A 56-year-old man with a history of alcohol abuse is hospitalized due to aspiration pneumonia of the right lower lung. While being treated, he develops severe hypoxemia requiring transfer to the intensive care unit for initiation of mechanical ventilation. Temperature is 37.1 C (98.8 F), blood pressure is 120/62 mm Hg, and pulse is 95/min. Pulmonary examination is significant for bilateral crackles. There is no jugular venous distension or peripheral edema. Repeat chest x-ray demonstrates diffuse, bilateral alveolar opacities. Initial ventilator settings demonstrate a tidal volume of 510 mL (8 mL/kg of predicted body weight), positive end-expiratory pressure of 10 cm H₂O, respiratory rate of 35/min, and FiO₂ of 80%. Tidal volume is reduced to 380 mL (6 mL/kg of ideal body weight) over several hours; the other ventilator parameters are unchanged. Plateau pressure is 27 cm H₂O. Arterial blood gas results are as follows: What is the best next step in management of this patient?

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11ec193a_98e6_c22d_853d_77942e5a98b1_MD0002_00 This patient with pneumonia has developed significant hypoxemia, a PaO₂/FiO₂ <300 (65/0.8=81), and diffuse alveolar infiltrates on chest x-ray, consistent with acute respiratory distress syndrome (ARDS). Ventilator settings in ARDS are adjusted to achieve adequate oxygenation while minimizing ventilator-induced lung injury. Reduced lung compliance of affected lung tissue causes overdistension of healthy alveoli at high-volume ventilation (>8 mL/kg). However, low-tidal-volume ventilation (LTVV; 4-6 mL/kg of ideal body weight) that maintains a plateau pressure of ≤30 cm H₂O significantly reduces mortality and ventilator-dependent days by reducing barotrauma to remaining healthy lung tissue. A consequence of LTVV is alveolar hypoventilation resulting in hypercapnia and respiratory acidosis. Generally, pH ≥7.25 is well-tolerated in patients without contraindication to increased carbon dioxide levels (eg, cerebral edema, seizures; "permissive hypercapnia") or acidosis (eg, vasodilatory shock). Serial blood gases should be performed to ensure that hypercapnic acidosis develops gradually and remains at an acceptable level, as in this patient. Due to the magnitude of benefits from LTVV, it is sometimes recommended that bicarbonate drips be started temporarily to maintain an acceptable pH if needed, rather than making ventilator changes that would violate LTVV parameters. (Choice B) Another important complication of LTVV is alveolar collapse. This is avoided by increasing positive end-expiratory pressure (PEEP) in patients with moderate-to-severe ARDS (PaO₂/FiO₂ <200). A drawback of high PEEP is an increase in peak and plateau pressures. Because this patient's plateau pressure is within the recommended range (≤30 cm H₂O), there is no need to reduce PEEP. (Choice C) This patient's arterial oxygen tension is already within the recommended range. When possible, the FiO₂ should be decreased to <60% to reduce the risk of oxygen toxicity. (Choice D) Although increasing the respiratory rate increases ventilation and is expected to lower paCO₂, a rate of >35/min is not advisable as it can result in breath stacking and auto-PEEP. Permissive hypercapnia is preferred. (Choice E) Historically, ventilator settings were adjusted in an attempt to normalize the arterial blood gas results (eg, eliminate hypercapnia), including raising tidal volumes to >6 mL/kg. However, persistent tidal volumes >6 mL/kg are more likely to cause lung injury due to elevated pressures, and should be avoided. Educational objective: Ventilator-induced lung injury is a major cause of mortality and morbidity in acute respiratory distress syndrome. Injury can be avoided by using low tidal volume ventilation (4-6 mL/kg of ideal body weight) and maintaining a plateau pressure of ≤30 cm H₂O. The resulting hypercapnia is permissible as long as pH is ≥7.20 and the patient lacks contraindications. __________ References: Surviving sepsis campaign: international guidelines for management of severe sepsis and septic shock: 2012. (http://www.ncbi.nlm.nih.gov/pubmed/23353941)

A 56-Year-Old Man with a History of Alcohol Abuse Is