Change in characteristics of human vagal sigmoidal baroreflex curve during conscious state and general anesthesia

Volatile anesthetics have been shown to decrease baroreflex. In humans, cardiovagal baroreflex curve is best described to be sigmoidal and analysis of sigmoid model can provide more information, such as threshold (THR), saturation (SAT), maximal gain (Gmax), operating range (OR, difference between THR and SAT), operational point (OP, relative position of resting set point within the OR) in addition to the linear gain. We compared the changes in the sigmoid model of the systolic blood pressure (SBP)-RR interval relation between awake state and general anesthesia.

Sigmoid curve was obtained using the 'modified Oxford technique' (bolus sodium nitroprusside followed by bolus phenylephrine) to perturb SBP in awake group and 2 vol% sevoflurane anesthesia group. Radial arterial beat-by-beat SBP and raw RR interval data points were plotted and then binned in 2 mmHg SBP increments. Curve fittings were performed using symmetric four-parameter sigmoid model and asymmetric five-parameter sigmoid model. Gmax was calculated from the first derivative of the logistic function.

Compared with awake control, general anesthesia caused decreases of linear gain, Gmax and OR. RR interval THR and SBP THR were unaffected, whereas SBP SAT increased and RR interval SAT decreased, resulting from reduced linear gain and curvature parameter. Resting set point fell and OP shifted significantly to THR region.

Sigmoidal baroreflex curve revealed diminished gains, OR and OP during general anesthesia. Moreover, our results demonstrate that the analysis of sigmoid baroreflex arc across the entire baroreflex range provide more unique information beyond that of linear gain.