Transdiaphragmatic pressure

Pimax values depend on a level of motivation and comprehension of the maneuver that is often not obtainable in mechanically ventilated patients (59). In contrast to the voluntary nature of the Pimax technique, transdiaphragmatic pressures elicited by single stimulations of the phrenic nerves—or twitch pressure—are independent of patients motivation and eliminate the influence of the central nervous system (Fig. 4) (53). 

Figure 7 Twitch transdiaphragmatic pressure elicited by phrenic nerve stimulation (upper panel) and FRC (lowerpanel) in a patient with severe emphysema before (left) and after (right) lung volume reduction surgery. The increase in transdiaphragmatic pressure after surgery was partly due to a decrease in the operating lung volume as demonstrated by the decrease in FRC. Abbreviation FRC, functional residual capacity. Source Data from Ref. 128.

Measures of transdiaphragmatic pressure and nonvolitional tests of RMS are more accurate and may be indicated when other tests are difficult to interpret in the context of clinical trials (27). 

Figure 5 Transdiaphragmatic twitch pressure recorded in mechanically ventilated patients recovering from an episode of acute respiratory failure. Box represents range of transdiaphragmatic twitch pressures recorded in ambulatory patients with severe COPD. Most mechanically ventilated patients had evidence of diaphragmatic weakness [data from Ref. 62 (open circles), and from Ref 61 (closed circles)]. Abbreviation COPD, chronic obstructive pulmonary disease. Source From Refs. 44.

Watson AC, Hughes PD, Louise HM, et al. Measurement of twitch transdiaphragmatic, esophageal, and endotracheal tube pressure with bilateral anterolateral magnetic phrenic nerve stimulation in patients in the intensive care unit Crit Care Med 2001 29(7) 1325-1331. Cattapan SE, Laghi F, Tobin MJ. Can diaphragmatic contractility be assessed by airway twitch pressure in mechanically ventilated patients Thorax 2003 58(l) 58-62. 

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