Supplementary Material for: The effect of body position on responses to rapid bilateral anterolateral magnetic phrenic nerve stimulation in healthy humans (PosiStim)

Introduction: Mechanical ventilation can induce lung injury and diaphragmatic dysfunction. Diaphragmatic activation via rapid bilateral anterolateral magnetic phrenic nerve stimulation (rBAMPS) may attenuate both conditions. Given a change in body position during mechanical ventilation alters patient prognosis, this study compared inspiratory responses and side-effects to rBAMPS in different body positions in healthy volunteers.

Methods: 17 healthy participants underwent three blocks of 1-s rBAMPS at 25Hz starting at 20% of maximal stimulator output with increases in 10% increments (until participant cessation or 100%) while lying semirecumbent, lying supine, and while seated. Esophageal and gastric balloon catheters were used to assess transdiaphragmatic pressure (Pdi). Tidal volume (VT) was calculated from flow recorded with a pneumatometer. Discomfort, pain, and paresthesia were assessed via numerical visual scales. Tolerability to rBAMPS was assessed as the highest stimulator output achieved in each position.

Results: Body position significantly affected both VT (P = 0.007) and Pdi,mean (P = 0.044). The semirecumbent position resulted in higher VT and Pdi,mean compared to the supine (+ 2.6 mLkg-1 ideal body weight [IBW]; + 2.5 cmH2O) and sitting (+ 1.7 mLkg-1 IBW; + 0.5 cmH2O) positions. The lowest stimulator output tolerability was observed in the sitting position, and the highest stimulator output tolerability in the semirecumbent position. However, differences in tolerability were not reflected in changes of any sensory rating.

Conclusion: rBAMPS could produce physiological resting breathing in semirecumbent, supine and sitting positions. The semirecumbent position may optimize the stimulus to the diaphragm, while the supine position may limit VT and lung overdistension.