Few areas of critical care medicine have been the subject of as much investigative attention or clinical concern as the set of problems grouped under the label acute respiratory distress syndrome (ARDS). This syndrome, first formally described in 1967,1 continues to be recognized clinically as a rapidly developing impairment of pulmonary oxygen exchange accompanied by diffuse infiltrates and altered respiratory system mechanics that cannot be attributed solely to hydrostatic forces. Fueled by better characterization of innate pathophysiology and of iatrogenic factors, considerable progress has been made in recent years toward reducing the adverse consequences of this condition. Yet, after more than four decades, active debate continues regarding key elements of the ventilatory prescription and appropriate therapeutic targets.
From the outset, mechanical ventilation with positive pressure has been essential in addressing the life-threatening gas exchange abnormalities and otherwise unsustainable workloads associated with ALI. Only in the relatively recent past, however, has there been clear documentation that the tidal pressures of mechanical ventilation can impact morbidity and survival.2,3 This awareness has caused a conceptual shift away from attempting to restore normal blood gases at the costs of high pressure and toward adopting the avoidance of preventable iatrogenicity (“lung protection”) as the first priority.
Many aspects of the debate concerning appropriate ventilator management of this group of conditions can be traced to the heterogeneity of the patient population, to our still imperfect comprehension of the mechanisms of ventilator-associated lung injury (VILI) and to the relative imprecision of the criteria upon which the label ARDS and/or acute lung injury (ALI) is assigned.4 Despite an incomplete and still evolving understanding, a rich experimental and clinical database—much of it collected over the past two decades—allows for the development of a rational set of principles upon which to formulate an effective ventilation strategy.5–7 Definitive answers for many important clinical questions related to this topic are not available; what is presented here reflects a pathophysiology-guided approach to accomplish essential clinical objectives while avoiding VILI (Table 29-1).
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Table 29-1: Conceptual Principles in Acute Respiratory Distress Syndrome Ventilation
- ARDS is a heterogeneous problem
- Between patients
- Over time
- Between lung regions
- Risk for ventilator-induced lung injury is proportional to transalveolar pressure
- Lung recruitment is essential to avoid ventilator-induced lung injury
- The chest wall influences regional lung volumes, tolerated pressures, and recruitability
In the clinical setting, mechanical ventilation ensures adequate oxygenation of arterial blood, provides sufficient oxygen transport to vital organs and tissues, assists in eliminating carbon dioxide, relieves excessive burdens placed upon the respiratory muscles, helps maintain alveolar stability, and allows therapeutic measures that require controlled ventilation. Despite its undeniable value, however, mechanical ventilation also has the potential to inflict adverse clinical outcomes. The task of accomplishing ventilation safely in patients with injured lungs is made far ...