Physiologic Stress During Walking and Cycling Early After Cardiac Surgery

Purpose: To characterize cardiac effort and heart rate (HR) responses during the 6-minute walk test (6MWT) early after cardiac surgery, compare HR patterns between walking and cycle ergometry, and evaluate differences by sex, frailty, and atrial fibrillation status. Methods: We analyzed a publicly available PhysioNet dataset of adults ≥65 years with pre-frailty/frailty undergoing cardiac surgery who completed a 6MWT and symptom-limited cycle ergometry with continuous single-lead electrocardiogram (ECG) monitoring (Polar H10). Cardiac effort was defined as total 6MWT heartbeats divided by distance walked (beats/m). Associations with cycle-ergometry performance were assessed using Spearman correlations with bootstrap CI; paired t tests compared HR responses between modalities; subgroup differences were evaluated using Welch t tests and multivariable linear regression. Results: Of 80 enrolled participants, 66 (83%) had analyzable 6MWT ECG recordings (mean age 73 ± 5 years; 67% male; 18 ± 8 days post-surgery). Mean 6-minute walking distance (6MWD) was 300 ± 79 m, and mean cardiac effort was 1.62 ± 0.52 beats/m. In 38 participants with paired cycle-ergometry data, HR responses were similar between walking and cycling. Greater 6MWD was associated with better cycle-ergometry performance, while higher cardiac effort was associated with lower cycle-ergometry performance. In multivariable analysis, female sex was independently associated with higher cardiac effort ( β = .34 beats/m; P = .042), atrial fibrillation with lower average and end-of-test HR ( P ≤ .035), and greater frailty with shorter 6MWD ( P = .036). Conclusions: Cardiac effort is a feasible ECG-derived metric that complements 6MWD by integrating heart rate response with functional performance early after cardiac surgery. In this frail, predominantly beta-blocked cohort, HR responses during walking closely mirrored cycle ergometry.