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. 2021 Jan 25;16(1):e0245927. doi: 10.1371/journal.pone.0245927

Utility of SpO2/FiO2 ratio for acute hypoxemic respiratory failure with bilateral opacities in the ICU

Yosuke Fukuda 1,*, Akihiko Tanaka 1, Tetsuya Homma 1, Keisuke Kaneko 1, Tomoki Uno 1, Akiko Fujiwara 1, Yoshitaka Uchida 1, Shintaro Suzuki 1, Toru Kotani 2, Hironori Sagara 1
Editor: Aleksandar R Zivkovic3
PMCID: PMC7833145  PMID: 33493229

Abstract

Acute hypoxemic respiratory failure (AHRF) with bilateral opacities causes fatalities in the intensive care unit (ICU). It is often difficult to identify the causes of AHRF at the time of admission. The SpO2 to FiO2 (S/F) ratio has been recently used as a non-invasive and alternative marker of the PaO2/FiO2 (P/F) ratio in acute respiratory failure. This retrospective cohort study was conducted from October 2010 to March 2019 at the Showa University Hospital, Tokyo, Japan. We enrolled 94 AHRF patients who had bilateral opacities and received mechanical ventilation in ICU to investigate their prognostic markers including S/F ratio. Significant differences were observed for APACHE II, S/F ratio, PaO2/FiO2 (P/F) ratio, and ventilator−free-days at day 28 for ICU mortality, and for age, S/F ratio, P/F ratio, duration of mechanical ventilation, and ventilator−free days at day 28 for hospital mortality. Multivariate logistic regression analysis showed that the S/F ratio was significantly and independently associated with the risk of death during in ICU (p = 0.003) and hospitalization (p = 0.002). The area under the receiver operating characteristic curves (AUC) based on the S/F ratio were significantly greater than those based on simplified acute physiology score (SAPS) II and sequential organ failure assessment (SOFA) for ICU mortality (0.785 in S/F ratio vs. 0.575 in SAPS II, p = 0.012; 0.785 in S/F ratio vs 0.594 in SOFA, p = 0.021) and for hospital mortality (0.701 in S/F ratio vs. 0.502 in SAPS II, p = 0.012; 0.701 in S/F ratio vs. 0.503 in SOFA, p = 0.005). In the subanalysis for bacterial pneumonia and interstitial lung disease groups, the AUC based on the S/F ratio was the greatest among all prognostic markers, including APACHE II, SAPS II, and SOFA. The S/F ratio may be a useful and noninvasive predictive prognostic marker for acute hypoxemic respiratory failure with bilateral opacities in the ICU.

Introduction

Acute hypoxemic respiratory failure (AHRF) documented by the presence of bilateral opacities on X-ray or computer tomography (CT) is a life-threatening condition in the intensive care unit (ICU) [1]. Acute respiratory distress syndrome (ARDS) is a representative condition of AHRF with bilateral opacities that accounts for almost 10% of all ICU admissions and 23% of cases with mechanical ventilation [2]. However, it is often challenging to differentiate ARDS from non-ARDS on admission despite medical history interviews, physical findings, and examinations. In the LUNG SAFE study [2], a multinational prospective cohort study of ARDS, only 60% of ARDS cases were detected by clinicians [2]. They reported that only 34% of ARDS patients who met the Berlin ARDS criteria on admission were identified [2]. Although it is often difficult to define the patient’s prognosis owing to the challenge in recognizing and making a differential diagnosis of ARDS on admission, it is crucial to identify prognostic predictors to develop a treatment strategy in patients with the mechanically ventilated AHRF.

