Underestimation of cardiac vagal control in regular exercisers by 24-hour heart rate variability recordings

To examine whether ceiling effects at long inter beat intervals (IBIs)cause an underestimation of cardiac vagal control in regular exercisers by time and frequency-domain measures of respiratory sinus arrhythmia (RSA).

24-hour ECG and respiration recordings were performed in 26 regularly exercising subjects, actively engaged in aerobic training for the past year, and enrolled in supervised training in the six weeks pre-study, and in 26 age- and sex-matched non-exercisers. Sleep and waking levels of cardiac vagal control were estimated by RSA obtained through the peak-valley method, by the standard deviation of the IBIs, the root mean square of successive IBIs, and the high frequency IBI spectral power.

In 11 of the exercisers the IBI-RSA relationship was characterized by a quadratic relationship. This reflected a ceiling effect at very long IBI values attained by regular exercisers, particularly during the nighttime recording. Irrespective of this ceiling effect, RSA as well as other heart rate variability (HRV) measures was still significantly larger in the exercisers with a quadratic IBI-RSA relationship than in non-exercisers or exercisers with a linear IBI-RSA relationship.

We conclude that a subgroup of regular exercisers is characterized by a low heart rate paired to high levels of cardiac vagal control. In these exercisers, vagal control is underestimated from HRV measures in ambulatory recordings. Inspection of the IBI-RSA relationship should be routinely added when HRV measures are used to index cardiac vagal control.