Similarities between behavior of respiratory muscles in breath-holding and in elastic loading

Breath-holding subjects often exhibit involuntary contractions of respiratory muscles which are much stronger and faster than the efforts they would make during unrestricted breathing at the same level of CO2 and O2. To gain a better understanding of the genesis of these contractions, we compared them with the respiratory response to external elastic loading. Normal men rebreathed a mixture of 8% CO2 in oxygen against no load, elastic loads of 25 and 75 cm H2O/L, and held their breath, equivalent to an elastic load of 226 cm H2O/L. At iso-CO2, increasing loads led to progressively smaller tidal volumes, inspiratory flow rates and ventilation. However, respiratory muscles were progressively activated by the loads, as indicated by increasing occlusion pressure, so that inspiratory flow rate and ventilation were defended much better than could be expected if no neural compensation occurred. The pattern of respiratory muscle activity in breath-holding was qualitatively similar to that in elastic loading, and seemed quantitatively to be an extreme form of reaction to a large load. The reduction in inspiratory time and therefore of peak inspiratory pressure and ratio of inspiratory to total time with very large loads could be viewed as an adaptive response to limit respiratory muscle fatigue.