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KEY POINTS

KEY POINTS

  • The Lung Injury Score (LIS) is based on the chest x-ray, hypoxemia, positive end-expiratory pressure (PEEP), and respiratory compliance, and each component is scored from 0 to 4.

  • The Berlin definition of acute respiratory distress syndrome (ARDS) (2012) includes timing (within 1 week of a known clinical insult), bilateral opacities on chest x-ray, respiratory failure not explained by cardiac failure or fluid overload, and mild, moderate, or severe hypoxemia.

  • A consistent histopathologic feature of ARDS is neutrophil infiltration of the pulmonary microvasculature, interstitium, and alveoli.

  • With ARDS, it is not unusual to see greatly diminished lung compliance on the order of 10 to 30 mL/cm H2O.

  • The current standard of care for ARDS is lower tidal volume ventilation and the use of incremental inspired oxygen–PEEP combinations to achieve oxygenation goals.

  • PEEP is used in ARDS to allow for adequate oxygen delivery to tissues while reducing the concentration of inspired oxygen to nontoxic (below 0.6) levels.

  • The most common type of extracorporeal life support ([ECLS]; or extracorporeal membrane oxygenation [ECMO]) is venovenous, where blood is withdrawn from and returned to the venous system.

  • Inhaled nitric oxide (iNO) is a selective pulmonary vasodilator with no systemic side effects.

  • Any use of “rescue” steroids for late ARDS must be individualized and, optimally, delivered before disease day 14.

  • The mortality of ARDS in several randomized controlled trials over the past two decades has decreased to 20% to 25%.

INTRODUCTION

The maintenance of gas exchange may be tenuous in the injured patient because of dysfunction in three key elements of the respiratory system. First, the central nervous system (CNS) may be impaired, resulting in inadequate respiratory drive, or inability to maintain patent proximal airways. Second, injury to the torso can produce changes in compliance, ineffective respiratory effort, and pain that impact the patient’s ability to complete the work of breathing. Third, primary and secondary insults to the lung result in ineffective gas exchange. In practice, it is common for patients to suffer simultaneous insults affecting all three elements. Respiratory failure that relates primarily to injury to the CNS is discussed at length in other chapters and will not be extensively covered here. This chapter will focus on insults that affect work of breathing and gas exchange. The syndrome of acute respiratory distress syndrome (ARDS) and current management are the major focus.

HYPERCAPNIC PULMONARY FAILURE

The neurohormonal response to injury (see Chapter 63) results in a remarkable increase in cellular metabolism. This creates a substantial increase in carbon dioxide (CO2) production that must be matched by increased elimination from the lungs. While a resting adult eliminates 200 cm3/kg/min of CO2, postinjury hypermetabolism results in CO2 production in the range of 425 cm3/kg/min.1 Thus, the minute ventilation required to maintain eucapnia may rise from a resting rate of approximately ...

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