Validity and reliability of measurement of peripheral oxygen saturation during the 6-Minute Walk Test in patients with systemic sclerosis

Peripheral oxygen saturation (SpO2) using the fingers may have important limitations due to Raynaud’s phenomenon and sclerodactyly in patients with systemic sclerosis (SSc). Sensors located at more central body positions may be more accurate as these as less prone to Raynaud attacks. To determine the validity and reliability of the SpO2 measured at the finger, forehead, and earlobe during the 6-Minute Walk Test (6MWT). Eighty two patients with SSc had an arterial line placed while performing the 6MWT. Peripheral oxygen saturation was simultaneously measured by finger, forehead, and earlobe sensors and compared to the arterial oxygen saturation (SaO2) measured before and after the 6MWT. 40 patients repeated the 6MWT one week later to determine re-test reliability. We used Bland–Altman plots to display the agreement between SpO2 and SaO2. The intraclass correlation coefficient for repeated measurement of minimum SpO2 was calculated. The mean difference between SpO2 and SaO2 after the 6MWT was − 3% (SD: ± 5), 0% (SD: ± 2), and 1% (SD: ± 2) for the finger, forehead, and earlobe, respectively. The minimum SpO2 measured at the finger demonstrated the poorest re-test reliability: The ICC (95% CI) showed good agreement using the ear and forehead probe (ICCear = 0.89 [95% CI 0.80; 0.94]; ICCforehead = 0.77 [95% CI 0.60; 0.87]), while a modest reliability was found using the finger probe (ICCfinger = 0.65 95% CI [0.43; 0.80]). SpO2 should be measured using either the earlobe or forehead during the 6MWT in patients with SSc. Clinical Trials.Gov (NCT04650659). Supplementary Information The online version contains supplementary material available at 10.1007/s00296-024-05532-5.


Introduction
Systemic sclerosis (SSc) is a progressive chronic connective tissue disease characterized by microvasculopathy and extensive fibrosis in the skin and internal organs.The disease has an increased mortality, with SSc associated interstitial lung disease (SSc-ILD) and pulmonary arterial hypertension (SSc-PAH) accounting for the majority of SSc-related deaths [1].The peripheral microvasculopathy in SSc leads to poor perfusion of the fingers, which manifest as Raynaud's phenomenon, digital ulcers, and poor healing.Overt symptoms of fibrosis include thickness of the skin, dry skin and contractures of the joints [2].Many patients with SSc have decreased exercise tolerance, which may have multiple aetiologies, in which musculoskeletal disease, internal organ involvement, and deconditioning may play a role [3].The disease progression is highly variable, and accurate markers of disease activity is essential for qualified management of the disease.
The 6-Minute Walk Test (6MWT) is a standardized noninvasive sub-maximum exercise test.During the 6MWT, the distance (6MWD), effort, and peripheral oxygen saturation (SpO 2 ) are registered [4].The test is primarily used as an outcome measure of clinical SSc trials, to monitor treatment response in patients with pulmonary involvement, and as a measure of functional capacity in general [5].
Exercise-induced desaturation during the 6MWT is associated with the degree of dyspnea, diffusion capacity for carbon monoxide and the extent of lung fibrosis HRCT in patients with SSc [6][7][8][9].Furthermore, SpO 2 desaturations have been associated with progression of SSc-ILD and poorer prognosis in patients with SSc [10,11].
While digital sensors are commonly used to measure SpO 2 during the 6MWT, these measurements may have important limitations in patients with SSc due to disease related microangiopathy, Raynaud's phenomenon, sclerodactyly and motion artifacts during the 6MWT [12].Consequently, finger probe pulse oximetry may cause inaccurate measures of SpO 2 , and there may be substantial variation of SpO 2 measurements in patients with SSc [13,14].Indeed, several authors author advocate for measuring Sp0 2 at the forehead in patients with SSc [5,7].Still, the evidence for measuring Sp0 2 at more central locations is based on only a single study examining the re-test reliability of SpO 2 measurement during the 6MWT in a small cohort of patients with SSc [7].
We aimed to determine the validity and re-test reliability of peripheral oxygen saturation measured at the finger, forehead, and earlobe as compared with blood gas analysis during the 6MWT in patients with SSc.

