Non-invasive ventilation (NIV) is extensively used in critical care settings and emergency departments for a variety of aetiologies but specially for acute respiratory failure (ARF). Recommendations based on the GRADE methodology were addressed on several conditions such as exacerbation of chronic obstructive pulmonary disease (COPD), cardiogenic pulmonary oedema, de novo hypoxaemic respiratory failure, immunocompromised patients, chest trauma, palliative care, post-operative care, weaning and post-extubation period. NIV eliminates morbidity related to the endotracheal tube and use of sedatives so it reduces intensive care unit (ICU) acquired pneumonia, diaphragmatic atrophy, ICU acquired weakness and delirium. On the other hand, the harmful effects of spontaneous breathing through the intensity of inspiratory effort may follow a critical increase in respiratory drive, thus producing uncontrolled tidal change in dynamic transpulmonary pressure (PLdyn) that would increase the risk of injury to the dependent lung and predispose the patient to the onset of self-inflicted lung injury (SILI). High positive end-expiratory pressure (PEEP) renders spontaneous effort non injurious. P-SILI may worsen the clinical outcome of patients who require endotracheal intubation after having received noninvasive respiratory support. The underlying mechanisms of SILI are heterogeneous and include the pendelluft phenomenon, increased transvascular pressure gradient aggravating alveolar damage, excessive diaphragmatic loading with impaired systemic oxygen delivery and muscle injury. Therefore, measuring the level of inspiratory effort is recommended.

Esophageal manometry is a precise estimate of the changes in pleural pressure and is considered the gold standard to measure respiratory effort. Tonelli et al. measured tidal change in esophageal pressure (ΔPes) in patients with acute hypoxic de novo respiratory failure on NIV and demonstrated a median baseline value of ΔPes of 34 cmH2O that was significantly reduced within the first 2 hours of ventilation in patients who were successful in the NIV trial, whereas those failing the NIV trial did not show a significant reduction. However, esophageal manometry is rarely available bedside in acute settings on severe patients with respiratory distress so other ways of measuring inspiratory effort have been assessed, such as nasal pressure swings or the patient's respiratory effort against the occluded airway (ΔPocc). The latest was demonstrated on invasive mechanical ventilation patients. Lopez Navas et al. tried to correlate the inspiratory pressure-time product (PTPinsp) from transdiaphragmatic pressure to a novel expiratory occlusion method of 0.2 s in healthy volunteers with NIV on different settings; however, their results through Bland-Altman analysis of PTPinsp revealed mean differences between -4.22 and 7.57 cmH2O (SD 0.77- 8.52) and considerable differences between subjects. Moreover, Dargent A, et al. explored the feasibility of a noninvasive respiratory drive evaluation using ventilator-derived data as P0.1, clinical information and diaphragm ultrasound in COVID 19 patients on CPAP session with 5 cmH2O. They showed that P0.1 was achievable during NIV with a median value of 4.4 \[2.7-5.1\] cmH2O and not correlated with leaks, though they were small (5 \[4-7\] l/min); nevertheless, P0.1 was not accurate at predicting the risk of intubation but it was limited by its small sample size. In addition, P0.1 has been previously evaluated (with other physiological parameters) on NIV in COPD patients to predict post-extubation respiratory distress. They reported that only P0.1 recorded 1 h after the discontinuation of mechanical ventilation followed by 30 minutes of 4 cmH2O pressure support ventilation, was significantly different between the patients with and without respiratory distress (4.2 vs 1.8, p \< 0.01). Nonetheless, there are no studies that measured bedside the pressure generated by the respiratory muscles during NIV.

The aim of this proof-of-concept physiological study was to describe the correlation between ΔPocc measured on the ventilator and ΔPes in healthy subjects with NIV.