An observational study of simultaneous pulse oximetry and arterial oxygen saturation readings in intensive care unit/high dependency unit in COVID-19 patients

Abstract

Background: The coronavirus-2019 (COVID-19) pandemic has increased the use of pulse oximeters worldwide. It has become an inevitable tool in the monitoring of the disease. However, the accuracy of pulse oximeters in COVID-19 has not been established.

Aims and Objectives: The aims of the study were to examine the relationship between oxygen saturation as measured by pulse oximeter (SpO2) and oxygen saturation measured by arterial blood gas analysis (SaO2) measurements in COVID-19 patients admitted to the intensive care unit (ICU)/high dependency unit (HDU) and to assess the ability of SpO2 readings to detect low SaO2 and low oxygen tension in COVID-19 patients.

Materials and Methods: This prospective observational study was conducted in the COVID-19 ICU and high dependency unit of a tertiary care hospital in Bengaluru, India. All patients admitted with confirmed COVID-19, meeting the eligibility criteria, were included in the study. We assessed bias and limits of agreement between paired samples of oxygen saturation from pulse oximetry (SpO2) and arterial oxygen saturation from blood gas analysis (SaO2).

Results: The sample mean difference SpO2-SaO2 is −0.86% (bias) and the 95% confidence interval for the mean difference was −1.67 and −0.04. The lower limit of agreement was −7.32 with a 95% confidence interval (−8.74, −5.91). The upper limit of agreement was 5.61 with a 95% confidence interval of 4.19 and 7.02.

Conclusion: SpO2 values are not completely dependable in estimating SaO2 in COVID-19 patients in ICU/HDU; therefore, arterial blood gas analysis measurement of oxygen saturation has to be done depending on the clinical scenario CTRI (CTRI/2020/11/029035).

Full text

18 Asian Journal of Medical Sciences | Mar 2022 | Vol 13 | Issue 3
INTRODUCTION
The ongoing coronavirus pandemic has caused pain
and suffering all across the globe. Although huge leaps
have been made in the elds of medicine in the past
century, the pandemic has proven to be a great challenge
to humankind. The understanding of the virus and its
mechanism is not complete and still eludes scientists.
The world is struggling and to date, no drugs have been
proven to be curative and no vaccine has been found to
have 100% efcacy.
Pulse oximeters are used widely across the globe to monitor
patients and to guide people when to seek medical help
when they are in home isolation. They are widely used in
various clinical settings for decision-making regarding the
severity of the disease and titration of oxygen therapy as it
is a simple and non-invasive method compared to arterial
blood gas analysis which is invasive and requires trained
staff and equipment and can be done only in a clinical
