BACKGROUND: Vagal cardiac function plays an important role in cardiovascular homeostasis. The purpose of this study was to investigate the association between vagal cardiac function and changes in blood pressure induced by phenylephrine and by thigh cuff deflation after low-dose atropine (LDA)(2µg/kg) administration.

METHODS: Beat-to-beat changes in R-R intervals (RRI) and systolic blood pressures (SBP) were measured in 33 healthy volunteers during spontaneous and controlled (15 min^-1) breathing before and after LDA administration. The RMSSD (root mean square of successive differences of RRI), pNN50 (proportion of successive RRI > 50 ms in relation to the total RRI), standard deviation 1 (SD1) from Poincare plots, power spectral densities of heart rate variability (HRV) and SBP variability, and spontaneous baroreflex sensitivity (BRS) by transfer function analysis were assessed. Acute hypertension was induced by phenylephrine (2 µg/kg), whereas acute hypotension was induced by thigh cuff deflation.

RESULTS: RMSSD, pNN50, SD1 of Poincare plots, and the high frequency (HF) power of HRV increased after LDA administration as did spontaneous BRS. Moreover, acute hypertension induced by phenylephrine was significantly attenuated (15.9 ± 1.9 to 10.8 ± 3.1 mmHg, P = 0.004) after LDA administration. However, acute hypotension induced by thigh cuff deflation was not significantly changed (13.4 ± 3.9 to 11.9 ± 4.2 mmHg, P = 0.62) after LDA administration. Changes in SBP during acute hypertension induced by phenylephrine were significantly correlated with changes in the HF power of HRV (r = -0.60, P < 0.001).

CONCLUSIONS: Increasing vagal cardiac function induced by LDA attenuated increased SBP during acute hypertension induced by phenylephrine, but not decreased SBP during acute hypotension induced by thigh cuff deflation in healthy awake subjects.