Effects of noradrenaline on human vagal baroreflexes

BACKGROUND. Baroreflex sensitivity (BRS) is depressed in conditions associa ted with high sympathetic nerve activity in proportion to circulating norad renaline (NA) levels. Despite the prognostic importance of measurements of BRS in patients, there is little information on how high NA levels affect a rterial baroreflex function. AIM. To understand better the role of NA in cardiovascular homeostasis. METHODS. We gave incremental intravenous NA infusions (at 50 and 100 ng/kg/ min) to 12 healthy young men. We measured RR intervals and photoplethysmogr aphic arterial pressures and estimated BRS with cross-spectral and sequence methods during metronome-guided respiration at 0.25 Hz. RESULTS. The high NA infusion rate significantly increased respiratory-freq uency (0.15-0.40 Hz) RR interval spectral power and decreased low-frequency (0.04-0.15 Hz) systolic pressure spectral power compared with baseline lev els (P < 0.05 for both). Cross-spectral BRS increased from an average (+/- SD) baseline level of 17.3 +/- 6.6 to 34.1 +/- 20.8 ms/mmHg at the high NA infusion rate (P < 0.05). Sequence BRS values did not increase significantl y during NA infusions. The percentage of sequences with parallel changes in systolic pressures and RR intervals decreased progressively from a baselin e level of 16.0 +/- 12.9 to 10.1 +/- 7.4 during the low NA infusion rate an d to 6.2 +/- 6.2% during the high rate (P < 0.05 and 0.01, respectively). CONCLUSIONS. Increases in circulating NA to high physiological levels do no t depress BRS but interfere with the close baroreflex-mediated coupling tha t is usually present between arterial pressure and heart rate.