THE LOAD OF INSPIRATORY MUSCLES IN PATIENTS NEEDING MECHANICAL VENTILATION

We studied 31 consecutive mechanically ventilated patients with acute respiratory failure in two stages: (1) During spontaneous breathing th rough the respirator, switching from full mechanical assistance to con tinuous positive airway pressure mode with 0 cm H2O pressure. We measu red maximum inspiratory pressure and continuously monitored the patter n of breathing. After 8 to 25 min, none of the patients were able to s ustain spontaneous breathing and mechanical ventilation was required t o resume. (2) Subsequently, during mechanical ventilation, by manipula ting the variables of the ventilator we simulated the pattern of spont aneous breathing the patients had just before the re-institution of me chanical ventilation. We assessed the respiratory mechanics by the con stant flow end-inspiratory and end-expiratory occlusion method. Intrin sic positive end-expiratory pressure was present in 29 patients. The r atio of the mean inspiratory pressure per breath over the maximum insp iratory pressure (PI/PImax), as well as Ppeak/PImax, had excessively h igh mean values, equal to 0.42 +/- 0.11 and 0.56 +/- 0.10, respectivel y. Pressure-time index was 0.14 +/- 0.04. When we plotted the PI/PImax and Ppeak/PImax against the dynamic increase in FRC, we found that th e PI/Plmax of 13 patients (42%) and the Ppeak/PImax of 25 of 31 patien ts (81%) were placed above a hypothetical critical line, representing the critical inspiratory pressures above which fatigue may occur. In a ddition, almost all patients were gathered around the critical line. W e conclude that during discontinuation from mechanical ventilation (MV ) almost all patients breathe against a high inspiratory load and thei r inspiratory muscles perform work that may lead to fatigue.