Mechanisms of hypercapnia in patients with chronic obstructive pulmonary disease (COPD)

RESUMO

A higher ventilatory drive evaluated by the inspiratory occlusion pressure (Poc) and a respiratory pattern characterized by smaller tidal volume (VT) and higher breathing frequency (f) was detected in patients with chronic obstructive pulmonary disease (COPD), in relation to normals. The purpose of this study was to identify the possible mechanisms involved in the development of hypercapnia in those patients, at rest and during exercise. We have studied 11 normocapnic (PaCO2 ( 45 mmHg) and 9 hypercapnic (PaCO2 > 45 mmHg) COPD patients. As expected, no difference in the ventilatory response and neural drive was detected between the two groups. However, the hypercapnic patients have higher values of serum HCO-3 and lower values of PaO2 at rest and values of the ratio dead volume to tidal volume (VD/VT) significantly higher at rest (0.67 vs. 0.55) and during exercise (0.54 vs. 0.38) in relation to normocapnic individuals. There was also a significant positive correlation at rest (r = 0.66*) and during exercise (r = 0.65*; *p < 0.05), between PaCO2 and VD/VT, identifying a decreased alveolar ventilatory efficiency, important in the development of hypercapnia in those patients. when the COPD patients were divided into two distinct groups (PaCO2 ( 40 and ( 50 mmHg), a respiratory pattern characterized by higher f and smaller VT was detected in the hypercapnic group during exercise. In conclusion, a higher VD/VT linked to alterations of the respiratory pattern (lower VT) and to inequalities of ventilation/perfusion (high V/Q areas), seems to explain the hypercapnia of our COPD patients, since the ventilatory response and neural drive were similar in normo and hypercapnic patients.