Acute respiratory distress syndrome (ARDS) is a distinct type of respiratory failure that manifests as acute, diffuse lung inflammation that causes hypoxemia and is associated with high mortality. Early recognition of the syndrome and the prompt use of appropriate ventilator and adjunct strategies to manage the disease is essential in reducing critical care morbidity and mortality. This chapter will cover how ARDS is a common cause of hypoxemia in the critically ill patient and will go over strategies for the early diagnosis of the disease. Later, the chapter will also go over strategies to manage and help these patients recover, including the key tenets of mechanical ventilatory support and the indications for noninvasive ventilatory support. Finally, adjunct strategies to help clinicians struggling with refractory hypoxemia will be reviewed, including the use of recruitment maneuvers, open lung ventilation, paralysis, proning, and extracorporeal membrane oxygenation (ECMO), among other modalities.
DEFINITION AND CAUSES OF HYPOXEMIA
Hypoxemia is defined as low levels of oxygen in the blood; more specifically, it is frequently defined as a level of partial pressure of oxygen that is less than 60 mm Hg in arterial blood. This cutoff is physiologically significant as there is rapid dissociation of oxygen from hemoglobin below this value that ultimately leads to a significantly reduced oxygen carrying capacity.1 In the critical care unit, ARDS is always high on the differential for a patient with acute hypoxemia.
Hypoxemia is divided into five distinct etiologies: decreased inspired levels of oxygen, hypoventilation, impaired diffusion, ventilation perfusion mismatch, and right to left shunt. Clinically, both a low level of inspired oxygen and hypoventilation can be excluded in the critical care unit due to the clinician controlling a patient’s minute ventilation and fraction of inspired oxygen with a ventilator. Impaired diffusion is also rarely a cause of hypoxemia in the mechanically ventilated patient due to oxygen being a perfusion and not diffusion limited gas. As a result, most causes of hypoxemia in critically ill patients are due to ventilation perfusion mismatch and shunt (Figure 10-1).
Schematic showing causes of hypoxemia. (Adapted from Glenny, RW. Teaching ventilation/perfusion relationships in the lung. Advances in Physiology Education. 2008;32(3):192-195.)
ARDS causes hypoxemia by causing alveolar filling from a diffuse inflammatory process, resulting in ventilation perfusion mismatch and shunt as described below.
PATHOGENESIS, DEFINITION, AND EPIDEMIOLOGY OF ARDS
The hypoxemia driven by ARDS is caused by the leaking of protein rich fluid from lung capillaries into their adjacent alveoli, a process driven by the inflammatory response to the original inciting event. The increase of fluid and inflammation in the alveoli and interstitial space in turn decreases the ability of oxygen to move from the alveoli to the capillaries, thereby causing hypoxemia.