changes in cortisol concentration following hyperbaric exposures has been contradictory, possibly due to the inclusion of many confounding factors. Therefore, the aim of this study was to document short- and long-term cortisol responses following repeated water immersions arid/or exposure to raised partial pressure of oxygen under controlled conditions. Thirty-two Navy divers (31 ± 7 [19-44] years; mean ± SD) were exposed to one of three resting thermoneutral experimental conditions at a pressure of 1.

Cumulative effects of repeated exposure to pO2 = 200 kPa (2 atm).

Even asymptomatic exposures to elevated oxygen partial pressure (pO2) can influence subsequent exposures. Dry chamber dives of three hours' duration at pO2 of 200 kPa were conducted to examine cumulative effects. Experiments were single (n = 27), or paired exposures with surface intervals (SIs) 15 to 17 hours (n = 30), six hours (n = 33), or three hours (n = 36).

Cardiovascular and autonomic responses to physiological stressors before and after six hours of water immersion.

The physiological responses to water immersion (WI) are known; however, the responses to stress following WI are poorly characterized. Ten healthy men were exposed to three physiological stressors before and after a 6-h resting WI (32-33°C): 1) a 2-min cold pressor test, 2) a static handgrip test to fatigue at 40% of maximum strength followed by postexercise muscle ischemia in the exercising forearm, and 3) a 15-min 70° head-up-tilt (HUT) test. Heart rate (HR), systolic and diastolic blood pressure (SBP and DBP), cardiac output (Q), limb blood flow (BF), stroke volume (SV), systemic and calf or forearm vascular resistance (SVR and CVR or FVR), baroreflex sensitivity (BRS), and HR variability (HRV) frequency-domain variables [low-frequency (LF), high-frequency (HF), and normalized (n)] were measured.

Exercise carbon dioxide (CO2) retention with inhaled CO2 and breathing resistance.

Combined effects on respiratory minute ventilation (VE)--and thus, on end-tidal carbon dioxide partial pressure (P(ET)CO2)--of breathing resistance and elevated inspired carbon dioxide (CO2) had not been determined during heavy exercise. In this Institutional Review Board-approved, dry, sea-level study, 12 subjects in each of three phases exercised to exhaustion at 85% peak oxygen uptake while V(E) and P(ET)CO2 were measured. Participants inhaled 0%, 1%, 2% or 3% CO2 in air, or 0% or 2% CO2 in oxygen, with or without breathing resistance, mimicking the U.

Physiologically acceptable resistance of an air purifying respirator.

Physiologically acceptable limits of inspiratory impediment for air purifying respirators (APRs) were sought.Measurements on 30 subjects included pressure in, and flow through, an APR, and respiratory and cardiovascular variables. Exercise with and without APR included ladder climbing, load lift and transfer, incremental running and endurance running, with endurance at 85% peak oxygen uptake.

Validity and reliability of diastolic pulse contour analysis (windkessel model) in humans.

The present study assessed (1) the impact of the measurement site (lower versus upper extremity) on the corresponding compliance variables and (2) the overall reliability of diastolic pulse contour (Windkessel-derived) analysis in normal and hypertensive subjects. Arterial tonograms were recorded in the supine position from the radial and posterior tibial arteries in 20 normotensive (116+/-12/68+/-8 mm Hg) and 27 essential hypertensive subjects (160+/-16/94+/-14 mm Hg). Ensemble-averaged data for each subject were fitted to a first-order lumped-parameter model (basic Windkessel) to compute whole-body arterial compliance (C(A)) and to a third-order lumped-parameter model (modified Windkessel) to compute proximal compliance (C(1)) and distal compliance (C(2)).