HFO To ECMO in Refractory Hypoxemia

Refractory Hypoxemia was introduced in the definition of ARDS since 1967 by Ashbaugh: “Heavy lung”Low compliance “Refractory hypoxemia”. Refractory hypoxemia is defined as: PO2 lower than 100 mmHg at FiO2 equal to 1, i.e. P/F ratio lower than 100, in analogy with Berlin severe ARDS. Low gas exchange is related to low VA/Q and shunt. To improve oxygenation 3 mechanisms: 1. Opening and keeping open recruited/perfused alveoli; 2. Diversion of blood flow; 3.Adding oxygen to the blood. Any time you increase mean airway pressure oxygenation improves, recruitment occurs and CO decreases. Shunt improvement correspond to recruitment when VA increases, PO2 increases and PCO2 decreases. Shunt improvement correspond to flow diversion when VA does not changes, PO2 increases and PCO2 decreases at smaller extent. HFO is performed at mean airway pressure such as to approach the total lung capacity (25 to 30 cmH2O) Oxygenation improves due to: Recruitment, blood flow diversion, blood flow reduction. However HFO was not found associated to improved survival. In refractory hypoxemia always consider the prone position, the best cure of hypoxemia.