key: cord-0839126-hgv6iy2i
authors: Wiles, M. D.; El‐Nayal, A.; Elton, G.; Malaj, M.; Winterbottom, J.; Gillies, C.; Moppett, I. K.; Bauchmuller, K.
title: The effect of patient ethnicity on the accuracy of peripheral pulse oximetry in patients with COVID‐19 pneumonitis: a single‐centre, retrospective analysis
date: 2021-09-20
journal: Anaesthesia
DOI: 10.1111/anae.15581
sha: 40523e19b9171e22129bd4eed55958d74f4a48cc
doc_id: 839126
cord_uid: hgv6iy2i

Pulse oximetry is used widely to titrate oxygen therapy and for triage in patients who are critically ill. However, there are concerns regarding the accuracy of pulse oximetry in patients with COVID‐19 pneumonitis and in patients who have a greater degree of skin pigmentation. We aimed to determine the impact of patient ethnicity on the accuracy of peripheral pulse oximetry in patients who were critically ill with COVID‐19 pneumonitis by conducting a retrospective observational study comparing paired measurements of arterial oxygen saturation measured by co‐oximetry on arterial blood gas analysis (SaO(2)) and the corresponding peripheral oxygenation saturation measured by pulse oximetry (S(p)O(2)). Bias was calculated as the mean difference between SaO(2) and S(p)O(2) measurements and limits of agreement were calculated as bias ±1.96 SD. Data from 194 patients (135 White ethnic origin, 34 Asian ethnic origin, 19 Black ethnic origin and 6 other ethnic origin) were analysed consisting of 6216 paired SaO(2) and S(p)O(2) measurements. Bias (limits of agreement) between SaO(2) and S(p)O(2) measurements was 0.05% (−2.21–2.30). Patient ethnicity did not alter this to a clinically significant degree: 0.28% (1.79–2.35), −0.33% (−2.47–2.35) and −0.75% (−3.47–1.97) for patients of White, Asian and Black ethnic origin, respectively. In patients with COVID‐19 pneumonitis, S(p)O(2) measurements showed a level of agreement with SaO(2) values that was in line with previous work, and this was not affected by patient ethnicity.

Pulse oximetry is used widely to titrate oxygen therapy in patients who are critically ill. As pulse oximetry is inexpensive, non-invasive and can be used continuously, it provides a convenient way to monitor the oxygenation of patients via indirect measurement of the saturation of arterial blood. The US Food and Drug Administration recommends that pulse oximetry devices should be validated by directly comparing values with oxygen saturations measured by co-oximetry of an arterial blood sample [1] . Most pulse oximeter validation studies have been performed using healthy volunteers; those that have involved patients who are critically ill have shown variable agreement with arterial blood saturations measured by co-oximetry [2] [3] [4] .

Pulse oximetry is also used as a triage tool in pneumonitis. However, there are concerns regarding the accuracy in this disease [5] and in patients who have a greater degree of skin pigmentation [6] [7] [8] . This is important as patients from non-White ethnic backgrounds have been disproportionally affected by COVID-19 infection, with a greater risk of critical illness as a result [9, 10] . We aimed to determine the impact of patient ethnicity on the accuracy of peripheral pulse oximetry in patients who were critically ill with COVID-19 pneumonitis.

We conducted a retrospective, observational study of arterial blood oxygen saturation measurement in patients with confirmed COVID-19 pneumonitis who had been admitted to our tertiary general critical care unit with hypoxaemic respiratory failure. The purpose of the study was to quality-assure local practice with respect to saturation targets, due to concerns that peripheral oxygen saturations may be under-estimating the degree of hypoxaemia. The study was registered prospectively with the local clinical effectiveness unit as a service review and was deemed not to require formal ethical approval. Patients were identified by cross-referencing the critical care clinical information system (MetaVision ICU; iMDsoft â , Tel Aviv, Israel) and the local Intensive Care National Audit and Research Centre (ICNARC) case-mix programme database. 

