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INTRODUCTION
A pulse oximeter is a non-invasive medical device used to monitor patients’ oxygen saturation. The peripheral oxygen saturation (SpO2) measured by this device is an estimate of the oxygen saturation in the artery (SaO2). In general, a pulse oximeter uses dual-wavelength absorption spectrophotometry to measure SpO2 [1]. The wavelengths are selected to provide the best separation of absorbance values of oxyhemoglobin and deoxyhemoglobin [1]. The ratio of the two absorbance values is used to calculate SpO2 values.
Since a pulse oximeter is used for continuous monitoring of oxygenation and plays an important role in the early detection of hypoxia, the SpO2 accuracy of a pulse oximeter should be validated for patient safety. The performance of a pulse oximeter is evaluated based on clause 201.12.1 of the ISO 80601-2-61:2011 (E) guidelines [2], and a controlled desaturation study is conducted within the range of 70-100% of SaO2 by administering air-nitrogen-CO2 mixtures to a patient or volunteer. Pulse oximetry equipment consists of a monitor (electronics, display, and operator-equipment interface), probe, and cable extender, and its performance needs to be evaluated according to international standards. The use of different sensors with the same pulse oximeter monitor is considered to be a difference warranting re-evaluation of its reliability. In particular, the United States of America Food and Drug Administration (USFDA) requires a controlled desaturation study to evaluate the performance of pulse oximeter equipment before it can be sold in the United States [3].
The aim of this study was to determine whether the MP570T pulse oximeter (MEK-ICS Co., Ltd., Korea) with various finger probes would provide clinically reliable SpO2 readings over a range of 70-100% SaO2 during non-motion conditions.
MATERIALS AND METHODS
Volunteer population
The study had a single-center, non-randomized design. This study was conducted in a normal environment at the Intensive Clinical Research Room of the author’s hospital according to ISO 14155: 2011 guidelines and the clinical trial guidelines of the pulse oximeter (Ministry of Food and Drug Safety, Korea), which complied with ISO 80601-2-61:2011 (E) guidelines [2]. This study was approved by our Institutional Review Board (no. 2016-0069) and was registered in an international clinical trials registry platform (http://cris.nih.go.kr, no. KCT0004854). Written informed consent was obtained from all volunteers. The inclusion criteria for the participants were: aged 20-50 years, carboxyhemoglobin < 3%, methemoglobin < 2%, and total hemoglobin concentration > 10 g/dl. The exclusion criteria were: a known history of respiratory or cardiovascular disease, smoking habits, evidence of pregnancy, history of syncope, diabetes mellitus, skin diseases that can affect SpO2 values, such as onychomycosis [4], and body mass index ≥ 35 kg/m2. The volunteers were fully informed of the study protocols and completed health assessment questionnaires before enrollment.
Procedure for controlled desaturation study
Volunteers were monitored using end-tidal carbon dioxide partial pressure and fraction of inspired oxygen by using Carescape® B850 (GE Healthcare, USA). Each volunteer was placed in a semi-Fowler’s position and connected to a breathing circuit to administer the nitrogen-air-carbon dioxide mixtures. A nose clip was applied to prevent breathing of room air. For frequent blood sampling, an arterial cannula was placed in the radial artery of each volunteer. Seven pulse oximeter probes were simultaneously attached to each volunteer’s fingers. The following models were used in this study: (1) WA-100 reusable finger probe (MEDNIS Co., Ltd., Korea), (2) MDNA disposable finger probe (MEDNIS Co., Ltd.), (3) IS-1011 disposable finger probe (Insung Medical Co., Ltd., Korea), (4) CJ340NA disposable finger probe (CHUN JI IN Medical Co., Ltd., Korea), (5) NellcorTM OxiMax DS-100A reusable finger probe (Medtronic, USA), (6) NellcorTM OxiMax MAX-N disposable finger probe (Medtronic), and (7) OXI-PRO DA disposable finger probe (Bio-Protech Inc., Korea). The same reusable probes were used for all volunteers, and a new disposable probe was used for each volunteer. To minimize light interference from outside, both hands were covered with a blanket. An air warmer (Bair Hugger™, 3M™, USA) was applied to the hands to prevent hypoperfusion. To obtain the SpO2 value to be referenced when determining the target plateau, a reusable pulse oximeter finger probe (OxiMax® N-600x, Medtronic) was also used. The probes were placed on the fingers of the hand with the arterial catheter as follows: CJ340NA disposable finger probe (thumb), MDNA disposable finger probe (index finger), OXI-PRO DA disposable finger probe (middle finger), NellcorTM OxiMax MAX-N disposable finger probe (ring finger). The probes were placed on the fingers of the other hand as follows: IS-1011 disposable finger probe (thumb), OxiMax® N-600x (index finger), WA-100 reusable finger probe (middle finger), NellcorTM OxiMax DS-100A reusable finger probe (ring finger). Each volunteer was exposed to various levels of induced hypoxia from 70-100% of SaO2. Each plateau of oxygen saturation was maintained for at least 30 s until stabilization, after which 1 ml of arterial blood was drawn into a heparinized syringe (Fig. 1). The study period consisted of two rounds of hypoxia, and the volunteers were maintained on room air between each round. SaO2 measurements using a CO-oximeter (ABL90 FLEX, Radiometer Medical A/S, Denmark) were used as a reference for the SpO2 accuracy.
Methods of performance evaluation
The measured SaO2 values were matched with the corresponding SpO2 values. Reference SaO2 data outside the range of 73-97% were excluded from the analysis [2]. The SpO2-SaO2 pair data were analyzed according to the following statistics.
