COMPARISON OF CERVICAL MAGNETIC AND TRANSCUTANEOUS PHRENIC-NERVE STIMULATION BEFORE AND AFTER THRESHOLD LOADING

Brief supramaximal stimulation of the phrenic nerves (twitch) is consi dered a promising technique to detect diaphragmatic fatigue in humans. However, the most commonly employed methodology (transcutaneous stimu lation) is technically difficult. Cervical magnetic stimulation is a r ecently described technique that is potentially simpler and may obviat e some of the problems inherent with transcutaneous stimulation. The p urpose of this study was to determine the ability of cervical magnetic stimulation to evaluate diaphragmatic function. Accordingly, we measu red transdiaphragmatic pressure (Pdi) during transcutaneous and cervic al magnetic stimulation of the phrenic nerves before and after a poten tially fatiguing task; inspiratory threshold loading to task failure. During threshold loading, subjects generated approximately 60% of thei r maximal esophageal pressure with each breath until they could no lon ger reach the target pressure. At least 10 twitches were obtained duri ng both transcutaneous and magnetic stimulation before and 10, 30, 60, and 120 min after threshold loading. Control twitch Pdi was significa ntly larger during magnetic stimulation compared with transcutaneous s timulation: 39.3 +/- 3.0 (mean +/- SE) versus 27.4 +/- 2.3 cm H2O, p < 0.0005. This increase in twitch Pdi was solely due to the esophageal component. Following threshold loading, a significant reduction in tra nscutaneous twitch Pdi was seen in only three of the 10 subjects. Mean transcutaneous twitch Pdi fell only slightly from 27.4 +/- 2.3 during control to 25.1 +/- 2.2 cm H2O at 10 min after loading (p < 0.004). I n contrast, magnetic twitch Pdi was significantly reduced in nine of t he 10 subjects following threshold loading. Mean magnetic twitch Pdi f ell from 39.3 +/- 3.0 during control to 31.1 +/- 3.0 cm H2O at 10 min after loading (p < 0.0001). The average fall in twitch Pdi post-loadin g (expressed as a percentage of the control value) was significantly g reater for magnetic stimulation compared with transcutaneous stimulati on: 21.0 +/- 3.1 versus 7.8 +/- 2.9%, p < 0.0001. In summary: (1) in t he fresh state, twitch Pdi is larger with magnetic stimulation compare d with transcutaneous stimulation, and (2) transcutaneous and cervical magnetic twitch Pdi are affected differently by threshold loading to task failure.