We have some tools for the prediction of the patient clinical outcomes with AHRF in the ICU [3]. The Acute Physiology and Chronic Health Evaluation (APACHE) II score was developed in 1985 and constitutes a scoring system that is used to evaluate the severity of illness in the ICU [4], and it is significantly associated with mortality in patients with acute lung injury/ARDS patients [5]. Kao et al. reported that APACHE II is a useful tool for predicting the 60-day mortality in cases of influenza pneumonia-related ARDS [6]. Similarly, the simplified acute physiology score (SAPS) II and sequential organ failure assessment (SOFA)—developed to assess organ failure in the intensive care unit [3]—have also been reported to be useful prognostic indicators in acute respiratory failure [7, 8]. Moreover, pulse oximetry saturation is continually monitored in the ICU as a noninvasive tool for assessing patients' respiratory status. The PaO2/FiO2 (P/F) ratio that defines the severity of ARDS according to the Berlin criteria [9] has an established correlation with the SpO2/FiO2 (S/F) ratio [10, 11]. It was suggested that the S/F ratio may be useful as an alternative marker for the P/F ratio in ARDS [7, 12]. However, the indicators that are useful concerning in-hospital and ICU deaths have been debated. Specifically, now that coronavirus disease-2019 (COVID-19) is rampant worldwide, a simple index that can be monitored continually is needed, even in clinical settings that are resource-limited [13].

This study aimed to investigate which indicators would be useful for patients with the mechanically ventilated AHRF with bilateral opacities in the ICU.

Materials and methods

This retrospective cohort study was conducted from October 2010 to March 2019 at Showa University Hospital, Tokyo, Japan. The eligibility criteria included age ≥ 18 years, use of mechanical ventilation (positive end-expiratory pressure (PEEP) exceeding 5 cmH2O), bilateral opacities on chest X-ray or CT on ICU admission, and an examination of arterial blood gas analysis 24 h after admission. Bilateral opacities on chest X-ray or CT were independently judged by two respiratory specialists according to the Berlin definition of ARDS [9]. When there was disagreement between the specialists on the decision, they discussed it and reached a consensus. Patient characteristics and data, including age, sex, body mass index (BMI), smoking history, length of stay, duration of mechanical ventilation, vital signs, fluid intake/output balance, chest radiographic findings, blood examination findings, and comorbidities, were obtained from the medical records of the patients. Additionally, we evaluated the severity of disease using APACHE II, SAPS II, SOFA, the P/F ratio, and S/F ratio at 24 h after ICU admission [14]. We followed up the patients up to 60 days after admission. The settings of mechanical ventilation were adjusted the FiO2 to maintain SpO2 at 90–95% and PEEP exceeding 5cmH2O according to ARDSNet mechanical ventilation strategy [15] by each physician in charge.

All data were not fully anonymized before we accessed them. The date range during which patients’ medical records/samples were assessed was April 2019 to December 2019. After publishing a notice that stating that the research would be based on the patient's clinical information on the website of the Showa University Ethics Committee, we obtained informed consent in the form of opt-out. The Showa University Ethics Committee approved this study and the opt-out consent mechanism (approval number: 2795). The study complied with the principles of the Declaration of Helsinki.

Statistical analysis was performed using JMP (Version15, SAS Institute, Minato-ku, Tokyo, Japan). All data are presented as median (range), or number (percentage), as required. Differences between categorical variables were analyzed using the Fisher's exact test with 95% confidential interval (CI) statistics and continuous variables were analyzed using the Mann–Whitney U test. Receiver operating characteristic (ROC) curves were plotted to estimate the cutoff values for diagnosis and we conducted the likelihood ratio test for goodness of fit. P values less than 0.05 were considered statistically significant.

Results

A total of 572 patients admitted to ICU were screened (Fig 1). According to the exclusion criteria, we excluded the following patients: 224 patients who did not receive invasive mechanical ventilation, 168 patients who did not have bilateral opacities on X-ray or CT, 50 patients who did not have P/F ratio results at 24 h post-admission, eight patients who died within 24 h of admission to the ICU, two patients who had acute heart failure, and two patients who had an unknown outcome. We included only the first admission to the ICU of the same patient during the study period. Finally, 94 patients were enrolled in this study (Fig 1).

Fig 1. The study flow diagram in this study.

Fig 1

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572 patients in intensive care unit (ICU) were screened. After excluding the patients who met the exclusion criteria, 94 patients were enrolled in current study.

