|Year : 2020 | Volume
| Issue : 1 | Page : 51-56
Accuracy of pulse oximetry in comparison with arterial blood gas
Khadeja M Mohamed Elhossieny
Department of Anesthesia and Surgical Intensive Care, Faculty of Medicine, Zagazig University, Zagazig, Egypt
|Date of Submission||07-Nov-2018|
|Date of Acceptance||10-Sep-2019|
|Date of Web Publication||16-Apr-2020|
MD Khadeja M Mohamed Elhossieny
Department of Anesthesia and Surgical Intensive Care, Faculty of Medicine, Zagazig University, Elglaa/Zagazig, 4511
Source of Support: None, Conflict of Interest: None
Background Oxygen plays a principal role in aerobic respiration. Oxygenation is tested by pulse oximetry, which is considered the “fifth vital sign” of health status. It is widely used in critical care medicine to measure arterial oxygen saturation (SaO2).
Aim To evaluate the factors affecting the accuracy of pulse oximetry (SpO2) relative arterial oxygen saturation SaO2.
Patients and methods The study included 70 patients admitted to emergency and surgical ICU. They were subjected to full history taking, complete physical examination and laboratory investigations, including complete blood count, liver and kidney function tests, taking sample of arterial blood gases, and at the same time, recording the readings of pulse oximetry.
Results This study shows that SpO2 is accurate relative to SaO2 in arterial blood gases as the difference between SpO2 and SaO2 was less than ±3%. Many factors affected accuracy, such as mechanical ventilation, vasopressor, oxygen mask (P≥0.047), age (P≥0.045), and corresponding hypoxia (P=0.002), whereas the results were insignificant regarding sex and sepsis (P=0.568 and 0.660, respectively).
Conclusion The pulse oximeter remains a valuable tool in intensive care patients, but an awareness of its limitations such as old age more 65 years, mechanical ventilation, vasopressors support, and oxygen mask is important, as they were the impact factors affecting its accuracy. It is an important component of enhancing the quality of intensive care.
Keywords: arterial blood gas, acid–base balance, oxygen saturation, pulse oximetry
|How to cite this article:|
Mohamed Elhossieny KM. Accuracy of pulse oximetry in comparison with arterial blood gas. Res Opin Anesth Intensive Care 2020;7:51-6
|How to cite this URL:|
Mohamed Elhossieny KM. Accuracy of pulse oximetry in comparison with arterial blood gas. Res Opin Anesth Intensive Care [serial online] 2020 [cited 2020 May 31];7:51-6. Available from: http://www.roaic.eg.net/text.asp?2020/7/1/51/282597
| Introduction|| |
Oxygen is elixir of life; it plays a principal role in aerobic respiration. Oxygenation is tested by different methods, such as pulse oximetry or arterial blood gas (ABG) sampling. Pulse oximetry is considered to be the fifth vital sign of health status. It is widely used in critical care medicine to noninvasively measure arterial oxygen saturation (SaO2) .
The oximeter is composed of a sensor device consisting of two light sources, red and infrared, and photo detector to measure absorption of visible light. Most monitors also display the heart rate .
Pulse oximetry is the continuous noninvasive measurement of blood oxygen saturation. It is useful in any setting when oxygenation is unstable, including intensive care, operating and recovery room, and emergency and hospital ward, and to determine the effectiveness or need for oxygen supply .
Advantages of pulse oximetry include simplicity of use and the ability to provide continuous and immediate record of oxygen saturation measurement and heart rate. It can also provide an early warning of hypoxemia ,,.
The accuracy of pulse oximetry (SpO2) to estimate SaO2 in critically ill patients has yielded mixed results . Certain studies of critically ill patients have demonstrated that multiple factors influence the SpO2, whereas others have not, especially in ICU patients ,.
An ABG is a blood test that was done by sampling blood from an artery to determine the pH of the blood, the partial pressure of carbon dioxide, oxygen, electrolyte, and the bicarbonate level .
ABG analysis is the gold standard to obtain information about oxygenation, ventilation, and acid–base status of critical patients. It is used to evaluate many respiratory and metabolic conditions . The radial artery is usually the vessel of choice for arterial cannulation. Larger arteries are preferred when the risk of thrombosis is high or when the expected duration of cannulation is greater than 7 day .
| Patients and methods|| |
After ethical committee approval and patient informed consent were obtained, we conduct our study on 70 patients at emergency and surgical ICU, Zagazig University Hospitals, in the period from May 2016 to May 2017. We performed this study to confirm and compare the factors affecting the efficacy of pulse oximeter in relation to ABG.
