Cardiovascular profile of pimobendan, a benzimidazole-pyridazinone derivative with vasodilating and inotropic properties

Intravenous infusions of 0.01-0.1 mg•kg^-1 min^-1 of pimobendan, a benzimidazole-pyridazinone derivative in pigs with normal coronary circulation caused dose-dependent changes in heart rate (10-35%), left ventricular systolic pressure (-5 to -45%), left ventricular filling pressure (-20 to -40%) but had only a minor effect on the maximum rate of rise of left ventricular pressure (max LVdP/dt; 10-20%). The decrease in mean arterial blood pressure was primarily due to systemic vasodilation; peripheral resistance and cardiac output decreased by up to 40 and 14%, respectively. Vasodilation occurred in several vascular beds, but was particularly pronounced in the adrenals, stomach, small intestine and myocardium. Although the increase in myocardial blood flow favoured the epicardium, vascular conductance in both the endo- and epicardial layers was significantly increased. Myocardial O₂ consumption (MVO₂) was not affected despite the increase in heart rate. Bolus injections of 0.1-0.5 mg•kg^-1 pimobendan produced similar changes in all haemodynamic variables, except max LVdP/dt which now increased by 30-70%. As in the infusion experiments, cardiac output tended to decrease due to a pronounced reduction in ventricular preload probably as a result of venodilation and the consequent reduction in cardiac filling. However, in animals where max LVdP/dt and cardiac output were reduced and pre- and/or after-load were increased by partial occlusion of the left anterior descending coronary artery, pimobendan clearly increased both max LVdP/dt and cardiac output. Pretreatment with propranolol did not modify any of the cardiovascular responses to pimobendan, thereby excluding the involvement of a beta-adrenoceptor mechanism. Pimobendan is thus a compound with vasodilator and positive inotropic properties that improves cardiac output in animals with severe myocardial ischaemia. The finding that the mild tachycardia caused by pimobendan was not accompanied by an increase in MVO₂ warrants investigation to evaluate its usefulness in the treatment of heart failure.