Mastering PF Ratios: A Key to Determining VV ECMO Candidacy in ARDS

Acute Respiratory Distress Syndrome (ARDS) is a severe lung condition that leads to significant hypoxemia, characterized by low levels of oxygen in the blood despite high levels of supplemental oxygen. One crucial metric used to assess the severity of ARDS is the PF ratio, which stands for the ratio of arterial oxygen partial pressure (PaO2) to the fraction of inspired oxygen (FiO2).

What is the PF Ratio?

The PF ratio is calculated using the following formula:

PF Ratio = PaO2/FiO2

- PaO2: The partial pressure of oxygen in arterial blood, measured in mmHg.

- FiO2: The fraction of inspired oxygen, expressed as a decimal (e.g., 100% oxygen is 1.0).

 

Classification of ARDS Based on PF Ratio

- Mild ARDS: PF ratio between 200 and 300

- Moderate ARDS: PF ratio between 100 and 200

- Severe ARDS: PF ratio less than 100

The lower the PF ratio, the more severe the hypoxemia, indicating a greater impairment in the lungs' ability to oxygenate blood.

Using PF Ratios to Determine VV ECMO Candidacy

Venovenous ECMO (VV ECMO) is a life-saving intervention for patients with severe respiratory failure who do not respond to conventional therapies. VV ECMO provides respiratory support by oxygenating the blood outside the body, allowing the lungs to rest and heal.

Candidacy for VV ECMO in ARDS:

- Severe ARDS: Patients with a PF ratio less than 100, despite optimal mechanical ventilation and other supportive measures, are considered for VV ECMO. This indicates a critical level of hypoxemia that warrants the use of advanced support.

- Refractory Hypoxemia: Persistent hypoxemia despite high levels of PEEP (Positive End-Expiratory Pressure) and FiO2 suggests that conventional ventilation is insufficient, making VV ECMO a viable option.

Clinical Decision-Making

The decision to initiate VV ECMO involves a comprehensive evaluation of the patient's condition, including:

- Severity of hypoxemia as indicated by the PF ratio

- Duration of mechanical ventilation

- Overall health status and potential for recovery

- Presence of contraindications for ECMO

VV ECMO can significantly improve oxygenation and provide a bridge to recovery for patients with severe ARDS. Early identification of candidates based on PF ratios and other clinical factors is crucial for timely and effective intervention.

References

LITFL (Life in the Fast Lane). (n.d.). PaO2/FiO2 Ratio (P/F Ratio). Retrieved from https://litfl.com/pa02-fi02-ratio-p-f-ratio/

EMCrit. (n.d.). EMCrit 343 - CV-EMCrit: Who to Refer for VV ECMO - Patient Selection and When to Call for Help. Retrieved from https://emcrit.org/emcrit/cv-emcrit-who-to-refer-for-vv-ecmo-patient-selection-and-when-to-call-for-help/

Critical Care. (n.d.). Ultra-lung-protective ventilation and biotrauma in severe ARDS patients on veno-venous extracorporeal membrane oxygenation: a randomized controlled study. Retrieved from https://ccforum.biomedcentral.com/articles/10.1186/s13054-019-2482-5