Hypoventilation

Underlying causes can be at the level of the CNS, diminishing the drive to breathe, or in the respiratory pump with disease affecting the chest wall or inspiratory muscles, or lung abnormalities associated with increased dead space or severe ventilation-perfusion mismatch.

Alveolar ventilation is insufficient to meet metabolic demands, with elevations in arterial carbon dioxide tension and decrease in PaO2.

Nervous system or respiratory muscle problems impair inspiratory muscle force and limit tidal volume and include spinal cord injuries, myasthenia gravis, amyotrophic lateral sclerosis and myopathies.

Obesity, kyphoscoliosis, previous thoracotomy and ankylosing spondylitis decrease respiratory system compliance and increased the work of breathing.

Chronic obstructive pulmonary disease and interstitial lung disease increase work of breathing and impair gas exchange.

With disordered respiratory drive there are defects in the metabolic respiratory control system whereby abnormalities in blood gases and cerebral acid-base status are not sensed or does not result in appropriate brainstem response and can occur with encephalitis, malignancy, stroke, hypothyroidism and use of opiates and sedatives.

Occurs when central drive to breathing is decreased, which is most commonly secondary to medications.

When minute ventilation drops, increased CO2 displaces oxygen, and decreases oxygen available for capillary uptake, and this occurs because combined partial pressures of all the gases within an alveolus must equal atmospheric pressure.

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