Neural respiratory drive in healthy subjects and in COPD

Subjects

In total, 100 healthy subjects (mean±sd age 40.3±17.4 yrs (range 18–79 yrs); 56% male; ethnicity: 54% Chinese, 36% white European, 10% other) and 30 COPD patients (age 66.6±7.82 (52–88) yrs; 76.0% male; ethnicity: 63% white European, 37% Chinese; FEV₁ 34.8±13.9% predicted) were studied. The subjects’ age, height, weight and body mass index (BMI) were documented. Spirometry (FEV₁ and slow vital capacity (VC)) and inspiratory capacity (IC) were also measured in COPD patients. Informed consent was taken and the study was performed in accordance with Local Research Ethics Committee (King’s College Hospital, London, UK) procedures.

Instrumentation and signal processing

EMG_di recordings were made from the crural diaphragm using multipair oesophageal electrode catheters, as previously described 13. Further details of the electrode design, positioning and signal processing are given in the online supplementary material.

EMG_di recordings at rest and during maximal inspiratory manoeuvres

Recordings were made sitting upright in a chair, with a nose-clip in place for all measurements except sniff nasal pressure. To record EMG_di during resting breathing, subjects sat quietly in a relaxed posture for ≥5 min, until ≥2 min of stable, consistent EMG_di signals had been recorded. Airflow was measured through a mouthpiece connected in series to a pneumotachograph. EMG_di was then recorded during four inspiratory manoeuvres: 1) maximal inspiration to total lung capacity (TLC); 2) maximal static inspiratory effort at functional residual capacity (FRC) against a closed valve 16; 3) maximal sniff from FRC; and 4) maximum voluntary ventilation for 15 s (“sprint MVV”). Manoeuvres 1–3 were repeated at least three times, until the investigator was satisfied that a truly maximum effort had been performed. The sprint MVV was performed once only.

Calculation of resting EMG_di

The raw signal was converted to root mean square (RMS; Powerlab Chart v5.4 software, ADInstruments, Chalgrove, UK), using a time constant of 50 ms and a moving window. The maximum RMS-EMG_di value during 100-ms subdivisions of each breath was then determined, manually selecting EMG_di signals falling between QRS complexes of the ECG artefact. The mean maximum RMS-EMG_di per breath over two representative 30-s subdivisions of the whole recording was then calculated.

Calculation of EMG_di % max

EMG_di signals recorded during each of the maximum inspiratory manoeuvres were converted to RMS. The largest RMS-EMG_di value calculated by analysis of these recordings was labelled “RMS-EMG_di,peak”. EMG_di % max for each subject was then calculated as the mean maximum RMS-EMG_di per breath as a percentage of RMS-EMG_di,peak.

Assessment of intra- and interobserver reproducibility of EMG_di % max

In total, 10 healthy subjects were studied on two occasions >24 h apart, at the same time of day. The intraobserver reproducibility of EMG_di % max measurements was assessed by comparing the results of a single investigator’s (C.J. Jolley) analysis of measurements made on two separate days. The interobserver reproducibility of EMG_di % max measurements was assessed by comparing the results of two investigators’ (C.J. Jolley and C. Reilly) analysis of a single set of measurements in five of these subjects.

Bilateral anterolateral magnetic phrenic nerve stimulation

Bilateral anterolateral magnetic phrenic nerve stimulation (BAMPS) was performed using two double circular 43-mm coils (P/N 9784-00; Magstim Co.,Whitland, UK) placed anterolaterally over the left and right phrenic nerves, as previously described 17. The coils were powered by a Magstim 200 stimulator (Magstim Co.). During the study, subjects were seated upright in a chair with a nose-clip in place. Stimulation was performed at end-expiration with the abdomen unbound. BAMPS was performed at 80, 85, 90, 95 and 100% maximum stimulator output (MSO), to determine supramaximality. The amplitude of the CMAP_di,MS was measured as the peak–trough amplitude, as previously described 18. The interoccasion coefficient of variation (CV) in the previous study was 8.6% 18.