Determining the relation between studied parameters was done using correlation coefficient. P value >0.05 is considered non-significant. P value <0.05 is considered significant. ROC curve; For the construction of the ROC curve, relations between sensitivity and specificity for various cut off points were plotted The area under the curve (AUC) provides an index of the overall discriminative ability of the test Determining the relation between studied parameters was done using correlation coefficient. P value >0.05 is considered non-significant. P value <0.05 is considered significant. ROC curve; For the construction of the ROC curve, relations between sensitivity and specificity for various cut off points were plotted The area under the curve (AUC) provides an index of the overall discriminative ability of the tt.
Age above 16 years old and both sexes were the inclusion criteria.
Patients below 16 years old, history of smoking owing to high level of carboxy hemoglobin, and significant jaundice (bilirubin ≥40 μmol) were the exclusion criteria.
Skin pigment (tattoo, juindice, dirt) and painted fingernails (nail polish).
The following procedures were carried out for every patient included in this study: medical and surgical history was taken, and clinical and laboratory investigations were performed, including complete blood count, liver and kidney function tests, and ABG.
The reference parameters were as follows: accuracy when difference between SaO2 and SpO2 was less than ±3% , hypoxia when SaO2 less than 95% , sepsis when temperature more than 38°C or less than 36°C, heart rate more than 90 beats/min, and white blood cells count more than 12 000 cells/µl or less than 4000 cells/µl .
Shock diagnosis is mostly clinical, based on evidence of insufficient tissue perfusion, heart rate more than 100, and systolic blood pressure less than 90 mmHg .
The pulse oximetry (Monitor Nellcor Pulse Oximetry DS-100A Adult Finger Sensor) value was recorded at the time of the ABG withdrawal and analyzed on advice COBAS B 221.
Before blood sampling, we must examine blood supply to hand, and this was done by Allen and modified Allen test as follows: one hand is examined at a time. The hand is elevated and the patient is asked to clench their fist for about 30 s. Pressure is applied over the ulnar and the radial arteries so as to occlude both of them. Still elevated, the hand is then opened. It should appear blanched (pallor may be observed at the finger nails).
Ulnar pressure is released while radial pressure is maintained, and the color should return within 5–15 s. If color returns as described, Allen’s test is considered to be normal. If color fails to return, the test is considered abnormal, and it suggests that the ulnar artery supply to the hand is not sufficient. This indicates that it may not be safe to cannulate or needle the radial artery ,.
Palpate the radial artery, and then insert a needle using the other hand at an angle of 30 degrees. We should observe bright red flashback into the needle. Once the required amount of blood has been collected, quickly remove the needle. Immediately press down firmly with some gauze over the site for at least 5 min, to prevent hematoma formation. Place the syringe in the bag of ice (icing the sample will prevent any further metabolism of blood). Static analysis was done between SpO2 and SaO2 at the same time to detect the accuracy of SpO2 and factors affecting the accuracy, for example, age, oxygen supply, hypoxia, mechanical ventilation, and vasopressors.
| Results|| |
[Table 1],[Table 2],[Table 3] demonstrate there is no significant correlation between sex and SpO2.
[Table 4],[Table 5],[Table 6] shows that there is a significant correlation between patients on oxygen mask and SpO2.
|Table 4 Comparison between patient on oxygen mask and accuracy of pulse oximetry|
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|Table 5 Comparison between patients on mechanical ventilation and accuracy of pulse oximetry fixed FiO2 40%|
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|Table 6 Comparison between patients on vasopressor support and accuracy of pulse oximetry|
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[Table 7] and [Table 8] show that there is a highly significant correlation between hypoxia and SpO2 ([Figure 1]).
|Table 7 Correlation between hypoxic patients and accuracy of pulse oximetry|
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|Table 8 Comparison between septic patients and accuracy of pulse oximetry|
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| Discussion|| |
ABG analysis is the gold standard to obtain information about oxygenation, ventilation, and acid–base status of critical patients. It is used to evaluate many respiratory and metabolic conditions .
The SpO2 to estimate SaO2 in critically ill patients has yielded mixed results. Both the degree of inaccuracy, or bias, and its direction have been inconsistent. Although certain studies of critically ill patients have demonstrated that many factors influence the SpO2, others have not .
