Acute Respiratory Failure

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.

image Causes of Hypoxic Respiratory Failure

image Hypercarbic Respiratory Failure

PaCO2 is inversely proportional to alveolar ventilation; thus, PaCO2 increases when the elimination of carbon dioxide is decreased because of a decrease in minute ventilation. PaCO2 also increases if minute ventilation remains constant but carbon dioxide production increases. Primary pulmonary diseases are the most common cause of hypercarbia, although nonpulmonary causes contribute to hypoventilation, increased PaCO2, and the need for mechanical ventilatory support.

Minute ventilation can be decreased owing to pulmonary or nonpulmonary factors. Pulmonary causes of impaired minute ventilation include large airway obstruction (e.g., due to the presence of a foreign body or laryngeal spasm), small airway obstruction (e.g., bronchospasm), and destruction of lung parenchyma (e.g., emphysema). Extrapulmonary causes of hypercarbia include neurologic and muscular problems. Neurologic problems include depression of central respiratory drive due to the pharmacologic effects of narcotics or sedatives; depression of respiratory drive as a consequence of stroke, intracranial hemorrhage, or head trauma (i.e., central alveolar hypoventilation); and impaired neuromuscular transmission due to phrenic nerve injury or spinal cord injury (C5 or higher), Guillain-Barré syndrome, myasthenia gravis, or the polyneuropathy of critical illness. Muscular weakness or skeletal abnormalities can cause a decrease in tidal volume and minute ventilation. Causes of hypoventilation secondary to musculoskeletal abnormalities are prolonged use of neuromuscular blocking agents, malnutrition, hypomagnesemia, hypokalemia, hypophosphatemia, kyphoscoliosis, rib fractures, and flail chest, to name several.

In rare cases, hypercarbia can be secondary to increased carbon dioxide production and relative hypoventilation due to overfeeding, since fat synthesis increases the rate of carbon dioxide production relative to the rate of oxygen consumption (respiratory quotient >1.0). Hypermetabolism, such as occurs with high fever or thyrotoxicosis, also is associated with increased carbon dioxide production and (in the setting of already impaired minute ventilation) can exacerbate hypercarbia.

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Jul 7, 2016 | Posted by in CRITICAL CARE | Comments Off on Acute Respiratory Failure

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