Relationship between minute ventilation and PaCO

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In normal exercising humans, the minute ventilation increases in proportion to both the metabolic rate and cardiac output. The alveolar ventilation increases because lung perfusion increases in proportion to the ventilation while the anatomical dead-space either decreases (due to larger tidal volumes) or remains unchanged. Thus both CO 2 production and alveolar ventilation increase during exercise, so that PaCO2 either remains the same or decreases slightly. In contrast, a normal subject on a positive-pressure ventilator requires an above-normal minute volume to maintain a normal PaCO2; this is generally attributed to an increase in dead-space ventilation.

When a normal non-exercising human increases minute ventilation spontaneously, the cardiac output and metabolic rate do not increase in proportion. The predicted relationships between minute volume and PaCO2 in non-exercising humans are listed in T§M§...,3,. Spontaneously breathing patients with an increased minute ventilation and PaCO2 significantly higher than predicted in Tab]— have either significantly increased dead-space ventilation or increased CO 2 production. The existence of a significant disparity between minute volume and PaCO2 should alert the clinician to the possibility that an acute dead-space-producing pathology (diminished cardiac output, pulmonary emboli) may be present.

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Table 3 Expected relationships between minute volume and PaCO2 in the normal non-exercising human

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Responses

  • russell
    How is minute ventilation compared to cardiac output?
    1 year ago
  • Jesse
    How is minute ventilation comparable to cardiac output (in terms of how it is calculated)?
    1 year ago
  • Geraldine
    How is minute ventilation related to cardiac output in calculations?
    1 year ago

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