Effects of a New Interface for NIV on Respiratory Drive

Around 30% of patients admitted to the Intensive Care Unit (ICU) are affected by hypoxemic Acute Respiratory Failure (hARF). The primary supportive treatment in hypoxemic patients is oxygen therapy, which is commonly delivered through nasal prongs or masks. New devices, able to deliver high-flow gas through a nasal cannula (HFNC), have been recently made available. HFNC delivers heated and humidified gas up to 60 L/min, with a fraction of inspired oxygen (FiO2) ranging from 0.21 to 1, via a wide bore soft nasal prong. Warming and humidification of the inspired gas prevent the adverse effects of cool dry gases on the airway epithelium and facilitate expectoration. HFNC also washes out exhaled carbon dioxide (CO2) from the pharyngeal dead space. HFNC has been shown an effective means to deliver oxygen therapy in many clinical conditions.

In healthy subject during spontaneous unassisted breathing, end-expiratory pharyngeal pressure is about 0.3 and 0.8 cmH2O, with open and closed mouth, respectively. Compared to unassisted spontaneous breathing, HFNC generates greater pharyngeal pressure during expiration, while in the course of inspiration it drops to zero, which limits the effectiveness of HFNC in patients with lung edema and/or collapse. By recruiting lung atelectatic regions, reducing venous admixture and decreasing the inspiratory effort, continuous positive airway pressure (CPAP) is likely more effective in these instances. Compared to noninvasive ventilation by application of an inspiratory pressure support, CPAP offers several advantages, which include ease of use and lack of patient-ventilator asynchrony.

CPAP may be applied either through mask or helmet. This latter is better tolerated than facial masks and allows more prolonged continuous CPAP application. When applying CPAP by helmet, however, heating and humidification of the inhaled gas is problematic because of condensation of water inside the interface, so called "fog effect". Moreover, in patients receiving CPAP by helmet some re-breathing occurs.

To overcome these limitations and combine the beneficial effects of HFNC and CPAP, the investigators designed a new device combining HFNC and helmet CPAP.

Recently, this combination was shown to be capable to provide a stable CPAP and effective CO2 washout from the upper airways with negligible CO2 re-breathing. Nonetheless, because of the complex interplay between CPAP and HFNC, the amount of truly applied airway pressure, diaphragm function and temperature inside the helmet might be affected to some extent. In 14 adult healthy volunteers, we found that adding HFNC to CPAP (as referenced to CPAP), 1) did not importantly alter either the pre-set airway pressure during inspiration or temperature inside the helmet; 2) increased expiratory airway pressure proportionally to the flow administered by HFNC, but to a lower extent than HFNC alone (as referenced to spontaneous breathing); 3) determined only slight modifications of the respiratory drive (as assessed through diaphragm ultrasound), compared to CPAP alone, 4) did not cause "fog effect" inside the helmet and 5) did not worsen comfort. We therefore suggested that adding heated humidified air through nasal cannula at a flow of 30 L/min during CPAP would probably be the best setting to be applied in patients with hypoxemic acute respiratory failure.

In patients with hARF, the use of noninvasive respiratory support (CPAP and non-invasive ventilation) is still debated. Patients receiving oxygen therapy, HFNC or CPAP/NIV maintain spontaneous breathing, which allows avoidance of sedation, thus limiting diaphragm dysfunction and delirium, permits easier mobilisation and prevents infections and ICU-acquired weakness. However, the maintenance of spontaneous breathing in patients with damaged lungs and high respiratory drive may result in global/regional pressure/volume changes possibly aggravating initial lung injury. This condition has been defined as patient self-inflicted lung injury (P-SILI). Indeed, respiratory drive is increased in patients with hARF. The high respiratory effort is one of the major determinants of increased transpulmonary pressure (Pl), which is the pressure acting across the lung. Pl represents the pressure alveoli are exposed to, and is considered among the most important determinants of P-SILI. Therefore, the reduction of Pl, across a decrease of the respiratory effort, might be advantageous in patients with hARF.

Investigators have therefore designed this pilot physiologic randomized cross-over study to investigate if, in patients with hARF, HFNC+CPAP reduces the respiratory effort, as compared to HFNC and CPAP (first outcome). Furthermore, we will assess the diaphragm activation, as assessed with ultrasound, gas exchange and patient's comfort among different settings (secondary outcomes).