Anterior magnetic phrenic nerve stimulation: laboratory and clinical evaluation

Objective: Anterior magnetic stimulation (aMS) of the phrenic nerves is a n ew method for the assessment of diaphragm contractility that might have par ticular applications for the clinical assessment of critically ill patients who are commonly supine. Design: We compared aMS with existing techniques for measurement of diaphra gm weakness and fatigue in 10 normal subjects, 27 ambulant patients with su spected 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 cmH(2)O, did not differ significantly from that obtained with bilateral supramaximal electrical stimulation (ES), 24. 9 cmH(2)O, or bilateral anterior magnetic phrenic nerve stimulation (BAMPS) , 27.3 cmH(2)O. 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 fal l in BAMPS-Tw Pdi (r(2) = 0.84, p = 0.03) indicating that aMS can detect di aphragm fatigue. In ambulant patients aMS agreed closely with existing meas ures of diaphragm strength. The maximal sniff Pdi correlated with both the aMS-Tw Pdi (r(2) = 0.60, p < 0.0001) and the BAMPS-Tw Pdi (r(2) = 0.65, p < 0.0001) and the aMS-Tw Pdi was a mean (SD) 2.2 (4.3) cmH(2)O less than BAM PS-Tw Pdi. In addition, aMS correctly identified diaphragm dysfunction in p atients studied on the ICU. Conclusions: We conclude that aMS is of clinical value for the investigatio n of suspected diaphragm weakness.