A UK study found that current NHS guidelines on blood oxygen levels, that between 94 and 98 per cent are 'normal', may be too high, with drops below 96 per cent correlated to higher risk of death.
The study led by Matthew Inada-Kim, acute physician, Royal Hampshire County Hospital and NHS national clinical lead for patient deterioration, also makes a strong case for all households to keep an oximeter as well as a thermometer in their medicine cupboard at home.
“We found that even small deflections in oxygen saturations of 1-2 per cent below 96 per cent confer an increased mortality risk, say the authors.
“Even patients with presenting oxygen saturations of 94-95 per cent, values regarded as within this normal range, had a significantly higher 30-day mortality rate (5.3 per cent) than those presenting with oxygen saturations higher than 95 percent ( 2.3 per cent).
The study looked at hundreds of thousands of people who had called an ambulance between March 1 and July 31 this year. It honed in on 1,080 with a confirmed Covid-19 diagnosis whose oxygen levels and outcome of their disease was available.
The team added some experts have argued a reading indicative of hospitalisation uses a cut-off of 92 per cent. But this would miss approximately one third of patients who go onto to suffer adverse outcomes, according to this study.
And the study found that oxygen saturations were the most predictive of death or ICU admission – more than age or pre-existing health problems.
The researchers said a lower threshold for hospital conveyance may be necessary for Covid-19 patients so they are not left at home, where they are at risk of getting sicker.
'We have demonstrated that even relatively minor derangements in peripheral oxygen saturation are an early warning of potential deterioration,' they said. 'Use of oxygen saturation as an indicator of disease severity may therefore underestimate the risk of leaving patients at home after ambulance assessment.'
In most cases of pneumonia, breathlessness is an early symptom of the disease and comes before hypoxia – the state in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level. But with Covid-19, hypoxia often comes first, known as 'silent hypoxia'; by the time breathlessness occurs, patients may be extremely sick without even knowing.
NHS England is believed to have purchased around 200,000 pulse oximeters for a scheme of home oxygen saturation reading, according to the British Medical Journal (BMJ). Patients with Covid-19 who don't need immediate hospital attention but are at high risk of developing serious symptoms, such as those over 65 years old or with a chronic health condition, are to be given the devices to use at home to help monitor if they are deteriorating.
Nigel Watson, chief executive, told The BMJ that the evidence was now fairly strong that if oxygen saturation fell to 93/94 per cent the mortality risk increased to around 13 per cent. If it fell below that level 'the risk would increase to about 28 per cent'.
The idea is to create 'virtual covid wards' of patients at risk and to monitor oxygen saturation through patients taking readings and relating these to their health teams, Mr Watson said. 'If it starts dropping and particularly goes to 94 per cent, then you'll potentially take some action, which may include [the patient] being admitted to hospital.'
Rob Barnett, chair of Liverpool Local Medical Committee, confirmed that in his area health officials were working on 'virtual covid wards'. He said: 'As we monitor more patients in the community, we will know what the tipping point is for them to get into hospital, which is likely to be earlier than we might do if we weren't monitoring them at all.' But he said even though early treatment is a benefit, it would mean more patients are in the hospital, adding strain to the service.
Validation of home oxygen saturations as a marker of clinical deterioration in patients
with suspected COVID-19
Authors: Matthew Inada-Kim; Francis P. Chmiel; Michael J. Boniface; Helen Pocock; John J. M. Black; Charles D. Deakin
medRxiv preprint 11 November 2006
ABSTRACT
Background
The early identification of deterioration in suspected COVID-19 patients managed at
home enables a more timely clinical intervention, which is likely to translate into improved outcomes. We undertook an analysis of COVID-19 patients conveyed by ambulance to hospital to investigate how oxygen saturation and measurements of other vital signs correlate to patient outcomes, to ascertain if clinical deterioration can be predicted with simple community physiological monitoring.
Methods
A retrospective analysis of routinely collected clinical data relating to patients conveyed to
hospital by ambulance was undertaken. We used descriptive statistics and predictive analytics to
investigate how vital signs, measured at home by ambulance staff from the South Central Ambulance
Service, correlate to patient outcomes. Information on patient comorbidities was obtained by linking
the recorded vital sign measurements to the patient’s electronic health record at the Hampshire
Hospitals NHS Foundation Trust. ROC analysis was performed using cross-validation to evaluate, in a
retrospective fashion, the efficacy of different variables in predicting patient outcomes.
Results
We identified 1,080 adults with a COVID-19 diagnosis who were conveyed by ambulance to
either Basingstoke & North Hampshire Hospital or the Royal Hampshire County Hospital (Winchester)
between March 1st and July 31st and whose diagnosis was clinically confirmed at hospital discharge.
Vital signs measured by ambulance staff at first point of contact in the community correlated with
patient short-term mortality or ICU admission. Oxygen saturations were the most predictive of
mortality or ICU admission (AUROC 0.772 (95 % CI: 0.712-0.833)), followed by the NEWS2 score
(AUROC 0.715 (95 % CI: 0.670-0.760), patient age (AUROC 0.690 (95 % CI: 0.642-0.737)), and
respiration rate (AUROC 0.662 (95 % CI: 0.599-0.729)). Combining age with the NEWS2 score (AUROC
0.771 (95 % CI: 0.718-0.824)) or the measured oxygen saturation (AUROC 0.820 (95 % CI: 0.785-0.854))
increased the predictive ability but did not reach significance.
Conclusions
Initial oxygen saturation measurements (on air) for confirmed COVID-19 patients
conveyed by ambulance correlated with short-term (30-day) patient mortality or ICU admission,
AUROC: 0.772 (95% CI: 0.712-0.833). We found that even small deflections in oxygen saturations of 1-
2% below 96% confer an increased mortality risk in those with confirmed COVID at their initial
community assessments.
[link url="https://www.medrxiv.org/content/10.1101/2020.11.06.20225938v1.full.pdf"]Full medRxiv preprint (Open access)[/link]