Effects of lung volume on diaphragm EMG signal strength during voluntary contractions

The use ofesophageal recordings of the diaphragm electromyogram (EMG) signalstrength to evaluate diaphragm activation during voluntary contractionsin humans has recently been criticized because of a possible artifactcreated by changes in lung volume. Therefore, the first aim of thisstudy was to evaluate whether there is an artifactual influence of lungvolume on the strength of the diaphragm EMG during voluntarycontractions. The second aim was to measure the required changes inactivation for changes in lung volume at a given tension, i.e., thevolume-activation relationship of the diaphragm. Healthy subjects(n = 6) performed contractions of thediaphragm at different transdiaphragmatic pressure (Pdi) targets (range20-160 cmH₂O) whilemaintaining chest wall configuration constant at different lungvolumes. The diaphragm EMG was recorded with a multiple-arrayesophageal electrode, with control of signal contamination andelectrode positioning. The effects of lung volume on the EMG werestudied by comparing the crural diaphragm EMG root mean square (RMS),an index of crural diaphragm activation, with an index of globaldiaphragm activation obtained by normalizing Pdi to the maximum Pdi atthe given muscle length(Pdi/Pdi_max@L) at thedifferent lung volumes. We observed a direct relationship between RMSand Pdi/Pdi_max@L independent of diaphragm length. The volume-activation relationship ofthe diaphragm was equally affected by changes in lung volume as thevolume-Pdi relationship (60% change from functional residual capacityto total lung capacity). We conclude that the RMS of the diaphragm EMGis not artifactually influenced by lung volume and can be used as areliable index of diaphragm activation. The volume-activationrelationship can be used to infer changes in the length-tensionrelationship of the diaphragm at submaximal activation/contractionlevels.

     

Crural diaphragm activation during dynamic contractions at various inspiratory flow rates

The purpose of this study was to evaluate the influence ofvelocity of shortening on the relationship between diaphragm activation and pressure generation in humans. This was achieved by relating theroot mean square (RMS) of the diaphragm electromyogram to thetransdiaphragmatic pressure (Pdi) generated during dynamic contractionsat different inspiratory flow rates. Five healthy subjects inspiredfrom functional residual capacity to total lung capacity at differentflow rates while reproducing identical Pdi and chest wall configurationprofiles. To change the inspiratory flow rate, subjects performed theinspirations while breathing across two different inspiratoryresistances (10 and 100 cmH₂O · l¹ · s),at mouth pressure targets of 10, 20, 40, and60 cmH₂O. The diaphragmelectromyogram was recorded and analyzed with control of signalcontamination and electrode positioning. RMS values obtained forinspirations with identical Pdi and chest wall configuration profileswere compared at the same percentage of inspiratory duration. Atinspiratory flows ranging between 0.1 and 1.4 l/s, there was nodifference in the RMS for the inspirations from functional residualcapacity to total lung capacity when Pdi and chest wall configurationprofiles were reproduced (n = 4). Athigher inspiratory flow rates, subjects were not able to reproducetheir chest wall displacements and adopted different recruitmentpatterns. In conclusion, there was no evidence for increased demand ofdiaphragm activation when healthy subjects breathe with similar chestwall configuration and Pdi profiles, at increasing flow rates up to 1.4 l/s.

     

Voluntary activation of the human diaphragm in health and disease

Intersubjectcomparison of the crural diaphragm electromyogram, as measured by anesophageal electrode, requires a reliable means for normalizing thesignal. The present study set out 1) to evaluate which voluntary respiratory maneuvers provide high andreproducible diaphragm electromyogram root-mean-square (RMS) values and2) to determine the relativediaphragm activation and mechanical and ventilatory outputs duringbreathing at rest in healthy subjects(n = 5), in patients with severechronic obstructive pulmonary disease (COPD,n = 5), and in restrictive patientswith prior polio infection (PPI, n = 6). In all groups, mean voluntary maximal RMS values were higher duringinspiration to total lung capacity than during sniff inhalation throughthe nose (P = 0.035, ANOVA). The RMS(percentage of voluntary maximal RMS) during quiet breathing was 8% inhealthy subjects, 43% in COPD patients, and 45% in PPI patients.Despite the large difference in relative RMS(P = 0.012), there were no differencesin mean transdiaphragmatic pressure (P = 0.977) and tidal volumes (P = 0.426). We conclude that voluntary maximal RMS is reliably obtainedduring an inspiration to total lung capacity but a sniff inhalationcould be a useful complementary maneuver. Severe COPD and PPI patientsbreathing at rest are characterized by increased diaphragm activationwith no change in diaphragm pressure generation.

