Can diaphragmatic contractility be assessed by airway twitch pressure in mechanically ventilated patients?

Background: In critically ill patients inspiratory muscle function may be assessed by measurements of maximal inspiratory airway pressure and the response of twitch transdiaphragmatic pressure (Pdi tw) to bilateral phrenic nerve stimulation. The first is limited by its total dependence on patient cooperation. Although the second approach is independent of patient volition, it is impractical because it requires oesophageal and gastric balloons. Because airway pressure is easily and non-invasively recorded in patients with artificial airways, we hypothesised that twitch airway pressure (Paw tw) reliably predicts Pdi tw and twitch oesophageal pressure (Poes tw) in mechanically ventilated patients. Methods: Thirteen mechanically ventilated patients recovering from an episode of acute respiratory failure received phrenic nerve stimulation at end exhalation. The rapid occlusion technique was used to record respiratory system mechanics. Results: Stimulations were well tolerated. Mean (SE) Paw tw at end exhalation was -8.2 (1.2) cm H2O and Poes tw and Pdi tw were -7.3 (1.1) and 10.4 (1.8) cm H2O, respectively. Stimulations produced a good correlation between Paw tw and Pdi tw (p<0.001), although the limits of agreement were wide. The results were similar for Poes tw. No relationship was found between the Paw tw/Poes tw ratio and respiratory system compliance or airway resistance. Paw tw reproducibility was excellent (mean coefficient of variation 6%, range 3-9%). Conclusions: Despite a good correlation between Paw tw and Poes tw, Paw tw did not reliably predict Poes tw or Pdi tw in mechanically ventilated patients. Nevertheless, the excellent reproducibility of Paw tw suggests that it may be a useful means of monitoring inspiratory muscle contractility in the routine care of mechanically ventilated patients.