Electrocardiographic interpretation of thiobarbiturate-induced dysrhthymias in dogs

The cardiac dysrhythmia occuring most commonly during intravenous administration of thiamylal sodium is ventricular bigeminy (ventricular premature depolarization coupled to the preceding sinus beat). Electrocardiographic tracings obtained during thiamylal-induced dyshythmias must be interpreted accurately if an accurate prognosis is to be give. Ventricular bigeminy seemed to originate in the ventricle, distal to the bundle of His. When ventricular premature depolarization was late in diastole, occuring simultaneiously with the next sinus-conducted impulse, a fusion beat resulted. The resultant ventricular bigeminy appeared as: (1) alternating ventricular premature dipolarizations, (2) an electrical alternans, and (3) alternating preexcitation syndrome. This dysrhythmia was associated with palpable alternating strong and weak frmoral arterial pulse. The likelihood that ventricular dysrhythmias will be caused by thiamylal is closely related to dose, concentration, and rate of administration. In a 23-month period, clinical occurrence of ventricular bigeminy during induction of anesthesia with thiamylal was approximately 4%.