Effects of respiratory muscle training on respiratory CO2 sensitivity in SCUBA divers.

Typically, ventilation is tightly matched to CO2 production. However, in some cases CO2 is retained (SCUBA diving). One factor behind hypoventilation in divers may be low respiratory CO2 sensitivity. If this is due to inadequate respiratory muscle performance it might be remedied by respiratory muscle training (RMT). We retrospectively investigated respiratory CO2 sensitivity prior to and after RMT in several groups of SCUBA divers. CO2 sensitivity (slope of expired ventilation as a function of inspired PCO2) was measured with a rebreathing technique in 35 subjects with diving experience. RMT consisted of either isocapnic hyperventilation or intermittent vital capacity breaths (twice/minute) against spring loaded breathing valves imposing static and resistive loads generating average inspiratory pressures of approximately 40 cmH2O and expiratory pressures of approximately 47 cmH2O; RMT was performed 30 min/day, 3 or 5 days/week for 4 weeks. Based on pre-RMT CO2 sensitivity the subjects were divided into three groups: low sensitivity: < 2 l/min/mmHg PCO2, normal: 2-4 l/min/mmHg, and high sensitivity: > 4 l/min/mmHg of inspired PCO2. The normal group had a Pre-RMT CO2 sensitivity of 2.88 +/- 0.60 and a post RMT sensitivity of 2.51 +/- 0.88 l/min/mmHg (Mean +/- SD, n = 19, p = n.s). Response in low sensitivity subjects increased from 1.41 +/- 0.32 to 2.27 +/- 0.53 (n = 10, p = 0.002,) while in the high sensitivity group it decreased from 5.41 +/- 1.25 to 2.90 +/- 0.32 l/min/mmHg (n = 6, p = 0.003). These preliminary findings showed that 46% of the subjects had abnormal sensitivity, and suggest that RMT may normalize it in hypo- and hyper-ventilating divers. If the present results are verified, RMT may be an effective means of enhancing safety in CO2 retaining divers.