Cardiovascular effects of intravenous pimobendan in dogs with acute respiratory acidosis

Abstract

Objective: Acidosis decreases myocardial contractile and myofibrillar responsiveness by reducing the calcium sensitivity of contractile proteins, which could reduce the effectiveness of pimobendan. We aimed to assess the cardiovascular effects of pimobendan in dogs subjected to acute respiratory acidosis.

Design: Randomized crossover study with a 2-week washout period.

Setting: University Laboratory.

Animals: Six healthy research Beagle dogs.

Interventions: Anesthetized dogs were administered 2 doses of IV pimobendan during conditions of eucapnia (Paco₂ 35-40 mm Hg) and hypercapnia (Paco₂ 90-110 mm Hg). Eucapnia was maintained by positive pressure ventilation and hypercapnia was induced by adding exogenous CO₂ to the anesthesia circuit. Heart rate (HR), systemic arterial blood pressure, cardiac output (CO), systemic and pulmonary vascular resistance (SVR and PVR, respectively), and pulmonary arterial pressure (PAP) were measured at baseline and 60 min after administering 0.125 mg/kg (low) and 0.25 mg/kg (high) pimobendan intravenously. Blood gas and biochemical analyses were performed at baseline and at the end of the experiment.

Measurements and main results: The median baseline blood pH was 7.41 (range: 7.33-7.45) and 7.03 (range: 6.98-7.09) under conditions of eucapnia and hypercapnia, respectively. The serum concentrations of epinephrine and norepinephrine and the HR, CO, and PAP were higher, and SVR was lower at baseline in hypercapnic dogs. Pimobendan dose-dependently increased CO in eucapnia (baseline: 3.6 ± 0.2 L/kg/m² [mean ± SE], low: 5.0 ± 0.4 L/kg/m² , high: 5.8 ± 0.5 L/kg/m² , P < 0.001) and hypercapnia (baseline: 4.9 ± 0.5 L/kg/m² , low: 5.8 ± 0.5 L/kg/m² , high: 6.2 ± 0.5 L/kg/m² , P < 0.001), and increased HR and decreased SVR and PVR under both conditions (P < 0.001). In hypercapnia, the degree of increase or decrease of these cardiovascular measurements (except for PAP) by pimobendan was less than that in the eucapnic dogs.

Conclusions: Pimobendan maintains function as an inodilator in anesthetized dogs with induced respiratory acidosis.

Keywords: canine; hypercapnia; hypercarbia; inodilator; phosphodiesterase inhibitor.