Statistical analysis

Ranges of EMG_di % max were expressed as 95% confidence intervals of the mean. Comparisons between healthy and COPD subjects were made using independent sample t-tests except comparisons of sex distributions, which were made using Fisher’s exact test. Values of p were considered to be significant at <0.05 level. Relationships between EMG_di and anthropometric or lung function variables were investigated by regression analysis. Intra- and interobserver reproducibility was assessed by calculating the CV and by Bland–Altman analysis 19.

Healthy subjects

The mean±sd EMG_di % max of the healthy group was 9.0±3.4%. The EMG_di % max was 9.2±3.4% for males and 8.8±3.3% for females (p = 0.53).

Correlations between EMG_di % max and age, height, weight and BMI are shown in table 2⇓. EMG_di % max was slightly higher in healthy subjects aged 51–80 yrs (26% of the total healthy subjects, 13 male) than in those aged 18–50 yrs (74% of the total, 43 male; 11.4±3.40 versus 8.16±2.92%, respectively; p = 0.001), although the overall linear correlation between EMG_di % max and age was weak (r = 0.34, p<0.001; see online supplementary material). There was no significant difference in RMS-EMG_di,peak between the older and younger cohorts (226.4±71.7 versus 250.3±67.4 μV, respectively). There were weak but significant negative correlations between EMG_di % max and absolute FEV₁ (r =  -0.34, p = 0.001), and between EMG_di % max and absolute VC (r =  -0.21, p = 0.04).

These data gave 95% confidence intervals of EMG_di % max of 7.5–8.8% in normal subjects aged 18–50 yrs, and 10.1–12.8% in subjects aged >50 yrs.

Sniff nasal inspiratory pressure (SNIP) was higher in the subjects aged 18–50 yrs than in those aged 51–80 yrs (91.4±22.3 versus 80.5±16.3 cmH₂O, respectively; p = 0.04), but the relationship between EMG_di % max and SNIP values was weak (r = 0.19, p = 0.06; n = 98). The difference between maximal inspiratory pressure (P_I,max) in those aged 18–50 yrs and those aged 51–80 yrs approached statistical significance (80.6±32.6 versus 69.5±20.7 cmH₂O, respectively; p = 0.09). There was no significant relationship between EMG_di % max and P_I,max (r = 0.10, p = 0.31).

Correlations between EMG_di % max and age, height, weight and BMI were similar in the white European and Chinese subgroups. Data comparing EMG_di % max values in white European and Chinese ethnic groups are provided in the online supplementary material.

CMAP_di,MS

CMAP_di,MS was assessed in 64 subjects. Supramaximality was judged to have been achieved when the mean CMAP_di,MS amplitude at 100% MSO was less than 5% greater than the highest mean CMAP_di,MS amplitude achieved at the lower stimulator outputs. Using these criteria, supramaximality was achieved in 92.8% of the subjects. The CMAP_di,MS amplitude achieved at 100% MSO values was recorded if supramaximality was not achieved.

Representative traces recorded during BAMPS are shown in the online supplementary material. The mean±sd CMAP_di,MS amplitude was 2.4±0.7 mV. The phrenic nerve conduction time (PNCT), defined as the time from the stimulation artefact to the onset of the CMAP, was 6.9±0.7 ms.

Relationships between CMAP_di,MS amplitude and RMS-EMG_di

Linear regression analysis revealed a positive correlation between each subject’s RMS-EMG_di,peak and CMAP_di,MS amplitude (r = 0.59, p<0.001; fig. 2⇓). The mean±sd RMS-EMG_di per breath (in µV) expressed as a percentage of CMAP_di,MS amplitude (RMS-EMG_di/CMAP_di,MS) was 0.9±0.4%. The relationships between RMS-EMG_di/CMAP_di,MS and age, height, weight and BMI were similar to those with EMG_di % max (table 2⇑).

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Fig. 2—

Relationship between amplitude of the diaphragm compound muscle action potential following bilateral anterolateral magnetic stimulation (CMAP_di,MS) and peak root mean square of spontaneous diaphragm electromyogram activity (RMS-EMG_di,peak). Linear regression analysis revealed a positive correlation between each subject’s RMS-EMG_di,peak and CMAP_di,MS amplitude (r = 0.59, p<0.001).

