The energy cost of breathing at depth and the effects of resistive respiratory muscle training
Simpson, Amber-Louise Elizabeth
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Background . Respiratory muscle training against resistance (RRMT) increases respiratory muscle strength and endurance, and swimming endurance at depth. We hypothesized that such effects result from RRMT reducing the high energy cost of ventilation at depth. Methods . Eight subjects breathed air in a hyperbaric chamber at a pressure equivalent to 55 fsw, before and after RRMT. They rested for 10 min, cycled an ergometer at 100W for 10 min, rested for 10 min, and then performed paced isocapnic (CO 2 admixture to inspired air) simulated exercise ventilation (ISEV). Results . Pre-RRMT minute-ventilation (VE) was 11.95 ± 3.09 L/min during rest, 47.74 ± 8.44 L/min during exercise, and was not different during ISEV (49.98 ± 10.41 L/min). Post-RRMT the values at rest, exercise and ISEV were not different. The end-tidal PCO 2 's were matched to the VE and were 34.65 ± 4.6 at rest, 44.49 ± 4.49 during exercise and 44.26 ± 2.54 mmHg during ISEV, and were not significantly different post-RRMT (33.58 ± 4.30, 43.59 ± 5.12, 43.40 ± 2.08 mmHg). Oxygen uptake (VO 2 ) was 0.32 ± 0.08 L/min at rest, 1.78 ± 0.15 during exercise and not different pre and post- RRMT, while during ISEV VO 2 decreased significantly from pre to post-RRMT (0.46 ±0.06 vs. 0.36 ± 0.11 L/min). The energy cost of ventilation (VO 2 /VE) was 0.027 ± 0.0063 L/L at rest and 0.038 ± 0.0067 during exercise, and did not change after RRMT; the values were significantly lower during ISEV (0.0094± 0.0021 L/L vs. 0.0074 ± 0.0023 L/L). Conclusion . The energy cost of ventilation, measured as VO 2 , at a simulated depth of 55 fsw was reduced significantly by RRMT. Whether this change was due to reduced work of breathing, and/or increased efficiency of the respiratory muscles (or some other factor) remains to be determined.