Pilot Study of Physiological Effect of High-Flow Nasal Cannula on Respiratory Pattern and Work of Breathing

Esophageal and gastric pressures will be measured with an esophageal ballon positioned at the lower third of the esophagus, filled with 0.5 mL of air and a gastric balloon filled with 1 mL of air. The proper position of balloons will be verified using the occlusion test as previously described. Transdiaphragmatic pressure (Pdi) is calculated as the difference between gastric (Pga) and esophageal (Pes) pressure. The pressure time integrals of the diaphragm and the other inspiratory muscles are calculated per breath (PTPdi/b and PTPes/b, respectively) and per minute (PTPdi/min and PTPes/min). Measurements will be collected at baseline, at each randomized HFNC and CPAP settings during the last 4 minutes of each 10 minutes session.

Inspiratory tidal volume (VTi), respiratory rate (RR), breath duration (Ttot), inspiratory time (Ti) and fractional inspiratory time (Ti/Ttot) will be determined using a Respiratory Inductive Plethysmography (RIP) system. This will measure the thoracic and abdominal excursion of the subjects via two inductive wires which are sewn into the elastic bands that encircle the thorax and abdomen. The acquired signals represent changes in cross-sectional area and, following calibration to determine the relative contribution of each signal, and volume calibration using spirometry, their weighted sum will reflect VTi. The RIP companion software will be used to derive RR, Ttot, Ti and Ti/Ttot on a breath by breath basis.

The oxygenation, the level of carbon dioxide, and the heart rate will be recorded using the Sentec transcutaneous monitoring system: a probe will be placed at the earlobe or on the forehead, and it will measure in a noninvasive way these parameters.