ECG 13

Signalment: Six-week-old female Alpaca (Cria)

Presenting complaint: Periods of tachycardia and weakness

50 mm/sec; 5 mm = 1 mV

This case is too complex for a quiz and an explanation works better. This animal has a bypass tract causing pre-excitation syndrome (Wolff-Parkinson-White syndrome). A bypass tract is a congenital piece of myocardium that bridges the atrial myocardium to the ventricular myocardium that conducts depolarization at a rate faster than the AV node. On the top trace this is manifested as a short PR interval with a slurred upslope to the QRS complex (a delta wave). This condition sets an animal up for a reentrant tachyarrhythmia. In humans and in dogs this condition can be treated by mapping the heart electrically and destroying the bypass tract with radiofrequency current.