Duration of QT interval in clinically normal dogs

The QT interval is the period from onset of the QRS complex to the end of the T wave. The QT interval is useful for monitoring drug (eg, quinidine) and electrolyte (eg, calcium) effects on the heart. It depends principally on heart rate (HR), and the relationship between QT interval and HR has been expressed for human beings and for dogs. The purpose of the study reported here was to quantify that relationship for dogs and to assess whether body weight also influenced QT interval. The ECG was recorded from 17 dogs, ranging in weight between 7 and 25 kg. Dogs were anesthetized with fentanyl/droperidol/ketamine, and HR was accelerated by administration of graded doses of atropine. A significant relationship was not found between QT interval and body weight. Despite changes in HR during sinus arrhythmia, a significant relationship was not found between QT and RR intervals. The QT interval vs HR accelerated by atropine was analyzed for all dogs and for small (7 to 10 kg, n = 5), medium (10 to 20 kg, n = 7), and large dogs (20 to 25 kg, n = 5). Equations relating QT interval to mean HR were calculated for each group. Our data may serve as a baseline with which to compare QT intervals from dogs with heart disease and/or electrolyte imbalance.