Measurement of twitch transdiaphragmatic, esophageal, and endotracheal tube pressure with bilateral anterolateral magnetic phrenic nerve stimulation in patients in the intensive care unit

Objective: In the critically ill, respiratory muscle strength usually has b een assessed by measuring maximum inspiratory pressure. The maneuver is vol itional, and results can be unreliable. The nonvolitional technique of bila teral anterolateral magnetic stimulation of the phrenic nerves, producing t witch transdiaphragmatic pressure, has been successful in normal subjects a nd ambulatory patients. In this study we used the technique in the intensiv e care unit and explored the measurement of twitch endotracheal tube pressu re as a less invasive technique to assess diaphragmatic contractility. Design: Clinical study to quantify diaphragm strength in the intensive care unit. Setting: patients from three London teaching hospital intensive care units and high-dependency units. Patients: Forty-one intensive care patients were recruited. Of these, 33 (2 0 men, 13 women) were studied. Interventions: Esophageal and gastric balloon catheters were passed through the anaesthetized nose, and an endotracheal tube occlusion device was plac ed in the Ventilation circuit, next to the endotracheal tube. Two 43-mm mag netic coils were placed anteriorly on the patient's neck, and the phrenic n erves were stimulated magnetically. Measurements and Main Results: On phren ic nerve stimulation, twitch gastric pressure, twitch esophageal pressure, twitch transdiaphragmatic pressure, and twitch endotracheal tube pressure w ere measured. Forty-one consecutive patients consented to take part in the study, and twitch pressure data were obtained in 33 of these. Mean transdia phragmatic pressure was 10.7 cm H2O, mean twitch esophageal pressure was 6. 7 cm H2O, and mean twitch endotracheal tube pressure was 6.7 cm H2O. The me an difference between twitch esophageal pressure and twitch endotracheal tu be pressure was 0.02 cm H2O. Correlation of the means of twitch endotrachea l tube pressure to twitch esophageal pressure was 0.93, and that for twitch endotracheal tube pressure to transdiaphragmatic pressure was 0.78. Conclusions: Transdiaphragmatic pressure can be measured in the critically ill to give a nonvolitional assessment of diaphragm contractility, but not all patients can be studied. At present, the relationship of twitch endotra cheal tube pressure to transdiaphragmatic pressure is too variable to relia bly represent a less invasive measure of diaphragm strength.