Effects of Breathing Resistance on Resting Ventilatory Sensitivity to CO2
Technical rept. Jun 2013-Feb 2014
NAVY EXPERIMENTAL DIVING UNIT PANAMA CITY FL
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Carbon dioxide partial pressure PCO2 is normally tightly controlled in blood and tissues. However, if the respiratory controller also protects the respiratory muscles by optimizing for their loading, ventilatory response to CO2 may be damped when resistance to breathing increases. This has been observed during heavy exercise with resistive breathing loads. Also observed has been a lack of correlation between PCO2 at the end of the heavy exercise and resting ventilatory sensitivity to CO2. We hypothesized that the disparity was caused by a change in the effective ventilatory sensitivity to CO2 with resistance in the breathing circuit. In 16 Navy divers, ventilatory sensitivity to CO2 was measured at rest using Reads method. Measurements were made with the basic rebreathing circuit and with moderate resistance on the inspiratory, expiratory, or both sides. The resistance elements were designed to generate work of breathing per tidal volume of 1 kPa when minute ventilation was 100 Lmin. Resistance caused no systematic change in ventilatory sensitivity. However, the distribution of sensitivities, while not different from that for the general population, included some divers with very low values. Those divers may be at risk of hypercapnia, but a correlation between resting dry measurements and in-water exercise measurements cannot be established.
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