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. 2021 Aug 9;11(1):124.
doi: 10.1186/s13613-021-00908-3.

P/FP ratio: incorporation of PEEP into the PaO2/FiO2 ratio for prognostication and classification of acute respiratory distress syndrome

Sunitha Palanidurai  1 , Jason Phua  2   3   4 , Yiong Huak Chan  5 , Amartya Mukhopadhyay  2   3   4   6
Affiliations

P/FP ratio: incorporation of PEEP into the PaO2/FiO2 ratio for prognostication and classification of acute respiratory distress syndrome

Sunitha Palanidurai et al. Ann Intensive Care. .

Abstract

Background: The current Berlin definition of acute respiratory distress syndrome (ARDS) uses the PaO2/FiO2 (P/F) ratio to classify severity. However, for the same P/F ratio, a patient on a higher positive end-expiratory pressure (PEEP) may have more severe lung injury than one on a lower PEEP.

Objectives: We designed a new formula, the P/FP ratio, incorporating PEEP into the P/F ratio and multiplying with a correction factor of 10 [(PaO2*10)/(FiO2*PEEP)], to evaluate if it better predicts hospital mortality compared to the P/F ratio post-intubation and to assess the resultant changes in severity classification of ARDS.

Methods: We categorized patients from a dataset of seven ARDS network trials using the thresholds of ≤ 100 (severe), 101-200 (moderate), and 201-300 (mild) for both P/F (mmHg) and P/FP (mmHg/cmH2O) ratios and evaluated hospital mortality using areas under the receiver operating characteristic curves (AUC).

Results: Out of 3,442 patients, 1,057 (30.7%) died. The AUC for mortality was higher for the P/FP ratio than the P/F ratio for PEEP levels > 5 cmH2O: 0.710 (95% CI 0.691-0.730) versus 0.659 (95% CI 0.637-0.681), P < 0.001. Improved AUC was seen with increasing PEEP levels; for PEEP ≥ 18 cmH2O: 0.963 (95% CI 0.947-0.978) versus 0.828 (95% CI 0.765-0.891), P < 0.001. When the P/FP ratio was used instead of the P/F ratio, 12.5% and 15% of patients with moderate and mild ARDS, respectively, were moved to more severe categories, while 13.9% and 33.6% of patients with severe and moderate ARDS, respectively, were moved to milder categories. The median PEEP and FiO2 were 14 cmH2O and 0.70 for patients reclassified to severe ARDS, and 5 cmH2O and 0.40 for patients reclassified to mild ARDS.

Conclusions: The multifactorial P/FP ratio has a greater predictive validity for hospital mortality in ARDS than the P/F ratio. Changes in severity classification with the P/FP ratio reflect both true illness severity and the applied PEEP strategy.

Trial registration: ClinialTrials.gov-NCT03946150.

Keywords: Acute respiratory distress syndrome; Mortality; PaO2/FiO2 ratio; Positive end-expiratory pressure.

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Conflict of interest statement

The authors declare that they have no competing interest.

Figures

Fig. 1
Fig. 1
Change of severity classifications when P/FP ratio is used instead of P/F ratio for severities. Severe refers to a ratio of ≤ 100, moderate refers to a ratio of 101–200, mild refers to a ratio of 201–300, non-ARDS refers to a ratio of > 300 mmHg or mmHg/cmH2O. Green ovals represent patients who were reclassified to a more severe category. Blue ovals represent patients who were reclassified to a milder category. Red ovals represent patients whose categories remained unchanged. Definition of abbreviations: ARDS  acute respiratory distress syndrome; P/F  ratio of the partial pressure of arterial oxygen (PaO2) to the fraction of inspired oxygen (FiO2); P/FP = (PaO2 * 10)/(FiO2 * positive end-expiratory pressure)
Fig. 2
Fig. 2
Scatter plots of patients according to P/F ratio and P/FP ratio. Each circle represents individual patients on a certain applied PEEP; each colour represents a different PEEP setting. Green bars represent patients who were classified by the P/FP ratio in each severity. Blue bars represent patients who were classified by the P/F ratio in each severity. Red bars represent patients whose categories remained unchanged. Definition of abbreviations: FiO2 = fraction of inspired oxygen; PEEP = positive end-expiratory pressure; P/F = ratio of the partial pressure of arterial oxygen (PaO2) to FiO2; P/FP = (PaO2 * 10)/(FiO2 * PEEP)
Fig. 3
Fig. 3
Receiver operating characteristic curves for the P/F ratio and the P/FP ratio for hospital mortality at different PEEP thresholds. AUC area under the curve; NS not significant, PEEP positive end-expiratory pressure, P/F = ratio of the partial pressure of arterial oxygen (PaO2) to the fraction of inspired oxygen (FiO2); P/FP = (PaO2 * 10)/(FiO2 * PEEP); CI confidence interval
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