Comparison of phenylephrine bolus and infusion methods in baroreflex measurements

Abstract

Phenylephrine (PE) bolus and infusion methods have both been used to measure baroreflex sensitivity in humans. To determine whether the two methods produce the same values of baroreceptor sensitivity, we administered intravenous PE by both bolus injection and graded infusion methods to 17 normal subjects. Baroreflex sensitivity was determined from the slope of the linear relationship between the cardiac cycle length (R-R interval) and systolic arterial pressure. Both methods produced similar peak increases in arterial pressure and reproducible results of baroreflex sensitivity in the same subjects, but baroreflex slopes measured by the infusion method (9.9 +/- 0.7 ms/mmHg) were significantly lower than those measured by the bolus method (22.5 +/- 1.8 ms/mmHg, P less than 0.0001). Pretreatment with atropine abolished the heart rate response to PE given by both methods, whereas plasma catecholamines were affected by neither method of PE administration. Naloxone pretreatment exaggerated the pressor response to PE and increased plasma beta-endorphin response to PE infusion but had no effect on baroreflex sensitivity. Thus our results indicate that 1) activation of the baroreflex by the PE bolus and infusion methods, although reproducible, is not equivalent, 2) baroreflex-induced heart rate response to a gradual increase in pressure is less than that seen with a rapid rise, 3) in both methods, heart rate response is mediated by the vagus nerves, and 4) neither the sympathetic nervous system nor the endogenous opiate system has a significant role in mediating the baroreflex control of heart rate to a hypertensive stimulus in normal subjects.

Phenylephrine-induced elevations in arterial blood pressure are attenuated in heat-stressed humans

“…Blood pressure returned to predrug levels within 5-10 min after the end of phenylephrine infusion. This observation, coupled with findings that there were no differences in the increase in blood pressure in response to repeated phenylephrine challenges separated by 30 min (25), suggests that the first infusion of phenylephrine did not affect the responses of the second infusion. Therefore, the blunted increase in blood pressure during the heat stress phenylephrine challenge was due to factors associated with the heat stress and not due to repeated phenylephrine infusion challenges.…”

Baroreflex Gain Predicts Blood Pressure Recovery During Simulated Ventricular Tachycardia in Humans

“…Although the use of bolus injections is the standard approach for assessing baroreflex function, the response to vasoactive drug infusion has also been used and is similarly impaired in disease processes such as congestive heart failure. 15 The use of a nitroprusside infusion probably resulted in smaller gain estimates than would have been produced by a bolus injection, as suggested by the study of Sullebarger et al 16 ; however, it is unlikely that this influenced the conclusions of this study. Second, the use of infusion produced a sustained hypotension similar to that experienced during the simulated VT (pacing) and thus is a reasonable model of the baroreflex stimulus experienced during these tachyarrhythmias.…”

Cyclooxygenase inhibition and baroreflex sensitivity in humans

“…These stepwise infusions of vasoactive medication may produce baroreceptor resetting or adaptation (14) as well as chronotropic effects that include both parasympathetic and sympathetic components (22). This may explain why methods utilizing steady-state drug infusions compared with bolus infusions of vasoactive medications, which result in primarily vagally mediated changes in cardiac period, do not correlate (36). Second, the vasoactive medication used to increase BP was norepinephrine.…”