DIAPHRAGM PERFORMANCE DURING MAXIMAL VOLUNTARY VENTILATION IN CHRONICOBSTRUCTIVE PULMONARY-DISEASE

In normal subjects 2 min of maximal voluntary hyperventilation results in failure of tension generation and low-frequency fatigue of the dia phragm. Patients with severe chronic obstructive pulmonary disease (CO PD) do not develop diaphragm fatigue during exhaustive treadmill exerc ise despite excessive inspiratory muscle loading and we hypothesized t hat they might be relatively resistant to the development of diaphragm fatigue during maximal ventilation. In six patients with severe COPD (mean FEV(1) 0.671) we therefore loaded the diaphragm using 2 min of m aximal isocapnic ventilation (MIV). initial mean ventilation was 28.6 L/min and diaphragm pressure-time product (PTPdi) 602 cm H2O . s/min; these values were sustained throughout MIV without significant decline . Mean twitch transdiaphragtime after MIV. Compared with normal subjec ts previously studied in our laboratory (Hamnegard, C.-H., et al. Eur. Respir. J. 1996;9:241-247) the reduction in PTPdi was disproportionat ely greater than the reduction in Tw Pdi. We conclude that, unlike nor mal subjects, 2 min of MIV causes neither failure of diaphragm perform ance nor low-frequency diaphragm fatigue in patients with severe COPD. It is likely that the diaphragm makes a relatively limited contributi on to the generation of maximal levels of ventilation in severe COPD.