Effects of mental task on heart rate variability during graded head-up tilt

In this study, we used spectral analysis of heart rate variability (HRV) to estimate the changes in autonomic control in response to disparate stimuli produced by mental task and graded head-up tilting. The low frequency (LF) component of HRV provided a quantitative index of the sympathetic and parasympathetic (vagal) activities controlling the heart rate (HR), while the high frequency (HF) component of HRV provided an index of the vagal tone. We studied 17 healthy male subjects (21-25 yr of age) who were placed on a tilt-table and the graded tilt-protocol involved tilted sine angles 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0. These tilt-protocols were repeated with or without the mental task, which consisted of auditory distinctive reaction-time tasks. The basal autonomic mode against the graded head-up tilt was characterized by reciprocal changes in sympathetic and vagal tones. There were significant increases of HR corresponding to the mental task with lower tilt-angle, albeit the changes with higher tilt angles were not significant. Furthermore, there were increases and decreases of the LF component induced by the mental task at lower and higher tilt-angles, respectively. These results revealed that the different responses of HR and LF component against the same tasks could be derived from the alterations of autonomic mode during gradual changes in autonomic control.