Acute respiratory distress syndrome (ARDS) is the acute onset of hypoxemia and bilateral pulmonary edema. ARDS is defined by the Berlin’s criteria.1 ARDS is a heterogenous disorder.

The PaO2/FIO2 ratio is a widely used bedside index of adequacy of oxygenation and to categorize disease severity in ARDS but it is a poor independent predictor of mortality in ARDS.2,3

Increased dead space ventilation has consistently been shown to be associated with increased mortality and disease progression in patients with acute respiratory distress syndrome (ARDS).4,5 Yet in day to day practice, dead space measurements are seldom performed. This is mainly due to problems associated with measuring dead space. The traditional method of calculating physiological dead space is cumbersome and requires a large chamber (Douglas Bag) for the collection of mixed expired gas.6 Volumetric Capnography is simple method but it is not integrated as standard in most commonly used ventilators and installation incurs additional expense. In addition, measured mixed expired PCO2 requires correction for compressed ventilator gas that contaminates the expired volume.7

Ventilatory ratio (VR) has been described as a bedside index that is easy to calculate and which monitors ventilatory efficiency8. VR is a product of measured expired minute ventilation(VE measured) and measured arterial PaCO2 (PaCO2 measured) normalized to a preset ventilator standard established from nomograms. VR is a unitless ratio.

Where, VE measured is the measured minute ventilation (ml/min), PaCO2 is the measured arterial partial pressure of carbon dioxide (mmHg), PBW- Predicted body weight 100- ideal minute ventilation (ml/min), 37.5- ideal PaC02 .

Ventilatory ratio correlates well with VD/VT in ARDS and higher values at baseline are associated with increased risk of adverse outcomes. VR is not affected by administration ofneuromuscular blocker.9

Most of the literature about VR is by retrospective studies and post hoc analysis. We plan to prospectively analyse VR, P/F ratio and their derived composite as a measure of severity and for bedside prognostication in ARDS.

Hypothesis: In this study we hypothesized that P/F ratio and VR combined are better predictor of mortality than either of them alone

Research questions: Is the combination of P/F ratio and VR, a better predictor of mortality than either of them alone in  ARDS 

Aims and Objectives: Aim of the study: To determine the predictive value of Ventilatory ratio, PaO2/Fio2 and their combination in outcome of ARDS

Detailed methodology: Patient will be recruited once admitted to ICU. The ventilator settings and various adjustments will left at the discretion of attending intensivist. Usually ARDSnet protocol is followed. Anthropometry (age, sex, weight, height and predicted body weight), respiratory variables (tidal volume, respiratory rate, minute ventilation, SpO2, PaCO2, P/F ratio, PEEP), MAP and APACHE II score will be noted. The goal of oxygenation is to target a peripheral blood oxygen saturation (SpO2) between 88% and 95% measured by pulse oximetry, or a partial pressure of oxygen (PaO2) of 55–80 mmHg measured by arterial blood gas analysis.

On admission arterial blood gas will be analysed and P/F ratio will be calculated. Within 30 minutes of ABG, ventilatory ratio will be calculated by measuring minute ventilation and measured PaCO2 and incorporating them into the formula i.e, 

              Where, VE measured is the measured minute ventilation (ml/min)

                             PaCO2 is the measured arterial partial pressure of carbon dioxide (mmHg)

                             PBW- Predicted body weight 

                            100- ideal minute ventilation (ml/min)

                            37.5- ideal PaCO2

Baseline VR will be taken after 1hour of stabilization of patient. Subsequently various measurements of VR and P/F ratio will be taken on days 1-4 and day 7 after 30 minutes ofchanges in ventilator settings/respiratory parameters/ any interventions during the ICU length of stay. Worst values of VR and P/F ratio will be taken for analysis out of the multiple measurements. All patients are to be followed up until 28 days after the admission.

Data analysis plan:

Continuous variables will be presented as Mean ± SD with 95% confidence interval. Quantitative variables will be compared by using student t-test if the data follows normal distribution. Otherwise non- parametric test (Mann Whitney U test) will be used. Chi-squared test for proportions will be used to test statistical differences between frequencies of categorical variables.The association between VR, P/F ratio and their combination with mortality will be assessed using univariate and multivariate regression.Kaplan-Meir survival curves will be plotted and log rank test will be used to compare survival curves.Two sided p-value of less than 0.05 will be considered significant.We will perform all analyses using Statistical Package for Social Sciences (SPSS, version 23.0 for windows)