Mechanisms affecting exercise ventilatory inefficiency-airflow obstruction relationship in male patients with chronic obstructive pulmonary disease

Abstract

Background: Exercise ventilatory inefficiency is usually defined as high ventilation (V ˙ E $$ \dot{\mathrm{V}}\mathrm{E} $$) versus low CO2 output (V ˙ CO 2 $$ \dot{\mathrm{V}}\mathrm{CO}2 $$). The inefficiency may be lowered when airflow obstruction is severe because V ˙ E $$ \dot{\mathrm{V}}\mathrm{E} $$ cannot be adequately increased in response to exercise. However, the ventilatory inefficiency-airflow obstruction relationship differs to a varying degree. This has been hypothesized to be affected by increased dead space fraction of tidal volume (VD/VT), acidity, hypoxemia, and hypercapnia. Methods: A total of 120 male patients with chronic obstructive pulmonary disease were enrolled. Lung function and incremental exercise tests were conducted, and V ˙ E $$ \dot{\mathrm{V}}\mathrm{E} $$ versus V ˙ CO 2 $$ \dot{\mathrm{V}}\mathrm{CO}2 $$ slope (V ˙ E / V ˙ CO 2 S $$ \dot{\mathrm{V}}\mathrm{E}/\dot{\mathrm{V}}\mathrm{CO}2\mathrm{S} $$) and intercept (V ˙ E / V ˙ CO 2 I $$ \dot{\mathrm{V}}\mathrm{E}/\dot{\mathrm{V}}\mathrm{CO}2\mathrm{I} $$) were obtained by linear regression. Arterial blood gas analysis was also performed in 47 of the participants during exercise tests. VD/VT and lactate level were measured. Results: VD/VTpeak was moderately positively related to V ˙ E / V ˙ CO 2 S $$ \dot{\mathrm{V}}\mathrm{E}/\dot{\mathrm{V}}\mathrm{CO}2\mathrm{S} $$ (r = 0.41) and negatively related to forced expired volume in 1 sec % predicted (FEV1%) (r = - 0.27), and hence the FEV1%- V ˙ E / V ˙ CO 2 S $$ \dot{\mathrm{V}}\mathrm{E}/\dot{\mathrm{V}}\mathrm{CO}2\mathrm{S} $$ relationship was paradoxical. The higher the V ˙ E / V ˙ CO 2 S $$ \dot{\mathrm{V}}\mathrm{E}/\dot{\mathrm{V}}\mathrm{CO}2\mathrm{S} $$, the higher the pH and PaO2, and the lower the PaCO2 and exercise capacity. V ˙ E / V ˙ CO 2 I $$ \dot{\mathrm{V}}\mathrm{E}/\dot{\mathrm{V}}\mathrm{CO}2\mathrm{I} $$ was marginally related to VD/VTrest. The higher the V ˙ E / V ˙ CO 2 I $$ \dot{\mathrm{V}}\mathrm{E}/\dot{\mathrm{V}}\mathrm{CO}2\mathrm{I} $$, the higher the inspiratory airflow, work rate, and end-tidal PCO2peak. Conclusion: 1) Dead space ventilation perturbs the airflow- V ˙ E / V ˙ CO 2 S $$ \dot{\mathrm{V}}\mathrm{E}/\dot{\mathrm{V}}\mathrm{CO}2\mathrm{S} $$ relationship, 2) increasing ventilation thereby increases V ˙ E / V ˙ CO 2 S $$ \dot{\mathrm{V}}\mathrm{E}/\dot{\mathrm{V}}\mathrm{CO}2\mathrm{S} $$ to maintain biological homeostasis, and 3) the physiology- V ˙ E / V ˙ CO 2 S $$ \dot{\mathrm{V}}\mathrm{E}/\dot{\mathrm{V}}\mathrm{CO}2\mathrm{S} $$ - V ˙ E / V ˙ CO 2 I $$ \dot{\mathrm{V}}\mathrm{E}/\dot{\mathrm{V}}\mathrm{CO}2\mathrm{I} $$ relationships are inconsistent in the current and previous studies. Trial Registration: MOST 106-2314-B-040-025.