Patient demographic information was presented in Table 1. The median patient age was 72 (62–80) years, and 75.5% were male. The mean APACHE II, SAPS II, and SOFA scores were 29 (24–35.8), 62 (52.3–75), and 14 (11.3–15), respectively. The median S/F and P/F ratios were 192 (152–245), and 180 (121–236). The S/F ratio and the P/F ratio were significantly correlated (Fig 2. S/F ratio = 60.9 + 0.75 × P/F ratio (p < 0.001, r = 0.87)). The disease profile of the 94 patients was as follows: bacterial pneumonia accounted for 38 patients, interstitial lung diseases (ILD) for 36, viral or pneumocystis pneumonia for nine, aspiration pneumonia for four, and others for seven. All patients with bacterial pneumonia, viral pneumonia, and pneumocystis pneumonia were septic.

Table 1. Clinical characteristics in total patients.

Total patients
n = 94
Age, year 72 (62–80)
Sex, male/female (% of male) 71 / 23 (75.5)
BMI, kg/m2 22.2 (19.6–24.4)
APACHE II 29 (24–35.7)
SAPS II 62 (52.3–75)
SOFA 14 (11.2–15)
S/F ratio 192 (152–245)
P/F ratio 179 (121–236)
Use of systemic corticosteroids, No. (%) 71 (75.6)
Use of sivelestat sodium, No. (%) 29 (30.9)
Use of vasopressor, No. (%) 53 (56.4)
Length of stay in hospital, days 31 (19.2–58.7)
Length of stay in ICU, days 10 (6.3–21.5)
Duration of mechanical ventilation, days 9 (5–24.5)
Ventilator—free days at day 28, days 15.5 (0–22.2)
Comorbidities, No. (%)
Hypertension 45 (47.9)
COPD 28 (29.8)
Malignancy 27 (28.7)
Diabetes mellitus 25 (26.6)
Coronary artery disease 16 (17.1)
Chronic kidney disease 9 (9.6)
Autoimmune disease 9 (9.6)
Diagnosis, No. (%)
Bacterial pneumonia 38 (40.4)
Interstitial lung disease 36 (38.2)
Viral/Pneumocystis pneumonia 9 (9.5)
Aspiration pneumonia 4 (4.2)
Others 7 (10.6)
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Data are presented as median (range), or number (percentage). APACHE acute physiology and chronic health evaluation, BMI body mass index, COPD chronic obstructive pulmonary disease, ICU intensive care unit, SAPS simplified acute physiology score, SOFA sequential organ failure assessment.

Fig 2. The relationship between the SpO2/FiO2 (S/F) and PaO2/FiO2 (P/F) ratios.

Fig 2

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S/F ratio showed significant linear correlation. S/F ratio = 60.9 + 0.75 × P/F ratio (p < 0.001; r = 0.87).

Table 2 showed differences between survivors and nonsurvivors in the ICU and hospital. Significant differences were observed for APACHE II, S/F ratio, P/F ratio, and ventilator−free-days at day 28 for ICU mortality (p = 0.027, p < 0.001, p < 0.001, p < 0.001, respectively), and for age, S/F ratio, P/F ratio, length of mechanical ventilation, and ventilator−free days at day 28 for hospital mortality (p = 0.002, p = 0.001, p = 0.002, p < 0.001, p < 0.001, respectively).

Table 2. Clinical characteristics between survivor or non-survivor in the hospital and ICU.

