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Biomedical subjects
J Moxham
Publications and source records attributed to J Moxham.
Measurement of respiratory muscle strength.
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Diaphragm↗
Unilateral magnetic stimulation of the phrenic nerve.
BACKGROUND: Electrical stimulation of the 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 of one 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. RESULTS: In the five normal subjects supramaximal stimulation 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 H2O with 35 mm coil; 14.1 (3.8) cm H2O with 43 mm coil; 10.0 (1.7) cm H2O with electrical stimulation). Spread of the magnetic field to the opposite phrenic nerve produced a small amplitude contralateral diaphragm EMG measured from skin surface electrodes which reached a mean of 15% of the maximum EMG amplitude produced by ipsilateral stimulation. Similarly, in six patients with EMG activity recorded directly from needle 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 phrenic nerve was rapidly achieved and well tolerated. In the 54 patients unilateral magnetic TwPDI was more closely related than unilateral electrical TwPDI to transdiaphragmatic pressure produced during maximum sniffs and cervical magnetic stimulation. Unilateral magnetic stimulation eliminated the problem of producing a falsely low TwPDI because of technical difficulties in locating and adequately stimulating the nerve. Eight patients with unilateral phrenic nerve paresis, as indicated by a unilaterally elevated hemidiaphragm on a chest radiograph and maximum sniff PDI consistent with hemidiaphragm weakness, were all accurately identified by unilateral magnetic stimulation. CONCLUSIONS: Unilateral magnetic phrenic nerve stimulation is easy to apply and is a reproducible technique in the assessment of hemidiaphragm contractility. It is well tolerated and allows hemidiaphragm contractility to be rapidly and reliably assessed because precise positioning of the coils is not necessary. This may be particularly useful in patients. In addition, the anterolateral positioning of the coil allows the use of the magnet in the supine patient such as in the operating theatre or intensive care unit.
Adult↗
Mouth pressure in response to magnetic stimulation of the phrenic nerves.
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 of the 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 13.7 cm H2O (range 11.3-16.1) and TW POES was 13.3 cm H2O (range 10.4-15.9) with a mean (SD) difference of 0.4 (0.81) cm H2O. 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 H2O. The relation between TW PM and TW PDI was less close but was well described by a linear function. In patients with diaphragm weakness (low sniff PDI) TW PM was < 10 cm H2O. CONCLUSIONS: TW PM reliably reflects TW POES and can be used to predict TW PDI in normal subjects and patients without lung disease. TW PM may therefore be a promising non-invasive, non-volitional technique for the assessment of diaphragm strength.
Adult↗
The effects of oxygen and dopamine on renal and aortic blood flow in chronic obstructive pulmonary disease with hypoxemia and hypercapnia.
Renal blood flow is reduced in patients with chronic respiratory failure caused by chronic obstructive pulmonary disease (COPD), and changes in renal hemodynamics are likely to be important in the pathogenesis of the edematous state of cor pulmonale. We therefore examined the hypothesis that this renal vasoconstriction is reversible by comparing the effects of oxygen therapy and the renal vasodilator dopamine on renal hemodynamics in both hypoxemic patients with COPD and those who were also hypercapnic. We assessed renal hemodynamics noninvasively with color-flow Doppler ultrasound. In order to validate the technique we recorded renal hemodynamics in a group of healthy volunteers before and during a dopamine infusion, and in a subgroup we simultaneously measured effective renal plasma flow (ERPF) with para-aminohippurate clearance. In the healthy volunteers there was a 22.5% rise in time-adjusted mean arterial velocity (Tamx) measured by Doppler compared with a 22% rise in ERPF with dopamine. This rise was significant (p < 0.05). In hypoxemic, normocapnic subjects Tamx rose by 25% with oxygen (p < 0.005), 20% with dopamine (p < 0.005), and 24% with both therapies. There was no significant change in aortic flow whether the subjects received air, oxygen, or dopamine (p = 0.77). In the hypercapnic patients there was no significant change in aortic or renal velocities while receiving oxygen or dopamine (p = 0.85 and 0.86). We conclude that color-flow Doppler velocity measurements can accurately detect changes in renal blood flow. Oxygen and dopamine are equipotent renal vasodilators in hypoxic COPD, but the effect is not additive.(ABSTRACT TRUNCATED AT 250 WORDS)
Adult↗
Exhaustive treadmill exercise does not reduce twitch transdiaphragmatic pressure in patients with COPD.
