# Introduction cute lung injury and ARDS are terrible syndromes with high mortality and morbidity [1][2] . It is estimated that 30 to 60% of all PICU admitted patient require mechanical ventilation and of these patient up to 25% may have ALI and 5 to10% may have ARDS. With the implementation of lung-protective ventilation strategies overall morbidity and mortality have improved signifycantly for both adult and children with ALI and ARDS [3][4] . Based on American European consensus conference (AECC) in 1994: Diagnostic criteria for acute lung injury(ALI) and ARDS requiring acute onset of disease, chest radiograph demonstrating bilateral pulmonary infiltrates, lack of significant left ventricular dysfunction and 2 2 Fio Pao (PF) ratio ? 300 for ALI or ? 200 for ARDS 5 . The first three components can be established with clinical history or noninvasively tools such as chest radiograph or echocardiography. However PF criteria require arterial blood sampling [6][7] . Concerns about anemia following blood sampling and a movement to minimally invasive approaches have led to reduction blood gas measurements in critically ill patient [8][9] . however studies in ARDS and ALI patient are lacking .Furthermore SF threshold values could be used for diagnosing ARDS and ALI [6][7][8][9][10] . Pulse oximetry is the most commonly utilized technique to monitor Oxygenation.Noninvasive and safe. It indirectly measures arterial hemoglobin O 2 Saturation by differentiating oxy hemoglobin form deoxygenated hemoglobin using their respective light absorption at wave lengths of 660 nm (red) and 940 nm (infra red) [11][12] . Pulse oximetry is used:1-detection of hypoxia.2-preven tion of hyperoxia. 3-for weaning from mechanical ventilation 4-titration of Fio 2 9-13 . In most PICU daily arterial blood sampling to calculate the PF ratio often impossible,then calculate the SF ratio and replacement by PF ratio for diagnose ARDS or ALI is non invasive and affordable 14 . Using SF ratio determine the degree of hypoxemianon invasively without the need for arterial blood sampling 7 . In this study we computed the relationship between SF and PF ratio in critically ill patient with ALI and ARDS. We hypothesize that the continuously available and noninvasively SF ratio can be used instead the PF ratio in diagnosis of ALI and ARDS. # II. # Methods In this cross sectional study 70 children with ARDS or ALI that admitted in Tabriz children's hospital PICU, Iran between 2012 and 2013 were studied. In Patient with ARDS or ALI under Mechanical ventilation with same Fio 2 , Pao 2 measared with Arterial blood sampling and Spo 2 measured with pulse oximetry and charted with in 5 min of each other.Computed SF and PF ratio. Inclusion criteria were children with ARDS or ALI and acute onset of disease and chest radiograph demonstrating bilateral pulmonary infiltrates, consistent with pulmonary edema. Exclusion criteria were children with pulmonary edema due to heart failure and congenital heart disease and Anatomic anomalies of lung or air ways. # III. # Statistical Analysis Statistical analyses were performed using the Statistical Package for Social Sciences, version 17.0 (SPSS,Chicago,Illinois). Quantitative data were pre sented as mean ± standard deviation (SD), while qualitative data were demonstrated as frequency and percent (%). The categorical parameters were compared by (? 2 ) tests, and the continuous variables were compared by independent t test. A p value of <0.05 was considered statistically significant. Relationship between SF and PF, described by linear regression equation. ROC curves were plotted to determine the sensitivity and Specificity of the SF threshold values correlating with PF of 200 (ARDS) and 300 (ALI). IV. # Results Of 70 children enrolled in this study, included 38 patient female (54.3%) and 32 patient male (45.7%) with a mean age of 32+ 5 months (mini mum 2 and maximum 144 months). A total of 70 data pairs 56 (80%) met the PF ratio criteria for RADS and 14(20%) met the PF criteria for ALI. The median time difference between charted values of Spo 2 and Pao 2 pairs was 5 min . Table (1) demonstrates baseline findings of the patients enrolled in the study. Age was no significantly relationship with SF ratio. Pvalue = 0.81 and was no significantly relationship with PF ratio Pvalue=0.99. Sex was no significantly relationship with SF ratio Pvalue = 0.77 and was no significantly relationship with PF ratio Pvalue =0.06. In general , SF ratio could be predicted well from PF ratio, described by the linear regression equation SF =57+0.61 PF. Based on this equation a PF ratio of 300 corresponds to an SF ratio of 235 and PF ratio of 200 to an SF ratio of 181. Pvalue <0.001[Fig1] The ALI SF cut off of 235 had 57% sensitivity and 100% specificity and ARDS cut off of 181 had 71% sensitivity and 82% specificity. In general, the SF ratio had excellent discrimination ability for ARDS( AUC=0.86) [Fig2] and good discrimination ability for ALI and ARDS (AUC=0.89) [Fig3]. # Discussion Acute lung injury (ALI) and ARDS significant causes of morbidity and mortality for patients admitted to PICU 15 . The routine use of pulse oximetry and capnography has led to reduce ABG measurements. In most PICU 16 , Pulse oximetry a