Independent responses of baroreceptors and blood pressure to low bolus dose of phenylephrine in fully conscious human volunteers

Background: Clinical examination of baroreflex in the Oxford method, using α₁-adrenergic stimulation, assumes that abrupt increase in blood pressure (BP) stimulates baroreceptors with cause and effect relationship. We can now quantify baroreceptor output independently using the NeuroScope. Aim: To conduct a pilot study and measure BP and baroreceptor output independently and simultaneously during α₁-adrenergic stimulation to establish causal relationships between the two. Methods: Healthy volunteers (4 males) aged 24.57 ± 0.09 years (mean ± SEM) were recruited in the study. Beat-to-beat BP was measured using the Finapres BP Monitor (BOC Health Care, Englewood, CO), cardiac vagal tone (CVT) and cardiac sensitivity to the baroreflex (CSB), representing baroreceptor output, were measured using the NeuroScope (MediFit Diagnostics Ltd, London) as previously described (Julu, et al. 2003; Al-Rawi, et al. 2004). Heart rate was derived from ECG R-R intervals and all these parameters were event-synchronized and time-stamped using the ITCMD^© (MediFit Diagnostics Ltd, London) for study of causal relationships. Results: There were variable and independent responses of BP, CVT, CSB and R-R interval to phenylephrine at doses <100 μg. At these low doses of phenylephrine there were consistent and significant increases in CVT, but not BP. CVT increased by 196% at 25 μg and 100% at 50 μg (p < 0.05) while systolic BP increased by 13% at 25 μg (p < 0.05) and only 10% at 50 μg (p > 0.05). Conclusions: Our results suggest responses of baroreceptor output independent to BP at threshold stimulation, which is inconsistent with cause-response relationship.