Temporal changes in complexity of cardiovascular regulation during head-up tilt test by entropic measures of fluctuations of heart period intervals and systolic blood pressure

Dynamical aspects of cardiovascular regulation in the head-up tilt test are revealed through the properties of the approximate entropy and sample entropy of increments between subsequent heart period intervals and increments between subsequent systolic blood pressure values. Marked differences are observed in the dynamical response to the tilt test of healthy subjects prone to spontaneous fainting and those who are less susceptible to fainting.