setting. Pulse oximeter can be a standalone device or it
can be part of multiparameter monitors. Oxyhemoglobin
absorbs more light in infrared band and deoxyhemoglobin
An observational study of simultaneous
pulse oximetry and arterial oxygen saturation
readings in intensive care unit/high
dependency unit in COVID-19 patients
Nadia Rose1, Ramya B Sriram2, Karthik GS3, Sowmya MJ4, Sudheer R5
1Assistant Professor, 2Post Graduate Resident, 3,4,5Professor, Department of Anaesthesiology, Rajarajeswari Medical
College and Hospital, Bengaluru, Karnataka, India
Submission: 04-12-2021 Revision: 03-02-2022 Publication: 01-03-2022
Address for Correspondence:
Nadia Rose, Assistant Professor, Rajarajeswari Medical College and Hospital, Bengaluru, Karnataka, India. Mobile: 9513454567
E-mail: dr.nadia.rose@gmail.com
Background: The coronavirus-2019 (COVID-19) pandemic has increased the use of pulse
oximeters worldwide. It has become an inevitable tool in the monitoring of the disease.
However, the accuracy of pulse oximeters in COVID-19 has not been established.
Aims and Objectives: The aims of the study were to examine the relationship between oxygen
saturation as measured by pulse oximeter (SpO2) and oxygen saturation measured by arterial
blood gas analysis (SaO2) measurements in COVID-19 patients admitted to the intensive care
unit (ICU)/high dependency unit (HDU) and to assess the ability of SpO2 readings to detect low
SaO2 and low oxygen tension in COVID-19 patients. Materials and Methods: This prospective
observational study was conducted in the COVID-19 ICU and high dependency unit of a
tertiary care hospital in Bengaluru, India. All patients admitted with conrmed COVID-19,
meeting the eligibility criteria, were included in the study. We assessed bias and limits
of agreement between paired samples of oxygen saturation from pulse oximetry (SpO2)
and arterial oxygen saturation from blood gas analysis (SaO2). Results: The sample mean
difference SpO2-SaO2 is −0.86% (bias) and the 95% condence interval for the mean
difference was −1.67 and −0.04. The lower limit of agreement was −7.32 with a 95%
condence interval (−8.74, −5.91). The upper limit of agreement was 5.61 with a 95%
condence interval of 4.19 and 7.02. Conclusion: SpO2 values are not completely dependable
in estimating SaO2 in COVID-19 patients in ICU/HDU; therefore, arterial blood gas analysis
measurement of oxygen saturation has to be done depending on the clinical scenario CTRI
(CTRI/2020/11/029035).
Key words: COVID-19; Oxygen saturation from ABG; SpO2
Access this article online
Website:
http://nepjol.info/index.php/AJMS
DOI: 10.3126/ajms.v13i3.41218
E-ISSN: 2091-0576
P-ISSN: 2467-9100
Copyright (c) 2022 Asian Journal of
Medical Sciences
This work is licensed under a Creative
Commons Attribution-NonCommercial
4.0 International License.
ORIGINAL ARTICLE ASIAN JOURNAL OF MEDICAL SCIENCES
ABSTRACT

