C Co om mp pa ar ri is so on n o of f c ce er rv vi ic ca al l m ma ag gn ne et ti ic c s st ti im mu ul la at ti io on n a an nd d b bi il la at te er ra al l p pe er rc cu ut ta an ne eo ou us s e el le ec ct tr ri ic ca al l s st ti im mu ul la at ti io on n o of f t th he e p ph hr re en ni ic c n ne er rv ve es s i in n n no or rm ma al l s su ub bj je ec ct ts s We compared cervical magnetic stimulation with conventional supramaximal bilateral percutaneous electrical stimulation in nine normal subjects. We measured oesophageal pressure (Poes), gastric pressure (Pgas) and transdiaphragmatic pressure (Pdi). The maximal relaxation rate (MRR) was also measured. The mean magnetic twitch Pdi was 36.5 cmH 2 O (range 27-48 cmH 2 O), significantly larger than electrical twitch Pdi, mean 29.7 cmH 2 O (range 22-40 cmH 2 O).The difference in twitch Pdi was explained entirely by twitch Poes, and it is possible that the magnetic technique stimulates some of the nerves to the upper chest wall muscles as well as the phrenic nerves. We compared bilateral, rectified, integrated, diaphragm surface electromyographic (EMG) responses in three subjects and found results within 10% in each subject, indicating similar diaphragmatic activation. The within occasion coefficient of variation, i.e. same subject/same session, was 6.7% both for magnetic and electrical twitch Pdi. The between occasion coefficient of variation, i.e. same subject/different days, was 6.6% for magnetic stimulation and 8.8% for electrical. There was no difference between relaxation rates measured with either technique.We conclude that magnetic stimulation is a reproducible and acceptable technique for stimulating the phrenic nerves, and that it provides a potentially useful alternative to conventional electrical stimulation as a nonvolitional test of diaphragm strength.

Background -Electrical stimulation ofthe phrenic nerve is a useful non-volitional method of assessing diaphragm contractility. During the assessment of hemidiaphragm contractility with electrical stimulation, low twitch transdiaphragmatic pressures may result from difficulty in locating and stimulating the phrenic nerve. Cervical magnetic stimulation overcomes some of these problems, but this technique may not be absolutely specific and does not allow the contractility ofone hemidiaphragm to be assessed. This study assesses both the best means of producing supramaximal unilateral magnetic phrenic stimulation and its reproducibility. This technique is then applied to patients. Methods -The ability of four different magnetic coils to produce unilateral phrenic stimulation in five normal subjects was assessed from twitch transdiaphragmatic pressure (TWPDI) measurements and diaphragmatic electromyogram (EMG) recordings. The results from magnetic stimulation were compared with those from electrical stimulation. To determine whether the magnetic field affects the contralateral phrenic nerve as well as the intended phrenic nerve, EMG recordings from each hemidiaphragm were compared during stimulation on the same side and the opposite side relative to the recording electrodes. The EMG recordings were made from skin surface electrodes in five normal subjects and from needle electrodes placed in the diaphragm during cardiac surgery in six patients. Similarly, the direction of hemidiaphragm movement 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 phrenic stimulation, maximum sniff PDI, and TwPDI during cervical magnetic stimulation.

Background -Skeletal muscle twitch responses may be transiently increased by previous contractions, a phenomenon termed twitch potentiation. The aim of this study was to examine the extent and time course of diaphragmatic twitch potentiation and its relationship to both the magnitude and duration of the preceding voluntary diaphragmatic contraction. Methods -Twitch transdiaphragmatic pressure (PDI) was measured in six normal subjects, before and after voluntary diaphragm contractions of 100%, 75%, 50%, and 25% of maximum PDI (PDImax) sustained for five and 10 seconds. Results -Twitch PDI was significantly increased after 100%, 75%, and 50% contractions. Following maximal contractions sustained for 10 seconds the mean increase in twitch PDI was 52%. Following 50% contractions sustained for five seconds the mean increase in twitch height was 28%. In all runs twitch PDI returned to rested levels within 20 minutes. Conclusions -Twitch potentiation can be substantial, even following submaximal contractions, and must be taken into account when twitch pressure is used to assess diaphragm contractility.

Background -Diaphragm strength can be assessed by the measurement of gastric (Tw PGA), oesophageal (Tw POES), and transdiaphragmatic (Tw PDI) pressure in response to phrenic nerve stimulation. However, this requires the passage of two balloon catheters. A less invasive method of assessing diaphragm contractility during stimulation of the phrenic nerves would be of clinical value. A study was undertaken to determine whether pressure measured at the mouth (Tw PM) during magnetic stimulation ofthe phrenic nerves accurately reflects Tw POES, and to investigate the relations between Tw PM and Tw PDI; and also to see whether glottic closure and twitch potentiation can be avoided during these measurements. Methods -Eight normal subjects and eight patients with suspected respiratory muscle weakness without lung disease were studied. To prevent glottic closure magnetic stimulation of the phrenic nerves was performed at functional residual capacity during a gentle expiratory effort against an occluded airway incorporating a small leak. Tw PDI, Tw POES, and Tw PM were recorded. Care was taken to avoid potentiation of the diaphragm. Results -In normal subjects mean Tw PM was 137 cm H20 (range 11 3-16 1) and Tw POES was 13 3 cm H20 (range 10.4-15.9) with a mean (SD) difference of 0-4 (0-81) cm H20. In patients mean Tw PM was 9 1 cm H2O (range 0.5-18.2) and Tw POES was 9 3 (range 0-7-18-7) with a mean (SD) difference of -0-2 (0.84) cm H20. It is easily applied, well tolerated and reproducible, and therefore suitable for the sequential assessment of diaphragm contractility in both normal subjects and patients.7The purpose of this study was to investigate the relation between mouth pressure and POES and PDI during magnetic stimulation of the phrenic nerve roots in normal subjects and patients whilst keeping the glottis open and avoiding potentiation of the diaphragm. Methods STUDY POPULATION

P Po or rt ta ab bl le e m me ea as su ur re em me en nt t o of f m ma ax xi im mu um m m mo ou ut th h p pr re es ss su ur re es s The mouth pressure meter measures and displays as a digital read-out peak pressure for inspiratory and expiratory efforts. It samples the signal at 16 Hz, and an integral microprocessor is programmed to determine and display the maximum pressure averaged over one second both during inspiratory and expiratory manoeuvres (PImax and PEmax, respectively). A fine bore catheter connecting the mouthpiece of the mouth pressure meter to a Validyne pressure transducer enabled simultaneous measurement of pressure, which was analysed by LabVIEW, running on a Macintosh Quadra 700 computer. We studied 13 normal subjects and 11 patients with respiratory disease. Each subject performed inspiratory and five expiratory efforts. The values displayed from the mouth pressure meter were manually recorded.The mouth pressure meter reliably and accurately measured peak pressure and maximal pressure both for inspiratory and expiratory efforts in normals and patients. The mean±SD difference when compared with the Validyne method was 0.19±0.12 and -0.04±0.12 kPa, for PImax and PEmax, respectively. This portable device should be useful to measure mouth pressures, not only in the routine lung function laboratory but also at the bedside and in the clinic.