Diaphragm electromyogram measured with unilateral magnetic stimulation

Abstract

The purpose of this study was to establish the phrenic nerve conduction time (PNCT) for magnetic stimulation and further assess the relatively new technique of anterior unilateral magnetic stimulation (UMS) of the phrenic nerves in evaluating the diaphragm electromyogram (EMG).An oesophageal electrode was used to record the diaphragm compound muscle action potential (CMAP) elicited by supramaximal percutaneous electrical phrenic nerve stimulation (ES) and UMS from eight normal subjects. The oesophageal electrode used for recording the CMAP was positioned at the level of the hiatus and 3 cm below. The diaphragm CMAP was also recorded from chest wall surface electrodes in five subjects.All of the phrenic nerves could be maximally stimulated with UMS. A clear plateau of the amplitude of the CMAP was achieved for the right and left phrenic nerves. The mean amplitudes of the CMAP recorded from the oesophageal electrode were, for the right side, 0.740.29 mV (meanSD) for ES and 0.760.30 mV for UMS with maximal power output, and for the left side 0.880.33 mV for ES and 0.800.24 mV for UMS. PNCT measured by the oesophageal electrode with ES and UMS with maximal output were, for the right side, 7.00.8 ms and 6.90.8 ms, respectively, and for the left side 7.81.2 ms and 7.71.3 ms, respectively. However, the CMAP recorded from chest wall surface electrodes with UMS was unsuitable for the measurement of PNCT.The results suggest that unilateral magnetic stimulation of the phrenic nerves combined with an oesophageal electrode can be used to assess diaphragmatic electrical activity and measure the phrenic nerve conduction time. Eur Respir J 1999; 13: 385±390. Measuring the phrenic nerve conduction time (PNCT) and the diaphragm electromyogram (EMG) in response to phrenic nerve stimulation provides useful information for the assessment of diaphragm function and in the diagnosis of neuromuscular disease. PNCT measured with conventional electrical stimulation (ES) of phrenic nerves is a well-established technique [1±3]. However, this measurement is not widely used because ES can be technically difficult [4±6]. To overcome the problems of ES, cervical magnetic stimulation (CMS) [5,6] and unilateral magnetic stimulation (UMS) of the phrenic nerve [4] have been developed. These techniques are both painless and easy to apply. However, the latencies [6,7] and amplitudes [5] of the diaphragm compound muscle action potential (CMAP) measured with CMS are variable and different to those produced by ES. To evaluate the technique of UMS chest wall surface electrodes have previously been used to record the CMAP, and the PNCT was found to be shorter than that measured with ES [4]. The PNCT measured with an oesophageal electrode using UMS has not been reported, although it is considered that the diaphragm EMG recorded from an oesophageal electrode is more specific than when using surface electrodes [1]. The diaphragm EMG recorded from chest wall electrodes can be contaminated by extradiaphragmatic muscle activity [2]; therefore, the PNC...

Effect of lung volume on the oesophageal diaphragm EMG assessed by magnetic phrenic nerve stimulation

“…Even with great care maximal stimulation may not be achieved constantly [13] since stimulating-electrode position is so critical. In contrast, with bilateral anterolateral magnetic stimulation the phrenic nerves can be reliably maximally stimulated [9,10]. The ability to easily achieve maximal phrenic nerve stimulation makes magnetic stimulation a useful investigative tool.…”

“…Bilateral magnetic stimulation of the phrenic nerves was performed using two 43 mm figure of eight coils (P/N 9784-00) powered by Magstim 200 stimulators (Magstim Co. Ltd., Whitland, Dyfed, UK). The coils were placed anterolaterally on each side of the neck over the phrenic nerves [9,10]. Five to seven supramaximal twitches were obtained at each lung volume.…”

Assessment of diaphragm paralysis with oesophageal electromyography and unilateral magnetic phrenic nerve stimulation

“…Once the phrenic nerve is located, it can be difficult; with ES, to sustain maximal stimulation because the position and direction of the stimulating electrode is critical, whatever the stimulus intensity. In contrast, UMS uses a large surface area for stimulation and the phrenic nerve can be easily and reliably stimulated [3]. Because it is easy to apply and maximally stimulates the phrenic nerve, UMS is likely to be useful in clinically assessing patients with diaphragm paralysis.…”

“…The coil was placed anterolaterally on the neck over the phrenic nerve [3]. Subjects were seated upright in a chair and stimulation was performed at resting end-expiration.…”

“…Unilateral magnetic phrenic nerve stimulation (UMS) is an easy and reliable method of supramaximally stimulating the phrenic nerve, but the diaphragm CMAP recorded from chest wall surface electrodes can be contaminated by the coactivation of other chest wall muscles [3±6]. The diaphragm CMAP recorded from an oesophageal electrode is less contaminated [3,7]. However, conventional single-pair oesophageal electrodes can be difficult to optimally position at the diaphragm electrical activity region (EARdi) [8] and an absent CMAP in response to phrenic nerve stimulation can be simply due to the recording electrode being distant from the diaphragm.…”