Study population and study design
We conducted a cross-sectional study at the Department of Rheumatology at Aarhus University Hospital in Denmark from 27 July 2021 to 21 December 2021 involving adult patients diagnosed with SSc according to the ACR/EULAR 2013 criteria [15].
Patients were excluded in case of recent or ongoing pneumonia, pregnancy, a diagnose of connective tissue overlap syndrome [16] or in case of severe physical or mental comorbidity, which prevented the performance of the 6MWT.Patients were allowed to use supplemental oxygen or walking aid during the 6MWT if needed.

6MWT and measurements of oxygen saturation
The 6MWT was performed at room temperature by the same investigator (ALE) according to the American Thoracic Society guidelines [4].Patients had acclimatised and rested for minimum 20 min before test start.SpO 2 , 6MWD and Borg dyspnoea score were collected [4,17].Raynaud's attacks during the 6MWT were noted in case the patient fingers turned white and/or blue.SpO 2 was continuously measured during the 6MWT by pulse oximeters (Vyntus® WALK, Nonin Model 3150, Viare Medical, Germany) using sensors at the finger, earlobe and forehead [18].The pulse oximeters measured SpO 2 as integers.The accuracy of the SpO 2 measurement (interval of 70-100%) with low perfusion was ± 2%.At the first visit, an arterial line was placed in all patients by a trained anesthesiologist (HH or PJ) on the opposite arm of the finger oximetry sensor.Arterial blood was drawn immediately before (pre-exercise) and after (postexercise) the 6MWT and analysed with a blood gas analyser (ABL800, Radiometer Medical, Brønshøj, Denmark).
A subgroup of patients (n = 40) repeated the 6MWT one week later without an arterial line.At visit 2 the earlobe and finger probe were placed on the same earlobe and finger as in visit 1 (Supplementary Fig. 1).

Data quality
The oximeters were electronic and paired with tablets via Bluetooth, which generated a graph of the continuous measurement of SpO 2 during the 6MWT (Supplementary Fig. 2).The quality of data was assessed by ALE using the following pre-specified criteria for exclusion of data (Supplementary Table 1).
(1) Technical error in collection or transfer of data from pulse oximeter and tablet (i.e., no readings from pulse oximeter or no data transfer from pulse oximeter to tablet at time of arterial blood gas test).
(2) Technical error in performance or analyse of arterial blood gas test.
In case of doubt, a consensus was reached in cooperation with KS.
All patients had Nailfold Videocapillaroscopy (NVC) images recorded of the 2nd to 5th finger.The capillary density was assessed with the 90-degree method from minimum 3 images per patient.The general capillary density was defined as the mean capillary density of the available pictures from the same hand.

Patient-related outcome measures
Patients answered two self-reporting questionaries: Raynaud's attacks the last month, including the Raynaud's Condition Score (RCS) [20] and burden of ischaemic ulcers, and the Scleroderma Health Assessment Questionnaire (SHAQ) [21].

