Biomedical subjects

Michael I Polkey

Publications and source records attributed to Michael I Polkey.

At least 19 recordsLinked to original sources

PubMed2005

Does symptom-limited cycle exercise cause low frequency diaphragm fatigue in patients with heart failure?

BACKGROUND: Reduced diaphragm contractility occurs in some healthy subjects when they exercise to exhaustion. This indicates low frequency fatigue, which may contribute to task failure. We hypothesised that patients with congestive heart failure (CHF) might be especially vulnerable to the development of low frequency diaphragm fatigue after exhaustive exercise. AIMS: To study the effect of exhaustive incremental cycle exercise on diaphragm contractility in patients with CHF. METHODS: 12 patients with CHF with an ejection fraction of 36.5 +/- 7.3% and 12 healthy age-matched control subjects performed an incremental cycle test to exhaustion. The unpotentiated twitch transdiaphragmatic pressure (twitch Pdi) in response to bilateral anterolateral magnetic phrenic nerve stimulation (BAMPS) was measured before and after exercise. RESULTS: Twitch Pdi at baseline was 20.2 +/- 6.7 cm H2O in the CHF group and 20.3 +/- .3 cm H2O in the controls (p = 0.957). 25 and 35 min post exercise the values were 19.9+/-5.4 and 20.0+/-5.1 cm H2O in the CHF group and 20.6 +/- 4.3 and 21.2 +/- 3.4 cm H2O in the control group; neither change was significant (F(2,27) = 0.007, p = 0.993; F(2,33) = 0.144, p = 0.866, respectively). CONCLUSION: When patients with CHF cycle to exhaustion, low frequency fatigue of the diaphragm does not occur, and this is unlikely to be an important factor limiting exercise capacity of such patients.

Diaphragm↗

PubMed2005

Effect of lung volume reduction surgery for emphysema on diaphragm function.

Preoperative prediction of a successful outcome following lung volume reduction surgery (LVRS) for emphysema is imperfect. One mechanism could be improvement in respiratory muscle function yet controversy exists regarding the magnitude and mechanism of such an improvement. Therefore, we measured diaphragm strength in 18 patients before and after LVRS. Mean (S.D.) FRC fell from 6.53 to 5.40 l (p = 0.0001). Mean sniff transdiaphragmatic pressure increased from 76 to 87 cm H2O (14%, p < 0.03) and mean twitch transdiaphragmatic pressure (Tw Pdi) increased by 2.5 cm H2O at 3 months (12%, p = 0.03). There was a highly significant increase in twitch esophageal pressure (Tw Pes) (60%, p < 0.0001), which was maintained at 12 months (46% increase, p = 0.0004). No change was observed in quadriceps twitch tension in nine subjects in whom it was measured. After LVRS the ratio Tw Pes:Tw Pdi increased from 0.24 to 0.37 at 3 months (p = 0.0003) and 0.36 at 12 months (p = 008). Low values of Sn Pdi, Sn Pes, Tw Pes and a high RV/TLC ratio were the preoperative variables most predictive of improvement in shuttle walking distance. We conclude that LVRS improves diaphragm function primarily by alteration of lung volume. Patients with poor diaphragm function and high RV/TLC ratio preoperatively are most likely to benefit from the procedure.

Adult↗

PubMed2005

Respiratory effects of combined truncal and abdominal support in patients with spinal cord injury.

OBJECTIVE: To determine whether a custom girdle, designed to provide truncal stability and abdominal support, will improve pulmonary function, enhance inspiratory muscle activity, and reduce the sensation of respiratory effort in patients with spinal cord injury (SCI). DESIGN: Pulmonary function, transdiaphragmatic pressure time product (PTP di ), twitch (Tw Pdi) and maximal transdiaphragmatic pressures (Pdi), and perception of respiratory effort (Borg Rating of Perceived Exertion score) were measured with and without an abdominal girdle in a seated position. SETTING: Rehabilitation hospital. PARTICIPANTS: Ten patients with posttrauma SCI (injury level, C5-T6). INTERVENTION: Application of the abdominal girdle. MAIN OUTCOME MEASURES: Borg score and measures of lung volumes, dynamic abdominal compliance, and Tw Pdi and maximal Pdi. RESULTS: Wearing of the girdle was associated with a lower Borg score (P = .002) and reduced functional residual capacity (P = .006) but increased inspiratory capacity (P = .02) and forced vital capacity (P = .02). Although there was a decrease in dynamic abdominal compliance (P < .001) and an increase in PTP di (P = .02), this was accompanied by an increase in both Tw Pdi (P = .02) and maximal Pdi (P = .03). CONCLUSIONS The custom girdle reduced the sensation of respiratory effort in patients with SCI by optimizing the operating lung volumes and decreasing abdominal compliance, which enhanced diaphragm performance.

