Comparison of three methods for the determination of baroreflex sensitivity in conscious rats

Baroreflex sensitivity was studied in the same group of conscious rats usin g vasoactive drugs (phenylephrine and sodium nitroprusside) administered by three different approaches: 1) bolus injection, 2) steady-state (blood pre ssure (BP) changes produced in steps), 3) ramp infusion (30 s, brief infusi on). The heart rate (HR) responses were evaluated by the mean index (mean r atio of all HR changes and mean arterial pressure (MAP) changes), by linear regression and by the logistic method (maximum gain of the sigmoid curve b y a logistic function). The experiments were performed on three consecutive days. Basal MAP and resting HR were similar on all days of the study. Brad ycardic responses evaluated by the mean index (-1.5 +/- 0.2, -2.1 +/- 0.2 a nd -1.6 +/- 0.2 bpm/mmHg) and linear regression (-1.8 +/- 0.3, -1.4 +/- 0.3 and -1.7 +/- 0.2 bpm/mmHg) were similar for all three approaches used to c hange blood pressure. The tachycardic responses to decreases of MAP were si milar when evaluated by linear regression (-3.9 +/- 0.8, -2.1 +/- 0.7 and - 3.8 +/- 0.4 bpm/mmHg). However, the tachycardic mean index (-3.1 +/- 0.4, - 6.6 +/- 1 and -3.6 +/- 0.5 bpm/mmHg) was higher when assessed by the steady -state method. The average gain evaluated by logistic function (-3.5 +/- 0. 6, -7.6 +/- 1.3 and -3.8 +/- 0.4 bpm/mmHg) was similar to the reflex tachyc ardic values, but different from the bradycardic values. Since different wa ys to change BP may alter the afferent baroreceptor function, the MAP chang es obtained during short periods of time (up to 30 s: bolus and ramp infusi on)re more appropriate to prevent the acute resetting. Assessment of the ba roreflex sensitivity by mean index and linear regression permits a separate analysis of gain for reflex bradycardia and reflex tachycardia. Although t wo values of baroreflex sensitivity cannot be evaluated by a single symmetr ic logistic function, this method has the advantage of better comparing the baroreflex sensitivity of animals with different basal blood pressures.