Serial physiologic studies of a chronic obstructive pulmonary disease patient in acute respiratory failure: clues for weaning?

A patient with severe chronic obstructive pulmonary disease was studied during acute respiratory failure. On the day of intubation his respiratory rate was 42, the tidal volume 295 ml, and the maximal inspiratory pressure 8 cm H2O. These parameters improved with rest by mechanical ventilation to 16, 620 ml, and 30 cm H2O, respectively, on the day of successful weaning. Daily tidal volumes correlated significantly with maximal inspiratory muscle pressures (r = 0.936; p less than 0.001). Respiratory system compliances and resistances were measured by the inflation, the end-inspiratory occlusion, and the interrupter methods. In general, inflation compliance and occlusion compliance were comparable and significantly smaller than the interrupter compliance (p less than 0.002 and p less than 0.003, respectively), whereas inflation resistance and occlusion maximal resistance were also comparable but significantly smaller than the interrupter resistance (p less than 0.0008 and p less than 0.0006, respectively). The former was due to increased hysteresis of the pressure volume curves and the latter due to expiratory compression of airways. The compliance was low, and the resistance was high on the day of intubation and became much higher and lower, respectively, on the day of successful extubation. These physiological changes were associated with weaning difficulty. We conclude that respiratory failure and weaning are complex physiologic events under the influence of muscle strength, lung mechanics, gas exchange, and control of breathing. Therefore, prediction of weaning success based upon one or two measured parameters as has been done is probably inadequate in difficult patients.