The Magnitude of the Postural Challenge Impacts on the Exponential Decay of the Baroreflex Impulse Response

We hypothesize that a postural challenge can affect the bandwidth of the cardiac baroreflex. The study estimated the impulse response function (IRF) of cardiac baroreflex via a model-based approach applied to spontaneous fluctuations of heart period (HP) and systolic arterial pressure (SAP). The exponential decay constant of the IRF was taken as an estimate of the bandwidth of the HP-SAP relationship: the higher the exponential decay constant, the wider the bandwidth. The IRF of the HP-SAP link was estimated in 17 healthy humans (age: 21–36 yrs, median <tex>$=29yrs;8$</tex> males) during graded head-up tilt with tilt table inclination at 0, 15, 30, 45, 60, 75 and 90 degrees. Each tilt session was preceded by a supine resting period and followed by a recovery. A bivariate autoregressive model with exogenous input was utilized to describe the dependence of HP variations on SAP changes. The exponential decay constant was calculated by fitting the IRF absolute value with a mono-exponential function. We found that the exponential decay constant gradually decreased with tilt table angles. This finding is compatible with a reduced bandwidth of the cardiac baroreflex, likely linked to the gradual vagal withdrawal associated with the magnitude of the challenge.