Anesthesia : Acute Intrinsic Pulmonary Disorders

ACUTE INTRINSIC PULMONARY DISORDERS

Acute intrinsic pulmonary disorders include pulmo-nary edema (including the acute respiratory distress syndrome [ARDS]), infectious pneumonia, and aspiration pneumonitis.

Preoperative Considerations

Reduced lung compliance in these disorders is pri-marily due to an increase in extravascular lung water, from either an increase in pulmonary capillary pres-sure or pulmonary capillary permeability. Increased pressure occurs with left ventricular failure, whereas fluid overload and increased permeability are pres-ent with ARDS. Localized or generalized increases in permeability also occur following aspiration or infectious pneumonitis.

Anesthetic Considerations

A. Preoperative Management

Patients with acute pulmonary disease should be spared elective surgery. In preparation for emer-gency procedures, oxygenation and ventilation should be optimized preoperatively to the greatest extent possible. Fluid overload should be treated with diuretics; heart failure may also require vaso-dilators and inotropes. Drainage of large pleural effusions should be considered. Similarly, massive abdominal distention should be relieved by naso-gastric compression or drainage of ascites. Persistent hypoxemia may require mechanical ventilation.

B. Intraoperative Management

Selection of anesthetic agents should be tailored to each patient. Surgical patients with acute pulmonary disorders, such as ARDS, cardiogenic pulmonary edema, or pneumonia, are critically ill; anesthetic management should be a continuation of their pre-operative intensive care. Anesthesia is most often provided with a combination of intravenous and inhalation agents, together with a neuromuscular blocking agent. High inspired oxygen concentra-tions and PEEP may be required. The decreased lung compliance results in high peak inspiratory pressures during positive-pressure ventilation and increases the risk of barotrauma and volutrauma. Tidal volumes for these patients should be reduced to 4–6 mL/kg, with a compensatory increase in the ven-tilatory rate (14–18 breaths/min), even if the result is an increase in end-tidal CO₂. Airway pressure should generally not exceed 30 cm H ₂O. Airway pressure release ventilation may improve oxygenation in the ARDS patient. The ventilator on the anesthesia machine may prove inadequate for patients with severe ARDS because of its limited gas flow capabili-ties, low pressure-limiting settings, and the absence of certain ventilatory modes. A more sophisticated intensive care unit ventilator should be used in such instances. Aggressive hemodynamic monitoring is recommended.