Hypoventilation

Overview
  • Hypoventilation results from a decrease in total lung ventilation below normal and is a basic pathophysiological cause of hypoxemia. Below we discuss how hypoventilation can yield reductions in arterial oxygen tension as well as describe how this cause of hypoxemia can be distinguished from other etiologies. Finally, we detail some of the pathophysiological sequelae which result from isolated hypoventilation.
Pathogenesis
  • Overview
    • Major reductions in total lung ventilation yield corresponding reductions to the rate of alveolar ventilation. Reduced rates of alveolar ventilation then profoundly affect the composition of alveolar air both in terms of its carbon dioxide and oxygen tension.
  • Effects on Alveolar Carbon Dioxide
    • As discussed in alveolar carbon dioxide, the partial pressure of CO2 in the alveolus is inversely proportional to the rate of alveolar ventilation. When alveolar ventilatory rates decline, the rate at which CO2 is eliminated by the lungs correspondingly decreases, thus yielding increased partial pressures of alveolar CO2 (PACO2). Consequently, hypoventilation is always accompanied by increased alveolar carbon dioxide values and in turn increased arterial carbon dioxide values, yielding hypercapnia and potentially respiratory acidosis.
  • Effects on Alveolar Oxygen
    • Declining alveolar ventilation rates also reduce the rate at which alveolar air is refreshed with oxygen-rich external air. As discussed in alveolar oxygen, this yields a decrease in the partial pressure of oxygen in the alveolar space (PAO2). This can be quantitatively appreciated from the "Alveolar Gas Equation" described in the alveolar oxygen page which demonstrates how increased values for alveolar carbon dioxide (PACO2) will result in reduced values for alveolar oxygen (PAO2). Because the alveolar oxygen tension determines the arterial partial pressure of oxygen, a hypoventilating individual can display sufficient decreases in their arterial oxygen tension to be considered hypoxemic.
Diagnosis
  • A-a Gradient
    • As described in oxygen pulmonary gas exchange, lower alveolar partial pressures of oxygen will reduce the oxygen diffusion gradient between the alveolar space and the blood in the pulmonary capillaries. This will result in a slowing in the rate of oxygen diffusion from the alveolar space to the pulmonary capillaries; however, in the absence of other overt pathology, the partial pressures of oxygen in these two compartments should equalize by the end of the pulmonary capillaries. Consequently, in a healthy individual at high elevations, the A-a Gradient will be normal.
  • Response to Oxygen Therapy
    • As observed from the "Alveolar Gas Equation" described in the [alveolar oxygen]] page, an increase in the inspired partial pressure of oxygen (PIO2) will increase the partial pressure of alveolar oxygen. Consequently high tension oxygen therapy should help correct the PAO2 of an individual suffering from isolated hypoventilation.
Etiologies
  • Overview
    • A large variety of etiologies can prevent the body in achieving its normal ventilatory rate and can be categorized according to the step at which the normal ventilatory process is blocked. Normal ventilation is initially organized by the brain, is coordinated by efferent signals conducted to the respiratory muscles, which then modify the biomechanics of the chest wall to induce inward and outward movement of airflow.
  • Central Ventilatory Defects
    • Voluntary Breath Holding
    • Pharmacological central respiratory depression by opiates
  • Conduction Defects
    • Spinal Cord Injury
    • Poliomyelitis: A complication of Polio Virus infection
    • Guillain-Barre Syndrome
  • Biomechanical Defects
  • Airflow Defects