is now available in most children's hospital and used routinely and shows oxygenation status, easier and continuously than Arterial blood sampling [17][18] . Pulse oximetry prevents Arterial blood sampling and cost for ABG analysis 19 . Using SF ratio for diagnose of ALI and ARDS lead to identification of undiagnosed cases of these syndromes 20 . SF ratio may be useful in many organ failure scores, such as lung injury scores 21 , multi organ dysfunction score 22 ,sequential organ failure assessment 23 ,instead PF ratio to estimate the degree of hypoxemia. In this study Included 70 patient with ALI or ARDS Pao 2 and Spo 2 measured with the same Fio 2 computed SF and PF ratio. We seen the relationship between SF and PF ratio was described following equation SF=57+0.61 PF and SF ratio threshold value for ALI was 235 and for ARDS was 181 corresponded of PF ratio 300 and 200. In the similar study khemani et al who used pediatric data. They found than an SF cutoff of 201 could predict PF criteria for ARDS with 84% sensitivity and 78% specificity and an SF of 263 could predict ALI with 93% sensitivity and 43% specificity 24 . In adult patients, the one study by Rice et al They found than an SF cut off of 235 could predict for ARDS with 85% sensitivity and 85% specificity and SF cut off of 315 could predict for ALI with91% sensitivity and 56 % specificity 25 . In this study, we assessed relationship between age and sex with PF and SF ratio. we measured Pao 2 and Spo 2 in maximum 5 min. The SF ratio thresholds determined in this study were based on PF ratio proposed by the AECC. There are certainly limitation to the this study: First, ABG and pulse oximetry measurements were close in time to each other (median 5min). Given that changes in Spo 2 and Pao 2 may happen quickly. Second, we did not control for PH, Hemoglobin, Paco 2 , temperature,that maybe influenced by the relationship between Spo 2 and Pao 2 . However non invasively SF ratio can be used for Diagnosis of ALI or ARDS. # VI. # Conclusion According to this study SF ratio is a reliable non invasive and continuously available marker for PF ratio for diagnose children with ALI or ARDS. Then can be replaced pulse oximetry by Arterial blood sampling. According to complication of Arterial blood sampling such as Anemia, Bleeding, in critical illness, Pulse oximetry can be used instead Arterial blood sampling. 12![Figure 1 : S/F ratio vs P/F ratio scatterplot for the derivation data set. The line represents the best fit linear relationship SF=57+0/61PF (P<0/001)](image-2.png "Figure 1 :Figure 2 :") ![](image-3.png "") Abstract-Background: Diagnostic criteria for acute lunginjury(ALI) and ARDS requiring acute onset of disease, chestradiograph demonstrating bilateral pulmonary infiltrates, lackof significant left ventricular dysfunction and2 2 Pao (PF) ratio FioResults: The relationship between SF and PF ratio wasdescribedbythefollowingregressionequationSF=57+0/61PF (P<0/001). SF ratios of 181 and 235corresponded of PF ratio 300 and 200. The ALI SF cutoff of235 had 57% sensitivity and 100% specificity, and ARDS, SFcutoff of 181 had 71% sensitivity and 82% specificity.Conclusion: SF ratio is a reliable noninvasive marker for PFratio to identify children with ALI or ARDS and can be replacedpulse oximetry by arterial blood sampling.Keywords: ARDS, ALI,2 2 pao Fio, pulse oximetry.Abbreviations: Pao 2 : Arterial partial pressure of oxygen ,Spo 2 : pulse Oximetric oxygen saturation, ARDS: AcuteRespiratory Distress syndrome, ALI: Acute lung injury,Fio 2 : Fraction of inspiratory oxygen, SF=2 Fio spo 2ratio,PF=2 Fio Pao 2ratio,ABG= Arterial blood gas analysis,PICU=Pediatric Intensive Care unit, Paco 2: Arterialpartial pressure of carbon dioxid, Sao 2= Arterial oxygensaturation.? 1MAX-MINMNAEPao 2 / Fio 2298-46155±61Spo2/Fio 2248-77152±47Spo 299-7194±4Fio 2100-4067±18Pao 2176-4196±25age144-232±5 * The acute respiratory distress syndrome LBWare MAMatthay N Engl J Med 342 2000 * The American-European Consensus Conference on ARDS: definitions,mechanisms, relevant outcomes, and clinical trial coordination GRBernard AArtigas KLBrigham Am J Respir Crit Care Med 149 1994 * Quality improvement report: linking guideline to regular feedback to increase appropriate requests for clinical test; blood gas analysis in intensive care PMerlani PGarnerin MDiby BMJ 323 2001 * ?Thoracic SoG-The American European Consensus Confe rence on ARDS Definitions, mechanisms GRBernard Artigas Brigham American Journal 1994 * Practice guideline for arterial blood gas measurement in the intensive care unit decreases numbers and increases appropriateness tests CSPilon MLeathley RLondon Crit Care Med 25 1997 * Meta-analysis of arterial oxygen saturation monitoring by pulse oximetry in adults LAJensen JEOnyskiw NGPrasad Heart Lung 27 1998 * Acute respiratory distress syndrome: under recognition by clinicians and diagnostic accuracy of three clinical definitions NDFerguson FFrutos-Vivar EstebanA Crit Care Med 33 2005 * Control of blood gas measurements in intensive-care units DRoberts POstryzniuk ELoewen Lancet 337 1991 * Do changes in pulse oximeter oxygen saturation predict equivalent changes in arterial oxygen saturation? 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