Arms (accuracy root mean square)
SpO2 accuracy was evaluated by the Arms for the overall range.
where SpO2ij is the ith measured SpO2 value and SRi is the ith measured standard reference value.
Bias
The average difference was calculated to show the bias of the device being tested compared to the reference. The mean bias was calculated for the overall range.
Precision
Precision is a measure of the scatter to be expected in multiple measurements taken with the same pulse oximeter equipment at a given oxygen saturation, considering both the variation among patients and the repeatability of the device. Precision was calculated for the overall range.
where SpO2fit,i is the value of the curve fitted to the test data at the ith reference oxygen saturation value (SRi).
Of these parameters, the pass/fail criteria of pulse oximeter equipment are determined by Arms. Over a range of 70-100% of SaO2, Arms was less than or equal 4% in non-motion conditions.
RESULTS
A total of 12 volunteers were enrolled in the study, and all volunteers were included in the analysis. The characteristics of these volunteers are presented in Table 1. The mean ± SD time for volunteers to participate in the current study from the first blood sampling to the last blood sampling was 25.8 ± 3.1 min. Of the 293 SpO2-SaO2 pairs obtained from these volunteers, 18 pairs were excluded from the analysis because they were outside the defined SaO2 range. Therefore, 275 pairs were included in the analysis (73 ≤ SaO2 [%] < 80: n = 91; 80 ≤ SaO2 < 90: n = 99, 90 ≤ SaO2 ≤ 97: n = 85). Performance comparison of each probe within the range of 73-97% of SaO2 is summarized in Table 2. Two of the seven probes (CJ340NA disposable finger probe and NellcorTM OxiMax MAX-N disposable finger probe) did not meet the performance evaluation criteria. Observed SpO2 versus measured SaO2 in two pieces of pulse oximeter equipment with the lowest (WA-100 reusable finger probe) and the highest (CJ340NA disposable finger probe) Arms are shown in Fig. 2. In the case of the CJ340NA probe, it was confirmed that overall SpO2i values corresponding to SRi values were high, which means that it is overestimated. For the two cables connected by the MP570T pulse oximeter, Bland-Altman plots and the relationship between SRi and bias are presented in Figs. 3 and 4, respectively. There were no adverse events observed throughout the study period.
DISCUSSION
The pulse oximeter is a piece of medical equipment that is commonly used in clinical settings [1], but few medical practitioners understand the accuracy evaluation criteria. Accuracy data obtained from controlled desaturation studies are not required for pulse oximeter equipment in Korea. However, the USFDA requires these data from companies desiring to sell pulse oximeter equipment in the United States [3]. The common manufacturing literature claim for Arms for pulse oximeters is ± 2–3% over the range of 70–100% SpO2 [5]. The total number of acceptable SpO2-SaO2 pair data obtained from controlled desaturation studies should be sufficient to statistically validate the specified SpO2 accuracy. Typically, at least 10 volunteers are recruited, and at least 20 arterial blood samples per volunteer are obtained and analyzed with at least 200 data pairs. Moreover, the distribution of the SRi values should have a similar density over the entire required range; for example, the ranges of 70-79%, 80-89%, and 90-100% SaO2 should each have approximately 1/3 of the total data. In general, the lower the SaO2, the higher the value of Arms. In the case of WA-100 reusable finger probe, the values of Arms were 4.06 for 70-79% SaO2, 2.32 for 80-89%, and 1.40 for 90-100%, respectively.
Particularly, the skin complexion of the study participants should be specified because this affects the accuracy of SpO2, with dark skin pigmentation resulting in an overestimation of arterial oxygen saturation especially at a low saturation in some pulse oximeters [6]. For some pulse oximeters, at 60–70% SaO2, SpO2 overestimated SaO2 (bias ± SD) by 3.56 ± 2.45% in darkly pigmented subjects, compared with 0.37 ± 3.20% in lightly pigmented subjects (P < 0.0001) [6]. Additionally, another previous study revealed the effect of skin pigmentation on bias [7]. For this reason, the USFDA recommended that controlled desaturation studies should include subjects with a range of skin pigmentation, including at least 2 darkly pigmented subjects or 15% of the subject pool, whichever is larger [3]. For this reason, some Africans participated in the current clinical trial.
On pulse oximeter accuracy evaluation, sex can be a factor affecting bias. In an earlier study, sex was found to be a significant univariate predictor for bias [7]. This may be because female has a smaller pulsatile signal detected by the probe due to the relatively smaller finger size. However, in general, female volunteers have lower hemoglobin levels than male volunteers. Female’s hemoglobin levels were also significantly lower in this study ((female [n = 6]: 13.1 g/dl; male [n = 6]: 14.9 g/dl; Student’s t-test, P = 0.001). Because low hemoglobin levels were prevalent in female, it was not possible to statistically separate the contributions of sex and low hemoglobin to oximeter bias [7]. In the case of the WA-100 reusable finger probe, the Arms did not show much difference between sexes within the range of 73-97% SaO2 (male: 2.55%; female: 3.07%).
Through this study, it was confirmed that not only the pulse oximeter but also the connected probes are important for evaluating accuracy. Sensor type was a predictor of bias in SpO2 estimates at low SaO2 levels [7]. A prior study also revealed that some of the sensors on the market may be inaccurate [5]. Therefore, it is necessary to check the performance of the pulse oximeter equipment (pulse oximeter and probe) in the clinical field.
In conclusion, these results provide supporting evidence that the SpO2 accuracy of the MP570T pulse oximeter with WA-100 reusable, MDNA disposable, IS-1011 disposable, NellcorTM OxiMax DS-100A reusable, and OXI-PRO DA disposable finger probes pass an Arms specification of ≤ 4% over the range of 70-100% SaO2, under non-motion conditions.
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