Hypoventilation occurs when the alveolar ventilation decreases relative to production of CO 2. The relationship between production and excretion is apparent from the
following equation, which indicates that when alveolar ventilation ( VA) falls and C02 production ( VC02) remains constant, the arterial PaC02 must increase:
If it is assumed that the respiratory ratio R remains constant, the alveolar gas equation further demonstrates that, for each unit rise in arterial PCO2, alveolar PO2 decreases by an approximately constant amount:
where PIO2 is the partial inspiratory oxygen tension. This equation also indicates that the hypoxemia of hypoventilation is readily overcome by administration of supplemental oxygen; inspiration of pure oxygen increases alveolar PO2 to 102 kPa, while arterial PCO2 rarely rises to levels above 14 kPa. The interrelationship of these variables is shown in Fig 1.
Fig. 1 Interrelationship between alveolar oxygen and carbon dioxide concentrations, pH, and oxygen saturation at varying minute ventilation. (Reproduced with permission from W§§L(199.8).)
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