Anterior magnetic phrenic nerve stimulation: Laboratory and clinical evaluation

Abstract

Objective: Anterior magnetic stimulation (aMS) of the phrenic nerves is a new method for the assessment of diaphragm contractility that might have particular applications for the clinical assessment of critically ill patients who are commonly supine. Design: We compared aMS with existing techniques for measurement of diaphragm weakness and fatigue in 10 normal subjects, 27 ambulant patients with suspected diaphragm weakness and 10 critically ill patients. Setting: Laboratory and intensive care unit of two university hospitals. Results: Although aMS was not demonstrably supramaximal in normal subjects, the mean value of twitch transdiaphragmatic pressure (Tw Pdi) obtained at 100% of stimulator output, 23.7 cm H2O, did not differ significantly from that obtained with bilateral supramaximal electrical stimulation (ES), 24.9 cm H2O, or bilateral anterior magnetic phrenic nerve stimulation (BAMPS), 27.3 cm H2O. A fatiguing protocol produced a 20 % fall in aMS-Tw Pdi and a 19 % fall in BAMPS-Tw Pdi; the fall in aMS-Tw Pdi correlated with the fall in BAMPS-Tw Pdi (r2 = 0.84, p = 0.03) indicating that aMS can detect diaphragm fatigue. In ambulant patients aMS agreed closely with existing measures of diaphragm strength. The maximal sniff Pdi correlated with both the aMS-Tw Pdi (r2 = 0.60, p < 0.0001) and the BAMPS-Tw Pdi (r2 = 0.65, p < 0.0001) and the aMS-Tw Pdi was a mean (SD) 2.2 (4.3) cmH2O less than BAMPS-Tw Pdi. In addition, aMS correctly identified diaphragm dysfunction in patients studied on the ICU. Conclusions: We conclude that aMS is of clinical value for the investigation of suspected diaphragm weakness.