Surface EMG-based quantification of inspiratory effort: a quantitative comparison with Pes
Authors: Graßhoff Jan , Petersen Eike , Farquharson Franziska , Kustermann Max , Kabitz Hans-Joachim , Rostalski Philipp , Walterspacher Stephan 
Published: December 2021

Background

Inspiratory patient effort under assisted mechanical ventilation is an important quantity for assessing patient–ventilator interaction and recognizing over and under assistance. An established clinical standard is respiratory muscle pressure$$\textit{P}_{\mathrm{mus}}$$Pmus, derived from esophageal pressure ($$\textit{P}_{\mathrm{es}}$$Pes), which requires the correct placement and calibration of an esophageal balloon catheter. Surface electromyography (sEMG) of the respiratory muscles represents a promising and straightforward alternative technique, enabling non-invasive monitoring of patient activity.

Methods

A prospective observational study was conducted with patients under assisted mechanical ventilation, who were scheduled for elective bronchoscopy. Airway flow and pressure, esophageal/gastric pressures and sEMG of the diaphragm and intercostal muscles were recorded at four levels of pressure support ventilation. Patient efforts were quantified via the$$\textit{P}_{\mathrm{mus}}$$Pmus-time product ($${\mathrm{PTP}}_{\mathrm{mus}}$$PTPmus), the transdiaphragmatic pressure-time product ($${\mathrm{PTP}}_{\mathrm{di}}$$PTPdi) and the EMG-time products (ETP) of the two sEMG channels. To improve the signal-to-noise ratio, a method for automatically selecting the more informative of the sEMG channels was investigated. Correlation between ETP and$${\mathrm{PTP}}_{\mathrm{mus}}$$PTPmuswas assessed by determining a neuromechanical conversion factor$$\textit{K}_{\mathrm{EMG}}$$KEMGbetween the two quantities. Moreover, it was investigated whether this scalar can be reliably determined from airway pressure during occlusion maneuvers, thus allowing to quantify inspiratory effort based solely on sEMG measurements.

Results

In total, 62 patients with heterogeneous pulmonary diseases were enrolled in the study, 43 of which were included in the data analysis. The ETP of the two sEMG channels was well correlated with$${\mathrm{PTP}}_{\mathrm{mus}}$$PTPmus($$\textit{r}={0.79\pm 0.25}$$r=0.79±0.25and$$\textit{r}={0.84\pm 0.16}$$r=0.84±0.16for diaphragm and intercostal recordings, respectively). The proposed automatic channel selection method improved correlation with$${\mathrm{PTP}}_{\mathrm{mus}}$$PTPmus($$\textit{r}={0.87\pm 0.09}$$r=0.87±0.09). The neuromechanical conversion factor obtained by fitting ETP to$${\mathrm{PTP}}_{\mathrm{mus}}$$PTPmusvaried widely between patients ($$\textit{K}_{\mathrm{EMG}}= {4.32\pm 3.73}\,{\hbox {cm}\hbox {H}_{2}\hbox {O}/\upmu \hbox {V}}$$KEMG=4.32±3.73cm2O/μV) and was highly correlated with the scalar determined during occlusions ($$\textit{r}={0.95}$$r=0.95,$$\textit{p}<{.001}$$p<.001). The occlusion-based method for deriving$${\mathrm{PTP}}_{\mathrm{mus}}$$PTPmusfrom ETP showed a breath-wise deviation to$${\mathrm{PTP}}_{\mathrm{mus}}$$PTPmusof$${0.43\pm 1.73}\,{\hbox {cm}\hbox {H}_{2}\hbox {O}\,\hbox {s}}$$0.43±1.73cm2Osacross all datasets.

Conclusion

These results support the use of surface electromyography as a non-invasive alternative for monitoring breath-by-breath inspiratory effort of patients under assisted mechanical ventilation.