Effect of trazodone on heart rate, heart rate variability, and QT-intervals in dogs

To determine the effect of trazodone and placebo on heart rate (HR), heart rate variability, and QT intervals in dogs after oral administration of trazodone or placebo.

Twenty healthy adult client-owned dogs were included in the study. Dogs were randomized in a double-blind, placebo-controlled crossover trial. Dogs received trazodone (6 mg/kg q 8 h) or placebo for 24 h, during which a 24-h ambulatory electrocardiogram (ECG) (Holter) was recorded. Diagnostic ECGs and clinician-assessed behavior scores were obtained before and after the 24-h study period. Owners also scored the dog's behavior changes during each of the 24-h study periods. Following a minimum one-week washout period, dogs received the alternate study drug, and all procedures were repeated. Linear and mixed models were used for statistical analyses.

Dogs receiving trazodone had higher average HRs (P=0.035), higher minimum HRs (P<0.001), and reduced HR variability parameters (P<0.001) on the Holter recordings. Electrocardiogram-derived QT interval was not different between groups. Dogs receiving trazodone appeared calmer based on owner assessments (P<0.001) and behavior scores (P=0.006).

Blood pressure and trazodone blood levels were not measured, making it impossible to determine the effect of these variables on the HR and ECG findings.

Trazodone increased HR and decreased HRV. Possible explanations include a previously described anticholinergic effect or a possible decrease in blood pressure causing a reflex response. No demonstrable effect on QTi was identified in this cohort of healthy dogs using standard trazodone dosing.