Fractal cardiac activity regulation during exercise and delirium risk in older adults

Delirium, acute confusion marked by fluctuations of attention and concentration, is associated with increased risk for poor long-term health outcomes, including Alzheimer's disease (AD), institutionalization, and mortality. Cardio-autonomic control is implicated in modulating response to acute stressors, and alteration is separately shown to predict mortality. Heart rate (HR) fluctuations display fractal correlations (similar temporal autocorrelation across different time scales) that reflect a dynamic interplay between sympathetic and vagal outflows, where disruption is thought to contribute to neuronal damage. We hypothesized that fractal cardiac activity regulation FCAR (α₁) correlations in HR predicts incident delirium.

Prospective cohort study using UK Biobank data.

Community-based participants undergoing ECG-monitored exercise testing.

41,109 adults (mean age 58.0 years) free from delirium or AD at baseline who completed a 7-minute stationary bike test (2009-2010) and had ≥1 hospitalization by March 2021.

Detrended fluctuation analysis was used to derive short-term fractal scaling exponents (α₁) from ECG data. Cox proportional hazard models examined the association between FCAR (α₁) and incident delirium, adjusting for demographics, lifestyle, comorbidities cognition, and exercise parameters.

Over a median of 11.4 years (IQR 11.3-11.5), 442 cases (11 per 1000) developed delirium. FCAR (α₁) was normally distributed, (mean 1.25 ± 0.35 (SD)). Compared to those in the highest FCAR (α₁) quartile (>1.50), participants in the lowest quartile (<1.01) had a 40% increased risk of delirium (HR 1.40; 95% CI: 1.05-1.88; p = 0.02). This risk increase was equivalent to being 3.3 years older in the fully adjusted models.

Lower FCAR (α₁) during exercise predicted higher long-term risk of delirium, suggesting a potential prognostic indicator associated with brain-heart resilience. Future research should examine its role in shared vulnerability to delirium and neurodegenerative disorders.