Human cardiac autonomic responses to head-up tilting during 72-h starvation

Starvation may change autonomic nervous system activity and sensitivity such that a greater vagal withdrawal may occur during a sympathetic challenge. Six healthy humans endured a 3-day, water-only fast, during which participants were subjected to passive 80° head-up tilt testing twice on each day (a.m. and p.m.). Heart rate, heart rate variability (HRV), ventilation \( (V^{\prime}_{\text{E}} ) \), and respiration (\( V^{\prime}_{{{\text{O}}_{ 2} }} \) and \( V^{\prime}_{{{\text{CO}}_{ 2} }} \)) were recorded during supine rest and head-up tilting. On Day 1 (a.m.), supine heart rate was 46.0 ± 3.3 beats min⁻¹, increasing to 51.6 ± 7.4 beats min⁻¹ on Day 3 (p.m.). On Day 1 (a.m.), supine high frequency HRV was 57.9 ± 31.6_NU, increasing to 69.5 ± 21.3_NU on Day 3 (p.m.). Tilt-induced increases in heart rate were greater following starvation (10.5 ± 7.8 vs. 16.1 ± 8.6 beats min⁻¹), and tilt-induced decreases in high frequency HRV were greater following starvation (−4.1 ± 27.7 vs. −28.0 ± 20.8_NU). Supine V′CO₂ remained unchanged, whereas V′O₂ increased and respiratory exchange ratio decreased (0.91 ± 0.10 vs. 0.80 ± 0.05). Greater vagal withdrawal and elevated heart rate induced by head-up tilting during starvation may indicate increased autonomic sensitivity.