Reduced diaphragm contractility has been described in normal subjects after whole body endurance exercise, and it indicates low frequency fatigue (LFF); it is unknown whether LFF is of clinical importance. We therefore studied the effect of treadmill exercise to exhaustion on diaphragm contractility in six patients with severe chronic obstructive pulmonary disease (COPD) (mean FEV1, 0.71, 27% predicted). The subjects first performed a short (control), treadmill walk and then, after resting, a second walk to a state of severe dyspnea. Cervical magnetic stimulation of the phrenic nerve roots was performed at the start of the study and 20 and 30 min after each walk. The twitch transdiaphragmatic pressure (Tw Pdi) was reproducible (mean coefficient of variation, 5.3%; range, 2 to 12.5%). Mean Twi Pdis were 18.4 cm H2O at baseline and 19.6 cm H2O and 19.2 cm H2O 20 and 30 min after the control walk. At the same times after the exhaustive walk, mean Tw Pdis were 19.6 and 20.4 cm H20. Tw Pdi was not reduced by exhaustive treadmill walking (p > 0.9), and a power calculation showed that the study had a 95% chance of detecting a 10% fall at the 5% significance level. We conclude that Tw Pdi is not reduced when patients with severe COPD walk to a state of extreme breathlessness and that therefore low frequency fatigue of the diaphragm does not occur.
Aged↗
Noninvasive ventilation in the management of decompensated COPD.
Decompensated chronic obstructive pulmonary disease (COPD) is likely to remain a common medical emergency for the foreseeable future. Decompensation occurs when the capacity of the respiratory muscle pump is less than the load placed upon it. Even with optimal medical therapy, the mortality rate remains high. Noninvasive positive pressure ventilation (NIV) is a new technique, which is increasingly used as an alternative to mechanical endotracheal ventilation (ETV). In this review we discuss the existing literature, consider unresolved areas for future research and describe practice in our own hospital.
Humans↗
The effect of lung volume on transdiaphragmatic pressure.
Diaphragm strength can be assessed by measurement of transdiaphragmatic pressure (Pdi) in response to stimulation of the phrenic nerves. The length-tension relationship of the diaphragm can be studied by measuring twitch Pdi over the range of lung volume. Previous studies of the relationship between lung volume and diaphragm strength have used the technique of electrical stimulation of the phrenic nerves. In these studies, the phenomenon of twitch potentiation has not been taken into account. It has previously been shown that prior contraction of the diaphragm can greatly enhance the twitch response, thus affecting the measurements. The aim of this study was to investigate the relationship between unpotentiated twitch Pdi and lung volume for volumes ranging from residual volume (RV) to total lung capacity (TLC) in normal subjects. Great care was taken to avoid muscle potentiation. For this purpose, we stimulated the phrenic nerves with a magnetic stimulator. In addition, we used positive pressure to inflate the lungs to high lung volumes. The impact of twitch potentiation on the length-tension relationship was investigated by subjects making maximum inspiratory efforts prior to phrenic nerve stimulation. The unpotentiated twitch Pdi decreased in a linear fashion with increasing lung volume over the full range of vital capacity by 0.54 kPa.L-1. Potentiation increased twitch Pdi by 40% at FRC and the effect was similar, in absolute terms, at all lung volumes. In relative terms, the effect of potentiation became greater as lung volume increased, and more than doubled twitch Pdi at TLC. With increasing lung volume, there is a linear fall in unpotentiated twitch Pdi with a slope that is less steep, over the same range of absolute lung volume, than previously reported. When assessing diaphragm strength by the twitch technique, it is essential to control for lung volume and equally important to control for twitch potentiation.
Adult↗
Respiratory muscle function in patients with asbestos-related pleural disease.
