Physiologic determinants of prolonged mechanical ventilation in patients after major surgery

The aim of the study was to evaluate the physiologic determinants of ventilator dependency in patients who underwent major surgery.

In this observational study, 43 stable tracheostomized patients undergoing prolonged ventilation (>14 days) were evaluated. Diaphragmatic muscle function was assessed invasively by the tension-time index of the diaphragm (TTdi), an indicator of diaphragm endurance time. The TTdi was calculated as transdiaphragmatic pressure/maximum transdiaphragmatic pressure × inspiratory time/total respiratory time and was recorded either when weaning from mechanical ventilation had finally been successful (n = 28 patients) or at the end of the fifth week in those patients in whom weaning failed (FW) (n = 15). Furthermore, the characteristics of survivors (n = 33) were compared with those of nonsurvivors (n = 10).

Successfully weaned patients had a lower breathing frequency/tidal volume or rapid shallow breathing index compared with FW patients (93.9 ± 45.5 vs 142.4 ± 60.3, respectively; P < .005). The TTdi was significantly higher in FW than in successfully weaned patients (0.107 ± 0.050 vs 0.148 ± 0.059; P < .023) and in nonsurvivors than in survivors (0.106 ± 0.046 vs 0.174 ± 0.058, P < .0001, respectively). A transdiaphragmatic pressure/maximum transdiaphragmatic pressure ratio of more than 40% was an independent predictor of mortality, whereas an increased frequency/tidal volume ratio and TTdi were independent predictors of weaning failure.

Difficult-to-wean patients after major surgery have overall a limited diaphragm endurance time, in particular, FW breathe very close to the fatigue threshold, and they adopt a rapid shallow breathing respiratory pattern to avoid crossing this threshold.