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Biomedical subjects

J Moxham

Publications and source records attributed to J Moxham.

At least 73 records · Page 4Linked to original sources
PubMed1999

An in vivo comparison of a catheter mounted pressure transducer system with conventional balloon catheters.

In the assessment of respiratory muscle function balloon catheters have been widely used for pressure measurements. However, this type of investigation is poorly tolerated by acutely ill patients. This study assessed the performance of a possible alternative, a catheter-mounted miniature pressure transducer (CMT). The assessment consisted of a laboratory study of the linearity, frequency response, and stability of gain and baseline of the CMT system, and an in vivo study directly comparing the CMT and balloon catheter systems in seven normal subjects for a range of respiratory manoeuvres. These were: 1) maximal inspiratory and expiratory pressures against a closed airway, 2) twitch transdiaphragmatic pressure elicited by cervical magnetic phrenic nerve stimulation, and 3) tidal breathing, sniffs and coughs in five body positions. The agreement of the two systems was analysed for measurements of 1) absolute pressures, 2) magnitude of changes in pressure, and 3) rate of change of pressure (maximum relaxation rate after sniff manoeuvres). The CMT system was linear, with a high frequency response and stable gain, but showed baseline drift. The two systems agreed well for measurements of change and rate of change of pressure, but less well for measurements of absolute pressure. The CMT system tested is potentially useful for studies of acute changes in respiratory pressures, or studies of respiratory muscle strength, but would be less useful where accurate measurements of absolute pressures are required.

Catheterization↗
PubMed1999

Respiratory aspects of neurological disease.

Neurological disease may result in respiratory dysfunction; however the manifestations of respiratory dysfunction in such patients may be atypical because of wider effects of their underlying condition. In the present review we have considered separately acute neuromuscular respiratory disease (as well as aspects of respiratory muscle function relevant to intensive care), chronic neuromuscular respiratory disease, sleep related disorders, respiratory consequences of specific neurological diseases, and neurological features of respiratory disease. Approaches to specific clinical problems are discussed; in many instances this can be expedited by close cooperation with a respiratory physician. We suggest that management of respiratory dysfunction in neurological disease depends critically on three factors: firstly, knowledge of when respiratory dysfunction is likely to occur; secondly, maintaining a high index of clinical suspicion (specifically apparently vague symptoms should not be uncritically attributed to the underlying neurological condition); and, thirdly, the pursuing of appropriate investigations.

Brain Diseases↗
PubMed1999

Effect of brachial plexus co-activation on phrenic nerve conduction time.

BACKGROUND: Diaphragm function can be assessed by electromyography of the diaphragm during electrical phrenic nerve stimulation (ES). Whether phrenic nerve conduction time (PNCT) and diaphragm electrical activity can be reliably measured from chest wall electrodes with ES is uncertain. METHODS: The diaphragm compound muscle action potential (CMAP) was recorded using an oesophageal electrode and lower chest wall electrodes during ES in six normal subjects. Two patients with bilateral diaphragm paralysis were also studied. Stimulations were deliberately given in a manner designed to avoid or incur co-activation of the brachial plexus. RESULTS: For the oesophageal electrode the PNCT was similar with both stimulation techniques with mean (SE) values of 7.1 (0.2) and 6.8 (0.2) ms, respectively (pooled left and right values). However, for surface electrodes the PNCT was substantially shorter when the brachial plexus was activated (4.4 (0.1) ms) than when it was not (7.4 (0.2) ms) (mean difference 3.0 ms, 95% CI 2.7 to 3.4, p<0.0001). A small short latency CMAP was recorded from the lower chest wall electrodes during stimulation of the brachial plexus alone. CONCLUSIONS: The results of this study show that lower chest wall electrodes only accurately measure PNCT when care is taken to avoid stimulating the brachial plexus. A false positive CMAP response to phrenic stimulation could be caused by inadvertent stimulation of the brachial plexus. This finding may further explain why the diaphragm CMAP recorded from chest wall electrodes can be unreliable with cervical magnetic stimulation during which brachial plexus activation occurs.

Action Potentials↗
PubMed1999

Functional magnetic stimulation of the abdominal muscles in humans.