Rose, et al.: SpO2 and SaO2 relationship in COVID-19
Asian Journal of Medical Sciences | Mar 2022 | Vol 13 | Issue 3 19
in red band. Pulse oximeter works by estimating oxygen
saturation from pulse oximetry (SpO2) from this differential
absorption of red (660 nm) and infrared (940 nm) light.1
Some coronavirus-2019 (COVID-19)-positive patients
have profound hypoxemia, but respiratory distress shown
by them may not correspond to their oxygen levels, that is,
they might not be exhibiting signs of distress as expected.2
This peculiar feature of COVID-19 patients can make
timely referral to the intensive care units (ICUs) difcult.
Knowing the relationship between SpO2, arterial oxygen
saturation from blood gas analysis (SaO2), and arterial
oxygen tension (PaO2) will greatly help in timely oxygen
therapy for COVID-19 patients.
Hence, we conducted this observational study in COVID-
19-positive patients admitted to intensive care and
high dependency units (HDUs) due to COVID-related
complications. Ebmeier et al.,3 have shown that there could
be clinically important difference in agreement between
SpO2 and SaO2 readings in non-COVID patients in ICU.
Some studies have shown that SpO2 can be approximated
to the blood oxygen saturation levels obtained from arterial
blood gas analysis but few other studies have shown to
have results that are contradictory.4-6 Philip et al., in their
study, noted that the agreement between SpO2 and SaO2
was limited to a small degree in COVID-19 patients.7
Aims and objectives
The aims of our study were to examine the relationship
between oxygen saturation as measured by pulse oximeter
(SpO2) and oxygen saturation measured by arterial blood
gas analysis (SaO2) measurements in COVID-19 patients
admitted to ICU/HDU and to assess the ability of SpO2
readings to detect low SaO2 and low oxygen tension (PaO2)
in COVID-19 patients.
MATERIALS AND METHODS
This is a single-center prospective observational cross-
sectional study in the ICU and HDU of a tertiary care
hospital during the COVID-19 pandemic from November
2020 to February 2021. Our hospital was a major hospital
dedicated to COVID patients during the rst wave of the
pandemic in India. The Institutional Ethics Committee
approval was obtained and study registered in CTRI
(CTRI/2020/11/029035).
Criteria for admission of reverse transcription-polymerase
chain reaction conrmed COVID-19-positive patients
to the ICU included: Saturation <90% in room air,
adult respiratory distress syndrome, sepsis, or comorbid
conditions with concern for clinical deterioration.
Admission to HDU: Saturation <94% in room air
(90–94%), respiratory rate more than 24/min, tachycardia
more than 120/min, or any abnormal laboratory values.
Inclusion criteria
COVID-19-positive patients of age more than or equal to
18 years and ≤80 years admitted to ICU/HDU.
Exclusion criteria
Patients aged <18 years and >80 years, diagnosis of
methemoglobinemia, smokers, and patients with nail polish
were excluded from the study.
Informed consent was obtained for including patient’s data
in the study. Demographic data and comorbid conditions
of all patients were noted. All patients admitted to the
units were given routine monitoring of vitals and arterial
blood gas sampling was done when clinically indicated as
part of routine clinical management of COVID-19 and
any associated disease. No investigations were done solely
for the purpose of the study. Paired recording of SpO2
and SaO2 was done simultaneously.3 The SpO2 value on the
monitor at the time when the blood was seen to enter the
ABG collection syringe was noted for simultaneous reading
and the sample was immediately analyzed using ABL80
FLEX blood gas analyzer, after removing air bubbles.
The blood gas analyzer was properly calibrated. The SpO2
recordings were done using Skanray Star 65 monitor with
Nellcor Nell 1 SpO2 monitor and Mindray Mec 2000
monitor with adult SpO2 sensor probes. The monitors were
calibrated by the biomedical department of our institution.
All SpO2 values were taken using nger probes. Finger
probe was placed in the opposite hand as that of arterial
blood gas sampling. The measurements were taken 3–4 h
after admission to the unit. Local factors inuencing pulse
oximeter readings and use of vasoactive drugs were noted.
Acute Physiology and Chronic Health Evaluation Ⅱ score8
was calculated within the rst 24 h of admission to the
unit. Type of oxygen therapy for COVID-19 respiratory
failure was noted along with FiO2 at the time of sampling.
Statistical analysis
All data collected were entered into Microsoft Ofce Excel
worksheet. Quantitative data were expressed as mean and
standard deviation. Qualitative data were expressed as
proportions. Bland–Altman method9 was used for assessing
agreement between SaO2 and SpO2. Statistical program R
was used for the statistical analysis.
RESULTS
Sixty-ve paired measurements were taken from 65 patients
admitted to the ICU and HDU. Thirty-nine patients
were from ICU and rest from HDU. Table 1 shows the
characteristics of patients. Comorbid conditions such as

