Relationship between breathlessness and hypoxic and hypercapnic ventilatory response in patients with COPD.

The purpose of this study is to examine the relationship between breathlessness and the ventilatory response to hypercapnia or hypoxia in patients with chronic obstructive pulmonary disease (COPD). Fifteen male patients (mean forced expiratory volume in one second (FEV1): 1.13 L) underwent tests to determine hyperoxic hypercapnic ventilatory response (HCVR) and isocapnic hypoxic ventilatory response (HVR) with simultaneous quantification of breathlessness by modified Borg scale. The ventilatory output was evaluated by the ratio of minute ventilation (V'E) divided by measured maximal voluntary ventilation (MVV). The magnitude of HCVR or HVR was assessed as the slope value of the V'E/MVV-end-tidal carbon dioxide pressure (PET,CO2) or arterial oxygen saturation (Sa,O2) regression line, respectively. The breathlessness during the tests was evaluated not only linearly in relation to V'E/MVV, but also at given levels of PET,CO2 or Sa,O2. The mean value of the breathlessness at two different levels of ventilation was greater during HVR than during HCVR, suggesting that hypoxia is dyspnogenic independently of ventilatory stimulation. The HCVR was inversely correlated with the breathlessness response to ventilation, while similar correlation was partly present for HVR. The HVR was positively correlated with the breathlessness at an Sa,O2 of 80%, whilst there was no such correlation between the HCVR and the breathlessness related to PET,CO2. Therefore, patients with a higher breathlessness related to increased ventilation had a lower HCVR and HVR, whilst those with a higher breathlessness with desaturation, which might include a direct influence of hypoxia, had a higher HVR. These findings suggest an interaction between ventilatory response and breathlessness during the test, which may partly include behavioural modulation of HCVR and HVR through the breathlessness in various ways, depending on the origin and nature of the sensation.