Heart Rate Response Predicts 6-Minutes Walking Distance in Pulmonary Arterial Hypertension

Because the 6-minute walking test (6MWT) is a self-paced submaximal test, the 6-minute walking distance (6MWD) is substantially influenced by individual effort level and physical condition, which is difficult to quantify. We aimed to explore the optimal indicator reflecting the perceived effort level during 6MWT. We prospectively enrolled 76 patients with pulmonary arterial hypertension and 152 healthy participants; they performed 2 6MWTs at 2 different speeds: (1) at leisurely speed, as performed in daily life without extra effort (leisure 6MWT) and (2) an increased walking speed, walking as the guideline indicated (standard 6MWT). The factors associated with 6MWD during standard 6MWT were investigated using a multiple linear regression analysis. The heart rate (HR) and Borg score increased and oxygen saturation (SO) decreased after walking in 2 6MWTs in both groups (all p <0.001). The ratio of difference in HR before and after each test (ΔHR) to HR before walking (HR) and the difference in SpO (ΔSpO) and Borg (ΔBorg) before and after each test were all significantly higher in both groups after standard 6MWT than after leisure 6MWT (all p <0.001). Multiple linear regression analysis revealed that ΔHR/HR was an independent predictor of 6MWD during standard 6MWT in both groups (both p <0.001, adjusted R = 0.737 and 0.49, respectively). 6MWD and ΔHR/HR were significantly lower in patients than in healthy participants (both p <0.001) and in patients with cardiac functional class III than in patients with class I/II (both p <0.001). In conclusion, ΔHR/HR is a good reflector of combined physical and effort factors. HR response should be incorporated into 6MWD to better assess a participant's exercise capacity.