Electronic Supplement Material

Dyspnea and surface inspiratory electromyograms

in mechanically ventilated patients

Matthieu Schmidt ; Félix Kindler ; Stewart B Gottfried ;

Mathieu Raux ; Francois Hug;Thomas Similowski ; Alexandre Demoule

Electronic Supplement Material

Extended Methods

Patients and methods

The study was conducted in a 16-bed ICU within a 1600-bed university hospital. It was externally approved with regard to ethics and compliance to the French law on biomedical research (“Comité de protection des personnes – Ile de France VI"). Informed consent was obtained from the patients.

Patients. Intubated or tracheostomised patients were eligible for inclusion in the study if: 1) they had been mechanically ventilated with inspiratory pressure support (IPS) for at least 12 hours; 2) they had received no sedative, vasopressor or inotropic medication during the last 12 hours; 3) their Ramsay score was ≤ 3 [1]. 4) according to the validated ATICE scale [2] they were awake and able to obey to five commands (“open/close your eyes”, “look at me”, “open your mouth and put out your tongue”, “nod”, and “raise your eyebrows when I have counted up to five”). Patients were not included in the study when communication was likely to be difficult (auditory or visual impairment, insufficient command of French), when they were known to suffer from prior psychiatric or neurological disease, or when they presented with obvious delirium at the time of evaluation.

The study pertains to a convenience sample of twelve patients (Table 1).

Study protocol. The patients were studied lying at 30° with their head maintained in the neutral position. They were ventilated using a Servo-i ventilator (Maquet Critical Care, Solna, Sweden), with a high sensitivity flow trigger (1L.min-1) and a 100ms inspiratory slope. Positive end expiratory pressure (PEEP) was set at 4 cmH2O and the fractional concentration of oxygen (FiO2) was set to achieve a SpO2 of 92% to 96%. For all patients, the PEEP and FiO2 were kept constant throughout the four experimental conditions. Two levels of pressure support (PS) were sequentially applied in random order. A low PS level (LowPS) targeted a tidal volume of 6-8 ml/kg whereas a high PS level (HighPS) targeted a tidal volume of 8-12 ml/kg. Two expiratory trigger (ET) levels were also sequentially applied in random order: a high ET level (HighET) set at 30% of the peak inspiratory flow (50% in chronic obstructive pulmonary disease [COPD] patients) and a low ET level (LowET) set at 5% of the peak inspiratory flow (30% in COPD patients). Four distinct conditions were thus defined (HighPS-LowET, LowPS-LowET, HighPS-HighET and LowPS-HighET), and applied during four 20 minutes periods. The subsequent results pertain to the analysis of the last 10 minutes of each of these epochs.

Measurements.

Flow and pressure. Flow was measured with a heated Fleisch pneumotachograph (Hans Rudolph, Kansas City, MO, USA) placed between the endotracheal tube and the Y-piece of the ventilator circuit. Airway pressure was measured proximal to the endotracheal tube by a pressure transducer (DP 15-32, Validyne, Northridge, CA, USA).

Electromyography. The EMG signals were collected using surface electrodes (48 x 33mm, ref 31.1925.21, Kendall, Tyco Healthcare, Germany). Bilateral parasternal intercostal-target recordings were obtained from the second intercostal space, close to the sternum (see Fig E1 in the ESM). Bilateral scalene-targeted recordings were obtained in the posterior triangle of the neck at the level of the cricoid cartilage. The best side was retained for iEMG analyses. Alae nasi-targeted recordings were obtained by placing one electrode on each nostril (see Fig E2 in the ESM). The signals were first pre-amplified (gain of 0.5) and pre-filtered below 10Hz and above 1,000 Hz (Electronique du Mazet, Le Mazet Saint Voy, France).

EMG, flow and pressure were sampled at 2,000 Hz (PowerLab, AD Instrument, Hastings, UK) and stored on file for subsequent analysis.

Dyspnea and comfort. At the end of each study period, dyspnea was rated using a visual analog scale (VAS) [3-5], and respiratory comfort was evaluated using the Adaptation to the Intensive Care Environment scale (ATICE) [2].

EMG processing and data analysis. EMG signals were averaged according to Hug et al. [6] (Figure E1). In brief; inspiratory efforts were identified from the flow signal. The EMG signal was then truncated in as many epochs as there were inspiratory efforts, each epoch starting 1s before the beginning of the corresponding inspiratory efforts and ceasing 2s after its conclusion so as to contain the full inspiratory-related EMG activity. In the end, eighty consecutive iEMG epochs phase-locked to inspiration were root mean-squared (RMS) and ensemble-averaged. This produced a mean iEMG-RMS signal that was used to measure; 1) the electromechanical inspiratory delay (EMID) from iEMG onset to flow onset; 2) the maximum iEMG activity (EMGmax); 3) the iEMG area under the curve (EMGAUC). EMGmax and EMGAUC were expressed as the percentages of their maximum value as observed in any of the four conditions. The ratio of EMGAUC to tidal volume (Vt) was calculated as an index of neuro-mechanical coupling (EMGAUC/ Vt).

Ineffective efforts were defined as an abrupt airway pressure drop (≥ 0.5 cmH2O) simultaneous to a flow decrease (in absolute value) and not followed by an assisted cycle during the expiratory period [6, 7]. In case of a doubt on an artefactual variation of pressure and flow, the presence of a concomitant EMG activity on extra diaphragmatic signals recordings ascertained the reality of the ineffective triggering.