ICU Hospital
Survivor Nonsurvivor p value Survivor Nonsurvivor p value
(n = 75) (n = 19) (n = 58) (n = 36)
Age, year 71 (60–79.5) 76 (66–81.5) 0.256 68 (55.5–79) 76 (68–84) 0.002
Sex, male/female (% of male) 56 / 19 (74.6) 15 / 4 (78.9) 1.000 42 / 16 (72.4) 29 / 7 (80.6) 0.463
BMI, kg/m2 22.6 (20.0–24.7) 21.5 (19.4–22.8) 0.364 22.9 (20.5–25.3) 21.5 (18.9–23.0) 0.084
APACHE II 28 (24–34) 36 (25.5–40.5) 0.027 27.5 (24–34) 32 (25.5–38.3) 0.109
SAPS II 60 (51.5–74.5) 66 (56–77.5) 0.311 60.5 (51.5–76) 64.5 (52.7–74.2) 0.969
SOFA 13 (11–15) 14 (12.5–15) 0.206 13 (11.2–15) 14 (11.7–15) 0.972
S/F ratio 200 (163.3–250) 132.8 (114–189) <0.001 200 (165.5–250) 162.5 (117.2–303.3) 0.001
P/F ratio 193.2 (136.2–249) 119.2 (104.9–162.4) <0.001 192.4 (140.4–252.4) 138.1 (105.8–202.1) 0.002
Use of systemic corticosteroids, No. (%) 54 (72) 17 (89.4) 0.143 41 (70.7) 30 (83.4) 0.219
Use of sivelestat sodium, No. (%) 24 (32) 5 (26.3) 0.251 15 (29.5) 14 (32.6) 0.251
Use of vasopressor, No. (%) 42 (56) 11 (57.8) 1.000 33 (56.9) 20 (55.6) 1.000
Duration of mechanical ventilation, days 8 (5–25.5) 13 (7–24) 0.398 6.5 (4–17) 17 (11–36) <0.001
Ventilator—free days at day 28, days 20 (0–23) 0 (0–11) <0.001 21.5 (11–24) 0 (0–13.7) <0.001
Comorbidities, No. (%)
Hypertension 36 (48) 9 (47.3) 1.000 29 (50) 16 (44.5) 0.673
COPD 23 (30.6) 5 (26.3) 0.786 19 (32.7) 9 (25) 0.492
Malignancy 21(28) 6 (31.5) 0.781 14 (24.2) 13 (36.2) 0.246
Diabetes mellitus 22 (29.3) 3 (15.7) 0.383 17 (30.4) 8 (22.3) 0.483
Coronary artery disease 14 (18.6) 2 (10.5) 0.779 9 (15.6) 7 (19.5) 0.779
Chronic kidney disease 6 (8) 3 (15.7) 0.380 5 (8.7) 4 (11.1) 0.728
Autoimmune disease 8 (10.6) 1 (5.2) 0.681 7 (12.1) 2 (5.6) 0.475
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Data are presented as median (range), or number (percentage). APACHE acute physiology and chronic health evaluation, BMI body mass index, COPD chronic obstructive pulmonary disease, ICU intensive care unit, SAPS simplified acute physiology score, SOFA sequential organ failure assessment

We used multivariate logistic regression models to determine the risk factors associated with death in the ICU and hospital (Table 3). While only the S/F ratio was independently associated with mortality in the ICU (p = 0.002), age and the S/F ratio were significant variables associated with mortality in the hospital (p = 0.001, p = 0.002, respectively).

Table 3. Multivariate logistic regression analysis for risk of death in the hospital and ICU.

ICU Hospital
OR 95% CI p value OR 95% CI p value
Age 1.020 0.975–1.080 0.337 1.070 1.030–1.120 0.001
S/F ratio 0.982 0.970–0.994 0.002 0.987 0.978–0.995 0.002
APACHE II 1.040 0.968–1.120 0.284 1.010 0.948–1.080 0.762
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APACHE acute physiology and chronic health evaluation, ICU intensive care unit

The area under the ROC curves (AUCs) for hospital mortality are presented in Fig 3. The AUC of the S/F ratio was 0.785 (95% CI 0.67–0.899, p = 0.002) for ICU mortality and 0.701 (95% CI 0.59–0.813, p = 0.002) for hospital mortality. The AUC of the S/F ratio was significantly greater than that of SAPS II and the SOFA for ICU mortality (S/F ratio vs. SAPS II, p = 0.012; S/F ratio vs. SOFA, p = 0.021) and hospital mortality (S/F ratio vs SAPS II, p = 0.012; S/F ratio vs. SOFA, p = 0.005). Subgroup analysis of patients with bacterial pneumonia or ILD also showed that the S/F ratio had the highest AUC among the four prognostic factors (Fig 4A–4D: bacterial pneumonia, 0.971 (95% CI 0.907–1.000, p = 0.001) for ICU mortality; Fig 4A: 0.700 (95% CI 0.493–0.908, p = 0.046) for hospital mortality; Fig 4B: interstitial lung disease, 0.697 (95% CI 0.520–0.875, p = 0.113) for ICU mortality; Fig 4C: 0.720 (95% CI 0.539–0.902, p = 0.018) for hospital mortality; Fig 4D).

