Figure 36-19 Mechanisms of airways obstruction producing an upsloping phase III capnogram. In a normal, healthy person (upper panel), there is a narrow range of V̇A/ ratios with values close to 1. Gas exchange units therefore have similar PCO₂ and tend to empty synchronously, and the expired PCO₂ remains relatively constant. During the course of exhalation, the alveolar PCO₂ slowly rises as CO₂ continuously diffuses from the blood. This causes a slight increase in PCO₂ toward the end of expiration, and this increase can be pronounced if the exhalation is prolonged (see Fig. 36-18C ). In a patient with diffuse airways obstruction (lower panel), the airway pathology is heterogeneous, with gas exchange units having a wide range of V̇A/ ratios. Well-ventilated gas exchange units, with gas containing a lower PCO₂ , empty first; poorly ventilated units, with a higher PCO₂ , empty last. In addition to the continuous rise in PCO₂ mentioned previously, there is a progressive increase caused by asynchronous exhalation.