All patients exhibited a hypoxemic acute respiratory failure related to Coronavirus Disease 2019 and presented refractory hypoxemia under classical nasal supplemental oxygen. As recommended, they benefited from a HFNC device to obtain a SpO2 above 90% [5]. As recommended, benefited of an HFNC device to obtain a SpO2 above 90% [5]. In this study, we demonstrated that the simple addition of a surgical mask on the patient’s face increased significantly the oxygenation of these hypoxemic COVID-19 patients admitted in ICU.
All started with the observation that these hypoxemic patients increased their SpO2 directly after receiving the surgical mask over the HFNC, a request of our hospital hygiene department. Adding this mask on the patient’s face while receiving HFNC oxygenation increases all the oxygenation parameters compared to classical HFNC therapy, without clinically significant change of PaCO2. All the settings of the HFNC devices were kept constant during the experiment.
We did not give to our patients any specific recommendation concerning mouth opening or closing during the experimentation, but we observed often an opened mouth with an increased room intake. The observed improvement in oxygenation parameters could be explained not only by an increased oxygen concentration in front of the mask but also by a decrease of room air entrainment that is known to dilute the gas mixture with less inspired O2 concentration [9]. The mask would then play a filter role by increasing the positive effect of the HFNC device and by decreasing the negative effect of entrainment of room air. We confirmed the additional effect of surgical mask to HFNC device rather than a favorable spontaneous evolution as its removal directly induced a return to previous oxygenation parameters as measured by the SpO2. Interestingly, none of our patients presented subjective complaints of discomfort by adding this surgical mask on top of HFNC.
The HFNC oxygen therapy is a well-known technique allowing heated and humidified gas with a maximum flow rate of 70 L/min and an adjustable oxygen fraction (FiO2) from 21 to 100% [10]. A recent meta-analysis showed that patients admitted with an acute hypoxemic respiratory failure from diverse aetiologies could improve oxygenation with HFNC compared to conventional oxygen therapy evolving towards a reduced need for tracheal intubation [11]. Likewise, it was recently demonstrated that the addition of a double-trunk mask on HFNC improves oxygenation in acute respiratory failure patients [12]. Non-invasive respiratory support plays an essential role in the treatment of COVID-19 patient with acute respiratory failure without the need of an urgent endotracheal intubation even if HFNC has not been assessed much yet. However, in adult patients admitted in ICU for an acute hypoxemic respiratory failure despite conventional oxygen therapy, as mentioned above, the Surviving Sepsis Campaign COVID-19 suggests the use of HFNC over Non-invasive positive-pressure ventilation (NIPPV) [5]. Recent study even observed an HFNC-positive response in moderate hypoxemic patients while failure rate increased as long as the PaO2/FiO2 decreased [13].
Importantly, this study was designed to assess the efficacy of adding a surgical mask on HFNC device and not to prevent endotracheal intubation.
Reducing the breath dispersion distance and aerosol generation during high-flow nasal ventilation to prevent SARS-CoV-2 transmission is a major issue. However, in vitro data using lung model with smoke generator or manikin are rather reassuring on this point. By using the same study method and similar breathing patterns, in vitro studies suggested that droplet dispersal during HFNC therapy was limited to the proximal space of the face and the cannula with even less dispersal distance of exhaled smoke compared to traditional high-flow oxygen therapy systems including non-rebreathing or Venturi masks that are traditionally used in acute hypoxemic respiratory failure [14, 15]. By studying different manikin models, in vitro and clinical studies, a recent review reported scientific evidences that use of HFNC during this pandemia has probably not increased either dispersion or microbiological contamination into the environment than other oxygen devices [16]. Furthermore, clinical studies evaluating bacterial environmental contamination of patients admitted in ICU for bacterial pneumonia and treated either by HFNC device or by conventional oxygen mask did not find any significant difference in bacterial counts in air or contact surface [17]. These data support the fact that there is actually no scientific proof of an increased bio-aerosols dispersion through an HFNC device compared to conventional high-flow oxygen therapy. Moreover, computational fluid dynamic simulations reported that wearing a surgical mask over HFNC might reduce aerosol droplets dispersion [18].
Our study presents several limitations. We enrolled a limited number of patients as we only included them in the presence of the investigators. We focused essentially in improving the oxygenation parameters. In this regard, specific measurements, such as minute ventilation, were not performed. Also, we did not enroll severe COPD patients and these data might not be generalizable to this population. However, PaCO2 measurement did not show any clinically significant increase, and it is worth noting that HFNC is proposed to treat those patients at home [19]. We further believe that targeting an SpO2 of 90% would limit the risk of oxygen-induced hypercapnia. Finally, the exact FiO2 delivered by the system while using the face mask was not measured, as already proposed by other authors [8].