Statistical analysis
Categorial data are reported as counts and percentages, and continuous data as mean values and standard deviation (± SD) when normally distributed or otherwise as median values and interquartile interval (IQI).Data distribution was investigated Q-Q plots and histograms.
The agreement of the SpO 2 of the three anatomical sites was examined using Bland-Altman plots to display the difference between SpO 2 and SaO 2 (bias) at pre-exercise and at post-exercise [22], and the re-test reliability was examined using Bland-Altman plots to display the agreements between the minimum SpO 2 during the 6MWT, the 6MWD and Borg dyspnea score at visit 1 and visit 2. Furthermore, the overall accuracy of the peripheral oxygen measurement was calculated by the accuracy root mean square (A rms ) (√ [(bias) 2 + (precision) 2) [23].In accordance with the Food and Drug Administration recommendation, we used a cut-off of A rms < 3% as the acceptable accuracy of the SpO 2 measurements [24].
The intraclass correlation coefficient (ICC, 95% confidence interval [95% CI]) for repeated measurement of minimum SpO 2 was calculated based on an absolute agreement, two-way mixed effect model [25].The ICC values were interpreted using the following definitions: ICC < 0.5: poor reliability, ICC: 0.5-0.75:moderate reliability, ICC 0.75-0.9:good reliability, and ICC > 0.90: excellent reliability.Furthermore, we calculated the frequency of measurement error of SpO 2 at post-exercise for the finger, forehead, and earlobe sensors.Measurement errors was defined as values being ± 4% different from SaO 2 values.In explorative analysis, we examined the impact of demographic parameters, comorbidities, and SSc specific parameters on the risk of measurement errors of the SpO 2 .Statistical significance was tested using Student's t test, the nonparametric Mann-Whitney U test and Fisher's exact test.All analyses were carried out in Stata17, where p < 0.05 was considered statistically significant.

Ethical permissions
The

Patient characteristics and baseline characteristics
One hundred ninety-nine patients were screened for the study.Sixteen patients were excluded from the study, and 101 patients chose not to participate.In total, 82 patients participated in our study (Supplementary Fig. 3).
The patients had a median age of 58 years (IQI: 52-66), and 76% of patients were female (Table 1).Thirty-seven patients (45%) had either SSc-ILD or PAH.None of the included patients needed supplemental oxygen or walking aid during 6MWT.

Validity of peripheral oxygen measurement
The agreements between measurement of SpO 2 and SaO 2 are shown in Fig. 1 and Table 2.The Finger probe measurements underestimated the arterial saturation (SpO 2(pre-exercise) = − 0.7% and SpO 2(post-exercise) = − 3.3%).At post-exercise, the A RMS was 5.8% and we observed wide limits of agreement between measurement of SpO 2 and SaO 2 (95% limit of agreement: − 6-12%).
The earlobe and forehead both overestimated the arterial oxygen saturation (earlobe SpO 2(post-exercise) = 1.3 and forehead SpO 2(post-exercise) = 0.2%).At post-exercise, the A RMS was < 3% and the limit of agreement between measurement of SpO 2 and SaO 2 was narrower for the earlobe and forehead than that was seen for the finger probe (earlobe: 95% limits of agreement: -2.5% to 5.2% and forehead: 95% limits of agreement: -2.9% to 3.2%).

Measurement error of peripheral oxygen measurement
Measurement errors (> ± 4% difference between SpO 2 and SaO 2 values) of peripheral oxygen saturation are highlighted with a hollow circle in Fig. 1.At post-exercise, measurement errors were registered in 23%, 3%, and 1% of the patients by the finger, earlobe, and forehead probe, respectively.

