This study observed the effects of early intravenous low-doses of
metoprolol on cardiac sympathetic activities and electrophysiological properties in
myocardial infarction (MI) dogs. Thirty two mongrel dogs with the first diagonal branch of the left anterior descending coronary artery ligated were randomly divided into three groups: The low-dose group was given
metoprolol 0.6 mg/kg immediately by
intravenous injection (n=12); the target-dose group was given
metoprolol 1.6 mg/kg (n=12), and the control group was injected with
normal saline at the same dose of the target-dose group (n=8).
Norepinephrine (NE) and
epinephrine (E) levels in the coronary sinus (CS) blood as well as the ventricular effective refractory period (ERP) were all measured during the experiments. We found that NE and E concentrations in the three groups were all increased compared with the previous measurement before
ligation. ERP values after MI were significantly decreased in all three groups compared with the first measurements. The three groups all exhibited uneven shortness of ERP among different regions, with significant shortness in infarcted area. Furthermore, there was no difference between the low and target-dose of
metoprolol in the reduction of regional ERP, and the same effect was also observed in induced arrhythmias. In conclusion, a lower dose of
metoprolol performed similarly as target-dose in reducing the
catecholamine concentrations in dogs with MI. Our study demonstrated that a lower dose of
metoprolol may be reasonable compared with the target-dose in β-blocker
therapy due to similar effect and lower toxicity.