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 P_co2) 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 ∼ 40 cmH20 and expiratory pressures of ∼ 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 1/min/mmHg P_co2, normal: 2-41/min/mmHg, and high sensitivity: >4 1/min/mmHg of inspired P_C02 The normal group had a Pre-RMT CO₂ sensitivity of 2.88±0.60 and a post RMT sensitivity of 2.51±0.881/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 1/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.