UNILATERAL MAGNETIC STIMULATION OF THE PHRENIC-NERVE

Background - Electrical stimulation of the phrenic nerve is a useful n on-volitional method of assessing diaphragm contractility. During the assessment of hemidiaphragm contractility with electrical stimulation, low twitch transdiaphragmatic pressures may result Et om difficulty i n locating and stimulating the phrenic nerve. Cervical magnetic stimul ation overcomes some of these problems, but this technique may not be absolutely specific and does not allow the contractility of one hemidi aphragm to be assessed. This study assesses both the best means of pro ducing supramaximal unilateral magnetic phrenic stimulation and its re producibility. This technique is then applied to patients. Methods - T he ability of four different coils to produce unilateral stimulation i n five normal subjects was assessed from twitch transdiaphragmatic pre ssure (TwPDI) measurements and diaphragmatic electromyogram (EMG) reco rdings. The results from magnetic stimulation were compared with those from electrical stimulation. whether the magnetic the contralateral p hrenic nerve as well as the intended phrenic nerve, EMG recordings fro m each hemidiaphragm were compared during stimulation on the same side and the opposite side relative to the recording electrodes. The EMG r ecordings were made from skin surface electrodes in five normal subjec ts and from needle electrodes placed in the diaphragm during cardiac s urgery in six patients. Similarly, the direction of hemidiaphragm move ment was evaluated by ultrasonography. To determine the usefulness of the technique in patients the 43 mm mean diameter double coil was used in 54 patients referred for assessment of possible respiratory muscle weakness. These results were compared with unilateral electrical phre nic stimulation, maximum sniff PDI, and TwPDI during cervical magnetic stimulation. Results - In the five normal subjects supramaximal stimu lation was established for eight out of 10 phrenic nerves with the 43 mm double coil. Supramaximal unilateral magnetic stimulation produced a higher TwPDI than electrical stimulation (mean (SD) 13.4 (2.5) cm H2 O with 35 mm coil; 14.1 (3.8) cm H2O with 43 mm coil; 10.0 (1.7) cm H2 O with electrical stimulation). Spread of the magnetic field to the op posite phrenic nerve produced a small amplitude contralateral diaphrag m EMG measured from skin surface electrodes which reached a mean of 15 % of the maximum EMG amplitude produced by ipsilateral stimulation. Si milarly, in six patients with EMG activity recorded directly from need le electrodes, the contralateral spread of the magnetic field produced EMG activity up to a mean of 3% and a maximum of 6% of that seen with ipsilateral stimulation. Unilateral magnetic stimulation of the phren ic nerve was rapidly achieved and well tolerated. In the 54 patients u nilateral magnetic TwPDI was more closely related than unilateral elec trical TwPDI to transdiaphragmatic pressure produced during maximum sn iffs and cervical magnetic stimulation. Unilateral magnetic stimulatio n eliminated the problem of producing a falsely low TwPDI because of t echnical difficulties in locating and adequately stimulating the nerve . Eight patients with unilateral phrenic nerve paresis, as indicated b y a unilaterally elevated hemidiaphragm on a chest radiograph and maxi mum sniff PDI consistent with hemidiaphragm weakness, were all accurat ely identified by unilateral magnetic stimulation. Conclusions - Unila teral magnetic phrenic nerve stimulation is easy to apply and is a rep roducible technique in the assessment of hemidiaphragm contractility. It is well tolerated and allows hemidiaphragm contractility to be rapi dly and reliably assessed because precise positioning of the coils is not necessary. This may be particularly useful in patients. In additio n, the anterolateral positioning of the coil allows the use of the mag net in the supine patient such as in the operating theatre or intensiv e care unit.