Open Access

ONLINE FIRST: Silent hypoxaemia in COVID-19. What does it mean?

Open Access

ONLINE FIRST: Silent hypoxaemia in COVID-19. What does it mean?

KATRINA O. TONGA, EMILY STONE

Figures

© DMITRII MELNIKOV/ALAMY/ DIOMEDIA. MODEL USED FOR ILLUSTRATIVE PURPOSES ONLY
© DMITRII MELNIKOV/ALAMY/ DIOMEDIA. MODEL USED FOR ILLUSTRATIVE PURPOSES ONLY
Dr Tonga is a Staff Specialist in the Department of Thoracic Medicine, St Vincent’s Hospital, Sydney; Conjoint Senior Lecturer at St Vincent’s Clinical School, UNSW Sydney, Sydney; and Lecturer at the University of Sydney, Sydney. Dr Stone is a Senior Staff Specialist in the Department of Thoracic Medicine,
 St Vincent’s Hospital, Sydney; and Conjoint Lecturer at St Vincent’s Clinical School, UNSW Sydney, Sydney, NSW.

Abstract

A unique presentation of silent hypoxaemia – minimal dyspnoea associated with profound hypoxaemia – has been reported in patients with COVID-19. The management of silent hypoxaemia requires early recognition, as these patients are at risk of rapid deterioration and respiratory failure. Early recognition of this phenomenon can be challenging for patients who are being monitored in the community. However, pulse oximeters are a simple tool that can be useful for identifying patients with silent hypoxaemia in the community and in hospital.

Key Points

  • Silent hypoxaemia is an apparently unique phenomenon in patients with COVID-19 where profound hypoxaemia is demonstrated without dyspnoea or clinical symptoms of respiratory distress.
  • Patients with silent hypoxaemia are at risk of rapid deterioration and respiratory failure. Therefore, early detection is important.
  • Pulse oximetry can be used to estimate arterial oxygen saturation and allow for remote monitoring of patients with COVID-19 in the community when hospitalisation is not required, enabling follow up by healthcare providers (e.g. phone or telehealth consultations). Many outpatient programs are providing patients with equipment for self- monitoring in the home setting.
  • The underlying mechanisms of silent hypoxaemia are not understood and little is known about long-term sequelae.

The coronavirus disease 2019 (COVID-19) outbreak has led to a global pandemic affecting tens of millions of people and causing millions of deaths worldwide. In Australia, over 45,000 confirmed cases have occurred at the time of writing, and numbers continue to increase.1

COVID-19 is caused by infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).2 Patients with COVID-19 typically present with symptoms and signs related to respiratory tract infection.3 Fever, cough, dyspnoea and myalgia are common presenting symptoms; however, clinical manifestations related to other organ systems, including the gastrointestinal tract and cardiac, neurological and vascular systems, have also been reported.4,5 The spectrum of COVID-19 severity is variable – the range includes patients who are asymptomatic, those with mild and self-limiting upper respiratory tract symptoms, and patients who have severe disease with acute respiratory distress syndrome (ARDS) and/or respiratory failure and death.4,6

An unique presentation that has been widely reported in patients with COVID-19 is that of no or minimal dyspnoea associated with profound hypoxaemia.7-10 This phenomenon has been coined ‘happy hypoxaemia’ or ‘happy hypoxia’, which is somewhat misrepresentative because patients with COVID-19 are not happy but potentially very unwell.11 A more fitting description – ‘silent hypoxaemia’ or ‘silent hypoxia’ – has also been used.12 

Advertisement

What is silent hypoxaemia?

Silent hypoxaemia is characterised by significant hypoxaemia without the expected features of respiratory distress.9,10,12,13 The patient is alert and conscious but lacks awareness of the hypoxaemia and reports no dyspnoea. Concurrent computed tomography imaging of the lungs may demonstrate consolidation and/or ground glass opacification.14 An example is described in the Box.

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

Advertisement

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