Chapter 21. Pulmonary Diseases
A 35-year-old patient with history of heroin abuse was found unresponsive and developed aspiration pneumonia after an overdose. His pneumonia was complicated by acute respiratory failure, and he required intensive care unit (ICU) admission with intubation, after which he developed severe acute respiratory distress syndrome (ARDS) (chest radiograph shown below). His initial vent settings were volume assist-control ventilation with a tidal volume of 840 (12 mL/kg of ideal body weight), a respiratory rate of 12 breaths/min, a positive end-expiratory pressure (PEEP) of 10 cm H2O, and a fraction of inspired oxygen (FiO2) of 0.5. His plateau pressure is 38 cm H2O, pH is 7.40, partial pressure of carbon dioxide (PCO2) is 46 mm Hg, and arterial partial pressure of oxygen (PaO2) is 68 mm Hg. Given his current clinical picture, he is placed on the National Institutes of Health ARDS Network protocol, which reduces his tidal volume to 6 mL/kg of ideal body weight with an accompanying increase in respiratory rate. He maintains an acceptable partial pressure of arterial carbon dioxide (PaCO2) and pH while the ARDS protocol is instituted. What will be the effect on lung compliance and oxygenation in the next 48 to 72 hours?
A portable supine view of the chest demonstrates asymmetrical confluent bilateral patchy airspace opacities suggestive of acute respiratory distress syndrome.
A. Both lung compliance and oxygenation will improve.
B. Both lung compliance and oxygenation will worsen.
C. Lung compliance will improve, but oxygenation will not be affected.
D. Neither will be affected.
B. Based on the ARDS Network trial published in 2007, a mortality benefit was found by using a low tidal volume strategy of 6 mL/kg of ideal body weight. However, despite having a mortality benefit, both respiratory system compliance and PaO2/FiO2 ratios are worse in low tidal volume protocols. Although reduced tidal volumes and lower plateau pressures are considered to be lung protective in the setting of ARDS, they adversely affect lung physiology. With low tidal volumes, lung recruitment and minute ventilation are both negatively affected, resulting in permissive hypercapnia, increased atelectasis, poor lung compliance, and an increase in ventilation/perfusion (V/Q) mismatch. These effects are often viewed as an acceptable trade-off if ventilator-induced lung injury can be avoided. Despite these negative effects on lung physiology in the first 24 to 48 hours, after 72 hours, the differences between low tidal volume and high tidal volume strategies are minimal, and patients on low tidal volume protocols demonstrate improved survival.
A previously healthy 50-year-old man is admitted to the ICU for progressively worsening ...