Functional magnetic stimulation (FMS) of the thoracic nerve roots to simulate cough has been suggested as a treatment approach in patients unable to voluntarily activate the abdominal muscles. However, factors that could influence the efficacy of FMS in clinical use have not been evaluated. In the present investigation we studied train length, posture, and frequency to determine the optimal stimulation protocol. We also evaluated the use of a valve at the mouth to enhance glottic function and investigated whether lung volume at the time of stimulation would influence the tension generated by the abdominal muscles. Studies were performed using a Magstim rapid stimulator augmented by four booster packs in nine healthy subjects; we measured the change in gastric (DeltaPga(FMS)), esophageal (DeltaPes(FMS)), and mouth pressure and expiratory flow. With our apparatus pressure generation was maximized by having a train length of at least 300 ms and a frequency of 25 Hz. Posture and valve use were not important determinants of DeltaPga(FMS) or DeltaPes(FMS). Lung volume exerted only a minor influence on DeltaPga(FMS), but the ratio DeltaPes(FMS):DeltaPga(FMS) was increased at TLC compared with FRC. Expiratory flow was increased by adopting a seated posture and using an occlusion valve with an opening threshold close to the maximum DeltaPes(FMS) generated by the stimulus train; however, expiratory flow was susceptible to interference from glottic incoordination. Representative results (with train length 600 ms, 25 Hz, and 100% power, seated) were mean DeltaPga(FMS), 166 cm H(2)O; mean DeltaPes(FMS), 108 cm H(2)O; and mean expiratory flow, 311 L/min. We confirm that FMS of the abdominal muscles can generate a substantial positive intra-abdominal and intrathoracic pressure and, consequently, expiratory flow in normal subjects.

Abdominal Muscles↗
PubMed1999

Diaphragm strength in chronic heart failure.

Reduced respiratory muscle strength has been reported in chronic heart failure (CHF) in several studies. The data supporting this conclusion come almost exclusively from static inspiratory and expiratory mouth pressure maneuvers (MIP, MEP), which many subjects find difficult to perform. We therefore performed a study using measurements that are less dependent on patient aptitude and also provide specific data on diaphragm strength. In 20 male patients and 15 control subjects we measured MIP and MEP as well as esophageal and transdiaphragmatic pressure during maximal sniffs (Sn Pes, Sn Pdi) and cervical magnetic phrenic nerve stimulation (Tw Pdi). In a subgroup the response to paired phrenic nerve stimulation (pTw Pdi) at interpulse intervals from 10 to 200 ms (5 to 100 Hz) was also determined. As expected, MIP was significantly reduced in the CHF group (CHF, 69.5 cm H(2)O; control, 96.7 cm H(2)O; p = 0.01), but differences were much less marked for Sn Pes (CHF, 95.2 cm H(2)O; control, 104.8 cm H(2)O; p = 0.20) and MEP (CHF, 109.1 cm H(2)O; control, 135.7 cm H(2)O; p = 0.09). Diaphragm strength was significantly reduced (Sn Pdi: CHF, 123.8 cm H(2)O; control 143.5 cm H(2)O; p = 0.04. Tw Pdi: CHF, 21.4 cm H(2)O; control, 28.5 cm H(2)O; p = 0.0005). Paired phrenic nerve stimulation suggested a trend to increased twitch summation at 5 to 20 Hz in CHF, although this did not reach significance. We conclude that mild reduction in diaphragm strength occurs in CHF, possibly because of an increased proportion of slow fibers, but overall strength of the respiratory muscles remains well preserved.

Adult↗
PubMed1999

Effect of hypercapnia on maximal voluntary ventilation and diaphragm fatigue in normal humans.

Relatively little is known about the combined effects of hypercapnia and fatigue on the human diaphragm. We examined the effects of acute hypercapnia and fatigue in seven subjects by measuring changes in transdiaphragmatic pressure (Pdi) elicited by cervical magnetic stimulation after 2 min maximal voluntary ventilation (MVV) while breathing air and also with the inspired PCO(2) increased to 8% for 12 min before and during the MVV. Diaphragm strength was assessed before and at 0, 20, 40, 60, and 90 min after the MVV in both studies with the subjects breathing air. There was no difference in the level of ventilation for each run. Mean (+/- SD) twitch Pdi (TwPdi) fell significantly (p < 0.01) at 20 min after the control and hypercapnic MVV; (30.4 [7.8] to 27.0 [8.1] cm H(2)O control and 30.3 [4.1] to 27.3 [5.0] cm H(2)O CO(2)) and remained significantly (p < 0.01) below baseline. The changes in TwPdi at 20 to 90 min were not significantly different between the control and CO(2) runs. The decrease in TwPdi at 0 min after MVV, however, was greater (15%) in the hypercapnic run than in the control run (8.1%) (p < 0.05) when compared with baseline valves. Hypercapnia does not intensify long lasting fatigue but may reduce diaphragm contractility immediately after MVV.