Fig 3. Receiver Operating Characteristic (ROC) curves for ICU mortality and hospital mortality.

Fig 3

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AUC for ICU mortality was 0.784 (A, 95%CI 0.648–0.877) and for hospital mortality was 0.701 (B, 95% confidence interval (CI) 0.579–0.800) using S/F ratio. The AUCs based on the S/F ratio were significantly greater than those based on simplified acute physiology score (SAPS) II and sequential organ failure assessment (SOFA) for ICU mortality (0.785 in S/F ratio vs. 0.575 in SAPS II, p = 0.012; 0.785 in S/F ratio vs 0.594 in SOFA, p = 0.021) and for hospital mortality (0.701 in S/F ratio vs. 0.502 in SAPS II, p = 0.012; 0.701 in S/F ratio vs. 0.503 in SOFA, p = 0.005). Both the cutoff for hospital mortality and the cutoff for ICU mortality were 147.69.

Fig 4. ROC curves for ICU and hospital mortality in bacterial pneumonia and interstitial lung diseases group.

Fig 4

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In the bacterial pneumonia group, the AUC for ICU mortality was 0.971 (A, 95% CI 0.907–1.000) and for hospital mortality was 0.700 (B, 95% CI 0.493–0.908) using the S/F ratio. In interstitial lung diseases group, AUC for ICU mortality was 0.697 (C, 95% CI 0.520–0.875) and for hospital mortality was 0.720 (D, 95% CI 0.539–0.902) using the S/F ratio.

Discussion

Our findings underscored that S/F ratio was easy to obtain a superior prognostic factor for mechanically ventilated AHRF with bilateral opacities in the ICU to APACHE II, SAPS II, and SOFA.

We showed that the S/F ratio was significantly higher for survivors compared with nonsurvivors in the hospital and ICU. In the pediatric ICU, the S/F ratio was considered a useful and noninvasive marker because of difficulty with daily blood gas sampling to calculate the P/F ratio. Wong et al. enrolled 70 children with ARDS and recorded some parameters on days 1, 3, and 7 [16]. They concluded that a low S/F ratio on the day of diagnosis was associated with the number of ventilator-free days and 28 days free of ICU admission. Another report showed that the S/F ratio was a readily available marker for detecting a high risk of death in pediatric patients with acute hypoxemic failure [17]. Conversely, data on whether the S/F ratio predicted mortality in adult patients was limited. Bass et al. focused on the usefulness of combining the S/F ratio and lung ultrasound (LUS) for AHRF patients, particularly ARDS [18]. They reported that the combination of S/F ratio and LUS identified ARDS with a sensitivity of 91% and a specificity of 48% [18]. A study in Rwanda, one of the developing countries with limited health care resources, found that the Kigali modification of the Berlin definition, including the use of the S/F ratio as an alternative to the P/F ratio, was useful for the prediction of the prognosis of ARDS patients [19]. Sendagire et al. also showed that the modified SOFA score using the S/F ratio instead of the P/F ratio was useful for the prediction of ICU mortality in a resource-limited setting (e.g., settings in which blood gas analysis data were lacking) [20]. Therefore, we considered that the S/F ratio would be useful markers to predict the mortality of AHRF with bilateral opacities in ICU.

We inferred that the S/F ratio was comparable to other indices that predict the prognosis of AHRF with bilateral opacities. In clinical practice, it is difficult to distinguish between ARDS and acute exacerbation of interstitial lung disease in patients with respiratory failure immediately after admission to the ICU, therefore it is useful to find prognostic factors for AHRF. The S/F ratio is a single indicator, and few studies directly compared its usefulness to the combined indices of APACHE II, SAPS II, and SOFA. In a multicenter cohort study of adult ARDS patients in the United States, both lower S/F ratio and lower APACHE II scores were related to early intubation in the ICU [21]. In another report on ARDS, saturation-based predictors, including the S/F ratio, were independently associated with clinical outcomes, whereas the P/F ratio was not associated with these outcomes [22]. One explanation for this finding is attributed to the fact that saturation-based markers are more sensitive and are continually available for the prediction of hospital mortality owing to ARDS compared with blood sample-based markers. APACHE II, SAPS II, and SOFA are extremely useful in the ICU, but we believe that the S/F ratio is more intuitive and straightforward than other indices.