COPD patients

EMG_di % max in the COPD patients was 27.9±9.9%. This was significantly higher than EMG_di % max recorded in the 26 healthy controls matched for age, height, weight and BMI (11.4±3.4%; p<0.001; table 3⇓ and fig. 3⇓). COPD patients generated a smaller tidal volume (V_T) as a percentage of the predicted VC (VC_pred) per unit EMG_di % max ((V_T % VC_pred)/(EMG_di % max)) than the healthy controls (0.8±0.4 versus 1.4±0.6 arbitrary units, respectively; table 3⇓ and fig. 3⇓).

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Fig. 3—

Box-and-whisker plots comparing diaphragm electromyogram as a percentage of maximum (EMG_di % max) in 30 chronic obstructive pulmonary disease (COPD) patients with 26 healthy subjects matched for age, height, weight and body mass index. Comparisons are made using the independent samples t-test. The box length is the interquartile range. •: outliers, i.e. cases with values between 1.5 and 3 interquartile ranges from the upper or lower edge of the box.

All patients completed FEV₁ and VC measurements, and IC was measured in 20 patients. Significant correlations, best described by curve regression functions, were observed between EMG_di % max and FEV₁ % pred, VC % pred and IC % pred, and between (V_T % VC_pred)/(EMG_di % max) and FEV₁ % pred, VC % pred and IC % pred (table 4⇓, fig. 4⇓ and additional figures in the online supplementary material).

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Fig. 4—

Scatter plots showing correlations between diaphragm electromyogram as a percentage of maximum (EMG_di % max) and percentage predicted a) forced expiratory volume in one second (FEV₁), b) vital capacity (VC) and c) inspiratory capacity (IC), using curve estimation models. Regression coefficients (r² values) and p-values were as follows: a) r² = 0.40, p<0.001; b) r² = 0.61, p<0.001; c) r² = 0.28, p = 0.02.

Peak RMS-EMG_di values during different maximal inspiratory manoeuvres

Data are presented as median (interquartile range (IQR)), as the sprint MVV data were non-normally distributed. The data are also presented in tables S2 and S3 and in figure S3, in the online supplementary material.

The TLC and P_I,max manoeuvres yielded peak values most frequently in the healthy group (31% each), and the TLC manoeuvre yielded the highest RMS-EMG_di values on average in that group (median (IQR) 208.2 (98.7) μV). The sniff manoeuvre yielded peak values most frequently in the COPD group (33%) and yielded the highest RMS-EMG_di values on average in that group (170.6 (76.5) μV). The MVV manoeuvre yielded the lowest values in both groups (healthy 158.7 (78.4) μV, COPD 150.2 (97.2) μV) despite yielding the highest value in 26% of the COPD group.

There were no significant differences between RMS-EMG_di values when the manoeuvres were compared for the COPD group (using the Wilcoxon signed-rank test to compare values within the same subject). In the healthy group, significant differences were observed between all manoeuvres except sniff and P_I,max (p = 0.62).

Intrasubject and interobserver reproducibility of EMG_di % max in healthy subjects

Determinants of EMG_di % max

In general, levels of NRD increase when the load on the respiratory muscles increases relative to their capacity, i.e. if the load increases, the capacity of the muscles decreases, or a combination of these two changes. Levels of EMG_di % max can therefore be explained in terms of ventilatory mechanics, and the pathophysiological changes in ventilatory mechanics that occur with disease.

Healthy subjects

An average EMG_di % max of 9.0% in normal subjects, in whom it can be assumed that there is no neuromechanical dissociation, is consistent with the high levels of ventilatory reserve that are known to exist in healthy individuals. The slightly increased EMG_di % max observed in the older (51–80 yrs of age) cohort compared with that of subjects aged <50 yrs is likely to reflect the known “normal” changes in ventilatory mechanics occurring with increased age. Declines in FEV₁ 20, VC 21, respiratory muscle strength 22 and chest wall compliance 23 observed during healthy ageing all increase the load:capacity ratio of the respiratory muscle pump, reducing ventilatory reserve, and would explain the tendency to higher levels of EMG_di % max in the older age group. The findings of significant negative correlations between EMG_di % max and absolute FEV₁ and VC are consistent with this. The current observation that the correlation of SNIP and P_I,max with EMG_di % max is weak suggests that altered lung and chest wall mechanics are more important contributors to increased drive than reduced diaphragm contractility in the healthy older cohort.