Abdomen↗

PubMed2005

Motor control of the costal and crural diaphragm--insights from transcranial magnetic stimulation in man.

The costal and crural parts of the diaphragm differ in their embryological development and physiological function. It is not known if this is reflected in differences in their motor cortical representation. We compared the response of the costal and crural diaphragms using varying intensities of transcranial magnetic stimulation of the motor cortex at rest and during submaximal and maximal inspiratory efforts. The costal and crural motor evoked potential recruitment curves during submaximal inspiratory efforts were similar. The response to stimulation before, during and at 10 and 30 min after 44 consecutive maximal inspiratory efforts was also the same. Using paired stimulations to investigate intra-cortical facilitatory and inhibitory circuits we found no difference between the costal and crural response with varying interstimulus intervals, or when conditioning and test stimulus intensity were varied. We conclude that supraspinal control of the costal and crural diaphragm is identical during inspiratory tasks.

Action Potentials↗

PubMed2005

Chest radiography cannot predict diaphragm function.

The finding of hemidiaphragm elevation on a chest radiograph, in absence of an ipsilateral lung disease, is assumed to indicate severe hemidiaphragm dysfunction. To test this hypothesis we retrospectively reviewed chest radiograph findings and corresponding twitch transdiaphragmatic pressure (T(W)P(DI)) results from 42 (17 female, age range 22-79 years) consecutive patients who underwent phrenic nerve stimulation studies. Chest radiographs were independently reviewed in a blind manner by two radiologists. The interobserver agreement was moderate, the kappa value ranging from 0.48 (left hemidiaphragm) to 0.59 (lung parenchyma). Hemidiaphragm dysfunction was diagnosed if T(W)P(DI) of corresponding hemidiaphragm was less than 3.5 cm H2O. The prevalence of patients with an elevated unilateral hemidiaphragm on chest radiograph was 64% and of patients with unilateral paralysis judged by T(W)P(DI) was 24%. Sensitivity, specificity, positive and negative predictive values for chest radiograph, as a diagnostic test for unilateral diaphragm dysfunction were 0.90, 0.44, 0.33 and 0.93, respectively. We conclude that the isolated elevation of hemidiaphragm on chest radiograph is of little value in the diagnosis of unilateral hemidiaphragm paralysis, though the condition is unlikely if diaphragm elevation is absent.

Adult↗

PubMed2004

Effect of bronchoscopic lung volume reduction on dynamic hyperinflation and exercise in emphysema.

Endobronchial valve placement improves pulmonary function in some patients with chronic obstructive pulmonary disease, but its effects on exercise physiology have not been investigated. In 19 patients with a mean (SD) FEV(1) of 28.4 (11.9)% predicted, studied before and 4 weeks after unilateral valve insertion, functional residual capacity decreased from 7.1 (1.5) to 6.6 (1.7) L (p = 0.03) and diffusing capacity rose from 3.3 (1.1) to 3.7 (1.2) mmol . minute(-1) . kPa(-1) (p = 0.03). Cycle endurance time at 80% of peak workload increased from 227 (129) to 315 (195) seconds (p = 0.03). This was associated with a reduction in end-expiratory lung volume at peak exercise from 7.6 (1.6) to 7.2 (1.7) L (p = 0.03). Using stepwise logistic regression analysis, a model containing changes in transfer factor and resting inspiratory capacity explained 81% of the variation in change in exercise time (p < 0.0001). The same variables were retained if the five patients with radiologic atelectasis were excluded from analysis. In a subgroup of patients in whom invasive measurements were performed, improvement in exercise capacity was associated with a reduction in lung compliance (r(2) = 0.43; p = 0.03) and isotime esophageal pressure-time product (r(2) = 0.47; p = 0.03). Endobronchial valve placement can improve lung volumes and gas transfer in patients with chronic obstructive pulmonary disease and prolong exercise time by reducing dynamic hyperinflation.