Rose, et al.: SpO2 and SaO2 relationship in COVID-19
20 Asian Journal of Medical Sciences | Mar 2022 | Vol 13 | Issue 3
cardiovascular disease and diabetes mellitus were noted but
not used for further analysis. Modes of oxygen therapy
used for COVID-19 pneumonia were included in the
study. Table 1 shows patient characteristics.Table 2 shows
the values obtained from arterial blood gas analysis and
oxygen saturation from pulse oximeter.
Figure 1 shows the Bland–Altman plot for graphical
representation of the relationship between SpO2 and SaO2.
Bias and limits of agreement were calculated. The sample
mean difference SpO2-SaO2 was 0.86% (bias) and the 95%
condence interval for the mean difference is −1.67 and
−0.04. This indicates that the mean of SpO2 is less than the
mean of SaO2 for all COVID-19 patients in the world. The
lower limit of agreement was −7.32 with a 95% condence
interval (−8.74, −5.91). The upper limit of agreement was
5.61 with a 95% condence interval of 4.19 and 7.02.
We built a linear model to measure the relationship between
SpO2-SaO2 and mean arterial pressure (MAP) at the time
of sampling. The estimated coefficient of MAP was
0.044 with a P=0.246. The R2 coefcient was only 0.021
which indicated that MAP has no signicant association
between the differences SpO2 and SaO2. We next tested
the relationship with hematocrit values which also showed
no signicant association, estimate was 0.02, and P=0.808
and R2 was 0.00095.
Ability of pulse oximeter to detect hypoxemia-SpO2 ≤92%
had a specicity 87% and sensitivity 100% to detect SaO2
of 90% or less. The specicity was 84% and sensitivity was
50% for SpO2 ≤92% to detect PaO2 ≤60 mmHg. SpO2
≤90% showed specicity 95% and sensitivity 75% to detect
a SaO2 of 90% or less. SpO2 ≤90% had a specicity 93%
and sensitivity 50% to detect PaO2 ≤60 mmHg.
DISCUSSION
Pulse oximeters are being widely used during the pandemic
but evidence regarding the precision of pulse oximeter
in COVID-19 patients is limited. There are not many
studies addressing the same. Our ndings from these 65
COVID-19-positive patients admitted to ICU and HDU
show that the limits of agreement are suboptimal than
other studies although the bias is −0.86%. The negative
bias shows that SpO2 underestimates SaO2. Thirty-six
out of 65 patients had their SpO2 values <SaO2 and six
patients showed more than 5% difference between SpO2
and SaO2 in our study. Some studies have shown that
SpO2 overestimates SaO2 while some have shown opposite
results. Philip et al.,7 in their study on 30 patients recovering
from severe COVID-19 infection, noted suboptimal levels
of agreement between SpO2 and SaO2 and a bias of 0.4%.
Van de Louw et al.,10 have shown that SpO2 underestimates
SaO2 at low oxygen saturation in non-COVID patients.
They also noted a great difference between SpO2 and SaO2
in a study on 102 non-COVID patients in ICU. Seguin
et al.,11 in their study in non-COVID patients, noted that
SpO2 overestimated SaO2 and the limits of agreements
were also large.
Table 1: Characteristics of patients
Age (years), mean (SD) 57.72 (15.20)
Sex, n (%) Female 19 (29.23%)
APACHE Ⅱ score, mean (SD) 12.35 (5.35)
Vasopressors/Inotropes, n (%) 7 (10.7%)
Comorbid conditions, n (%)
Cardiovascular disease 14 (21.5%)
Diabetes mellitus 10 (15.4%)
Chronic kidney disease 3 (4.6%)
Modes of oxygen therapy, n (%)
NIV* 21 (32.31%)
Intubated 3 (4.62%)
NRBM+ 33 (50.77%)
HFNO++ 2 (3.08%)
Face mask 6 (9.23%)
*NIV: Non‑invasive ventilation, +NRBM: Non‑rebreather mask, ++HFNO: High‑ow
nasal oxygen, APACHE: Acute Physiology and Chronic Health Evaluation, SD:
Standard deviation
Table 2: Arterial blood gas analysis and pulse
oximeter values
SaO2, mean (SD) 96.38 (3.39)
PaO2, mean (SD) 106.8 (38.94)
pH, mean (SD) 7.43 (0.07)
SpO2, mean (SD) 95.52 (3.56)
SD: Standard deviation, SaO2: Arterial oxygen saturation, PaO2: Arterial oxygen
tension
Relation between SpO2 and SaO2
The purple segment gives the 95% condence interval
for the bias with the middle dashed line being the mean
bias, the pink segment gives 95% condence interval for
the lower limit of agreement, and green segment gives the
95% condence interval for the upper limit of agreement.
Figure 1: Bland–Altman plot

