Assessment of neonatal diaphragm function using magnetic stimulation of the phrenic nerves

A nonvolitional test to assess diaphragm strength in neonates has not been previously described. Our aim was to assess the feasibility of cervical (CM S) and anterior (AMS) magnetic stimulation of the phrenic nerves in neonate s. Double circular stimulating coils (90-mm) were used. For CMS, one coil w as placed over the cervical spine to bilaterally stimulate the phrenic nerv e roots, whereas for AMS the coils were placed on the anterolateral aspect of the neck to allow unilateral and bilateral stimulation. Diaphragm contra ctility was assessed as transdiaphragmatic pressure (Pdi) measured with bal loon catheters positioned in the midesophagus and stomach. Stimulus suprama ximality was assessed by examining diaphragm twitch Pdi (TwPdi) across a ra nge of stimulator outputs; 85, 90, 95, and 100% of maximum. Pressure signal s were measured by differential pressure transducer and displayed in real t ime on a computer. Patients were studied supine during sleep. CMS was perfo rmed on seven neonates (mean gestational age [GA] 38 wk, range 33 to 40 wk) and AMS on 18 neonates (mean GA 37 wk, range 32 to 41 wk). The mean (SD) T wPdi with CMS was 2.5 (0.8) cm H2O. CMS was not supramaximal; reducing the stimulator output below 100% caused marked reductions in TwPdi, also the sh ape of the pressure waveforms suggested that CMS may not have activated the diaphragm alone. Mean (SD) TwPdi with AMS was 4.5 (1.3) cm H2O on the left , 4.1 (0.9) cm H2O on the right, and 8.7 (3.9) cm H2O for bilateral stimula tion. The shape of the pressure waveforms suggested that AMS was more speci fic and a plateau in TwPdi at higher stimulator outputs indicated supramaxi mality. We conclude that AMS may provide a useful technique to assess diaph ragm function in the neonate.