On the other hand, the ventilation-perfusion ratios are altered, as evidenced by increased VD/VT ratios. From the time of administering pure oxygen hypercapnia is only weakly linked to changes in total ventilation (which, after a few minutes returns to its initial level) and only slightly to the correction of hypoxaemia and desaturation (Haldane effect). In chronic airflow obstruction, CO2 production and ventilation are normal thus the factor responsible for hypercapnia is essentially the fraction of total ventilation lost in the anatomical and alveolar (VD/VT ratio) dead space, whose effect on PaCO2 is all the more marked on account of the high starting point. The determinants of PaCO2 are metabolic (hence importance of alkalosis) and ventilatory. Hypercapnia is common in chronic respiratory failure (IRCO), and may be further increased in a significant way by oxygen therapy, used for severe hypoxaemia in acute exacerbations.
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