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

Phrenic nerve stimulation is often considered to be difficult and unreliable. The time taken for the phrenic nerves to be located and adequately stimulated was measured in 110 subjects, aged 21-89 years, 26 of whom had diaphragmatic weakness; and phrenic nerve conduction time was recorded in 76 of these individuals. Each phrenic nerve was stimulated transcutaneously in the neck with square wave impulses 0 I ms in duration at 1 Hz and 80-160 volts while diaphragmatic muscle action potentials were recorded with surface electrodes. The time taken to locate either phrenic nerve ranged from two seconds to 22 minutes (median 10 s). Both nerves were located in 83 of the 84 control subjects (99%) and in 21 of the 26 patients with diaphragmatic weakness (81%). Mean (SD) phrenic nerve conduction time in the control subjects was 6-94 (0-77) ms on the right and 6-61 (0 77) ms on the left. A weak relationship was found between conduction time and the subjects' age and height. Four out of 24 patients with diaphragmatic weakness had a prolonged phrenic nerve conduction time. Transcutaneous stimulation of the phrenic nerves was not a time consuming procedure, and it was well tolerated, reproducible, and successful in 95% of subjects.Assessment of phrenic nerve function is necessary in candidates for permanent diaphragm pacing' and may be required in the investigation of patients with diaphragmatic weakness. Phrenic nerve conduction time provides a sensitive indicator of phrenic nerve function when the nerves are affected either by local lesions or by generalised ieuropathies. Prolonged conduction time has been fo'upd in phrenic neuritis,2 in mediastinal tumour, after surgical trauma,3 and in peripheral neuropathies.4Although percutaneous phrenic nerve stimulation was described in 19515 as a means of providing ventilatory support and again in 19674 as a method of investigation, phrenic nerve studies have not gained wide acceptance. Failure to locate the nerves,6 7 and discomfort have been considered to be important problems. The purpose of this study was to establish how often, how quickly, and how reproducibly each phrenic nerve could be located by means of twitch stimulations, to see whether this investigation could be applied routinely. Studies were performed in control subjects to determine the normal range of phrenic nerve conduction time and in patients with

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...

Background -Slowing of the maximum relaxation rate (MRR) of inspiratory muscles measured from oesophageal pressure (POES) during sniffs has been used as an index of the onset and recovery of respiratory muscle fatigue. The purpose of this study was to measure MRR at the nose (PNASAL MRR), to investigate its relationship with POES MRR, and to establish whether PNASAL MRR slows with respiratory loading. Methods -Five normal subjects were studied. Each performed sniffs before and after two minutes of maximal isocapnic ventilation (MIV). In a separate session the subjects performed submaximal sniffs. POEs and PNASAL were recorded during sniffs and the MRR (% pressure falllO ms) for each sniff was determined. Results -Before MIV mean POES MRR was 8-9 and PNASAL MRR was 9-3. The mean (SD) difference between PNASAL MRR and POES MRR during a maximal sniff was 0-48 (0.34) (n=64) and during submaximal sniffs was 0-28 (0.46) (n = 526). The subjects showed a mean decrease in sniff POES MRR of27'4% (range 22-5-36%) after MIV and a similar reduction in sniff PNASAL MRR of28-5% (range 24.1-41-3%). Both returned to control values within 5-10 minutes.Conclusions -PNASAL MRR reflects POES MRR over a wide range of sniff pressures, PNASAL MRR of maximal sniffs reflects POES MRR in normal subjects at rest and foliowing MIV, so measurement ofPNASAL MRR may be a useful non-invasive method for measuring inspiratory muscle MRR, thereby providing an index of respiratory muscle fatigue.