Case Studies In Small Animal

Cardiovascular Medicine

Home Up ECG 1 ECG 2 ECG 3 ECG 4 ECG 5 ECG 6 ECG 7 ECG 8 ECG 9 ECG 10 ECG 11 ECG 12 ECG 13 ECG 14 ECG 15 ECG 16 ECG 17 ECG 18 ECG 19 ECG 20 ECG 21 ECG 22 ECG 23 ECG 24 ECG 25 ECG 26

ECG 13

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

Presenting complaint: Periods of tachycardia and weakness

This case is too complex for a quiz and an explanation works better. This animal has a bypass tract causing pre-excitation syndrome. Some may know it as 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. The slurring is called a delta wave and occurs as the ventricular myocardium is "pre-excited" via the bypass tract. This condition sets an animal up for a reentrant tachyarrhythmia. This occurs in the second trace and for a short time in the third trace. In the second trace the first complex is normal. It has a normal PR interval and no delta wave so the bypass tract during this beat is refractory and conduction only occurs from the atria to the ventricles via the AV node. When the ventricles are depolarized the depolarization wave reaches the bypass tract from the ventricular side. By this time the bypass tract has had time to fully repolarize and so is no longer refractory. Consequently, depolarization passes retrograde from a ventricle to an atrium, producing a negative P wave, which can be seen as a small negative complex between the QRS complex and the T wave (ST segment). This sets up a repetitive loop causing a supraventricular tachycardia. In the third trace you see a mixture of complexes, some of which utilize the bypass tract antegrade, some that utilize it retrograde to set up the tachycardia, and some where the depolarization goes retrograde through the bypass tract but finds the AV node refractory and so only a negative P wave appears in the ST segment but does not produce a reentrant loop and supraventricular tachycardia. In humans and in dogs this condition can be treated by "mapping" the heart electrically via catheter electrodes placed in the heart to identify where the bypass tract lies and then the bypass tract can be destroyed by passing radiofrequency current through the electrode catheter to heat the bypass tract to a temperature that destroys it.

50 mm/sec; 5 mm = 1 mV

Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.