The Correlations of the Six-minute Walk Test and Respiratory Functions in Chronic Obstructive Pulmonary Disease Patients with Chronic Hypercapnia

Background: Dyspnea and related disabling symptoms are common in chronic obstructive pulmonary disease (COPD) patients with chronic hypercapnia. Unfortunately, the indicators during the six-minute walk test (6MWT) for prediction of respiratory functions or exercise intolerance in severe COPD has been little investigated. The relationship between parameters during the 6MWT and respiratory functions was therefore assessed in COPD patients with chronic hypercapnia. Methods: In 2002 and 2003, 37 COPD outpatients with chronic hypercapnia performed the 6MWT, and their respiratory function was measured. Twenty-eight males and nine females with COPD (mean forced expiratory volume in the first second of 26.1% of the predicted value, SD 7.7%) and hypercapnia (mean PaCO ₂ of 55.5mmHg, SD 6.4mmHg) were recruited. All patients were tested to measure pulmonary function, respiratory drive (airway occlusion pressure at 100ms, P _0.1), and respiratory muscle strength on the first day. On the second day, arterial blood gas analysis and the 6MWT were performed. Pearson's correlation coefficient and regression analysis were used for data analysis. Results: The study showed that the six-minute walk distance (6MWD) was weakly correlated with the resting arterial oxygen partial pressure (PaO ₂) (r=0.349, p=0.034), expiratory muscle strength (Pemax) (r=0.358, p=0.030), and changes of dyspnea sensation ({increment}Borg) (r=0.385, p=0.019); furthermore, {increment}Borg was weakly correlated with Pemax (r=0.377, p=0.021). The oxygen saturation measured at the end of the 6MWT (ExSpO ₂) was significantly correlated with FEV ₁/FVC (r=-0.443, p=0.006), pH (r=0.375, p=0.022), arterial carbon dioxide partial pressure (PaCO ₂) (r=-0.470, p=0.003), PaO ₂ (r=0.664, p=0.000) and P _0.1 (r=-0.344, p=0.037). The results of the multiple linear regression with the 6MWD as the dependent variable revealed that PaO _2, Pemax, and {increment}Borg were significant determinants of the 6MWD (p=0.018, adjusted R ²=0.259). Conclusion: Measurement of the 6MWT demonstrated that a stronger association of exercise limitation is the value of {increment}Borg in COPD patients with chronic hypercapnia. Ventilation constraints, hypoxemia, hypercapnia, and respiratory drive might be associated with oxygen desaturation during the 6MWT in COPD patients with chronic hypercapnia.