Effects of noradrenaline on human vagal baroreflexes

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Abstract
BACKGROUND. Baroreflex sensitivity (BRS) is depressed in conditions associated with high sympathetic nerve activity in proportion to circulating noradrenaline (NA) levels. Despite the prognostic importance of measurements of BRS in patients, there is little information on how high NA levels affect arterial 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 photoplethysmographic 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-frequency (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 levels (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 msJmmHg at the high NA infusion rate (P < 0.05). Sequence BRS values did not increase significantly during NA infusions. The percentage of sequences with parallel changes in systolic pressures and RR intervals decreased progressively from a baseline level of 16.0 ± 12.9 to 10.1 ± 7.4 during the low NA infusion rate and 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 not depress BRS but interfere with the close baroreflex-mediated coupling that is usually present between arterial pressure and heart rate.