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Hypoventilation (also known as respiratory depression) occurs when ventilation is inadequate (hypo meaning "below") to perform needed respiratory gas exchange. [1] By definition it causes an increased concentration of carbon dioxide (hypercapnia) and respiratory acidosis. Hypoventilation is not synonymous with respiratory arrest, in which ...
Lung diseases that primarily cause abnormality in alveolar gas exchange usually do not cause hypoventilation but tend to cause stimulation of ventilation and hypocapnia secondary to hypoxia. Hypercapnia only occurs if severe disease or respiratory muscle fatigue occurs.
The main physiologic causes of hypocapnia are related to hyperventilation. Hypocapnia is sometimes induced in the treatment of medical emergencies such as intracranial hypertension [3] and hyperkalemia. Self-induced hypocapnia through hyperventilation is the basis for the dangerous schoolyard fainting game.
Alveolar hypoventilation (decreased minute volume due to reduced respiratory muscle activity, e.g. in acute neuromuscular disease); this form can also cause type 2 respiratory failure if severe. Diffusion problem (oxygen cannot enter the capillaries due to parenchymal disease, e.g. in pneumonia or ARDS).
The density of the breathing gas is higher at depth, so the effort required to fully inhale and exhale increases, making breathing more difficult and less efficient (high work of breathing). [13] [3] [18] Higher gas density also causes gas mixing within the lung to be less efficient, thus increasing the effective dead space. [4] [5]
Causes include hypoventilation, impaired alveolar diffusion, and pulmonary shunting. [8] This definition overlaps considerably with that of hypoxic hypoxia. Pulmonary hypoxia is hypoxia from hypoxemia due to abnormal pulmonary function, and occurs when the lungs receive adequately oxygenated gas which does not oxygenate the blood sufficiently.
An abrupt stop of pulmonary gas exchange lasting for more than five minutes may permanently damage vital organs, especially the brain. Lack of oxygen to the brain causes loss of consciousness. Brain injury is likely if respiratory arrest goes untreated for more than three minutes, and death is almost certain if more than five minutes.
Hypoventilation exists when the ratio of carbon dioxide production to alveolar ventilation increases above normal values – greater than 45mmHg. If pH is also less than 7.35 this is respiratory acidosis .