Statistical Analysis. The statistical analysis was performed using the Prism 4.01 software (GraphPad Software, San Diego, CA). Normality testing (Kolmogorov-Smirnov) consistently failed: results are therefore expressed as median [25-75 interquartile range] and non-parametric statistical tests were used. A Friedman analysis of variance was performed to compare the 4 ventilatory assistance conditions in terms of EMGmax , EMGAUC and EMGmin, followed, when appropriate, by a pairwise comparison using Dunn’s post-hoc test. Comparison between the EMID of the 3 muscles were performed using a Kruskal-Wallis test. The relationship between dyspnea and iEMG values, tidal volume or ineffective efforts, was examined using the Spearman’s correlation. The normalisation/denormalisation technique described by Poon [8] was used to account for the use of within-patients replications in the correlation calculations. Differences were considered significant when the probability p of a type I error was below 5%.

References

1. Ramsay MA, Savege TM, Simpson BR, Goodwin R (1974) Controlled sedation with alphaxalone-alphadolone. Br Med J 2: 656-659

2. De Jonghe B, Cook D, Griffith L, Appere-de-Vecchi C, Guyatt G, ThÃ©ron Vr, Vagnerre A, Outin H (2003) Adaptation to the Intensive Care Environment (ATICE): development and validation of a new sedation assessment instrument. Crit Care Med 31: 2344-2354

3. Lush MT, Janson-Bjerklie S, Carrieri VK, Lovejoy N (1988) Dyspnea in the ventilator-assisted patient. Heart Lung 17: 528-535

4. Bouley GH, Froman R, Shah H (1992) The experience of dyspnea during weaning. Heart Lung 21: 471-476

5. Powers J, Bennett SJ (1999) Measurement of dyspnea in patients treated with mechanical ventilation. Am J Crit Care 8: 254-261

6. Hug Fo, Raux M, Prella M, Morelot-Panzini C, Straus C, Similowski T (2006) Optimized analysis of surface electromyograms of the scalenes during quiet breathing in humans. Respir Physiol Neurobiol 150: 75-81

7. Georgopoulos D, Prinianakis G, Kondili E (2006) Bedside waveforms interpretation as a tool to identify patient-ventilator asynchronies. Intensive Care Med 32: 34-47

8. Poon CS (1988) Analysis of linear and mildly nonlinear relationships using pooled subject data. J Appl Physiol 64: 854-859

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Tables

Table E1. Respiratory rate, tidal volume, dyspnea intensity as assessed with the visual analogic scale (VAS) and adaptation to the Intensive Care Environment scale (ATICE).

HighPS / LowPS
LowET / HighET / LowET / HighET
Tidal volume (mL.kg-1) / 10.5 [8.1-11.7] / 8.8 [8.1-11.5] / 6.8 [6.2-7.5] *$ / 6.9 [6.1-7.9]*
Respiratory rate (breaths.min-1) / 20 [13-22] / 23 [20-29] / 24 [19-31] / 27 [24-34]*
Minute ventilation (L.min-1) / 12.1 [7.3-14.0] / 12.5 [10.0-13.9] / 12.3 [11.1-14.2] / 12.1 [8.3-12.9]
Dyspnea intensity (VAS, from zero to 10) / 0.0 [0.0-1.5] / 1.0 [0.0-4.0] / 5.0 [3.5-7.0]*$ / 4.0 [3.5-7.0]*$
ATICE scale / 19.0 [18.5-20.0] / 20.0 [19.5-20.0] / 18.0 [16.5-18.5] / 18.0 [15.5-19.0]$

Columns are median and bars are interquartile range.

* p <0.05 with LowPS-High-ET; $ p<0.05 with LowPS-LowET.

Table E2. Electromyographic (EMG) activity of the scalenes, intercostal parasternal, and Alae nasi muscles.

Scalenes / Parasternal intercostals / Alae nasi
HighPS / LowPS / HighPS / LowPS / HighPS / LowPS
LowET / HighET / LowET / HighET / LowET / HighET / LowET / HighET / LowET / HighET / LowET / HighET
EMGmax
(%) / 12
[0-33] / 51
[0-80] / 88
[80-99] * / 100
[77-100] * / 16
[0-34] / 20
[0-60] / 82
[78-89] / 100
[100-100] *$ / 10
[8-11] / 27
[7-56] / 81
[76-96]* / 100
[78-100]*
EMGAUC
(%) / 11
[0-33] / 41
[0-67] / 83
[78-99] * / 100
[63-100] * / 11
[0-33] / 22
[0-50] / 81
[76-96] * / 100
[76-96] *$ / 11
[5-16] / 35
[8-50] / 82
[80-100]* / 100
[78-100]*
EMGmin
(%) / 9
[0-32] / 43
[0-87] / 81
[72-85]* / 100
[74-100]* / 10
[0-29] / 26
[0-59] / 80
[60-96] / 100
[86-100]*$ / 12
[1-20] / 33
[8-62] / 82
[73-100]* / 100
[80-100]*$

HighPS, high pressure support level targeting a tidal volume of 8-12 ml/kg; LowPS, low pressure support level targeting a tidal volume of 6-8 ml/kg; High-ET, high expiratory trigger level (30% of the peak inspiratory flow, 50% in COPD patients); LowET, low expiratory trigger level (5% of the peak inspiratory flow, 30% in COPD patients); EMGmax, peak of electromyographic activity; EMGAUC, area under the EMG curve; EMGmin, EMG activity per minute calculated as EMGAUC x (respiratory rate + ineffective effort per minute).

EMGmax, EMGAUC and EMGmin were expressed as for each patient as a proportion of the maximum value measured in any the four conditions.

Data are median [interquartile range]. * p <0.05 with HighPS-LowET; $ p<0.05 with HighPS-HighET

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Figures

Figure E1. Principle of the electromyographic signal analysis (EMG)

EMGaverage, averaged electromyographic signal (6); RMS, average signal root-mean-square; EMGmax , peak of the EMG activity; EMGAUC, area under the curve of the EMG signal.

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