T-wave over sensing in a pacemaker-dependent dog under general anesthesia caused by a change in T-wave morphology during infusion of dopamine and norepinephrine.

A 13-year-old intact male mixed-breed dog with a left-sided adrenal gland mass presented for an anesthetized computed tomography scan to aid with surgical planning. The dog had previously undergone transvenous implantation of a single-chamber, right-sided permanent pacemaker placed for treatment of a high-grade second-degree atrioventricular block. The pacemaker mode was changed from variable rate response (ventricular pacing, ventricular sensing, inhibited response, and rate modulation) to fixed demand (ventricular pacing, ventricular sensing, and inhibited response) in preparation for anesthesia, and the rate was set to 95 beats minute-1. The dog was premedicated with intravenous (IV) butorphanol tartrate (0.3 mg kg-1) and anesthesia was induced with propofol (1.25 mg kg-1, IV) and midazolam (0.3 mg kg-1, IV). Anesthesia was maintained with isoflurane delivered in oxygen. Following induction of anesthesia, there was a decrease in the mean arterial blood pressure from 108 mmHg to 45 mmHg, which was effectively treated with a bolus of lactated Ringer's solution (5 mL kg-1, IV) and infusions of dopamine (12 μg kg-1 minute-1) and norepinephrine (0.1 μg kg-1 minute-1). After starting dopamine and norepinephrine infusions, mean arterial blood pressure increased to 80 mmHg, and the ventricular rate decreased to 60 beats minute-1. Interrogation of the pacemaker revealed a change in T-wave morphology, which resulted in T-wave oversensing. Decreasing the lead sensitivity of the pacemaker corrected oversensing and restored the paced ventricular rate to 95 beats minute-1.