MAXIMUM RELAXATION RATE OF THE DIAPHRAGM DURING WEANING FROM MECHANICAL VENTILATION

Background - The maximum relaxation rate (MRR; percentage fall in pressure/10 ms) of oesophageal (FOES) and transdiaphragmatic (PDI) pressure slows under conditions of loaded breathing, and has been measured previously in normal subjects. MRR has not been measured in intubated patients weaning from mechanical ventilation. Methods - Five postoperative patients who were expected to wean and nine patients who had previously failed were studied. FOES and PDI MRR, peak oesophageal pressure during spontaneous breathing, maximum oesophageal pressure, and the inspiratory duty cycle were measured at rest during mechanical ventilation, in the first two minutes of spontaneous breathing, and after reventilation in those patients who failed, or before extubation in those patients who succeeded. Results - At rest FOES MRR in intubated patients had a range of 5.6-11 and PDI MRR 6.9-10.0, with a coefficient of variation of 9.9% and 7.3% respectively. FOES and PDI MRR were similar before and after extubation in five postoperative patients, and FOES MRR was reflected by endotracheal MRR measured at the airway. In five patients who failed to wean FOES and PDI MRR slowed by 47% and 44%, and fully recovered after 10 minutes reventilation. In four patients who were successfully weaned MRR was unchanged during spontaneous breathing. At the time when MRR decreased, the respiratory muscles were heavily loaded in relation to their strength. Conclusions - Weaning failure occurs when the applied load exceeds the capacity of the respiratory muscles, and this is associated with a slowing of respiratory muscle MRR.