Tracheal tube pressure change during magnetic stimulation of the phrenic nerves as an indicator of diaphragm strength on the intensive care unit

Diaphragm strength can be assessed from twitch gastric (TwPgas), twitch oesophageal (TwPoes), and twitch transdiaphragmatic pressure (TwPdi) in response to phrenic nerve stimulation. This requires the passage of balloon catheters, which may be difficult. Changes in pressure measured at the mouth during phrenic nerve stimulation avoid the need for balloon catheters. We hypothesized that pressures measured at the tracheal tube during phrenic stimulation, could also reflect oesophageal pressure change as a result of isolated diaphragmatic contraction and, therefore, reflect diaphragm strength. We aimed to establish the relationship between twitch tracheal tube pressure (TwPet), TwPoes, and TwPdi in patients in the supine and sitting positions. The phrenic nerves were stimulated magnetically bilaterally, in 14 ICU patients while supine and on another occasion while sitting up at 45 degrees. In the sitting position mean TwPoes was 9.1 cm H(2)O and TwPet 11.3 cm H(2)O (mean(SD) difference -2.2 (SD 1.5)). In the supine position mean TwPoes was 8.1 cm H(2)O and TwPet 9.9 cm H(2)O (mean difference -1.8 (2.2)). The difference between TwPoes and TwPet was less at low twitch amplitude; less than +/-1 cm H(2)O below a mean twitch height of 8 cm H(2)O supine and 10 cm H(2)O sitting. Sitting TwPet was related to TwPoes r(2)=0.93 and TwPdi r(2)=0.65 (P<0.01). Supine TwPet was related to TwPoes r(2)=0.84 and TwPdi r(2)=0.83 (P<0.01). The mean within occasion coefficient of variation while sitting was TwPet=13.3%, TwPoes=13.9%, TwPdi=11.2%, and supine TwPet=11.6%, TwPoes=14.6%, TwPdi=11.8%. We conclude that TwPet reflects TwPoes during diaphragmatic stimulation and is worthy of further study to establish its place as a guide to the presence of respiratory muscle strength and fatigue.