Magnetic stimulation of the phrenic nerves to assess diaphragm strength on the Intensive Care Unit

Traditional methods of assessing respiratory muscle strength in the critically ill rely on some degree of co-operation from the patient, and are of limited use. We performed bilateral magnetic stimulation of the phrenic nerves, using two 43 mm magnetic coils placed anteriorly on the neck [1]. The transdiaphragmatic pressure change (TwPdi) was recorded using oesophageal and gastric balloon catheters in the conventional manner. Twitch endotracheal tube pressure (TwPett) was also recorded, which reflects twitch oesophageal pressure (TwPoes). The pressure readings, Poes, Pgas and Pett were displayed and recorded on a computer together with the calculated value for Pdi (Pgas-Poes).

Twenty critically ill patients were studied (12 male, eight female), with a mean age of 59 years. Average length of ICU stay prior to the study was 27 days. The mean TwPdi was 9.5 cmH₂O (range 1.0-29.3), mean TwPoes was 6.6 cmH₂O (range 0.5-22.9), and mean TwPett was 70 cmH₂O (range 0.0-26.3). The mean difference between TwPett and TwPoes was 0.4 cmH₂O, and the correlation of the means of TwPett to TwPoes was 0.93.

Diaphragm contractility can be assessed in the sedated ICU patient, by magnetic stimulation of the phrenic nerves. This technique is non-volitional and is reasonably well tolerated. Our data shows that diaphragm conctractility in the critically ill patient is considerably less than in the laboratory based control subject [1]. Also, we report a good correlation between TwPoes and TwPett, leading to the possibility of further simplification of the technique.