Abstract: Objective: We investigated the influence of β-receptor blocker metoprolol on return of spontaneous circulation (ROSC) during cardiopulmonary resuscitation (CPR) in rats with induced myocardial infarction (MI). Methods: Male Sprague-Dawley rats were randomly divided into three groups: the sham-operated group, the MI group without metoprolol, which was fed the vehicle, and the MI+metoprolol group receiving intragastric metoprolol. Each group was further divided randomly into three subgroups, depending on the dosage of epinephrine administered during subsequent CPR applied after the induction of asphyxial cardiac arrest. Results: The ROSC rate was significantly decreased in the low dose subgroup of MI group, unchanged in the medium dose subgroup of MI group, and significantly decreased in the high dose subgroup of MI group, compared with the same dose subgroup of sham-operated group. MI+metoprolol group had a lower ROSC rate than MI group in the medium dose subgroup, and a higher ROSC rate than MI group in the high dose subgroup. There was no difference in blood K⁺ values of successful rats between MI group and MI+metoprolol group. The rats with successful CPR had lower blood K⁺ values than rats with unsuccessful CPR in each of the three treatment groups. Conclusions: Metoprolol administered to MI rats over a long period significantly improved ROSC rates under an appropriate dose of epinephrine during CPR. An increasing high blood K⁺ value would attenuate the rate of a successful CPR.

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