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Chapter 137. Respiratory Failure

Eric M. Siegal, MD, SFHM

  1. How should a clinician rapidly assess the severity and stability of a patient with acute respiratory distress?

  2. How can principles of respiratory physiology be effectively applied to guide diagnosis and therapy?

  3. What are the indications and contraindications for noninvasive positive pressure ventilation (NPPV), and what outcomes does this intervention affect?

  4. When does a patient require advanced airway management?

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Case 137-1

A 53-year-old with COPD was hospitalized with acute pancreatitis. He had received substantial fluid resuscitation and parenteral morphine for severe epigastric pain. He became increasingly confused and somnolent, and his oxygen saturation acutely decreased to 82 percent on 2 liters per nasal cannula. On a nonrebreather face mask, his SpO2 increased to 98 percent. He appeared uncomfortable, was tachycardic, tachypneic, and moderately confused. His expiratory phase was prolonged, and breath sounds were nearly absent at the lung bases. He had moderate peripheral edema, and his epigastrium was moderately tender.

Acute respiratory failure, a common inpatient medical emergency, mandates rapid assessment of the patient and initiation of appropriate and potentially life-saving therapy. Treatment decisions often must be made with limited information and well before a definitive diagnosis can be established. In this time-pressured, high-risk environment, clinicians must develop a diagnostic and therapeutic schema that facilitates a rapid and comprehensive approach to the patient with acute respiratory failure. The four key steps to approaching the patient with acute respiratory failure are

  1. Rapid assessment of the severity of the presentation and of the patient's stability

  2. Determination of the likely cause or causes

  3. Initiation of treatment

  4. Assessment of the efficacy of treatment

Understanding the pathophysiologic underpinnings of acute respiratory failure simplifies and increases the accuracy of the diagnostic approach and the likelihood of choosing a successful therapeutic intervention. The five main etiologies of acute respiratory failure are

  • Shunt
  • Ventilation/perfusion (V/Q) mismatch
  • Impaired diffusion
  • Decreased mixed venous oxygen saturation
  • Alveolar hypoventilation

Shunt

Shunt occurs when a portion of pulmonary blood flow does not participate in gas exchange. Physiologic shunt, the most common cause of acute hypoxemia in the hospitalized patient, develops when pulmonary perfusion is inappropriately maintained to injured, atelectatic or collapsed alveolar units. A relatively small area of infiltrate or atelectasis may cause profound hypoxemia if there is significant shunt to that area. Anatomic shunts describe physical conduits that divert deoxygenated blood into the systemic circulation, as occurs with right to left intracardiac shunt or intrapulmonary shunt. In both anatomic and physiologic shunts, deoxygenated blood enters the systemic circulation. As the degree of shunt (shunt fraction) increases, the partial pressure of arterial oxygen (PaO2) becomes increasingly independent of the fraction of inspired oxygen (FiO2), and once the shunt fraction exceeds 50%, increasing FiO2 does not increase PaO2. In conditions characterized by high shunt fractions, the FiO2 can often be lowered without compromising ...

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