Abstract

Alveolar pressure generated by HFO has been inferred from nasopharyngeal pressure or EELV measurements using bioimpedance. We conducted a prospective observational study to verify in situ the alveolar pressure at different flows.

A high-precision pressure Millar® Mikro-Cath able to reach respiratory zone of human lung was endoscopically placed in 5 random  spontaneously breathing patients. They received first O2 through face mask 15 L/m, and then all randomly received HFO at 15, 30, 50 and 60 L/m for two minutes each. After each step cardiac rate, invasive blood pressure and arterial blood gases were collected. A surface electromyography sensor was placed to assess respiratory effort. We collected Positive End-Expiratory Pressure (PEEP), minimum Peak Inspiratory Pressure (PIP), difference between PEEP and PIP, respiratory rate (RR), Inspiratory time (Ti) and Expiratory time (Te)

Compared with face mask, at HFO 60L/m PEEP increased (4,46 cmH20 vs 8,24 p<,001). The presence of PEEP in subjects in face mask should be highlighted, this may be attributable to autoPEEP or expiratory effort. Te (1,80 sec vs 2,02 p 0,023) and PaO2 also increased. Ti (1,42 vs 1,14 p<,001) and RR (20,88 vs 20 p 0,017) decreased. As a potential P-SILI, pressure oscillations (PEEP-PIP) increased at HFO 60 L/m (9,09 vs 13,66 p<,001). No differences were found in haemodynamics or arterial blood gases apart from PaO2.

In conclusion, HFO 60 L/m improves PEEP and PaO2 without affecting haemodynamics or gas exchange. Caution may be paid with extreme flows of HFO, where P-SILI and WOB can be worsened, especially in obstructive and established lung damage subjects.