The aim of the work was to determine the factors affecting SpO2 relative to SaO2, as pulse oximetry is considered accurate when the difference between SaO2 and SpO2 is less than ±3% .
Our study shows significant correlation between age and SpO2. Collins and Bakheit  reported the predictive value of pulse oximetry was better in patients who were less than 65 years of age compared with those who were older, as lung capacity decreases with age.
Against our study, John et al.  showed that age was not a statistically significant variable in the multivariate analysis.
We also showed no significant correlation between sex and SpO2. In contrary to this study, John et al.  showed that two oximeters, Nonin 9700 with clip-on probe and the Masimo radical with the adhesive probe, did not show an effect of skin pigment, although both had a significant sex effect. This observation may relate to smaller finger size and a correspondingly smaller pulsatile signal detected by the sensor.
This study shows significant correlation between ventilated patients with fixed FiO2 40% and patients on oxygen mask with SpO2. Similar to our result, Jubran and Tobin  studied ventilator-dependent patients, and the bias±precision of oximetry was 1.7±1.2% for SaO2 values. In contrast to this study, Arakawa et al.  showed that SpO2 before oxygen administration in the oxygen supplementation group was 97.9±1.4%, which was nearly comparable to the 97.3±2.1% in the room air breathing group.
Arakawa et al.  showed that SpO2 before oxygen administration in the oxygen supplementation group was 97.9±1.4%, which was nearly comparable to the 97.3±2.1% in the room air breathing group, and this disagreed with our results. There was a significant lack of agreement between SpO2 measurements and SaO2. This was large enough for oxygen therapy titration. Moreover, Olive and Twentyman  found that SpO2 is an inappropriate measurement for determining oxygen administration or clinical decision making. This also has implications for the monitoring methods used to implement emergency oxygen therapy guidelines.
This study shows significant correlation between patients on vasopressor and SpO2. Moreover, Umesh et al.  show that readings obtained may be unreliable and may be unrecordable in patients receiving high-dose vasoactive drug infusions to counter hypotension. This may be referred to vasoconstrictor and decrease blood flow. In contrast to our study, Wilson et al.  using stratified analyses found in a retrospective study performed in an emergency department on patients with severe sepsis or septic shock that vasoactive agents did not affect the accuracy of SpO2. Moreover, Szaflarski and Cohen  found that the need for vasoactive drugs did not significantly affect the SpO2, as pulse oximeters are dependent upon arterial pulsatility, and vasopressors may theoretically decrease pulsatility secondary to arteriolar vasoconstriction.
This study shows highly significant correlation between hypoxia and SpO2. Similar to this study, Wilson et al.  found that the mean differences (SpO2–SaO2) in hypoxemic patients was 4.92% and in nonhypoxemic patients was 1.89%. Possible reasons for decreased pulse oximeter accuracy with hypoxemia include lack of reliable human calibration data during extreme hypoxia and an increased proportion of reduced hemoglobin in hypoxic states, which can exacerbate error in the absorption ratio.Our results show no significant correlation between sepsis and SpO2. Secker and Spiers  reported that pulse oximetry significantly underestimated SaO2 by a mean of 1.4% (P<0.001) in patients with septic shock, but this bias was not significantly different relative to those without septic shock. In contrast, Chan et al.  in a retrospective review of 88 patients with severe sepsis and septic shock found that SpO2 significantly overestimated SaO2 by nearly 5%. Factors that may account for a difference in the direction in which SpO2 is biased include differences in (a) the extent of fluid resuscitation and tissue perfusion, (b) sepsis-induced cardiac dysfunction, (c) sites interrogated by the pulse oximeter probe, (d) types of pulse oximeters and probes used, (e) vasoconstrictor use, and (f) the presence of other comorbid conditions that may spuriously affect SpO2 values unpredictably. Thus, a number of variables that occur in patients with severe sepsis, and septic shock makes it difficult to predict which direction SpO2 may be biased .
| Conclusion|| |
SaO2 could be reliably predicted from pulse oximeter saturation. The pulse oximeter remains a valuable tool in the intensive care patients, but an awareness of its limitations such as old age, mechanical ventilation, vasopressors support, and hypoxia is important, as these were the impact factors affecting its accuracy. It is an important components of enhancing the quality of intensive care.
We recommend further studies on a larger number and different patients to detect and confirm more impact factors affecting the accuracy of SpO2.
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Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8]