Adult↗
PubMed1999

Quantification of the esophageal diaphragm electromyogram with magnetic phrenic nerve stimulation.

Measurement of the amplitude of the esophageal diaphragm compound muscle action potential (CMAP) could be useful for the diagnosis and monitoring of neuromuscular disease. However, quantification of the diaphragm CMAP has been hampered by difficulty in positioning the esophageal electrode at the diaphragm's electrically active center and many investigators report arbitrary units rather than voltage. To quantify the esophageal diaphragm CMAP we designed a multipair electrode which we evaluated during unilateral magnetic stimulation. The esophageal catheter consisted of four sequential electrode pairs. Overall the electrode spanned 17 cm and covered the entire electrically active region of the diaphragm. The diaphragm CMAP was simultaneously recorded from the four pairs at distances of 40, 39, 38, and 37 cm from the nose to the proximal electrode pair. Studies were undertaken in 10 normal subjects and 10 patients with diaphragm dysfunction. The amplitude of the CMAP (peak to peak) was defined as the average of five twitches recorded from the optimal pair of electrodes. The amplitude of the diaphragm CMAP elicited by unilateral maximal magnetic stimulation was 1.45 +/- 0.35 mV (mean +/- SD) for the right side and 1.68 +/- 0.47 mV for the left. When measured on different occasions the coefficient of variation (CV) was 8.6%. The amplitude of the CMAP measured from dysfunctional hemidiaphragms was much less than that measured from normal subjects. This study suggests that the diaphragm CMAP can be quantified using an appropriate esophageal electrode.

Action Potentials↗
PubMed1999

Respiratory muscle strength in Cushing's syndrome.

The effect of Cushing's syndrome on respiratory muscle strength is unknown. Therefore, we studied 10 consecutive patients with severe Cushing's syndrome. The respiratory muscles were assessed using maximal inspiratory and expiratory mouth pressures (MIP, MEP), maximal sniff transdiaphragmatic pressures (max sniff Pdi), and maximal sniff esophageal pressures (max sniff Pes). Maximal quadricep strength was also assessed. The patients demonstrated an overall mean MIP 92 cm H(2)O, SD 19 (mean 105% of predicted; SD, 23%), mean MEP 134 cm H(2)O, SD 35 (mean 99% of predicted; SD, 25%), mean max sniff Pdi 107 cm H(2)O, SD 12 (mean 78% of predicted; SD, 10%) and mean max sniff Pes of 92 cm H(2)O, SD 11 (mean 92% of predicted; SD, 11%). Quadriceps muscle strength was reduced in all 10 patients: mean 26 kg, SD 9 (mean 49% of predicted strength, SD 21%). Respiratory muscle weakness was not found, despite the presence of severe quadriceps impairment. We conclude that major weakness of the respiratory muscles is not usual in Cushing's syndrome.

Adult↗
PubMed1999

Long-term recovery of diaphragm strength in neuralgic amyotrophy.

Diaphragm paralysis is a recognized complication of neuralgic amyotrophy that causes severe dyspnoea. Although recovery of strength in the arm muscles, when affected, is common, there are little data on recovery of diaphragm function. This study, therefore, re-assessed diaphragm strength in cases of bilateral diaphragm paralysis due to neuralgic amyotrophy that had previously been diagnosed at the authors institutions. Fourteen patients were recalled between 2 and 11 yrs after the original diagnosis. Respiratory muscle and diaphragm strength were measured by volitional manoeuvres as maximal inspiratory pressure and sniff transdiaphragmatic pressure. Cervical magnetic phrenic nerve stimulation was used to give a nonvolitional measure of diaphragm strength: twitch transdiaphragmatic pressure. Only two patients remained severely breathless. Ten of the 14 patients had evidence of some recovery of diaphragm strength, in seven cases to within 50% of the lower limit of normal. The rate of recovery was variable: one patient had some recovery after 2 yrs, and the rest took 3 yrs or more. In conclusion, in most patients with diaphragm paralysis due to neuralgic amyotrophy, some recovery of the diaphragm strength occurs, but the rate of recovery may be slow.