The effect of asbestos-related pleural disease (ARPD) on the generation of maximum respiratory pressure was investigated in 11 male patients with ARPD mean age 57 years, range 45-74, and mean duration of asbestos exposure of 9.9 years, range 5-16. There were three smokers, seven ex-smokers and one non-smoker. Breathlessness ranged from grade 1-3 on the MRC score. The extent of pleural disease was calculated using a score based on the ILO score for pleural disease. Full respiratory function tests, global respiratory muscle strength and diaphragmatic strength were assessed. Respiratory muscle strength, including diaphragm strength, was normal. Recoil pressure was high or at the upper limit of normal in four patients and correlated with chest radiograph score for pleural disease (r = 0.65, P < 0.02). There was no difference in either global respiratory muscle or diaphragmatic strength between patients with and without involvement of one or both costophrenic angles or between patients with mild or severe breathlessness. We conclude that respiratory muscle strength is not importantly reduced in ARPD, and it is unlikely that weakness contributes to breathlessness in these patients. By contrast reduced chest wall compliance is likely to be an important factor in breathlessness in some cases.
Aged↗
A comparison of different modes of noninvasive ventilatory support: effects on ventilation and inspiratory muscle effort.
The aims of noninvasive ventilation include the correction of hypoventilation and unloading of inspiratory muscles. Volume cycled flow generators, bi-level positive airway pressure and continuous positive airway pressure techniques have all been used with face and nasal masks. We have compared these modes of ventilatory support, administered by a nasal mask in stable, awake outpatients with chronic obstructive pulmonary disease or neuromusculo-skeletal disease in respect of their effects on ventilation, inspiratory muscle effort and oxygen saturation. There were no clinically significant differences between the volume cycled flow generator and bi-level positive airway pressure methods; compared to spontaneous ventilation, oxygen saturation increased and inspiratory muscle effort decreased. Tidal volume increased and respiratory rate reduced, the largest changes occurring with bi-level positive airway pressure. Only the volume cycled flow generator increased minute ventilation significantly. Ventilation and inspiratory muscle effort were unaffected by continuous positive airway pressure but oxygen saturation was lower than during spontaneous ventilation. In awake, stable outpatients acclimatised to nasal ventilation there were no clinically significant differences between volume cycled flow generator and bi-level positive airway pressure techniques, but continuous positive airway pressure was less effective.
Aged↗
CD4+ lymphocytopenia due to common variable immunodeficiency mimicking AIDS.
There are an increasing number of published reports of patients with acquired immunodeficiency without evidence of HIV infection, who have been labelled as having "idiopathic CD4+ lymphocytopenia". The case is reported here of a young man who presented with Pneumocystis carinii pneumonia (PCP), CD4+ lymphopenia, and hypogammaglobulinaemia attributable to common variable immunodeficiency (CVID). The presentation of this condition, with many of the clinical and laboratory features of AIDS, highlights CVID as a diagnosis to be considered in the differential diagnosis of CD4+ lymphocytopenia.
Acquired Immunodeficiency Syndrome↗
Maximum relaxation rate of the diaphragm during weaning from mechanical ventilation.
BACKGROUND: The maximum relaxation rate (MRR; percentage fall in pressure/10 ms) of oesophageal (POES) and transdiaphragmatic (PDI) pressure slows under conditions of loaded breathing, and has been measured previously in normal subjects. MRR has not been measured in intubated patients weaning from mechanical ventilation. METHODS: Five postoperative patients who were expected to wean and nine patients who had previously failed were studied. POES and PDI MRR, peak oesophageal pressure during spontaneous breathing, maximum oesophageal pressure, and the inspiratory duty cycle were measured at rest during mechanical ventilation, in the first two minutes of spontaneous breathing, and after reventilation in those patients who failed, or before extubation in those patients who succeeded. RESULTS: At rest POES MRR in intubated patients had a range of 5.6-11 and PDI MRR 6.9-10.0, with a coefficient of variation of 9.9% and 7.3% respectively. POES and PDI MRR were similar before and after extubation in five postoperative patients, and POES MRR was reflected by endotracheal MRR measured at the airway. In five patients who failed to wean POES and PDI MRR slowed by 47% and 44%, and fully recovered after 10 minutes reventilation. In four patients who were successfully weaned MRR was unchanged during spontaneous breathing. At the time when MRR decreased, the respiratory muscles were heavily loaded in relation to their strength. CONCLUSIONS: Weaning failure occurs when the applied load exceeds the capacity of the respiratory muscles, and this is associated with a slowing of respiratory muscle MRR.
