Validity of Spo2/Fio2 Ratio in Detection of Acute Lung Injury and Acute Respiratory Distress Syndrome


1 Pediatric Pulmonologist, Peiatric Health Research Center, Tabriz University of Medical Sciences, Tabriz, Iran.

2 Pediatrician, Tabriz University of Medical Sciences, Tabriz, Iran.

3 Pediatric Nephrologist, Tabriz University of Medical Sciences, Tabriz, Iran.


 One ofdiagnostic criteria for Acute Lung Injury and Acute Respiratory Distress Syndrome
is  pao2/fio2 (PF) ratio 300 for ALI or 200 for ARDS. This criteria requires invasive arterial sampling. Measurement of Spo2/Fio2 (SF) ratio by pulseoximetry may be a reliable non invasive alternative to the PF ratio.
Methods and Materials:
In a cross sectional study we enrolled 105 sample of patients with ALI or ARDS, to determine the Spo2/Fio2 (SF). Pao2 wasmeasured through arterial blood sampling and Spo2 measured with pulse oximetry and documented within 5 minutes of each other.
The relationship between SF and PF ratio was described by the following equation: SF=57+0/61PF (P<0/001). Spo2/Fio2 (SF) ratios of 181 and 235 can be substituted pao2/fio2 (PF)   ratio of 200 and 300 in ARDS and ALI respectively.  The ALI SF cutoff of 235 had 57% sensitivity and 100% specificity, and ARDS, SF cutoff of 181 had 71% sensitivity and 82% specificity.
Spo2/Fio2 (SF) ratio is a reliable noninvasive marker to determine children with ALI or ARDS and can be used instead of it.



It is estimated that 30 to 60% of all Pediatic Intensive Care Unit admitted patients  require mechanical ventilation ,and of these patient up to 25%  may have ALI and 5 to10% may have ARDS. It means that Acute lung injury(ALI)  and ARDS are terrible syndromes  with high mortality and morbidity(1and 2).With  implementation of lung-protective ventilation strategies ,overall morbidity and mortality have improved significantly for both adult and children with ALI and ARDS(3and 4). Based on American European consensus conference (AECC) in 1994  one of diagnostic criteria for acute lung injury(ALI) and ARDS is   (PF) ratio 300 for ALI or 200 for ARDS(5).But PF criteria requires arterial blood sampling(6and 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 and 9)

Pulse oximetry is the most commonly utilized technique to monitor Oxygenation, Noninvasive and safe. It indirectly measures arterial hemoglobin O2 Saturation by differentiating oxyhemoglobin form deoxygenated hemoglobin using their respective light absorption at wave lengths of 660 nm (red) and 940 nm (infra red)(10 and 11)

In most PICU, daily arterial blood sampling to calculate the PF ratio often is impossible,  then calculation of SF ratio and replacement it to PF ratio for diagnosis of  ARDS or ALI is non invasive and  affordable (12). Using SF ratio determine the degree of hypoxemia non invasively and without the need for arterial blood sampling (7).

In this study we hypothesized that the continuously available and non invasive SF ratio can be used instead  the PF ratio in diagnosis of ALI and ARDS.  

Material and Methods:

In a cross sectional study105 sample of  children with ARDS or ALI who were admitted in PICU of Tabriz children's hospital  between  2012 to  2014 were  studied. In Patient with ARDS or ALI under Mechanical ventilation, Pao2was measared through arterial blood sampling and Spo2 measured with pulse oximetry and documented within 5 minutes of each other.

Inclusion criteria were children with ARDS or ALI with acute onset of  disease and  chest  radiograph  demonstrating  bilateral  pulmonary infiltrates. Exclusion criteria were children with pulmonary edema due to heart failure and congenital heart disease and Anatomic anomalies of lung or air ways.

Quantitative data were presented as mean ± standard deviation (SD), while qualitative data were demonstrated as frequency and percent (%). We used (χ2) and (independent t test) for quantitative and qualitative data as well. P value of



Of  105 children  enrolled  in  this study 56  patients were  female  (53.3%) and  49  patient  male (46.7%) with a  mean  age  of  33+ 6  months  ( minimum 3 and  maximum   140 months).

from total  of 105  data  pairs,   86  (81%)  met the  PF  ratio  criteria  for  RADS  and  19(20%) met the  PF  criteria  for  ALI.

Sex had no significant relationship with SF ratio (P = 0.77) and PF ratio( P =0.06.) 

In general , Spo2/Fio2 (SF) ratio could be predicted well from pao2/fio2(PF)  ratio, using the linear regression equation:  SF =57+0.61 PF. Based on this equation a PF ratio of 300 corresponds to SF ratio of 235 and PF ratio of 200 to SF ratio of 181( P <0.001). 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(figure 1).

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)


Acute lung injury (ALI) and ARDS are significant causes of morbidity and mortality in patients admitted to Pediatric Intensive Care Unit13 . The routine use of pulse oximetry  and capnography has led to reduce Arterial Blood Gase  measurements In most Pediatric Intensive Care Unit14 , Pulse oximetry is now available in most children's hospital and  used routinely and  shows oxygenation status , easier and continuously than Arterial blood sampling15-16. Pulse oximetry prevents Arterial blood sampling and cost for ABG analysis17. Using SF ratio for diagnose of  ALI and ARDS lead to identification of  undiagnosed cases of these syndromes 18.

In this study we included 105 patients with ALI or ARDS. Pao2 and Spo2 measured with the same Fio2 and  computed SF and PF ratio. Our study indicated that the relationship between SF and PF ratio was described with following equation SF=57+0.61 PF, and SF ratio threshold value  for ALI was 235 and for ARDS and 181 corresponded of PF ratio 300 and 200.

In the similar study khemani et al  found than SF cut off  of  201 could predict PF criteria for ARDS with 84% sensitivity and 78% specificity and SF of 263 could predict ALI with 93% sensitivity and 43% specificity19.

In adult patients, in study by Rice et al They found that SF cut off of  235 could predict ARDS with 85% sensitivity and 85% specificity and SF cut off of 315 could predict ALI with 91%sensitivity and 56 % specificity(20).


This study indicates that Spo2/Fio2 (SF) ratio is a reliable, non invasive and available marker for diagnosis ALI or ARDS in critically ill children.


 This research was financially supported by Pediatric Health Research Center, Tabriz University of Medical Sciences, Tabriz, Iran.

Authors have no conflict of interest.


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