Case Studies In Small Animal

Cardiovascular Medicine

Case 10

Electrocardiogram

This electrocardiogram (ECG) shows three simultaneously recorded leads (I, II, and III). It reveals a basic rhythm that is sinus arrhythmia with a rate that varies between approximately 85 and 115 beats/minute. The mean electrical axis is normal (i.e., lead II has the tallest R waves) with normal appearing P-QRS-T complexes. Please note on subsequent screens that the left atrium and left ventricle in this dog are markedly enlarged yet there is no sign of that on this ECG. This is a frequent finding.

There are frequent QRS complexes (third, fifth, ninth, and twelfth) that come in too early (i.e., are premature). They have an abnormal configuration, have no P wave in front of them, are wider than the QRS complexes from the normal sinus beats, and have a T wave that is opposite in polarity to the normal T waves. What are they? They depolarize at a rate of about 166 beats/minute. (There are 9 mm between the last sinus beat and the premature beat so there are 0.36 seconds [9/25th of a second] between the normal and the premature beats. This translates into the premature beats depolarizing every 0.36 seconds or 166 beats/minute [60 seconds/minute divided by 0.36 seconds/beat] if they became repetitive). This means there is an ectopic site in the heart that is competing with the sinus node for pacing the heart and it is depolarizing at a rate of 166 beats/minute. In this dog, the ectopic site only depolarizes once each time and then stops so the sinus node takes over again.

Quiz

Correct Quiz

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