Adult↗
Effect of inspiratory pressure support on exercise tolerance and breathlessness in patients with severe stable chronic obstructive pulmonary disease.
BACKGROUND: In patients with chronic obstructive pulmonary disease exercise tolerance is commonly limited by breathlessness. These patients have an increased ventilatory load at rest which is exacerbated during exercise. The purpose of this study was to investigate the effect of supporting ventilation by non-invasive inspiratory pressure support (IPS) during submaximal treadmill exercise in such patients to see if they would experience less breathlessness and improve their exercise capacity. METHODS: Eight men with disabling breathlessness due to chronic obstructive pulmonary disease (COPD) (mean (SD) FEV1 0.73 (0.2) 1) were studied. Patients walked on a treadmill until their sensation of breathlessness, scored at one minute intervals, reached level 5 ("severe") on the 10-point Borg scale. Studies were performed with IPS (mean airway pressure 12-15 cm H2O), continuous positive airway pressure (CPAP 6 cm H2O), and with oxygen (2 l/min via a mask) in random order on three separate days. Each of these walks was compared with a control walk using a sham circuit (breathing air via an oxygen mask at 2 l/min from an unlabelled cylinder), and with a baseline walk in which patients walked freely on the treadmill. On cessation of exercise, distance achieved and a leg fatigue score were recorded. RESULTS: No patients stopped due to leg fatigue, all stopping only when their sensation of breathlessness had reached level 5 on the Borg scale. IPS improved median walking distance by 62% compared with the control walk (sham circuit). There was no change in walking distance with either CPAP or oxygen at 2 l/min. There was no difference between the control and the baseline walks. CONCLUSIONS: Inspiratory pressure support can reduce breathlessness and increase exercise tolerance to submaximal treadmill exercise in patients with COPD. This could have implications for the rehabilitation of these severely disabled patients.
Aged↗
Effect of oral theophylline on resting energy expenditure in normal volunteers.
BACKGROUND: The aim of this study was to investigate the contribution of regular treatment with oral theophylline to the increase in resting oxygen consumption observed in patients with chronic airflow limitation who are receiving bronchodilator therapy. METHODS: Resting oxygen consumption (VO2) and carbon dioxide production (VCO2) were measured in 10 normal subjects (six men, age 21-48 years, weight 50-85 kg) before and after 11 days of treatment with either placebo or theophylline in a double blind manner, in twice daily oral doses ensuring trough serum concentrations between 8.4 and 13.5 mg/l. An open canopy method was used to measure VO2 and VCO2 and in all test conditions this was extended for 60 minutes after an inhalation of 800 micrograms of salbutamol super-imposed on the background placebo or theophylline treatment. RESULTS: Resting VO2 and heart rate were increased during theophylline treatment compared with placebo by 6.5% and 8.4% respectively. Salbutamol inhalation transiently increased VO2, VCO2, and heart rate in all tests but this was not modified by background theophylline treatment. CONCLUSION: Oral theophylline treatment causes a sustained increase in resting oxygen consumption and heart rate but does not modify the metabolic response to acutely inhaled salbutamol.
Adult↗
Inspiratory muscle relaxation rate assessed from sniff nasal pressure.
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 fall/10 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 of 27.4% (range 22.5-36%) after MIV and a similar reduction in sniff PNASAL MRR of 28.5% (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 following MIV, so measurement of PNASAL MRR may be a useful non-invasive method for measuring inspiratory muscle MRR, thereby providing an index of respiratory muscle fatigue.
Air Pressure↗
Potentiation of diaphragmatic twitch after voluntary contraction in normal subjects.
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.
Adult↗
Pseudotumour of the lung caused by Mycobacterium malmoense infection in an HIV positive patient.
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Adult↗
Portable measurement of maximum mouth pressures.
We have compared a small portable mouth pressure meter (MPM) to our laboratory standard (LS) pressure recording equipment in order to evaluate this new device. 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.
Adult↗