Autonomic adaptation to repeated cold water ingestion: time-resolved HRV analysis

Heart rate variability (HRV) is a non-invasive marker of autonomic function, and cold-water ingestion effectively modulates HRV. However, no prior study has provided a unified time-resolved modeling framework capable of separating residual and stimulus-specific autonomic effects in repeated HRV stimulation paradigms.

Electrocardiograms from 39 healthy participants, exposed to two cold water ingestion (500 ml at  8[Formula: see text]C) separated by 30 min, were analyzed. After segmenting recordings into 5-minute windows, HRV variables in time, frequency, and fractal domains were extracted. Linear mixed-effects models with linear and quadratic time components were used to characterize both transient and sustained responses.

The first stimulus resulted in pronounced autonomic modulation: heart rate decreased by 8.8%, other time-domain HRV variables increased (SDNN: +27.8%, RMSSD: +42.4%, pNN50: +75.6%). Maximum responses reached within 17-30 min, with recovery times of up to 60 min. The second stimulus resulted in additional but attenuated responses (SDNN: +2.9%, RMSSD: +41.7%, pNN50: +76.2%), with shorter recovery times (25-32 min), suggesting partial physiological adaptation. Frequency domain responses were smaller in magnitude but additive: with successive stimuli, ln(LF) response (+5.8% to +6.7%), and ln(HF) response increased (+9.2% to +10.5%). Fractal analysis revealed a decrease in short-term scaling (Alpha 1: -8.1% to -8.8%) without significant changes in long-term dynamics (Alpha 2).

The proposed time-resolved modeling framework effectively separates residual from stimulus-specific HRV effects, enabling quantification of repeated autonomic responses and providing a standardized approach for future repeated HRV stimulation studies.