Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 89

In restrictive conditions, a reduced KCO suggests an intrapulmonary cause (e.g. fibrosis). In extrapulmonary causes (e.g. chest wall deformity, respiratory muscle weakness, obesity), the KCO tends to be elevated. (Reason: KCO is effectively telling us about the transfer of CO only in the alveoli that are ventilated. The non‐ventilated alveoli are effectively discounted because they don’t contribute to V_A. As the V/Q matching system will divert blood away from the non‐ventilated alveoli, the ventilated alveoli will have more than their normal share of blood. The greater blood volume increases CO absorption and thus gas transfer.)

In obstructive conditions, a reduced KCO suggests COPD (emphysema). In asthma, the KCO may be elevated. (Reason: Asthma does not affect every airway to an identical degree; there is therefore an exaggerated heterogeneity of ventilation. As discussed already, KCO is more heavily influenced by the well‐ventilated areas which, because of V/Q matching, have more than their fair share of perfusion.)

In the presence of normal spirometry, a reduced KCO is a strong indicator of intrinsic lung disease (affecting the pulmonary vasculature or alveoli; consider pulmonary hypertension or a combination of emphysema and fibrosis). (Reason: the effects of emphysema and fibrosis on FEV₁:FVC ratio cancel each other out though both cause a diminution in gas transfer.)