COPD

The results of the present study confirm the hypotheses that EMG_di % max would be higher in COPD patients than healthy subjects, and that the levels of EMG_di % max would be highest in patients with the most severe disease. High levels of EMG_di % max indicate that there is a relative increase in the RMS of EMG_di in COPD compared with healthy subjects, i.e. recruitment of larger numbers of diaphragm motor units and/or an increase in diaphragm motor unit firing rate in COPD. It is, in fact, well known that the firing frequency of motor neurons supplying both the diaphragm 24 and nondiaphragmatic muscles 25 is increased in COPD. However, De Troyer et al. 24 and Gandevia et al. 25 used needle electrodes, which is clearly not feasible in general clinical practice.

The increase in the RMS of EMG_di in COPD is likely to be the result of three main factors. First, the diaphragm must generate more pressure to achieve a given V_T, compared with healthy subjects. Increased airways resistance in COPD results in significant expiratory airflow limitation at rest, leading to gas trapping, which increases intrathoracic end-expiratory pressure. A positive end-expiratory pressure imposes a threshold load that must be overcome before inspiratory airflow can be generated. A reduction in chest wall compliance, as hyperinflation progresses, also contributes to the mechanical load associated with inspiration in severe disease. Hyperinflation is also due to a loss of elastic recoil in emphysema. Secondly, the maximum pressure-generating capacity of the diaphragm is reduced. Polkey et al. 26 demonstrated a linear negative correlation of twitch transdiaphragmatic pressure with increasing lung volume of 3.5 cmH₂O·L⁻¹. The ability of the diaphragm to generate transdiaphragmatic and oesophageal pressure is, therefore, reduced in COPD, and these changes are exaggerated with acute-on-chronic hyperinflation. Thirdly, patients with COPD need to generate increased absolute levels of ventilation to overcome ventilation/perfusion (V′/Q′) mismatch 27.

Although RMS-EMG_di,peak in COPD patients was 83% lower than in healthy subjects, a lower denominator is unlikely to explain the higher EMG_di % max in COPD, as correcting for this gives an average EMG_di % max in COPD of 23.5±8.2%, still significantly higher than EMG_di % max in the healthy group (p<0.001).

Significance of raised EMG_di % max and reduced (V_T % VC_pred)/(EMG_di % max) in COPD

The current finding that EMG_di % max is raised, and is negatively correlated with FEV₁, VC and the degree of hyperinflation at rest (described in terms of IC % pred) in COPD, reinforces the contention that, by providing a composite measure of ventilatory load and capacity, EMG_di % max could provide an alternative method of assessing COPD disease severity. This could be the focus of future studies in larger numbers of COPD patients, including a more detailed investigation of the relationship between EMG_di % max and other physiological measures, including hypoxaemia, hypercapnia and V′/Q′ mismatch, than were carried out in the present study.

(V_T % VC_pred)/(EMG_di % max) was lower in COPD patients than healthy subjects, reflecting neuromechanical uncoupling in COPD, and this correlated with disease severity. Neuromechanical dissociation has previously been demonstrated during exercise in COPD using EMG_di measurements 28, but the relationship with disease severity has not previously been documented. This observation also emphasises the value of EMG_di % max over other commonly used indirect measures of ventilatory drive, such as mouth occlusion pressure at 100 ms (P_0.1), or the amplitude of tidal oesophageal pressure swings, which will underestimate levels of NRD in COPD patients with the most neuromechanical dissociation.

