Heart rate variability: Response to graded head up tilt in healthy men.

BACKGROUND Heart rate variability is actually a misnomer for R to R variability in cardiac cycle. Variation in successive cycle length is called the heart rate variability (HRV). Head-up tilt is a model of studying cardiovascular haemodynamics, which reflects in heart rate variability (HRV). OBJECTIVES To study the effect of 10 degrees and 70 degrees head-up tilt on HRV. MATERIALS AND METHODS The study was done in the Department of Physiology using graded head up tilt (passive orthostatism). HRV measurement was done at 10 degrees and 70 degrees tilt and compared with supine using standardised methods on 30 consenting healthy males (age 25.37+/-3.89 years). The HRV variables across postures were compared by ANOVA and Bonferroni test. RESULTS The heart rate increased at 70 degrees compared to 10 degrees and supine (70.48+/-8.17 Vs 70.22+/-8.67 and 88.51+/-12.84 bpm, p<0.001). The 70 degrees tilt decreased vagal HRV indicators compared to 10 degrees and supine: SDNN (31.13+/-8.12 Vs 38.07+/-11.29 and 38.13+/-10.89 ms, p<0.05), RMSSD (20.06 +/-8.47 Vs 34.23+/-14.22 and 36.16+/-12.22 ms, p<0.001), NN50 count (13.03+/-20.58 Vs 45.07+/-44.44 and 55.27+/-44.10, p<0.01), pNN50 (3.28+/-6.08 Vs 14.06+/-15.65 and 16.65+/-14.23, p<0.01), HF power (197.20+/-143.76 Vs 218.17+/-155.85 and 216.87+/-150.98 Hz, p<0.05), HFnu unit (24.28+/-14.16 Vs 45.48+/-16.34 and 47.67+/-19.89, p<0.001). The 70 degrees tilt increased LF power% (197.20+/-143.76 Vs 218.17+/-155.85 and 216.87+/-150.98, p<0.001). LFnu unit (75.72+/-14.76 Vs 54.52+/-16.34 and 52.32+/-19.89, p<0.001), LF: HF (4.96+/-4.08 Vs 1.53+/-1.138 and 1.69+/-1.67, p<0.001) compared to 10 degrees and supine. CONCLUSION At 70 degrees tilt, HRV measures, reflecting vagal contribution to cardiac-cycle length, decreased with reciprocal increase in sympathetic activity compared to 10 degrees or supine leading to increase in sympathetic predominance. A 10 degrees tilt, which is almost equivalent to lying down with pillow, did not change HRV from supine. activity.