The Utility of Estimating the Shunt Fraction of Critical Patients with COVID-19 in determining the outcome of High Flow Nasal Cannula at Lung Center of the Philippines, a pilot study.

Introduction: High Flow Nasal Cannula (HFNC) enabled us to treat Acute Respiratory Distress Syndrome (ARDS) successfully on subsets of patients with COVID 19 without requiring invasive ventilatory support and with low mortality. Cautious use of HFNC is required to closely monitor the response in those patients with ARDS who develop clinical signs of respiratory failure. Currently the use of HFNC in Acute Hypoxemic Respiratory Failure is not well established due to conflicting studies. Several guidelines did not specify clear recommendations who is indicated and who will benefit on HFNC. Therefore the potential use of HFNC to provide an fi02 of up to 100% at shunt fraction of 0.1-0.3 may provide a significant increase in Pa02/fio2 ratio to correct the hypoxemia brought by ARDS but theoretically unclear with shunt fraction > 0.4. Objectives: This study investigated the initial shunt fraction as a parameter to determine if HFNC will benefit a specific group of patients to predict who will benefit from HFNC, prevent delayed intubations and consequently conserve resources. Methods: This was a retrospective, cohort study. Total of 205 Covid19 critical patients initially on HFNC admitted from July 1, 2020 to July 31, 2021 were included. Results: Forty-seven patients (22.9%) showed improved oxygenation and were successfully withdrawn from HFNC. In the binary logistic regression analysis, factors affecting mortality showed that age was the only variable predictive of in-hospital all-cause mortality. In the factors affecting intubation, logistic regression revealed greater shunt fraction would increase the odds of being intubated. Patients who have 30-40% shunt fraction had 3.8 times higher odds of being intubated and having a shunt fraction of >40% had 3.5 times the odds of being intubated in comparison with shunt fraction of <30%. Conclusions: HFNC has low success rate in covid19 critical patients but significantly showed benefit in those with shunt fraction <30%. Additionally, this study was not able to show the benefit of HFNC in terms of mortality and length of hospital stay. Shunt fraction is still worthwhile to use as a gauge but should be combined with clinical judgment and/or in conjunction with other parameters.