We showed the usefulness of the S/F ratio as a predictor of mortality in AHRF patients with bilateral opacities in bacterial pneumonia. Interestingly, while the S/F ratio exhibited excellent accuracy in predicting ICU mortality in bacterial pneumonia, it yielded a fair prediction accuracy of hospital mortality. A previous report showed that the lowest S/F ratio tertile (S/F ratio < 164) at ICU admission was associated with hospital mortality compared with the highest S/F ratio tertile (S/F ratio > 236) in the instances in which patients with severe sepsis and septic shock in the ICU were separated into three groups according to the S/F ratio [23]. Our results suggest that the S/F ratio may be a powerful prognostic indicator for both hospital mortality and ICU mortality in bacterial infection-based AHRF with bilateral opacities. In the interstitial pneumonia subgroup, the S/F ratio was a relatively useful measure compared with other measures with low accuracies for hospital and ICU mortality. While low P/F ratio, high positive end-expiratory pressure, age, and low APACHE III score values were associated with hospital mortality in ILD patients that required mechanical ventilation [24], there were no reports that examined the usefulness of the S/F ratio in ILD for hospital and ICU mortality. Our finding was new, but we thought that the S/F ratio's usefulness needs further study because of the poor–fair accuracy.

Our study is associated with several limitations. Firstly, this study was a retrospective cohort study. Because all data were obtained from medical records, selection bias was inherent. Secondly, the number of patients included in this study was not enough because it was a single-center study. We considered that further research is needed to answer the clinical question of whether the S/F ratio is prognostically useful in acute hypoxemic respiratory failure. Thirdly, this study included patients with SpO2 > 97%. The reason for this is that some patients remained on liberal oxygenation therapy with high SpO2 control, although we aimed to implement conservative therapy with a target of 90–95% SpO2 as a mechanical ventilator management [25]. It is known that the oxyhemoglobin dissociation curve is flat in situations where SpO2 >97%, by contrast, Kwack et al. reported that the S/F ratio in patients with SpO2 > 97% was inconsistently useful for predicting acute deterioration [26]. Fourthly, we found no evidence that indicated that the S/F ratio at ICU admission is useful. We must be cautious in our interpretation of SpO2, as FiO2 is often set high immediately after the start of mechanical ventilator management. Future research is needed to evaluate the S/F ratio's usefulness in earlier phases in the ICU.

Conclusions

In conclusion, our findings suggest that the S/F ratio may be useful for assessing the impact on clinical outcomes of the mechanically ventilated AHRF with bilateral opacities. When patients were admitted to the ICU, insufficient information is available. Some formulas exist and are used to predict clinical outcomes for patients, but most of them require scoring methods. Because the S/F ratio does not require complicated calculations, we believe the S/F ratio is simple, can be monitored, and useful for predicting any clinical outcomes in acute hypoxemia.

Supporting information

S1 Table. Database of patients characteristics.

(XLSX)

Click here for additional data file. (27KB, xlsx)

Acknowledgments

We gratefully thank the intensive care unit's medical staff at Showa University Hospital for assistance and contributions to our study. We thank enago (www.enago.jp) for English language editing.

Data Availability

All relevant data are within the manuscript and its Supporting Information files.

Funding Statement

The authors received no specific funding for this work.

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Decision Letter 0

Aleksandar R Zivkovic

21 Dec 2020

PONE-D-20-35947

Utility of SpO2/FiO2 ratio for acute hypoxemic respiratory failure with bilateral opacities in the ICU

PLOS ONE

Dear Dr. Yosuke,

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PLOS ONE

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Reviewer #1: The study assessed the SpO2/FiO2 ratio in ARDS.

Although the topic remains of interest, the manuscript should be highly modified.

First, the authors included only 94 patients within 10 years. The number of patients is very low and the study could not respond to the question.

Second, one third of patients had interstitail lung disease. In that setting diagnosis of ARDS is very difficult and most of those patients does not have ARDS.

3- the multivariate analysis is very strange including the same variable several time (APACHE2, SOFA and SAPS are related variables).