Discussion
This study showed that measurement of SpO 2 using the finger sensor was inaccurate and underestimated the SaO 2 .Furthermore, we demonstrated that SpO   The poor accuracy of finger measurements of SpO 2 may be explained by Raynaud's phenomenon and the peripheral microvasculopathy in patients with SSc, which lead to inaccurate measurements of SaO 2 due to poor perfusion and hypothermia of the fingers.Indeed, measurement error by the finger probe was primary seen in patients with Raynaud's attack during the 6MWT and was associated with the use of vasodilator treatment.Several studies have shown impaired perfusion and reduced oxygen delivery in the digital arteries in patients with SSc compared to healthy controls [26][27][28][29].Furthermore, the accuracy of On the other hand, a recent study reported that blood perfusion of the skin in the face was not different at rest in patients with SSc compared to healthy individuals [14].Thus, our findings support that oximetry areas that are not affected by Raynaud's phenomenon, should be used during the 6MWT in patients with SSc.
Swigris et al. examined the accuracy of finger SpO 2 measurement and the prognostic value of desaturation in 83 SSc patients during a cardiopulmonary exercise test [11].While this study found that the finger SpO 2 overestimated the SaO 2 , the limits of agreement for the mean difference of finger SpO 2 and SaO 2 were wide at maximum exercise as in our study.Furthermore, the study also found that desaturation defined as SpO 2 below 89% or SpO 2 fall > 4 points during maximal exercise was associated with a higher mortality.Therefore, it is a crucial finding in our study that the measurement error of the finger probe in a significant proportion of patients had SpO 2 values being ± 4% different from SaO 2 .
Only one previous study has examined the re-test reliability of peripheral oxygen saturation at different anatomical sites during the 6MWT in patients with SSc.Compared to our study, there was only moderate agreement of forehead and finger SpO 2 measurements during two 6MWT while the agreement for the earlobe SpO 2 was poor [7].Still, this study was small (N = 25), and it was only possible to obtain reliable measurements using the earlobe probe in a minority of patients (n = 7).In our study, the post-exercise Borg dyspnoea score and the 6MWD were similar at both visits which is in line with other studies that also found good reproducibility of the 6MWD in patients with SSc [7,30,31].
The main strength of this study was the large number of prospectively recruited SSc patients and the fact that we were able to compare measurements of continuous SpO 2 at three different anatomical locations with the arterial saturation as the gold standard.Still, due to the study setup, we were only able to measure arterial oxygen saturation pre-and post-exercise (averagely 20 s post-exercise) and, therefore, the lowest arterial oxygenation may have occurred during the 6MWT.In addition, we were not able to measure oxygen saturation in all patients due to either technical error in collection of data from pulse oximeters or failure in performing or analyzing the blood gas analysis.Last, due to the dynamic performance of the 6MWT, we may not have visually registered all cases of Raynauds' attacks.
In our study, 43% of the patients had signs of ILD which is similar to the proportion of SSc patients with pulmonary fibrosis in the EUSTAR database [32].Furthermore, the post-exercise Borg dyspnoea scores were similar to other studies of the 6MWT in patients with SSc [7,30,31,33,34].Still, our results are based on only a single center using a specific pulse oximeter.Hence, the disease characteristics of the SSc may be different in other setting and our results may not be representative for other pulse oximetry sensors.

Conclusion
The present study showed a high accuracy for measuring the SpO 2 using the earlobe or forehead during the 6MWT in patients with SSc.Furthermore, we demonstrated that finger measurement of SpO 2 has poor validity and that measurement error of SpO 2 was associated with markers of peripheral vasculopathy in SSc.The preferred method for monitoring SpO 2 in clinical SSc-trials and in the monitoring of SSc patients should be at the forehead or the earlobe.

Fig. 1
Fig. 1 Bland-Altman plots showing the validity of measurement of peripheral oxygen saturation at the finger, earlobe, and forehead.Black circles: values with ≤ ± 4% difference between SpO 2 and SaO 2 values.Hollow circle: values with > ± 4% difference between SpO 2 and SaO 2 values.Solid line: Mean difference between SpO 2 and SaO 2

Latest high-resolution CT-scan, n (%)
IQI Interquartile interval, mRSS modified Rodnan Skin Score, ACA Anti-Centromere Antibody, Anti-SCL-70 Anti-Topoisomerase I, SHAQ Scleroderma Health Assessment Questionnaire, FEV1 Forced Expiratory Volume in 1 s, FVC Forced Vital Capacity, DLCO Diffusing Capacity of Carbon Monoxide, ILD Interstitial lung disease, NYHA New York Heart Association, RCS Raynaud's Condition Score, COPD Chronic obstructive pulmonary disease and SSc Systemic sclerosis