Evaluating the relationship between respiratory muscle electromyography and ultrasound in healthy subjects

<bold>Introduction:</bold> Respiratory muscle function may be evaluated using non-invasive tests, including parasternal or diaphragm surface electromyography (EMG_para, EMG_di) and thoracic ultrasound (US). The relationship between respiratory muscle EMG and US indices has not previously been evaluated. <bold>Methods:</bold> Healthy subjects performed spirometry, sniff, maximal inspiratory and expiratory mouth pressures (SNIP, MIP, MEP). Tidal and sniff US_di excursion and end-tidal inspiratory parasternal thickness were assessed. EMG_para and US-guided EMG_di were performed to measure neural respiratory drive index (NRDI). Data are reported as mean (±SD) or median (IQR). Pearson or Spearman correlation analyses were performed as appropriate. <bold>Results:</bold> Twenty subjects (12 females, mean age 33.45±8.97 years) had: FEV₁ 99±12%pred, FVC 102±13%pred, SNIP 89±30 cmH₂O, MIP 86±35 cmH₂O, MEP 102±47 cmH₂O. NRDI_para was 113 (59-251)%·bpm and NRDI_di was 90 (40-184)%·bpm. Sniff US_di excursion was 2.48 (1.90-2.90) cm, tidal US_di was ­­­­­­1.97 (1.47-2.10) cm. Right and left parasternal thickness were 0.36±0.15 and 0.33±0.14 cm, respectively. Inverse correlations were found between NRDI_para and sniff US_di excursion (r=-0.58, p=0.01) and SNIP (r=-0.69, p=0.001), and NRDI_di and sniff US_di (r=-0.45, p=0.04) and SNIP (r=-0.46, p=0.04). SNIP was positively correlated to sniff US_di excursion (r=0.57, p=0.01). <bold>Conclusion:</bold> In healthy subjects, surface electromyography of parasternal intercostal muscles and diaphragm is related to US-measured diaphragm excursion. Evaluation of these non-invasive tests of respiratory muscle function in disease states where there is load capacity imbalance of the respiratory muscle pump is warranted.