Published in:
01-09-2018 | What's New in Intensive Care
Adjunct and rescue therapies for refractory hypoxemia: prone position, inhaled nitric oxide, high frequency oscillation, extra corporeal life support
Published in: Intensive Care Medicine | Issue 9/2018
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The mortality of severe acute respiratory distress syndrome (ARDS), defined with a PaO2/FiO2 ratio of 100 mmHg or less with at least 5 cm H2O of PEEP, still exceeds 40% [1]. Furthermore, although it is true that more ARDS patients die from multi-organ failure than hypoxemia per se, an important subgroup of severe ARDS do die from refractory hypoxemia often defined as a PaO2 < 60 mmHg despite FiO2 1.0 [2]. Therefore, rescue treatments aiming at preventing hazardous hypoxemia are crucial. In this paper, we will cover prone position, inhaled nitric oxide (iNO), high-frequency oscillation (HFO) and extracorporeal life support (ECLS) as interventions to treat refractory hypoxemia. For each of them, we will discuss mechanism of action to improve hypoxemia (Fig. 1), potential benefits and risks, and make a personal recommendation about use. Before moving towards these measures, clinicians should ensure the basics are attended to—intravascular volume optimized, cardiac output sufficient [3], and patient sedated and paralyzed. We assume that lung protective mechanical ventilation is ongoing including lower tidal volumes and sufficient PEEP, the latter of which should usually involve at least a trial of higher PEEP (15–20 cm H2O) [4]. When patients remain severely hypoxemic despite these measures, rescue therapy for refractory hypoxemia must be considered (Fig. 1).
Fig. 1
Mechanisms of action of adjunct therapies during severe ARDS on ventilation/perfusion \( \left( {\frac{{\dot{V}_{A} }}{{\dot{Q}}}} \right) \) distribution. Due to massive loss of aeration and increase in lung tissue as shown on the lung CT slice, hypoxemia results from large amounts of shunt and \( \frac{{\dot{V}_{A} }}{{\dot{Q}}} \) mismatch, which may be due to low alveolar ventilation \( \left( {\dot{V}_{A} } \right) \) as compared to lung perfusion \( \left( {\dot{Q}} \right) \) or to more lung perfusion relative to \( \dot{V}_{A} \). This latter occurs when hypoxic pulmonary vasoconstriction is impaired making the lung perfusion predominant in non-aerated lung regions. To improve \( \frac{{\dot{V}_{A} }}{{\dot{Q}}} \) matching and hence oxygenation, interventions may act on both sides of the equation. By increasing perfusion in well ventilated regions, inhaled nitric oxide (iNO) and intravenous fluid challenges redistribute lung perfusion and cardiac output (CO) to well-aerated areas and better match \( \frac{{\dot{V}_{A} }}{{\dot{Q}}} \). PEEP can increase aeration in non- or poorly ventilated areas thereby improving \( \frac{{\dot{V}_{A} }}{{\dot{Q}}} \). CO may decrease with PEEP, which also contributes to \( \frac{{\dot{V}_{A} }}{{\dot{Q}}} \) improvement. High frequency oscillation and recruitment maneuvers improve \( \frac{{\dot{V}_{A} }}{{\dot{Q}}} \) by increasing \( \dot{V}_{A} \). Prone position increases \( \dot{V}_{A} \) in the vertebral lung regions where \( \dot{Q} \) remains prevalent and, furthermore, makes the distribution of \( \frac{{\dot{V}_{A} }}{{\dot{Q}}} \) homogeneous across the lung
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