An ECG is essential if an arrhythmia
is detected on auscultation but may not be particularly helpful in some cases, e.g.
chronic valvular endocardiosis.
Left atrial enlargement may cause a prolonged P wave (greater than 0.04s in both
the dog and cat) called P mitrale. Prolonged P waves may be biphasic.
Right atrial enlargement (seen with tricuspid insufficiency) may cause an increased
P wave voltage (greater than 0.4 mV in the dog, greater than 0.2 mV in the cat) called
P pulmonale. Both P pulmonale and P mitrale may be present at the same time.
Left ventricular enlargement may result in tall R waves, and prolonged QRS complexes.
Slurring of the ST segment may be seen.
Right ventricular enlargement may result in right axis deviation and deep S waves
(greater than 0.35 mV in lead II), and positive T waves in lead V10.
Notching of the QRS complex (R wave) is often seen in geriatric dogs and is thought
to be caused by the presence of MIMIs.