Adult↗
PubMed1999

Diaphragm electromyogram measured with unilateral magnetic stimulation.

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, 0.74+/-0.29 mV (mean+SD) for ES and 0.76+/-0.30 mV for UMS with maximal power output, and for the left side 0.88+/-0.33 mV for ES and 0.80+/-0.24 mV for UMS. PNCT measured by the oesophageal electrode with ES and UMS with maximal output were, for the right side, 7.0+/-0.8 ms and 6.9+/-0.8 ms, respectively, and for the left side 7.8+/-1.2 ms and 7.7+/-1.3 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.

Action Potentials↗
PubMed1998

Measurement of sniff nasal and diaphragm twitch mouth pressure in patients.

BACKGROUND: Inspiratory muscle weakness is a recognised cause of unexplained dyspnoea. It may be suggested by the finding of a low static inspiratory mouth pressure (MIP), but MIP is a difficult test to perform, with a wide normal range; a low MIP may also occur if the patient has not properly performed the manoeuvre. Further investigation conventionally requires balloon catheters to obtain oesophageal (Poes) and transdiaphragmatic pressure (Pdi) during sniffs or phrenic nerve stimulation. Two non-invasive tests of inspiratory muscle strength have recently been described--nasal pressure during a maximal sniff (Sn Pnas) and mouth pressure during magnetic stimulation of the phrenic nerves (Tw Pmo). The use of these two tests in combination might identify patients without inspiratory muscle weakness who are unable to produce a satisfactory MIP< therefore avoiding the need for investigation with balloon catheters. METHODS: Thirty consecutive patients with clinically suspected inspiratory muscle weakness and a low MIP underwent both conventional (Sn Poes and Tw Pdi) and non-invasive testing (Sn Pnas and Tw Pmo). Weakness was considered to be excluded by a Sn Poes of > or = 80 cm H20 or a Tw Pdi of > or = 20 cm H20. The limit values used to test the hypothesis were Sn Pnas > or = 70 cm H20 or Tw Pmo > or = 12 cm H20. RESULTS: Inspiratory muscle weakness was excluded in 17 of the 30 patients. Fifteen of these would have been identified using Sn Pnas and Tw Pmo, with better results when the two tests were combined. The cut off values selected for Sn Pnas and Tw Pmo were shown by ROC plots to indicate normal strength conservatively, avoiding failure to detect mild degrees of weakness. No patient with global weakness was considered normal by Sn Pnas or Tw Pmo. CONCLUSIONS: In most patients with normal inspiratory strength and a low MIP, Tw Pmo and Sn Pnas used in combination can reliably exclude global inspiratory muscle weakness, reducing the number of patients who need testing with balloon catheters.

Adult↗
PubMed1998

Influence of acute lung volume change on contractile properties of human diaphragm.

The effect of stimulus frequency on the in vivo pressure generating capacity of the human diaphragm is unknown at lung volumes other than functional residual capacity. The transdiaphragmatic pressure (Pdi) produced by a pair of phrenic nerve stimuli may be viewed as the sum of the Pdi elicited by the first (T1 Pdi) and second (T2 Pdi) stimuli. We used bilateral anterior supramaximal magnetic phrenic nerve stimulation and a digital subtraction technique to obtain the T2 Pdi at interstimulus intervals of 999, 100, 50, 33, and 10 ms in eight normal subjects at lung volumes between residual volume and total lung capacity. The reduction in T2 Pdi that we observed as lung volume increased was greatest at long interstimulus intervals, whereas the T2 Pdi obtained with short interstimulus intervals remained relatively stable over the 50% of vital capacity around functional residual capacity. For all interstimulus intervals, the total pressure produced by the pair decreased as a function of increasing lung volume. These data demonstrate that, in the human diaphragm, hyperinflation has a disproportionately severe effect on the summation of pressure responses elicited by low-frequency stimulations; this effect is distinct from and additional to the known length-tension relationship.

Adult↗
PubMed1998

Diaphragm EMG measured by cervical magnetic and electrical phrenic nerve stimulation.

