Focal magnetic phrenic nerve stimulation. Small figure of eight-shaped magnetic coils can be used for focal stimulation (focal MS) of the phrenic nerve in the neck unilaterally or bilaterally, at the same point as stimulation by ES (114, 115). Bilateral focal MS is easily applied by an operator standing in front of the sub
ject and gives values for Pdi,tw and the Pga,tw/Pes,tw ratio that are close to ES (115), thus avoiding any problems associated with stimulating upper trunk muscles (see subsequent section). This technique could make bilateral PNS easier in supine patients, as in the ICU, because for ES the operator has to stand behind the patient and for CMS the subject's neck has to lie over the coil, which may be uncomfortable and impede optimal positioning. Unilateral focal MS may allow assessment of the mechanical properties of one hemidiaphragm alone (87, 114, 116).
Anterior magnetic stimulation. The possibility of evoking a bilateral diaphragm EMG response through anterior magnetic stimulation (antMS) with a 90-mm circular coil similar to that used for CMS, placed flat over the upper part of the sternum, has been described (106). Anterior magnetic stimulation is potentially a simple technique also applicable to supine subjects in difficult settings, but pressure responses remain to be evaluated.
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