Bronchoscopy↗

PubMed2004

Community pulmonary rehabilitation after hospitalisation for acute exacerbations of chronic obstructive pulmonary disease: randomised controlled study.

OBJECTIVE: To evaluate the effects of an early community based pulmonary rehabilitation programme after hospitalisation for acute exacerbations of chronic obstructive pulmonary disease (COPD). DESIGN: A single centre, randomised controlled trial. SETTING: An inner city, secondary and tertiary care hospital in London. PARTICIPANTS: 42 patients admitted with an acute exacerbation of COPD. INTERVENTION: An eight week, pulmonary rehabilitation programme for outpatients, started within 10 days of hospital discharge, or usual care. MAIN OUTCOME MEASURES: Incremental shuttle walk distance, disease specific health status (St George's respiratory questionnaire, SGRQ; chronic respiratory questionnaire, CRQ) and generic health status (medical outcomes short form 36 questionnaire, SF-36) at three months after hospital discharge. RESULTS: Early pulmonary rehabilitation, compared with usual care, led to significant improvements in median incremental shuttle walk distance (60 metres, 95% confidence interval 26.6 metres to 93.4 metres, P = 0.0002), mean SGRQ total score (-12.7, -5.0 to -20.3, P = 0.002), all four domains of the CRQ (dyspnoea 5.5, 2.0 to 9.0, P = 0.003; fatigue 5.3, 1.9 to 8.8, P = 0.004; emotion 8.7, 2.4 to 15.0, P = 0.008; and mastery 7.5, 4.2 to 10.7, P < 0.001) and the mental component score of the SF-36 (20.1, 3.3 to 36.8, P = 0.02). Improvements in the physical component score of the SF-36 did not reach significance (10.6, -0.3 to 21.6, P = 0.057). CONCLUSION: Early pulmonary rehabilitation after admission to hospital for acute exacerbations of COPD is safe and leads to statistically and clinically significant improvements in exercise capacity and health status at three months.

Acute Disease↗

PubMed2004

Demonstration of a second rapidly conducting cortico-diaphragmatic pathway in humans.

Functional imaging studies in normal humans have shown that the supplementary motor area (SMA) and the primary motor cortex (PMC) are coactivated during various breathing tasks. It is not known whether a direct pathway from the SMA to the diaphragm exists, and if so what properties it has. Using transcranial magnetic stimulation (TMS) a site at the vertex, representing the diaphragm primary motor cortex, has been identified. TMS mapping revealed a second area 3 cm anterior to the vertex overlying the SMA, which had a rapidly conducting pathway to the diaphragm (mean latency 16.7 +/- 2.4 ms). In comparison to the vertex, the anterior position was characterized by a higher diaphragm motor threshold, a greater proportional increase in motor-evoked potential (MEP) amplitude with voluntary facilitation and a shorter silent period. Stimulus-response curves did not differ significantly between the vertex and anterior positions. Using paired TMS, we also compared intracortical inhibition/facilitation (ICI/ICF) curves. In comparison to the vertex, the MEP elicited from the anterior position was not inhibited at short interstimulus intervals (1-5 ms) and was more facilitated at long interstimulus intervals (9-20 ms). The patterns of response were identical for the costal and crural diaphragms. We conclude that the two coil positions represent discrete areas that are likely to be the PMC and SMA, with the latter wielding a more excitatory effect on the diaphragm.

Adult↗

PubMed2004

Corticospinal control of respiratory muscles in chronic obstructive pulmonary disease.

