Answer:

The history of the patient’s illness provides key evidence for determining the cause of respiratory failure. The clinician should review the past medical history, anesthesia record, operative reports, and perform a careful physical examination. The integration of these data should generate a short list of diagnostic possibilities (Table 3.1). Laboratory testing can help exclude or confirm diagnoses.

Arterial blood gas analysis can help triage disease severity and identify alveolar hypoventilation, increased alveolar-arterial oxygen gradient, and acid-base status. Additional labs including complete blood count, metabolic profile, and coagulation parameters are an important part of the early evaluation. Chest x-rays are an essential part of the diagnostic approach to respiratory failure. However, their interobserver variability is high and their diagnostic accuracy is limited in acute respiratory failure.

Point-of-care diagnostic ultrasound can aid the diagnosis of acute respiratory failure (Table 3.2). In critically ill patients, pulmonary ultrasound is superior to chest x-ray in identifying pneumothorax, consolidation, pleural effusion, and lung edema. The use of basic echocardiography to identify right or left heart dysfunction can improve diagnostic accuracy. This approach can help distinguish cardiogenic from noncardiogenic causes of pulmonary edema and raise the suspicion for pulmonary embolism. Ultrasonography, like all clinical skills, requires expert training.

Pulmonary emboli (PE) are common in the postoperative period. PE cause tachycardia, tachypnea, and mild hypoxemia. Large PE may cause shock. An abrupt increase in arterial to end-tidal PCO₂ gradient can be a sign of increased alveolar dead space from PE. If a clinician suspects PE, a computed tomography (CT) angiography is the best test. Ventilation-perfusion scintigraphy is an alternative if CT angiography cannot be performed. Given the high case fatality rate of untreated PE, therapeutic anticoagulation is indicated when PE is strongly suspected or confirmed. Clinicians must carefully consider the risk of bleeding from surgery or neuraxial anesthesia sites.

Pneumonia is the most important postoperative infection. Surgery, hospitalization, and illness alter the host’s bacterial flora and lead to pneumonia with antibiotic-resistant organisms. A chest x-ray, arterial blood gas, and blood cultures should be obtained in all patients suspected of having health care-associated pneumonia. In the case of ventilator-associated pneumonia, clinical diagnosis may be inaccurate. Quantitative cultures of the lower respiratory tract obtained from bronchoalveolar lavage or protected brushings can assist the diagnosis.

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