Fig. 1.
Aging and alterations of heart rate properties. A: ECGs recorded in C57Bl/6 male mice at 4 and 24 mo of age. B: data for average RR interval duration. C: data for time-domain parameters of heart rate variability (HRV). SDRR, standard deviation (SD) of RR intervals; CVRR, coefficient of variation of RR intervals; RMSSD, square root of the mean of the squared differences between adjacent RR; m, months. D: frequency-domain parameters of HRV for aging mice. E: frequency bands normalized by total power. F: data for low frequency-to-high frequency (LF/HF) ratio. G: Poincaré plots obtained from ECGs recorded from male C57Bl/6 mice at 4 and 25 mo of age. RRn and RRn+1 axes span from 73 to 95 ms. H: data for nonlinear indexes. Quantitative data shown in B–F and H were obtained from mice at ~4 mo (n = 44), ~12 mo (n = 23), ~18 mo (n = 31), and ~24 mo (n = 30) of age. Data are presented as medians and interquartile ranges. *P < 0.05 vs. ~4 mo. **P < 0.05 vs. ~12 mo.