Spontaneous baroreflex measurement in the assessment of cardiac vagal control

Spontaneous baroreflex sensitivity (SBR) has been suggested to be a measure of tonic parasympathetic cardiac control. The decrease of SBR during vagal inhibition was proven before. In this study, we investigated the response of SBR during vagal activation by administering intravenous atropine and phenylephrine in eight and ten healthy volunteers respectively. Atropine was given at a rate of 0.5 μg/kg/min for 20 minutes and the infusion rate of phenylephrine was adjusted to increase the blood pressure 20 to 30 mmHg above baseline value. We found that SBR at first increased from 16.9 ± 9.5 to 41.5 ± 24.9 ms/mm Hg (p < 0.05) and then decreased to 8.9 ± 6.2 ms/mm Hg (p < 0.05 compared with the peak value) after the initiation of atropine infusion. SBR also increased significantly (27.2 ± 12.5 to 49.6 ± 11.3 ms/mm Hg, p < 0.01) during phenylephrine infusion. The authors propose SBR as a measure of cardiac vagal effect because SBR increases under vagal activation.