4-comparison of AUC were not adequat : the authors should use likelihood ratio test.

5- the authors demonstrated that P/F ratio and S/F ratio are correlate. However, P/F ratio is a well known prognosis factor during ARDS. However, the authors wrote that prognostic factors for these patients that are avalaible within the first 24 hours of admission have not been fully studied (in the abstract).

Reviewer #2: My main concern is about the novelty of the results presented. Indeed, it has been previously shown, in a larger population of ARDS patients that spo2/fiO2 predicts outcomes (PMID: 26271028). Therefore, why the authors think that their results are innovative and merits publication? This is an issue that should be clearly stated in the manuscript. In other words, it is not clear what these results add to the current knowledge.

The part regarding the patients included should be moved to the results section (page 7, last paragraph).

It is unicentric and retrospective with low sample size.

Tables and Figures are included in the middle of the results section and makes the reading difficult to follow.

In the first sentence of the discussion the authors said that “Our findings underscored that S/F ratio was an easy to obtain a superior prognostic 216 factor for AHRF with bilateral opacities in the ICU.”. I would suggest saying “mehcanically ventilated patients with ARDS”. All patients included have bilateral infiltrates, were mechanically ventilated and have PF ratio <300.

**********

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PLoS One. 2021 Jan 25;16(1):e0245927. doi: 10.1371/journal.pone.0245927.r002

Author response to Decision Letter 0

3 Jan 2021

Dear Reviewers

We noted our comments to the reviewer.

Reviewer #1

(Q1) The study assessed the SpO2/FiO2 ratio in ARDS. Although the topic remains of interest, the manuscript should be highly modified. First, the authors included only 94 patients within 10 years. The number of patients is very low and the study could not respond to the question.

(A1) As you point out, this is a single-center, retrospective study, and we agreed that the number of patients was insufficient to answer the clinical question. We added this fact in the manuscript as one of the limitations (Page 20, Line 296−299).

(Q2) Second, one third of patients had interstitail lung disease. In that setting diagnosis of ARDS is very difficult and most of those patients does not have ARDS.

(A2) We agree with your opinion. In this study, the target condition was an acute hypoxemic respiratory failure (AHRF) that was the heterogenous population including acute exacerbations in ILD and ARDS. Because it is difficult to make a definite diagnosis of ARDS at the time of admission to the ICU (PMID: 26903337), we focued on patients with AHRF. We believe that it is clinically useful to determine the prognostic markers for AHRF and this is one novel point in this study. We described this point in the introduction of original manuscript (Page 4 Line 67−Page 5 Line 72). In addition, we added a sentence in the discussion to emphasize this point (Page 18, Line 263−266).

(Q3) The multivariate analysis is very strange including the same variable several time (APACHE2, SOFA and SAPS are related variables).

(A3) Thank you for appropriate suggestion. We also thought that the types and number of factors used in the multivariate logistic regression analysis were extraordinary, so we reanalyzed the results by including three factors in Table 3: age, S/F ratio, and APACHE II (Page 14 Line 196−204).

(Q4) Comparison of AUC were not adequat : the authors should use likelihood ratio test.

(A4) We agree with your suggestion. We conducted a likelihood ratio test. The goodness of fit for each model was as follows: The S/F ratio for ICU mortality in all patients, p = 0.002; The S/F ratio for hospital mortality in all patients, p = 0.002; The S/F ratio for ICU mortality in patients with bacterial pneumonia, p = 0.001; The S/F ratio for hospital mortality in patients with bacterial pneumonia, p = 0.046; The S/F ratio for ICU mortality in patients with interstitial lung disease, p = 0.113; The S/F ratio for hospital mortality in patients with interstitial lung disease, p = 0.018 (Page 14 Line 206−Page 15 Line 217).

(Q5) The authors demonstrated that P/F ratio and S/F ratio are correlate. However, P/F ratio is a well known prognosis factor during ARDS. However, the authors wrote that prognostic factors for these patients that are avalaible within the first 24 hours of admission have not been fully studied (in the abstract).

(A5) Thank you for youre precise suggestion. We rewrote the sentences in the intoroduction section of the abstract that you pointed out. Moreover, We added a description in the methods section of the abstract (Page 2, Line 31−33; Page 3, Line 37).