The purpose of the study was to compare electrical stimulation (ES) and cervical magnetic stimulation (CMS) of the phrenic nerves for the measurement of the diaphragm compound muscle action potential (CMAP) and phrenic nerve conduction time. A specially designed esophageal catheter with three pairs of electrodes was used, with control of electrode positioning in 10 normal subjects. Pair A and pair B were close to the diaphragm (pair A lower than pair B); pair C was positioned 10 cm above the diaphragm to detect the electromyogram from extradiaphragmatic muscles. Electromyograms were also recorded from upper and lower chest wall surface electrodes. The shape of the CMAP measured with CMS (CMS-CMAP) usually differed from that of the CMAP measured with ES (ES-CMAP). Moreover, the latency of the CMS-CMAP from pair B (5.3 +/- 0.4 ms) was significantly shorter than that from pair A (7.1 +/- 0.7 ms). The amplitude of the CMS-CMAP (1.00 +/- 0.15 mV) was much higher than that of ES-CMAP (0.26 +/- 0.15 mV) when recorded from pair C. Good-quality CMS-CMAPs could be recorded in some subjects from an electrode positioned very low in the esophagus. The differences between ES-CMAP and CMS-CMAP recorded either from esophageal or chest wall electrodes make CMS unreliable for the measurement of phrenic nerve conduction time.

Action Potentials↗
PubMed1998

Expiratory muscle function in amyotrophic lateral sclerosis.

Few data exist concerning expiratory muscle function in amyotrophic lateral sclerosis (ALS). We studied 26 patients with ALS (16 with respiratory symptoms and 10 without) and measured the maximal static expiratory mouth pressure (MEP), the gastric pressure during a maximal cough (Cough Pga), and the gastric pressure after magnetic stimulation of the lower thoracic nerve roots (Tw Pga). These measurements were related to the ability to generate transient supramaximal flow during a cough (cough spikes), to arterialized capillary blood gases, and to inspiratory muscle strength. Vocal cord motion was examined endoscopically in 11 of the 16 symptomatic patients. Expiratory muscle weakness was related to inability to generate cough spikes with a threshold effect such that spikes were absent for Cough Pga < 50 cm H2O (p = 0.009) or Tw Pga < 7 cm H2O (p = 0.006) and was usually associated with inspiratory muscle weakness. However, in multivariate analysis, PaCO2 was only significantly associated with the maximal sniff esophageal pressure (p = 0.02). Symptomatic patients had significantly lower inspiratory muscle strength, whereas, of the expiratory muscle tests, only Tw Pga was significantly lower (p = 0.0009) in symptomatic patients. Abnormal vocal cord motion was observed in two of the 11 patients examined. We conclude that abdominal muscle weakness in ALS, when substantial, results in an inability to generate transient supramaximal flow during a cough. However, the primary determinant of both ventilatory failure and respiratory symptoms seems to be inspiratory muscle weakness.

Adult↗
PubMed1998

Maximum rate of change in oesophageal pressure assessed from unoccluded breaths: an option where mouth occlusion pressure is impractical.

The mouth occlusion pressure 100 ms after onset of inspiration (P0.1) is considered a clinically useful measure of the combined output of the respiratory centre and muscle pump. However, theoretical and practical difficulties can arise when using P0.1 in the assessment of patients with severe chronic obstructive pulmonary disease (COPD). It was hypothesized that the maximum rate of change in oesophageal pressure (dPoes,max/dt) may be an alternative to P0.1. To test this hypothesis P0.1 was compared with mean dPoes,max/dt measured from neighbouring unoccluded breaths in five normal subjects during CO2 rebreathing. In all subjects a close correlation was found between both dPoes,max/dt and P0.1 and carbon dioxide tension (PCO2). In six patients with severe COPD performing exhaustive treadmill walks, dPoes,max/dt was found to increase progressively with walking time. Mean dPoes,max/dt at the start was 6.2 cmH2O x 100 ms(-1) and at the finish was 18.7 cmH2O x 100 ms(-1) (p<0.03). In conclusion, the maximum rate of change in oesophageal pressure measured from unoccluded breaths could be an alternative in circumstances where it is not feasible to use measurements of the mouth occlusion pressure 100 ms after onset of inspiration.

Aged↗