Although there is no standardised definition for silent hypoxaemia, oxygen saturation measured by pulse oximetry (Sp02) and arterial oxygen pressure levels as low as 70% and 40 mm Hg, respectively, have been reported in asymptomatic patients with COVID-19.15 There are limited data on the severity of hypoxaemia in asymptomatic patients with COVID-19 and the prevalence of silent hypoxaemia is not known. Reports suggest the prevalence to be between 20 and 40%; however, this may not be a true reflection because concurrent oxygen saturation measurements and dyspnoea scores were not recorded.8 Moreover, some reports may have included patients without dyspnoea who had not yet developed hypoxaemia.7
Silent hypoxaemia poses a major risk to patients because the subjective sensation of dyspnoea and clinical signs of respiratory distress are not present. It may be associated with an increased risk of mortality and poor outcomes.16-18 Furthermore, up to one-third of patients with COVID-19 lung injury and without dyspnoea or signs of respiratory distress can rapidly develop severe disease with respiratory failure and subsequent ARDS.4
Diagnosis of silent hypoxaemia
Subjective dyspnoea measurements and self-reported symptoms of shortness of breath have been used to risk stratify patients with COVID-19. However, these are insufficient to accurately detect hypoxaemia in patients who are considered high risk for severe COVID-19; thus, objective tools are required.19
Pulse oximeters, a simple and noninvasive tool, can be used to estimate arterial oxygen saturation. These tools, which are cheap and easy to use, are typically available as a device that is placed over a finger. They allow for remote monitoring of patients with COVID-19 in the community when hospitalisation is not required.19-21 Further information is available from the RACGP website (see https://www.racgp.org.au/clinical-resources/covid-19-resources/other-health-issues/home-care-guidelines-patients-with-mild-covid-19 and https://www.racgp.org.au/download/Documents/Standards/RACGP-Standards-for-general-practices-5th-edition.pdf).
It is crucial to recognise factors that may result in measurement error and affect pulse oximeter accuracy. Factors to consider include rapid measurement fluctuation when the arterial oxygen pressure/tension falls on the steep portion of the oxygen dissociation curve and diminished pulsatile blood flow due to hypotension, vasoconstricting medications or peripheral vascular disease.20 The increased skin pigmentation of some individuals and the use of nail polish can also affect readings.20 Additionally, the accuracy may vary depending on the type of pulse oximeter device, particularly when oxygen saturation falls below 90%.20 Issues arising from incorrect placement of the pulse oximeter over a finger can be overcome by instead using an earlobe or forehead probe; however, these devices are not readily available and are more costly than finger pulse oximeters.
Arterial blood gas (ABG) analysis is a more invasive tool that can be used to accurately detect hypoxaemia. This involves measurements of the pH and partial pressures of oxygen and carbon dioxide in arterial blood and provides information about a patient’s acid-base balance, the effectiveness of gas exchange and state of their ventilatory control. An ABG analysis requires a blood sample, usually drawn from the radial artery, and is generally not feasible to perform in the community.