     

Chest wall muscle cross talk in canine costal diaphragm electromyogram

Sinderby, C., S. Friberg, N. Comtois, and A. Grassino.Chest wall muscle cross talk in the canine costal diaphragm electromyogram. J. Appl. Physiol.81(5): 2312-2327, 1996.The present paper describes the influenceof cross talk from the abdominal and intercostal muscles on the caninediaphragm electromyogram (EMG). The diaphragm EMG was recorded withbipolar surface electrodes placed on the costal portion of thediaphragm (abdominal side), aligned in the fiber direction, andpositioned in a region with a relatively low density of motor endplates. The results indicated that cross talk may occur in thediaphragm EMG, especially during conditions of loaded breathing andlight general anesthesia. The cross-talk signals showed characteristicsthat were entirely different from the diaphragm EMG. Although thediaphragm EMG was typical for signals recorded with electrodes alignedin the fiber direction, the cross-talk signals were characteristic ofthose obtained with electrode pairs not aligned in the direction of themuscle fibers. Alterations in electrode positioning, interelectrodedistance, and/or electrode surface area cannot guarantee theelimination of cross-talk signals, whereas spinal anesthesia at a highthoracic level will paralyze the sources of the cross talk and henceeliminate the cross-talk signals. By taking advantage of thedifferences in EMG signal characteristics for the diaphragm EMG andcross-talk signals, an index that has the capability to detect crosstalk was developed.

     

Diaphragm interference pattern EMG and compound muscle action potentials: effects of chest wall configuration

Beck, Jennifer, Christer Sinderby, Lars Lindström, andAlex Grassino. Diaphragm interference pattern EMG and compound muscle action potentials: effects of chest wall configuration. J. Appl. Physiol. 82(2): 520-530, 1997.The effect of chest wall configuration on the diaphragmelectromyogram (EMGdi) was evaluated in five healthy subjects with anesophageal electrode for both interference pattern EMGdi (voluntarycontractions) and electrically evoked diaphragm compound muscle actionpotentials (CMAPs). Diaphragm CMAPs (both unilateral and bilateral)were evaluated for the baseline-to-peak amplitude (Ampl), the time fromthe onset of the CMAP to first peak (T1), root mean square (RMS), andcenter frequency (CF) values of the CMAP power spectrum. CF values fromthe interference pattern EMGdi power spectrum were also calculated. ForCMAPs obtained at an electrode position least influenced by variationsinduced by electrode positioning, Ampl increased with diaphragmshortening from functional residual capacity (FRC) to total lungcapacity (TLC) by 101 and 98% (unilateral and bilateral,respectively). Bilateral CMAP RMS values increased 116% from FRC toTLC. CMAP T1 values decreased with diaphragm shortening from FRC to TLC by 1.1 and 2.1 ms for the unilateral and bilateral stimulations, respectively, and CF increased for the bilateral diaphragm CMAPs withdiaphragm shortening. CF values from the interference pattern EMGdi didnot show any consistent change with chest wall configuration. Thus CFvalues of the interference pattern EMGdi obtained with an esophagealelectrode can be considered reliable for physiological interpretation,at any diaphragm length (if electrode positioning and signalcontamination are controlled for), contrary to the diaphragm CMAPs,which are sensitive to changes in chest wall configuration. It isspeculated that the different results (over the effects of chest wallconfiguration on interference pattern EMGdi and diaphragm CMAPs) may bebecause of summation properties of the signals and how these influencethe EMG power spectrum.

     

Enhancement of signal quality in esophageal recordings of diaphragm EMG

Sinderby, Christer A., Jennifer C. Beck, Lars H. Lindström, and Alejandro E. Grassino. Enhancement of signalquality in esophageal recordings of diaphragm EMG. J. Appl. Physiol. 82(4): 1370-1377, 1997.The cruraldiaphragm electromyogram (EMGdi) is recorded from a sheet of muscle,the fiber direction of which is mostly perpendicular to an esophagealbipolar electrode. The region from which the action potentials areelicited, the electrically active region of the diaphragm(EAR_di) and the center of this region (EAR_{di ctr}) mayvary during voluntary contractions in terms of their position withrespect to an esophageal electrode. Depending on the bipolarelectrode's position with respect to theEAR_{di ctr}, the EMGdi isfiltered to different degrees. The objectives of the present study wereto reduce these filtering effects on the EMGdi by developing ananalysis algorithm referred to as the "double-subtraction technique." The results showed that changes in the position of theEAR_{di ctr} by ±5 mm withrespect to the electrode pairs located 10 mm caudal and 10 mm cephaladprovided a systematic variation in the EMG power spectrumcenter-frequency values by ±10%. The double-subtraction techniquereduced the influence of movement of theEAR_{di ctr} relative to theelectrode array on EMG power spectrum center frequency and root meansquare values, increased the signal-to-noise ratio by 2 dB, andincreased the number of EMG samples that were accepted by the signalquality indexes by 50%.