Remote Management of covid-19 using home pulse oximetry and virtual ward support.

Oxygen levels in covid-19

Low blood oxygen—technically, hypoxaemia but usually referred to as hypoxia—can be defined as a measured oxygen saturation below 94% in the absence (or below 88% in the presence) of chronic lung disease. In most patients who die of acute covid-19, the initial illness advances insidiously, sometimes with “silent hypoxia” (hypoxia without clinically perceptible symptoms of dyspnoea), leading to pneumonia followed by acute respiratory distress syndrome, usually in week 2. The underlying pathology in covid-19 related hypoxia is probably a ventilation-perfusion mismatch, caused by a combination of intrapulmonary shunting, loss of lung perfusion regulation, intravascular microthrombi, and reduced lung compliance leading to alveolar collapse.

Many patients hospitalised with acute covid-19 have severe hypoxia. Hypoxia, silent hypoxia, and the need for supplementary oxygen are all independent predictors of worse outcomes in covid-19.Novel prognostic tools such as the 4C score have shown the importance of identifying hypoxia early, and there are physiological reasons for managing the complication promptly and actively.

For all these reasons, UK guidelines recommend that assessment and monitoring of breathless, unwell, or high risk patients with suspected covid-19 should include pulse oximetry. Guidance published in January 2021 by the World Health Organization includes a provisional recommendation for “use of pulse oximetry monitoring at home as part of a package of care, including patient and provider education and appropriate follow-up.”

Pulse oximeters: practicalities and cautions

Home pulse oximeters have long been used in primary care settings—most usually for monitoring chronic lung disease and heart failure. Devices cost £15-35 ($20-48, €17-40) when sold commercially, but substantial savings can be made on bulk purchases. They are relatively simple and quick to use, though not everyone is able to understand or operate them.

Finger pulse oximeters are generally accurate and reliable (provided they have evidence of a quality standard such as ISO 80601-2-61:2017), however, most smartphone oximeters are unreliable and should not be used.  The Apple watch (Series 6 and above) has an inbuilt oximeter that operates on similar principles to a standard finger oximeter but is not designed (or licensed) for medical use (Tarassenko L, personal communication).

Pulse oximeter readings should be taken on a warm finger, with the patient in the upright (sitting) position and resting. The instrument should be left to stabilise for a minute before confirming the reading. Falsely low readings may occur if the patient’s peripheries are cold, if there is poor perfusion owing to (for example) hypotension, hypovolaemic shock, or cardiac failure, if the patient has tattoos, nail polish, or false nails, when the patient is supine (see below), and at low levels of oxygen saturation.2324 Readings may also be falsely low in patients with anaemia, sickle cell disease, and other haemoglobinopathies. This article relates mainly to those over 16 years, however, oximeters are accurate in children, though it is important to use a paediatric oximeter and to use it on a finger (not, for example, an ear lobe).

A study of hospital inpatients found that those with black or brown skin were three times as likely as those with white skin to have occult hypoxia—that is, pulse oximeter readings in the normal range but arterial blood gas levels in the hypoxic range. The oximeter reading gave false reassurance in 11.7% of patients with black or brown skin and 3.6% of those with white skin. However, other authors have found that inaccuracy of pulse oximeters in those with darker skin relates mainly to saturations below 90%—substantially below the level that would trigger a hospital referral. For the purposes of home monitoring, we suggest some rules of thumb to mitigate skin colour bias. First, ensure that the patient is using a quality-marked oximeter, since cheaper products marketed directly to the public may be less accurate. Second, take account of deviation from the patient’s own baseline if known. Third, take particular care to assess the patient holistically rather than relying solely on the oximeter readings. Finally, be prepared to act on any reading below the normal range in a patient with darker skin, regardless of ethnicity.

What is an abnormal pulse oximeter reading?

The normal range of oxygen saturation is 94-98% at rest in patients without chronic lung disease.1 A pulse oximeter reading of 92% or lower is one defining feature of “severe” disease in acute covid-19 (requiring urgent referral to hospital). Depending on the patient’s own normal range, a reading of 93-94% may indicate “moderate” disease (requiring prompt assessment, for example in a community clinic). Trend is important: a reading that is borderline and falling is far more concerning than one that is borderline and stable or rising. The pulse oximeter reading is part of a wider assessment of the patient: if there are red flags (such as central chest pain or blue lips), the patient should be fast-tracked to urgent care whatever the oximeter reading. A drop in saturation of 3% or more on exertion is considered abnormal and should prompt further assessment.

Patients with chronic lung disease often have a degree of hypoxia, in which case target saturation rates generally fall between 88% and 92%. These patients are often aware of what is normal for them. A drop of 3% or more below what is normal for the patient warrants further assessment and a drop of 4% or more may require hospital admission.

Nocturnal desaturation (a fall in the patient’s oximeter reading at night) of up to 3% occurs commonly; up to five brief episodes per night are considered normal (especially if there is some pre-existing lung disease) because of variation in depth of breathing during sleep. More prolonged or frequent desaturations may indicate obstructive sleep apnoea, though more specialist monitoring when not acutely unwell would be required to diagnose this.

Who should use a pulse oximeter at home?

Many people have purchased their own pulse oximeter for self-monitoring in the absence of risk factors or symptoms. Others already own an oximeter for monitoring a pre-existing condition such as heart failure. In a “straw poll” of 15 000 social media followers by the lead author, more than 60% said they had a pulse oximeter at home. Many patients with acute covid-19 appear able to learn how to use a pulse oximeter and are willing to take regular readings (see accompanying Patient Perspective).

Despite widespread enthusiasm for, and ownership of, pulse oximeters by lay people, official policy takes a more restrictive view of who needs to use one for covid-19 monitoring (eg, those over 65 and with relevant risk factors and comorbidities such as obesity, diabetes, or frailty). WHO guidance states that oximeters for home use should be considered “in symptomatic patients with covid-19 and risk factors for progression to severe disease who are not hospitalized.”

There is a hypothetical risk that use of a home oximeter may exacerbate anxiety. In the authors’ clinical experience, this is rare, and indeed home oximetry may be introduced to help manage anxiety during acute covid-19. A previous qualitative study by our own team found that both patients and relatives were reassured by home oximetry in long term condition monitoring.32 If patients find that home oximetry worsens their anxiety, they should not use it.

Virtual wards: a service model to support home monitoring

Pulse oximeters do not save lives, good clinical care does. There is a world of difference between the patient who is given a pulse oximeter and symptom diary and told to contact the health service if they deteriorate and the patient who is given the same equipment, shown how to use it, and who then receives regular calls from a healthcare professional.