The effect of topical ophthalmic 1% atropine on heart rate and rhythm in normal dogs

Objective To determine if topically administered ophthalmic atropine affects heart rate or rhythm in clinically normal dogs. Methods Two groups of 15 healthy dogs were evaluated, one consisting of dogs weighing 15 kg. Each dog was suited with a Holter monitor. At start time 0, dogs received one drop of ophthalmic 1% atropine solution, or one drop of sterile saline solution as a control in each eye, via random assignment. Each dog served as their own control. This procedure was repeated two more times, at 6-h intervals, for a total of three treatments over a 12-h period. Holter monitors recorded heart rate and rhythm for 24 h. Statistical analysis was performed to compare values between the groups. Dose dependent changes in cardiac parameters were evaluated. Results The mean heart rate and average minimum heart rate was significantly higher during the treatment period compared to the control period (8% and 13%, respectively). The mean number of hours with a heart rate  180 bpm did not differ significantly between the two groups. There was no evidence of dose dependence on heart rate when comparing small and large dogs. No significant differences in heart rhythm were noted between groups for measurable parameters. Conclusions Topically administered atropine causes a small but significant increase in heart rate in healthy dogs.