An Evolving Clinical Need: Discordant Oxygenation Measurements of Intubated COVID-19 Patients

Abstract

Since the first appearance of the severe acute respiratory syndrome corona virus 2 (SARS-CoV-2) earlier this year, clinicians and researchers alike have been faced with dynamic, daily challenges of recognizing, understanding, and treating the coronavirus disease 2019 (COVID-19) due to SARS-CoV-2. Those who are moderately to severely ill with COVID-19 are likely to develop acute hypoxemic respiratory failure and require administration of supplemental oxygen. Assessing the need to initiate or titrate oxygen therapy is largely dependent on evaluating the patient's existing blood oxygenation status, either by direct arterial blood sampling or by transcutaneous arterial oxygen saturation monitoring, also referred to as pulse oximetry. While the sampling of arterial blood for measurement of dissolved gases provides a direct measurement, it is technically challenging to obtain, is painful to the patient, and can be time and resource intensive. Pulse oximetry allows for non-invasive, real-time, continuous monitoring of the percent of hemoglobin molecules that are saturated with oxygen, and usually closely predicts the arterial oxygen content. As such, it was particularly concerning when patients with severe COVID-19 requiring endotracheal intubation and mechanical ventilation within one of our intensive care units were observed to have significant discordance between their predicted arterial oxygen content via pulse oximetry and their actual measured oxygen content. We offer these preliminary observations along with our speculative causes as a timely, urgent clinical need. In the setting of a COVID-19 intensive care unit, entering a patient room to obtain a fresh arterial blood gas sample not only takes exponentially longer to do given the time required for donning and doffing of personal protective equipment (PPE), it involves the consumption of already sparce PPE, and it increases the risk of viral exposure to the nurse, physician, or respiratory therapist entering the room to obtain the sample. As such, technology similar to pulse oximetry which can be applied to a patients finger, and then continuously monitored from outside the room is essential in preventing a particularly dangerous situation of unrealized hypoxia in this critically-ill patient population. Additionally, it would appear that conventional two-wavelength pulse oximetry may not accurately predict the arterial oxygen content of blood in these patients. This discordance of oxygenation measurements poses a critical concern in the evaluation and management of the acute hypoxemic respiratory failure seen in patients with COVID-19.

Keywords: Arterial blood gas; COVID-19; Hemoglobin; Oxygen saturation; Pulse oximetry; SARS-CoV-2.

Figure 1

Scatter diagram depicting the SpO₂ (left Y axis) and PaO₂ (right Y axis) of each of the 49 patients (X axis). The horizonal red line indicates a PaO₂ of 59 mmHg.