Basing Respiratory Management of COVID-19 on Physiological Principles

Abstract

The dominant respiratory feature of coronavirus disease (COVID-19) is arterial hypoxemia greatly exceeding abnormalities in pulmonary mechanics (decreased compliance) (1-3). Many patients are intubated and placed on mechanical ventilation early in their course. Projections on usage of ventilators has led to fears that insufficient machines will be available and even to proposals for using a single machine to ventilate four patients. The coronavirus crisis poses challenges for staffing, equipment, and resources, but it also imposes cognitive challenges for physicians at the bedside. It is vital that caregivers base clinical decisions on sound scientific knowledge to gain the greatest value from available resources (4). Patient oxygenation is evaluated initially using a pulse oximeter. Oxygen saturation as measured by pulse oximetry (Sp O 2) can differ from true Sa O 2 (measured with a CO-oximeter) by as much as 64% (5). Interpretation of readings of Sp O 2 above 90% becomes especially challenging because of the sigmoid shape of the oxygen dissociation curve. Given the flatness of the upper oxygen dissociation curve, a pulse oximetry reading of 95% can signify an arterial oxygen tension (Pa O 2) anywhere between 60 and 200 mm Hg (6, 7)-values that carry extremely different connotations for management of a patient receiving a high concentration of oxygen. Difficulties in interpreting arterial oxygenation are compounded if supplemental oxygen has been instituted before a pulmonologist or intensivist first sees a patient (the usual scenario with COVID-19). Assessment of gas exchange requires knowledge of fractional inspired oxygen tension (FI O 2); unless the patient is breathing room air, this is not knowable in a nonintubated patient. With a nasal cannula set at 2 L/min, FI O 2 ranges anywhere between 24% and 35% (8). Arterial blood gases yield a more precise measure of gas exchange. With knowledge of Pa O 2 , Pa CO 2 , and FI O 2 , the alveolar-toarterial oxygen gradient can be rapidly calculated. The alveolar-toarterial oxygen gradient enables more precise evaluation of the pathophysiological basis of hypoxemia than more widely used Pa O 2 /FI O 2 , because this ratio may reflect changes in PO 2 , FI O 2 , or both. Hypoxemia accompanied by a normal alveolar-to-arterial oxygen gradient and increase in Pa CO 2 signifies hypoventilation. Hypoventilation is uncommon with COVID-19. Instead, hypoxemia with COVID-19 is usually accompanied by an increased alveolar-to-arterial oxygen gradient, signifying

Early predictors of in-hospital mortality in patients with COVID-19 in a large American cohort

“…In other smaller case series, reported mortality associated with invasive ventilation was also dismal with 97% mortality in one cohort of 32 patients and 100% in a 17-patient series [ 19 , 20 ]. The utilization of invasive mechanical ventilation represents a complicated provider decision based on oxygen saturation, dyspnea, respiratory rate, chest x-ray, and other factors [ 33 ]. In our cohort this decision was also likely influenced by factors including guidelines supporting early intubation and cautioning the use of noninvasive mechanical ventilation strategies [ 34 ].…”

Hospital Preparedness for COVID-19: A Practical Guide from a Critical Care Perspective

“…As the number of critically patients increased and international guidelines were refined, high-flow nasal cannula was increasingly used, preferably in a negativepressure room (17), to possibly delay or avoid the need for intubation. The decision to intubate relies on the clinical judgment of the critical care physician and is based on factors such as hypoxemia, tachypnea, increased work of breathing, and gas exchange (18). These practices continue to develop as recommendations evolve.…”

Reducing aerosol dispersion by high flow therapy in COVID‐19: High resolution computational fluid dynamics simulations of particle behavior during high velocity nasal insufflation with a simple surgical mask

“…Newer information suggests important physiological phenotypes which may be better suited to non-invasive support than to invasive mechanical ventilation 25. The role of invasive mechanical ventilation as the "first-line" therapy after simple oxygen management fails has been brought into question 26. As such, the ability to limit the overall environmental exposure becomes important.…”