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.S. Navy, the CRU-103 regulator and MBU 23/P mask, and some effects of the interactions on the user. The data reported were collected during a larger research effort examining potential physiological and cognitive effects of low regulator inlet pressures. Seventeen participants completed a series of tasks under mild exercise while breathing 40% O2 (balance N2) from an MBU-23/P mask supplied by a CRU-103 regulator with supply pressures 10, 6, 4, and 2 psig (CRU-103 specifications are for inlet pressures from 5 to 120 psig). Variables measured included flow to the mask and pressures at the regulator supply, in the hose to the mask, and in the mask. In addition to restricting inspiratory flow, low inlet pressure to the CRU-103 caused a counterintuitive overshoot in gas delivery pressure at end-inspiration, a mean increase of 1.5 cm H2O between the 10- and 2 psig conditions. The added pressure to the exhalation valve increased the expiratory threshold, the pressure to start expiratory flow, by approximately 2 cm H2O, increasing the effort needed to exhale.

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.S. Navy's MK 16 rebreather underwater breathing apparatus (UBA). Compared to air baseline (0% inspired CO2 in air without resistance): (1) Oxygen decreased baseline V(E) (p < 0.01); (2) Inspired CO2 increased V(E) and P(ET)CO2 (p < 0.01); (3) Resistance decreased V(E) (p < 0.01); (4) Inspired CO2 with resistance elevated P(ET)CO2 (p < 0.01). In air, V(E) did not change from that with resistance alone. In oxygen, V(E) returned to oxygen baseline. End-exercise P(ET)CO2 exceeded 60 Torr (8.0 kPa) in three tests. Subjects identified hypercapnia poorly. Results support dual optimization of arterial carbon dioxide partial pressure and respiratory effort. Because elevated CO2 may not increase V(E) if breathing resistance and VE are high, rebreather UBA safety requires very low inspired CO2.

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. Resistance that did not alter minute ventilation (VE) was judged acceptable long-term. Acceptable short-term impediments were deduced from end exercise conditions. Proposed long-term limits are inspiratory work of breathing per tidal volume (WOBi/VT) ≤ 0.9 kPa and peak inspiratory pressure (P (i) peak) ≤1.2 kPa. Proposed short-term limits are: for VE ≤110 L min(-1), WOBi/VT ≤1.3 kPa and P (i) peak ≤ 1.8 kPa; and for VE >130 L min(-1), WOBi/VT ≤1.6 kPa. A design relation among VE, pressure­flow coefficients of an APR, and WOBi/VT is proposed. STATEMENT OF RELEVANCE: This work generalises results from one APR by considering the altered physiological parameters related to factors inhibiting exercise. Simple expressions are proposed to connect bench-test parameters to the relation between ventilation and work of breathing. Population-based recommendations recognise that those who need more air flow can also generate higher pressures.

Respiratory muscle training improves swimming endurance in divers.

Respiratory muscles can fatigue during prolonged and maximal exercise, thus reducing performance. The respiratory system is challenged during underwater exercise due to increased hydrostatic pressure and breathing resistance. The purpose of this study was to determine if two different respiratory muscle training protocols enhance respiratory function and swimming performance in divers. Thirty male subjects (23.4 +/- 4.3 years) participated. They were randomized to a placebo (PRMT), endurance (ERMT), or resistance respiratory muscle training (RRMT) protocol. Training sessions were 30 min/day, 5 days/week, for 4 weeks. PRMT consisted of 10-s breath-holds once/minute, ERMT consisted of isocapnic hyperpnea, and RRMT consisted of a vital capacity maneuver against 50 cm H(2)O resistance every 30 s. The PRMT group had no significant changes in any measured variable. Underwater and surface endurance swim time to exhaustion significantly increased after RRMT (66%, P < 0.001; 33%, P = 0.003) and ERMT (26%, P = 0.038; 38%, P < 0.001). Breathing frequency (f (b)) during the underwater endurance swim decreased in RRMT (23%, P = 0.034) and tidal volume (V (T)) increased in both the RRMT (12%, P = 0.004) and ERMT (7%, P = 0.027) groups. Respiratory endurance increased in ERMT (216.7%) and RRMT (30.7%). Maximal inspiratory and expiratory pressures increased following RRMT (12%, P = 0.015, and 15%, P = 0.011, respectively). Results from this study indicate that respiratory muscle fatigue is a limiting factor for underwater swimming performance, and that targeted respiratory muscle training (RRMT > ERMT) improves respiratory muscle and underwater swimming performance.