Patients with chronic obstructive pulmonary disease (COPD) face an increased respiratory load and in consequence have an elevated respiratory drive. We used transcranial magnetic stimulation (TMS) to investigate associated changes in corticospinal excitability both at rest and during voluntary facilitation at different levels of inspiratory effort. Diaphragm and abdominal motor thresholds were significantly lower in COPD than healthy controls, but the quadriceps response was the same. In patients there was a significant increase in diaphragm response from rest during 20% inspiratory efforts but no further increase with greater efforts. In controls there was a further stepwise increase at 40% and 60% of inspiratory effort. The cortical silent period was significantly shorter in COPD. Using paired stimulation to study intracortical inhibitory and excitatory circuits we found significantly less excitability of intracortical facilitatory circuits in patients at long (>7 ms) interstimulus intervals. These results suggest that there is a ceiling effect in motor control output to the respiratory muscles of patients with COPD.

Abdominal Muscles↗

PubMed2004

Angiotensin converting enzyme genotype and strength in chronic obstructive pulmonary disease.

Quadriceps muscle weakness is an important contributor to exercise limitation in patients with chronic obstructive pulmonary disease. The deletion allele of the angiotensin converting enzyme gene polymorphism has previously been associated with a greater response to strength training in healthy subjects and might, therefore, protect against detraining in these patients. In 103 stable outpatients (mean [SD] FEV(1) 34.4 [16.5] % predicted), the angiotensin deletion allele was associated with greater isometric quadriceps strength; mean (SD) 31.4 (10.8) kg for insertion homozygotes, 34.1 (13.0) kg for heterozygotes, and 38.3 (11.6) kg for deletion homozygotes (p = 0.04 linear trend). Adjusted for fat-free mass, the relationship was stronger (linear trend p = 0.007). There was no correlation between strength and genotype in a group of 101 age-matched healthy control subjects. Twitch quadriceps force in response to magnetic femoral nerve stimulation, measured in 39 patients, was also genotype dependent; 8.3 (2.6) kg for insertion homozygotes, 10.1 (3.6) kg for heterozygotes, and 12.4 (3.5) kg for deletion homozygotes (p = 0.002 linear trend). Body mass index and fat-free mass did not differ significantly between genotypes in either group. There was no association in either patients or control subjects between genotype and inspiratory muscle strength. In chronic obstructive pulmonary disease the deletion allele is associated with greater quadriceps strength independent of confounding factors.

Cohort Studies↗

PubMed2004

Depression of diaphragm motor cortex excitability during mechanical ventilation.

The effect of mechanical ventilation on the diaphragm motor cortex remains unknown. We assessed the effect of mechanical ventilation on diaphragm motor cortex excitability by measuring the costal and crural diaphragm motor-evoked potential (MEP) elicited by single and paired transcranial magnetic stimulation. In six healthy subjects, MEP recruitment curves of the costal and crural diaphragms were assessed at relaxed end expiration during spontaneous breathing [baseline tidal volume (Vt(baseline))] and isocapnic volume cycled ventilation delivered noninvasively (NIV) at three different levels of tidal volume (Vt(baseline), Vt(baseline) + 5 ml/kg liters, and Vt(baseline) + 10 ml/kg liters). The costal and crural diaphragm response to peripheral stimulation of the right phrenic nerve was not reduced by NIV. NIV reduced the costal and crural MEP amplitude during NIV (P < 0.0001) with the maximal reduction at Vt(baseline) + 5 ml/kg. Response to paired TMS showed that NIV (Vt(baseline) + 5 ml/kg) significantly increased the sensitivity of the cortical motoneurons to facilitatory (>9 ms) interstimulus intervals (P = 0.002), suggesting that the diaphragm MEP amplitude depression during NIV is related to neuromechanical inhibition at the level of motor cortex. Our results demonstrate that mechanical ventilation directly inhibits central projections to the diaphragm.

Abdomen↗

PubMed2004

Endobronchial volume reduction: a myth or a marvel?

