The next day, the patient is still in the PACU. The chest radiograph shows bilateral fluffy infiltrates, and the ABG is pH 7.32, PCO2 54 torr, and PO2 55 torr on CMV with a respiratory rate of 14 breaths per minute, a tidal volume of 550 mL, an FiO2 of 0.8 and a PEEP of 5 cm H2O. A pulmonary artery catheter is inserted, showing a pulmonary artery pressure (PAP) of 54/32 mmHg, pulmonary artery occlusion pressure (PAOP) of 13 mmHg, central venous pressure (CVP) of 12 mmHg, and a cardiac output (CO) of 5.4 L/min.
How would you manage this patient?
This patient presents with adult respiratory distress syndrome (ARDS). The criteria of the American-European Consensus Conference are:
· Acute onset ≤48 hours.
· Bilateral infiltrates on chest radiograph.
· PaO2/FiO2 ratio <200. [If the ratio is between 200 and 300, the syndrome is termed acute lung injury (ALI).]
· No cardiogenic pulmonary edema defined as a PAOP (“wedge”) <18 mmHg.
ARDS is remarkable for heterogeneous lung lesions, with normal alveoli, obliterated alveoli, and “recruitable” areas. Mortality remains >50%, but only 20% is due to respiratory failure. The main cause of death is multiple organ failure.
Obviously, in spite of the definition, a patient can have ARDS and fluid overload. In those cases, the treatment for ARDS should be administered concurrently with that of fluid overload.
The current trend in treating ARDS is based on the following:
· PEEP improves oxygenation by recruiting alveoli (increases functional residual capacity (FRC) and redistributing lung water. Its side-effects are hemodynamic (decrease in venous return) and volutrauma. “Best PEEP” can be determined by plotting a static pressure–volume curve, and is usually greater than 12.5 cm H2O. The “open lung” strategy aims at avoiding the repetitive opening and closing of recruitable alveoli.
· Low tidal volumes, 6 mL/kg of ideal body weight, is the only modality shown in a randomized trial to decrease mortality. Whether volume-controlled or pressure-controlled ventilation is used is unimportant. The aim is to limit the hyperinflation of normal alveoli. Plateau pressure is more important than peak pressure, the goal being to keep plateau pressure (including PEEP) below 30–35 cm H2O. Lowering the tidal volumes, even with an increased respiratory rate, often leads to an increase in the PCO2. The strategy of permissive hypercapnia consists in tolerating a higher PCO2 in order to prevent ventilator-induced lung injury. Keeping the pH >7.2 is recommended with sodium bicarbonate administration as needed.
· Prone positioning may lead to an improvement in oxygena-tion in up to 70% of patients that can persist even after return to the supine position. This strategy has not been shown to decrease mortality and is fraught with practical difficulties.
· Steroids are ineffective and even deleterious during the acute phase, but may be beneficial during the prolifera-tive phase (7–10 days), presumably by limiting fibrosis.
· Fluid management, is directed at keeping the patient as dry as tolerated from the hemodynamic and renal points of view, in attempt to decrease the extent of pulmonary edema. However, there is no evidence to support this approach.