Efficacy Of Respiratory Muscle Training To Mitigate Co2 Retention During Exercise And Increased Respiratory Load

CO2 retention and associated toxicity may limit military diving capability when using an underwater breathing apparatus (UBA). A major factor for CO2 retention during diving is UBA respiratory loads. Respiratory muscle training (RMT) may be a mitigation strategy for CO2 retention by improving respiratory muscle strength to better tolerate UBA respiratory loads, possibly lowering CO2 retention. PURPOSE: Examine the effect of RMT on CO2 retention during moderate exercise with a UBA-like respiratory load. METHODS: 19 of 25 planned subjects (age: 40 ± 9 yrs; 18 m, 1f) have completed the study. Testing occurred over three sessions: baseline, pre-RMT, and post-RMT. Baseline testing included a cycle ergometer peak oxygen consumption (V̇O2peak) test. Pre- and post-RMT testing sessions consisted of respiratory muscle strength tests (maximum inspiratory and expiratory pressure tests (MIP and MEP)) and a UBA-like load test (UBALT). The UBALT included ≤30 min cycling at 60% V̇O2peak with added inspiratory and expiratory resistance and 0.5% inspired CO2; max end tidal CO2 (ETCO2) and CO2 retention latency (≥ 50 mmHg for 1 min) were recorded, if it occurred. Between pre- and post-testing, subjects completed 3-5 RMT sessions per week for 4 weeks. RMT sessions consisted of forced vital capacity maneuvers while breathing against inspiratory and expiratory resistance once a min for 30 min. Resistance started at 50 cmH2O and increased by 10 cmH2O weekly. Paired t-tests were used to analyze changes in respiratory muscle strength and max ETCO2 and a mixed effects Cox regression was used to analyze changes in time to CO2 retention. Significance was p < 0.05. All data are mean ± SD. RESULTS: RMT increased MIP (Pre-RMT: -134 ± 28 cmH2O; Post-RMT: -152 ± 25 cmH2O; p < .01) and MEP (Pre-RMT: 170 ± 49 cmH2O; Post-RMT: 206 ± 51 cmH2O; p < .01). RMT did not reduce CO2 retention latency (median time Pre-RMT: 2.94 min; Post-RMT: 2.62 min; Hazard Ratio = 2.05; p = .11) but did increase max ETCO2 during UBALT (Pre-RMT: 57.8 ± 4.0 mmHg; Post-RMT: 59.7 ± 5.1 mmHg; p < .05). CONCLUSIONS: RMT is effective for increasing respiratory muscle strength; however, it may not be an effective mitigation strategy for CO2 retention during moderate exercise with UBA-like resistive effort. Supported by Naval Sea Systems Command Deep Submergence Biomedical Development Program Naval Sea Systems Command Deep Submergence Biomedical Development Program