In patients who are critically ill with COVID-19 pneumonitis, The US Food and Drug Administration sets out several standards for studies undertaken to test the accuracy of pulse oximeters including: ≥10 healthy subjects that vary in age and sex; ≥200 paired S p O 2 /SaO 2 measurements; and a range of skin pigmentations, including at least two darkly pigmented subjects or 15% of the study population (whichever is larger) [1] . Given the huge variation in age, ethnic origin, medical comorbidities and physiological derangements in patients admitted to critical care, this relatively low standard for calibration is surprising. The recommendations for acceptable accuracy are a root mean square difference between S p O 2 and SaO 2 measurements of <3% for finger and <3.5% for ear probes. This is calculated using the following formula: √(bias 2 +precison 2 ). In our study, all the patient groups were within this target accuracy specification: all patients 1.09%; patients of White, Asian and Black ethnic origin 1.08%, 1.13% and 1.56%, respectively.

The COVID-19 pandemic has highlighted significant differences between ethnic groups in terms of illness The bias and limits of agreement between SaO 2 and S p O 2 measurement are lower than that seen in other studies (summarised in Table 3 ). There are several possible reasons for this. First, it may be that S p O 2 measurement technology has improved over recent years. Second, all our patients were cared for in an ICU but were only receiving singleorgan support for a respiratory disease; this may have resulted in a greater focus on respiratory parameters by members of the nursing staff. Third, we did not study patients who were sedated and/or were receiving vasopressor infusions, which is not typical for many patients in an ICU.

Of interest is the fact that bias and limits of agreement between SaO 2 and S p O 2 measurement were not altered by the exclusion of patients who were normoxic (SaO 2 ≥94%). This is important as the degree of bias when the oxygen saturation is high-normal is of little clinical significance. For example, a 4% difference in measured S p O 2 and SaO 2 is unlikely to alter management when these are both ≥95%; however, the same 4% difference at saturations in the region of 88-92% risks either an unnecessary intervention (such as an arterial blood gas sample or increased respiratory support) or failure to recognise hypoxaemia. It is usual for S p O 2 to be monitored continuously and readings will be dynamic in nature. Thus, the degree of bias may vary throughout the monitoring period and may not be consistent or predictable. However, despite our ICU policy of targeting an SaO 2 88-92%, 5172 out of 6216 (82%) SaO 2 measurements were >92%, suggesting that most patients were receiving an excessive F I O 2 . This limits our ability to determine the bias between S p O 2 and SaO 2 at lower levels of oxygen saturation. Previous work has suggested that bias is increased at lower oxygen saturation levels (≤90%), especially in patients who have greater degrees of skin pigmentation [6, 7] .

Clinicians may wish to consider our findings when producing guidelines relating to S p O 2 targets in patients with COVID-19 pneumonitis. A target S p O 2 of 88% may risk occult hypoxaemia, given the accuracy and limits of agreement for this measurement, and a S p O 2 target of ≥90% may be of value in ward-based environments that do not have the ability to measure SaO 2 rapidly and repeatedly by sampling from arterial catheters.

There are some limitations to our study. First, this was a single-centre, retrospective study of a single disease state and all measurements were done using one type of equipment. As such, our findings should be seen as being hypothesis-generating and should not be extrapolated uncritically to other institutions, diseases or monitoring/ measurement devices. Second, in order to avoid the potential impact of variations in peripheral vasomotor tone on S p O 2 readings (either as a result of sedation or vasopressor administration), we did not study patients whose tracheas had been intubated and were receiving mechanical ventilation. Third, ethnic origin was determined by subjective, patient (or next of kin) self-identification. In addition, 70% of our study population was of White ethnic origin; the smaller number of patients of Asian and Black ethnic origin could have been a reason for the lack of precision around limits of agreement. Fourth, S p O 2 monitoring is dynamic and prone to artefact caused by poor probe positioning or patient movement. We attempted to minimise the effect of such errors by taking a mean of the S p O 2 readings in the 4-min period before SaO 2 measurement, but we cannot exclude the possibility that some artefactual readings may have been included in the analysis. However, this is a limitation of all studies that analyse real-time physiological measurement data retrospectively and reflects real-world data measurement conditions. We also did not correct for the presence of anaemia, acidaemia or skin temperature, all of which may have a small impact on bias measurement in oxygenation Table 3 Summary of previous studies assessing bias in the measurements of oxygen saturation by arterial blood gas analysis (SaO 2 ) and peripheral oxygen saturation (S p O 2 ). Where different ethnic groups or skin pigmentation were analysed these data are included. If not stated in the original study, then limits of agreement were calculated as 1.96 SD. Values are number or mean (SD). 

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