Reviewer #2:

(Q1) My main concern is about the novelty of the results presented. Indeed, it has been previously shown, in a larger population of ARDS patients that spo2/fiO2 predicts outcomes (PMID: 26271028). Therefore, why the authors think that their results are innovative and merits publication? This is an issue that should be clearly stated in the manuscript. In other words, it is not clear what these results add to the current knowledge.

(A1) Thank you for the appropriate suggestion and the article you presented. We believe there are two novelties in our study. First, our study populations are slightly different from previous studies because we included a group of patients with acute exacerbation of interstitial lung disease and ARDS as diseases causing acute hypoxic respiratory failure. Because it is difficult to make a definite diagnosis of ARDS at the time of admission to the ICU (PMID: 26903337), we focued on patients with AHRF. We believe that it is clinically useful to determine the prognostic markers for AHRF and this is one novel point in this study. Second, not many direct comparisons of the S/F ratio with multiple scoring systems (APACHE II, SAPS, SOFA), which proved useful as prognostic factors, have been performed. By conducting this comparison, we believe that we highlighted the simplicity and usefulness of the S/F ratio. However, we thought it was not easy to grasp the study's novelty in the current text as you pointed out. We reworded the first sentence of the discussion section and conclusion section (Page 16, Line 240−241; Page 21, Line 313).

(Q2) The part regarding the patients included should be moved to the results section (page 7, last paragraph).

(A2) Thank you for your valuable feedback. As per your suggestion, we moved the relevant part to the result section (Page 9, Line 148−Page10, Line 159) .

(Q3) It is unicentric and retrospective with low sample size.

(A3) We appreciate reviewer’s comments. As you mentiond, this retrospective study was conducted in a single center, and we thought that the number of patients was insufficient to answer the clinical questions. We added this point to the manuscript as one of the limitations (Page 20, Line 296−299).

(Q4) Tables and Figures are included in the middle of the results section and makes the reading difficult to follow.

(A4) Thank you for your suggestion. We inserted the Tables in the middle of the manuscript in accordance with the following PLOS ONE submission rules ”Place each table in your manuscript file directly after the paragraph in which it is first cited (read order). Do not submit tables in a separate file(s).” We also contacted the PLOS ONE office and were instructed to submit the manuscript in its current format. We apologize for any inconvenience this may cause and thank you for your cooperation.

(Q5) In the first sentence of the discussion the authors said that “Our findings underscored that S/F ratio was an easy to obtain a superior prognostic factor for AHRF with bilateral opacities in the ICU.”. I would suggest saying “mehcanically ventilated patients with ARDS”. All patients included have bilateral infiltrates, were mechanically ventilated and have PF ratio <300.

(A5) Thank you for your valuable feedback. While our study included ARDS patients, patients with acute exacerbation of interstitial lung disease were also selected. We did not consider these patients as ARDS. Moreover, we thought that it was difficult to determine whether a patient had ARDS or not at the time of ICU admission (PMID: 26903337). Therefore, we defined the patients who had bilateral infiltrates, were mechanically ventilated, and had a P/F ratio <300 as "acute hypoxemic respiratory failure." In light of this, we revised the sentences in discussion section you pointed out as follows: Our findings underscored that S/F ratio was easy to obtain a superior prognostic factor for mechanically ventilated AHRF with bilateral opacities in the ICU to APACHE II, SAPS II, and SOFA (Page 16, Line 240−241).

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Decision Letter 1

Aleksandar R Zivkovic

11 Jan 2021

Utility of SpO2/FiO2 ratio for acute hypoxemic respiratory failure with bilateral opacities in the ICU

PONE-D-20-35947R1

Dear Dr. Yosuke,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

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Kind regards,

Aleksandar R. Zivkovic

Academic Editor

PLOS ONE

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Reviewers' comments:

Acceptance letter

Aleksandar R Zivkovic

13 Jan 2021

PONE-D-20-35947R1

Utility of SpO2/FiO2 ratio for acute hypoxemic respiratory failure with bilateral opacities in the ICU

Dear Dr. Fukuda:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

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on behalf of

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