Recently, several techniques and devices have been suggested for achieving endobronchial volume reduction in patients with emphysema. The proposed devices include bronchial blockers, bronchial valves, glues, biomodulators, and stents for bronchial fenestration; all could be positioned only with flexible bronchoscopy. The current available evidence on safety and efficacy for these methods is based on animal studies and patient case series at best; therefore, all the results should be seen with caution. However, the preliminary work reviewed here suggests that most of the proposed techniques can work, and seem to be safer than surgical volume reduction. We do not yet have a proven, widely applicable endobronchial palliative treatment for severe emphysema, but we have a rapidly growing area of new research in interventional pneumology with preliminary results that challenge traditional medical thinking.

Journal Article↗

PubMed2004

Nutritional status is an important predictor of diaphragm strength in young patients with cystic fibrosis.

BACKGROUND: The effect of nutritional status and lung disease progression on diaphragm strength in young patients with cystic fibrosis remains unclear. OBJECTIVE: The aim of this study was to investigate the effect of nutritional status and airway obstruction on diaphragm strength. DESIGN: Twitch transdiaphragmatic pressure (Tw Pdi) obtained by bilateral anterior magnetic phrenic nerve stimulation, body mass index (BMI) z score, fat mass, fat-free mass (FFM), arm muscle circumference (AMC), forced expiratory volume in 1 s (FEV(1)), and functional residual capacity (FRC) were measured in 20 patients aged 15.1 +/- 2.8 y (x +/- SD). Values were expressed as a percentage of predicted values. RESULTS: Mean (+/-SD) Tw Pdi was 24.3 +/- 5.5 cm H(2)O. Univariate regression analysis showed positive correlations between Tw Pdi and nutrition scores (BMI z score: r = 0.63, P = 0.003; FFM: r = 0.47, P = 0.04; AMC: r = 0.45, P = 0.04), airway obstruction (FEV(1): r = 0.68, P = 0.001), and arterial oxygen partial pressure (r = 0.68, P = 0.001). Negative correlations were observed between Tw Pdi and dynamic hyperinflation (FRC: r = -0.65, P = 0.005) and arterial carbon dioxide pressure (r = -0.50, P = 0.03). Furthermore, stepwise regression analysis showed that Tw Pdi correlated with BMI z score (r = 0.75, P = 0.0002) and FEV(1) (r = 0.69, P = 0.001). CONCLUSIONS: Diaphragm strength is relatively well preserved in young patients with cystic fibrosis. However, the strength of the diaphragm decreases with the progression of malnutrition and airway obstruction.

Adolescent↗

PubMed2004

Quadriceps strength assessed by magnetic stimulation of the femoral nerve in normal subjects.

Magnetic stimulation of the femoral nerve is a new technique for assessment of quadriceps strength by measurement of twitch tension (TwQ), a test that is independent of subject motivation. In this study, we sought to establish better define normal values by measurement of TwQ in 45 normal subjects. Supramaximal stimulation, as judged by TwQ, was achieved in 43 of 45 subjects at a mean of 90% of maximum stimulator output. The mean TwQ was 7.3 kilograms force (kgf) (SD 2.3) for women and 9.8 kgf (SD 2.5) for men. The mean maximal voluntary contraction (MVC) force was 48.0 kgf (SD 9.2) for women and 70.1 kgf (15.2) for men. The mean TwQ/MVC ratio was 0.15 for both women and men. Significant correlations were observed between TwQ and weight (r = 0.33, P<0.03) and height (r = 0.32, P<0.03) but these were weak and only partially explained observed variance. Our data confirm that magnetic femoral nerve stimulation is generally acceptable to naive subjects. Values are provided as a function of weight in normal subjects.

Adult↗

PubMed2004

Apnea and history.

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Famous Persons↗

PubMed2004

Influence of immersion in water on muscle function and breathing pattern in patients with severe diaphragm weakness.

