Our study reinforce the value of using SpO2 and FiO2 as an noninvasive diagnostic tool for ARDS, We also set up a new diagnosis process base on SpO2 and FiO2, when a patients suspicious with ARDS, the SpO2 ≤ 97%, calculate SFR according to the formula: SFR = SpO2/FiO2, if SFR ≤ 352, ARDS may exist; when SpO2 > 97%, if high concentration inhaled oxygen is required, ie FiO2min > 39%, ARDS may exist. Meanwhile, We verified the accuracy of the model, The model has a very high sensitivity and specificity. Our finding could to enable clinicians to recognize ARDS timely and accurately, especially in developing countries which lacking sufficient medical resources. However, saturation-based measurements are easily available
in relatively resource-poor settings.
The establishment of the Berlin definition in 2012 made the diagnosis and classification of ARDS more accurate and standardized. However, repeated blood draws in the ICU environment may lead to iatrogenic anemia, the need for blood transfusions, an increased risk of infection, and an increase in hospital costs. In addition, as mentioned earlier, many underdeveloped regions and countries may lack the conditions for arterial blood gas. Therefore, a large number of recent studies[5, 9, 10] have confirmed that SFR can be used to evaluate ARDS patients. For example, in 2015 Kigali University Teaching Hospital proposed that if arterial blood gas and chest X-ray are difficult to achieve, SpO2/FiO2 ratio and lung ultrasound Can be used as a substitute for diagnosis of ARDS.
This study confirmed that there is a strong correlation between SFR and PFR, and it is statistically significant (r = 0.873, P < 0.0001),which means that this non-invasive methods can be used to estimate PaO2 and oxygenation index in clinical work. This is similar to the results of Katherine DesPrez, BA. They proposed that SFR can be used as a substitute for PFR and provide noninvasively prognostic information and assess the severity of ARDS. The linear regression equation of the relationship between SFR and PFR calculated in this study is PFR = 0.9162*SFR-21.39, which has a higher correlation coefficient, which shows that the formula in this study may be more in line with the linear relationship between SFR and PFR. The oxygenation index calculated by this formula may be more accurate. When SpO2 > 97%, statistical analysis shows that FiO2 is significantly correlated with PFR((R= -0.521,P < 0.0001)). In clinical applications,if the blood oxygen saturation is within the normal range due to the excessively high inspired oxygen concentration ,there maybe ARDS too.
In the oxygen dissociation curve, it can be seen that the curve drawn by the arterial blood oxygen saturation with the change of the arterial blood oxygen partial pressure is divided into three sections: upper, middle and lower. In the middle and lower part of the curve, the curve is steep, and SaO2 changes greatly with PaO2, which is conducive to hemoglobin releasing oxygen for tissue utilization. Therefore, separating the data with SpO2 ≤ 96% for linear regression, the correlation between the two is higher than before. In the upper part, the curve is flat, and SaO2 changes little with PaO2. Therefore, when SpO2 > 97%, the PFR calculated by the linear formula is not accurate. Through statistical analysis, there is a significant correlation between FiO2 and PFR at this time. when FiO2 more than 39%. The sensitivity of using FiO2 to diagnose ARDS is high, which is conducive to the early warning of ARDS. In the early research of Rice TW et al., they also explored the relationship between SpO2 and PaO2 and concluded that it is similar to this experiment, but they only analyzed the data of SpO2 ≤ 97%. They pointed out in the discussion that in the oxygen dissociation curve, the slope of SpO2 and PaO2 in the saturated state is almost zero, and a large change in PaO2 may cause little or no change in SpO2. So it excludes patients' data with high oxygen saturation. However, in clinical work, patients who rely on high concentration oxygen support,even though the monitor indicates that the blood oxygen saturation is normal, they may already have ARDS. In this case, if they are not detected and treated early, which may cause illness deterioration. In our research, if the minimum FiO2 (SpO2 > 97%) is greater than 39%, the patient may have early ARDS. The results of this study are of great significance to avoid ignoring the occurrence and development of ARDS. On the basis of the research results, a flow chart was drawn for the rapid diagnosis tool of high-risk patients (Fig. 3). Taking Berlin's definition as the gold standard for diagnosis, the diagnosis of the new diagnostic process has high sensitivity and specificity, indicating that the new diagnostic tool is suitable for early recognition of ARDS. However, a large sample of multiple centers is still needed for further verification.
However, it is worth mentioning that not all patients are suitable for condition assessment using this method. For example, in the case of carbon monoxide poisoning, peripheral circulatory failure, etc., it may not be possible to use finger pulse oxygen to monitor blood oxygen saturation, and blood gas analysis is the only way to rule out this disease.Transcutaneous blood oxygen saturation technology transmits two wavelengths of light (usually 660mm and 940mm) through the tissue to measure the absorbance change of each wavelength over time. The absorption of light by the tissue is cyclic, which is due to the circulation of the heart and the resulting pulsation of arterial blood into the tissue bed. However, COHb and O2Hb have similar absorption characteristics at 660mm. Therefore, the percutaneous oxygen saturation (SpO2) of patients with carb monoxide and peripheral circulatory failure must not be an accurate measurement of O2Hb. A recent observational study also confirmed that in patients using VV-ECMO, transcutaneous oxygen saturation is a poor oxygenation test. Due to the increase in COHb levels, as the VV-ECMO support time increases, Its reliability is reduced.This situation can also be seen in long-term smokers. Early studies have confirmed that the COHb content in the blood of long-term smokers will increase by 15%. Recent study has shown that carboxyhemoglobin saturation (SpCO) baseline levels are significantly higher in long-term smokers than in nonsmokers.Under these above circumstances, only blood gas analysis and percutaneous carbon monoxide analyzer can judge the patient's true oxygenation situation comprehensively.
There are some shortcomings in this study. First of all, it is a single-center observational study with a small sample size, so the new diagnostic tool needs to be verified by a multi-center large-sample study. In addition, we explored the linear relationship between SFR and PFR without considering the influence of other factors on the oxygen dissociation curve, such as pH, CO2, etc..This is because the purpose of this study is to explore the general relationship between the two, so that diagnostic tools can be used even when these confounders are unknown. Similarly, we did not conduct separate studies on different ventilation modes, such as ventilation mode and PEEF,etc.. Finally, this study only verified the efficacy of this new diagnostic tool, and did not explore its correlation with the prognosis of ARDS patients, which could be explored in later studies.