9 Acute Respiratory Failure

Acute respiratory failure is one of the leading causes of admission to an intensive care unit (ICU). Behrendt et al. reported that the incidence of acute respiratory failure requiring hospitalization was 137 per 100,000 population in the United States, and the median age of the patients was 69 years.¹ More recently, Ray et al. reported that 29% of patients presenting to an emergency department (ED) with acute respiratory failure require admission to an ICU.²

Acute respiratory failure can be secondary to either a failure of oxygenation (hypoxic respiratory failure), a failure of elimination of carbon dioxide (hypercarbic respiratory [ventilatory] failure), or both problems simultaneously. Chronic obstructive pulmonary disease (COPD) with acute exacerbation is the most common cause of ventilatory failure requiring ICU admission.

Causes of Hypoxic Respiratory Failure

Hypoventilation

The partial pressure of carbon dioxide in arterial blood (PaCO₂) increases when minute ventilation decreases. An increase in PaCO₂ decreases alveolar partial pressure of oxygen (PAO₂) because the carbon dioxide displaces oxygen in the alveoli. The relationship between PAO₂ and PaCO₂ is described by the alveolar gas equation:

The causes of hypoventilation are discussed below.

Ventilation/Perfusion Mismatch

Gas exchange is optimal when ventilation and perfusion in the lung are matched. A decrease in perfusion relative to ventilation (i.e., an increase in physiologic dead space) or a decrease in ventilation relative to perfusion (shunt) results in ventilation-perfusion () mismatching. Hypoxia occurs as a result of mismatching because of admixture of venous with arterial blood at the capillary level. mismatching is the most common cause of hypoxia in hospitalized patients. In contrast to hypoxemia caused by an anatomic shunt, hypoxemia caused by mismatching can be improved by administration of supplemental oxygen.

Shunt

Right-to-left shunting refers to the process whereby deoxygenated venous blood bypasses functioning alveolar-pulmonary capillary units and then mixes with oxygenated arterial blood. Right-to-left shunts can be caused by anatomic derangements, such as certain congenital cardiac malformations (e.g., atrial septal defect), but they can also occur when mismatching is so severe that a portion of pulmonary arterial blood flows through lung regions with essentially no ventilation. Potential causes of this sort of physiologic shunting include pneumonia, lung contusion, or severe congestive heart failure. Oxygenation cannot be improved with supplemental oxygen in patients with a true right-to-left shunt, irrespective of whether the shunt is caused by an anatomic or a functional derangement.

Diffusion Impairment

Thickening of the alveolar endothelial/epithelial barrier or a decrease in transit time in the pulmonary capillary bed (due to very high cardiac output) can impair diffusion of oxygen from the alveoli into the blood.

High Altitude

Barometric pressure decreases with increasing altitude; as a result, the partial pressure of oxygen in the ambient atmosphere decreases as well. Consequently, unless supplemental oxygen is provided, hypoxia is an inevitable consequence of respiration at high altitude.

Impaired Tissue Perfusion

When tissue perfusion is impaired, the cells attempt to maintain normal oxygen consumption by extracting more oxygen from the available blood supply. As a consequence, venous oxygen tension decreases. Unless the fractional pulmonary shunt flow is zero, the decrease in mixed venous oxygen tension inevitably leads to a decrease in arterial oxygen tension. Although low cardiac output or impaired blood flow to tissues can cause hypoxia, hypoperfusion per se is rarely a primary cause of clinically significant hypoxia. Nevertheless, hypoperfusion is a common factor that exacerbates the degree of hypoxia caused by other problems.

If the circulating concentration of carboxyhemoglobin or methemoglobin increases, the oxygen-carrying capacity of the blood decreases. Although arterial oxygen tension may be normal, arterial oxygen saturation is abnormally low because of the presence of Hb derivatives that are incapable of transporting oxygen.