STUDY OBJECTIVE: Dyspnea is a common symptom in patients with diaphragm weakness or paralysis. In particular, dyspnea may be aggravated by immersion. We hypothesized that immersion to the neck in water would decrease vital capacity and consequently increase the demand/capacity ratio of the respiratory muscles. DESIGN: Case series study. SUBJECTS: Seven patients with profound diaphragm weakness or paralysis proven by phrenic nerve stimulation, and seven normal control subjects. INTERVENTION AND MEASUREMENTS: We measured land-based and water-based spirometry, breathing pattern, and mouth occlusion pressures. RESULTS: We found that the patients could preserve minute ventilation despite a fall in vital capacity from a mean of 2.3 to 1.3 L, but this required an increased respiratory rate (RR) [21.4 to 26.7 breaths/min, p = 0.018]. We used mouth occlusion pressure 100 ms after the start of inspiration (P(0.1)) as an estimation of the drive to breath; P(0.1) increased from 1.4 to 3.9 cm H(2)O (p = 0.018) without significant change in tidal volume. CONCLUSIONS: Relative to control subjects, patients with diaphragm weakness have augmented drive to breathe in order to attempt to defend gas exchange. This conclusion is implied by the presevered minute ventilation with immersion, the augmented RR, and elevated P(0.1) relative to maximum static inspiratory pressure.

Adult↗

PubMed2003

Measurement of diaphragm loading during pressure support ventilation.

OBJECTIVE: The diaphragmatic pressure-time product (PTPdi) has been used to quantify the loading and unloading of the diaphragm. The validity of the relationship between PTPdi and diaphragm electrical activity (EMGdi) during pressure-support ventilation (PSV) is unclear. We examined this relationship. DESIGN AND SETTING: Physiological study in a physiology laboratory. SUBJECTS: Six healthy adults. INTERVENTIONS: Spontaneous breathing (SB) and two levels of PSV (6 and 12 cmH(2)O), breathing room air and incremental concentrations of carbon dioxide, sufficient to achieve an EMGdi signal of approximately 200% of baseline value. MEASUREMENTS AND RESULTS: We measured the electrical (EMGdi) and mechanical (PTPdi) activity of the diaphragm using oesophageal electrode and oesophageal and gastric balloon catheters. The relationship between EMGdi and PTPdi during SB was linear in five subjects and curvilinear in one. However, with PSV 12 cmH(2)O we observed that the relationship between EMGdi and PTPdi was 'left shifted'; specifically, for any given level of EMGdi the PTPdi was smaller with PSV 12 cmH(2)O than during SB. However, when PTPdi was converted to power (the product of pressure and flow) the tendency to left shift was largely reversed. CONCLUSIONS: We conclude that when assessing of diaphragm unloading during PSV flow measurements are required. Where flow is constant, PTPdi is a valid measure of diaphragm unloading, but if not these data may be used to make an appropriate correction.

Airway Resistance↗

PubMed2003

Effects of exhaustive incremental treadmill exercise on diaphragm and quadriceps motor potentials evoked by transcranial magnetic stimulation.

It is unknown whether changes in corticomotor excitability follow exercise in healthy humans. We hypothesized that a fall in the diaphragm and quadriceps motor-evoked potential (MEP) amplitude elicited by transcranial magnetic stimulation of the motor cortex would occur after an incremental exercise task. In 11 healthy subjects, we measured transdiaphragmatic pressure and isometric quadriceps tension in response to supramaximal peripheral magnetic nerve stimulation. MEPs were recorded from these muscles in response to transcranial magnetic stimulation. After baseline measurements, subjects performed a period of submaximal exercise (gentle walking). Measurements were repeated 5 and 20 min after this. The subjects then exercised on a treadmill with an incremental protocol to exhaustion. Transcranial magnetic stimulation was performed at baseline and at 5, 20, 40, and 60 min after exhaustive exercise, and force measurements were obtained at baseline, 20 min, and 60 min. Mean exercise duration was 18 +/- 4 min, and mean maximum heart rate was 172 +/- 10 beats/min. Twitch transdiaphragmatic pressure and twitch isometric quadriceps tension were not different from baseline after exercise, but a significant decrease was observed in diaphragm MEP amplitude 5 and 20 min after exercise (60 +/- 38 and 45 +/- 24%, respectively, of baseline, P = 0.0001). At the same times, the mean quadriceps MEPs were 59 +/- 39 and 74 +/- 32% of baseline (P < 0.0001 and P < 0.01, respectively). Studies using paired stimuli confirmed a likely intracortical mechanism for this depression. Our data confirm significant depression of both diaphragm and quadriceps MEPs after incremental treadmill exercise.

Adult↗