Rose, et al.: SpO2 and SaO2 relationship in COVID-19
Asian Journal of Medical Sciences | Mar 2022 | Vol 13 | Issue 3 21
Wilson–Baig et al.,12 noted that SpO2 underestimated arterial
blood gas saturation measurements in COVID-19 patients
(n=17), the probable reasons cited for this being tissue
hypoxia, different spectral properties of d-dimer and
ferritin, formation of complexes between the coronavirus
and hemoglobin and proposed that the situation of “happy
hypoxemia” noted in COVID-19 patients might be due
to these reasons. The limits of agreement values of our
study indicate that caution should be advised when oxygen
therapy is titrated solely based on SpO2 measurements.
The pulse oximeter value of 90% or less had poor
sensitivity in detecting low PaO2 (≤60 mmHg) and SaO2
of 90% or less in our study. This shows poor diagnostic
accuracy of SpO2 readings in estimating hypoxemia.
Sensitivity of SpO2 <90% to detect a PaO2 of <60 mmHg
in non-COVID patients was much higher in some studies
but low sensitivity has been shown in others. Pilcher et al.,13
showed a sensitivity of 88.6% and specicity 95.1% for
SpO2 < 90% to detect SaO2 <90% and a sensitivity of
70.5% and specicity of 98.2% to detect PaO2 <60 mmHg
in their study on non-COVID patients.
Our study has various strengths. Single paired measurement
was taken from each patient. The measurements were
taken simultaneously; the arterial blood gas analysis
was done immediately, so there was almost no time lag
between both measurements. As the measurements were
taken simultaneously, uctuations in oxygen levels14 which
could have happened over time were negated. This was
useful in improving the validity and removing any bias
from collecting data from the same patient by repeated
measurements.13
Ethnicity and skin color15,16 could affect the agreement
between SpO2 and SaO2 but all our patients were of same
South Indian ethnicity. SpO2 can overestimate SaO2,
especially when saturation is low in individuals who are
dark skinned.17
We excluded smokers, people with methemoglobinemia,
and patients with nail polish from our study thereby
avoiding some factors which could potentially affect the
pulse oximeter accuracy as seen in the previous studies.13,18,19
Other local factors which could affect SpO2 measurements
such as poor signal and motion artifacts were not observed
in any patient during measurement.
These findings are from a single hospital in a single
geographical area. More extensive studies with higher
sample size, different clinical situations, and with different
models of pulse oximeters have to be done to extrapolate
the ndings to other COVID-19-positive patients during
the pandemic. Different models and low-quality nger
pulse oximeter probes are widely available in the market and
are being used extensively as many hospitals are stretched
beyond their admission capacities.
Limitations of the study
There are some limitations for the study that has to be
considered. Original planned sample size using Yamane
equation20 was 100, considering 135 COVID-19 admissions
as population size and degree of error 0.05. However,
the admissions of COVID-19-positive patients decreased
during the study time as the rst wave of the pandemic
had already peaked; we were able to get data of 65 eligible
patients during the study period.
Values such as ferritin and d-dimers which could have
different spectral properties21 at 660 and 940 nanometers
as suggested by Wilson–Baig et al.,12 were not considered
in this study. Studying these values in COVID-19 patients
will aid in understanding the relation with SpO2 better, if
any exists.
CONCLUSION
Oxygen therapy and titration are mostly guided by pulse
oximeter in almost all COVID treatment centers as it is
non-invasive and simple method and offers continuous
monitoring. However, our study shows that SpO2 values
are not completely dependable in estimating SaO2 in
COVID-19 patients in ICU/HDU due to suboptimal limits
of agreement. Arterial blood gas measurements have to be
obtained depending on the clinical scenario of the patient.
ACKNOWLEDGMENT
Dr. Monisha, Dr. Kishore
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Authors Contribution:
NR- Conception,acquisition,analysis,interpretationofdata,drafting,approvalofnalversion,andagreestobeaccountableforallaspectsofwork;
RBS- Acquisition,drafting,approvalofnalversion,andagreestobeaccountableforallaspectsofwork;KGS- Analysis,revisingcritically,approvalofnal
version, and agrees to be accountable for all aspects of work; SMJ- Acquisition,interpretationofdata,drafting,approvalofnalversion,andagreestobe
accountable for all aspects of work; and SR- Interpretationofdata,revisingcritically,approvalofnalversion,andagreestobeaccountableforallaspectsofwork
Work attributed to:
Rajarajeshwari Medical College and Hospital, Mysore Road, Bengaluru - 560 060, Karnataka, India
Orcid ID:
Nadia Rose - https://orcid.org/0000-0002-1667-7337
Ramya B Sriram - https://orcid.org/0000-0001-8337-8241
Karthik GS - https://orcid.org/0000-0001-5102-1133
Sowmya MJ - https://orcid.org/0000-0002-0962-3234
Sudheer R - https://orcid.org/0000-0002-8706-7712
Source of Support: None, Conicts of Interest: None.