Chapter 17: Acute Respiratory Distress Syndrome

Objectives

1. Describe the clinical presentation of patients with acute respiratory distress syndrome (ARDS).

2. Discuss the potential of developing ventilator-induced lung injury in patients with ARDS.

3. List the indications for mechanical ventilation for patients with ARDS.

4. Discuss approaches used to set the ventilator for patients with ARDS.

5. Discuss common types of asynchrony in patients with ARDS.

6. Describe the role of lung recruitment and the setting of positive end-expiratory pressure (PEEP) by a decremental PEEP trial.

7. Discuss the management of severe refractory hypoxemia.

8. Describe the approach used to monitor patients with ARDS.

9. Describe the approach to ventilator liberation in patients with ARDS.

Introduction

Acute respiratory distress syndrome (ARDS) is a severe lung injury of diverse etiology. It is frequently related to sepsis and multiorgan failure and is associated with high mortality. ARDS results in diffuse alveolar damage, pulmonary microvascular thrombosis, aggregation of inflammatory cells, and stagnation of pulmonary blood flow. Because of the severity of illness of these patients, ARDS management consumes much time, energy, and resources in the intensive care unit (ICU). It is one of the most changeling causes of respiratory failure to manage and requires adherence to published guidelines.

Overview

Clinical Presentation

ARDS is characterized by hypoxemia of recent-onset, bilateral infiltrates on the chest radiograph, and PaO₂/FIO₂ less than or equal to 300, and no evidence of left heart failure. ARDS is categorized as severe (PaO₂/FIO₂ < 100), moderate (PaO₂/FIO₂ 100-200), and mild (PaO₂/FIO₂ > 200) with positive end-expiratory pressure (PEEP) more than or equal to 5 cm H₂O. Persistent ARDS requires assessment 24 hours after presentation on a PEEP more than or equal to 10 cm H₂O with an FIO₂ more than or equal to 0.5.

Evaluation of ARDS by chest computed tomography (CT) shows a heterogeneous disease with areas of consolidation, areas of collapse that are recruitable, and areas of normal lung tissue. Rather than considering ARDS as low-compliance lungs, the gas exchanging areas of the lungs should be considered of small volume when compared with normal lungs.

The pathology of ARDS progresses through two phases, although the process may resolve at any point in either phase. The first phase is characterized by an intense inflammatory response resulting in alveolar and endothelial damage, increased vascular permeability, and increased lung water. This phase lasts about 7 to 10 days and then frequently progresses to extensive fibrosis (phase 2). ARDS has been categorized as pulmonary (direct) and extrapulmonary (indirect) in origin. With pulmonary ARDS, there is direct injury to the lungs as occurs with aspiration, infectious pneumonia, trauma (lung contusion and penetrating chest injury), inhalation injury, near drowning, and fat embolism. With extrapulmonary ARDS, the initial injury is to ...