Evidence of alpha-adrenoceptor-mediated chronotropic action in children

We investigated alpha -adrenoceptor-mediated positive chronotropic action i n children using a pharmacological autonomic function test. Under cardiac v agal and sympathetic blockade, chronotropic responses to intravenous infusi on of an alpha -agonist, phenylephrine, were determined in 129 patients wit h suspected autonomic dysfunction. Heart rate changes to phenylephrine infu sion (0.4 mug/kg/min) for 5 minutes ranged from -2 to 50 beats/min (mean +/ - SD, 13.8 +/- 9.7 beats/min). alpha -Adre noceptor-mediated chronotropic ( APC) action more than 5 beats/min was observed in 88 (68%) subjects. APC wa s negatively correlated with age. Multivariate analysis showed that APC was correlated negatively with beta -sympathetic tone and positively correlate d with intrinsic heart rate. These results suggest that the alpha -adrenoce ptor-mediated chronotropic action is a developmental phenomenon and may pla y an important role in controlling heart rate in response to cardiac sympat hetic activation.