Bilateral CMAP_di,MS values

To the best of the current authors’ knowledge, there have been no previous studies that have compared CMAP_di,MS and RMS-EMG_di,peak in healthy subjects. Normal ranges of bilateral CMAP_di amplitudes and PNCT, recorded using an oesophageal electrode catheter following BAMPS, have also not previously been reported, principally because each phrenic nerve is usually assessed separately. The present findings of a mean±sd CMAP_di,MS amplitude of 2.4±0.7 mV and a PNCT of 6.9±0.7 ms are consistent, following extrapolation from bi- to unilateral measurements, with a unilateral CMAP_di,MS amplitude and PNCT of 1.45±0.35 and 6.9±0.9 ms (right) and 1.68±0.47 and 7.6±0.7 ms (left), as previously recorded in a similar manner 18.

Potential use of CMAP_di,MS amplitude to normalise EMG_di values

The finding that there are relationships between the amplitude of the volitional and nonvolitional EMG_di in healthy subjects suggests that the nonvolitional signal (CMAP_di,MS amplitude) may be used in place of the volitional RMS-EMG_di,peak when normalising resting EMG_di to maximum. This could be of particular importance during the assessment of patients on intensive care units (ICUs), who are unable to generate maximal volitional inspiratory efforts. Potential applications include prediction of weaning failure in patients with respiratory muscle load:capacity imbalance sufficient to impact critically on ventilatory reserve. Levels of EMG_di % max above the normal range would indicate that NRD had increased in response to an increase in ventilatory load with respect to the capacity of the respiratory muscles. Conversely, reductions in NRD, measured by assessing P_0.1, have been shown to be predictive of extubation failure on paediatric ICU 29. P_0.1 would, however, underestimate NRD in patients with disordered ventilatory mechanics, such as in COPD, where neuromechanical dissociation progresses exponentially as airflow obstruction and hyperinflation worsen 28. This approach would also allow the level of EMG_di activity at which neural-assist ventilators such as NAVA 15 are triggered to be defined as EMG_di % max, hence defining this threshold in terms of ventilatory reserve. Since ventilatory failure is the outcome of a critically low ventilatory reserve, this could prove to be a more appropriate approach than increasing NAVA support in response to changes from baseline EMG_di activity.

Potential clinical applications of EMG_di measurements to quantify NRD

The current study technique could be usefully applied to measure disease severity, progression and responses to treatment, in any disorder characterised by increased ventilatory load (e.g. airflow obstruction in asthma and COPD, reduced lung compliance in pulmonary fibrosis, or cardiac failure), reduced ventilatory capacity (in neuromuscular disease), or where there is a combination of both factors, such as in COPD, as herein discussed. The value of the method over other objective physiological measurements of disease severity, such as spirometry, or measurement of lung volumes, is that recording EMG_di gives a breath-by- breath measure of the load on the respiratory system, and can be used to provide measurements continuously during sleep without waking the patient 30, during exercise 28, and, as mentioned in the aforegoing discussion, could in addition be measured nonvolitionally in ventilated patients. The main factor limiting the translation of the technique to clinical practice is the acceptability of the oesophageal catheters to patients. However, in the current authors’ experience of the use of these and similar catheters to assess intrathoracic pressure in clinical practice, the catheters are acceptable in >95% of patients and are usually well tolerated.

In conclusion, the present study has demonstrated, in a large cohort of healthy subjects and patients with chronic obstructive pulmonary disease, that levels of neural respiratory drive, measured as diaphragm electromyogram as a percentage of maximum, are higher in patients with chronic obstructive pulmonary disease than in healthy subjects, and highest in patients with the most severe airflow obstruction and hyperinflation. Diaphragm electromyogram as a percentage of maximum therefore provides a composite measure of ventilatory load and capacity, and could provide a method of assessing chronic obstructive pulmonary disease severity. Normal ranges of diaphragm electromyogram as a percentage of maximum have also been established, which may be used for comparative data in future studies in patients with chronic obstructive pulmonary disease, or indeed any other cardiorespiratory disease, to further understanding of the pathophysiology of ventilatory failure. The current findings also demonstrate that nonvolitional activation of the diaphragm is of potential use in the assessment of diaphragm electromyogram as a percentage of maximum in patients who are unable to perform maximal volitional inspiratory manoeuvres.