ECG Interpretation for Practitioners

The most common electrocardiographic (ECG) findings of the dog will be discussed in an interactive way.

Indications for making an ECG are: bradycardia, tachycardia or irregular pulse, other then respiratory sinus arrhythmia. Because the sensitivity of the ECG is low for detection of chamber enlargement, echocardiography is currently the first choice diagnostic modality, when a cardiac murmur, a gallop rhythm or congestive right-sided heart failure are diagnosed. That is why only rhythm diagnoses will be discussed.

The P-waves result from atrial depolarization, whereas the QRS-complexes from ventricular depolarization. Because the depolarization wave of the atria is relatively small, the even smaller repolarization wave of the atria is not recognized on a surface electrocardiogram. However, the repolarization of the ventricles can always be seen as T-waves. In lead II, normal P-waves and normal QRS-complexes are positive. The polarity of the T-wave could be either positive or negative. Regardless of the configuration of the QRS and the T-wave, each QRScomplex should be followed by a T-wave and each Twave should be preceded by a QRS-complex. With other words, if the ventricles depolarize (= QRS-complex) they also have to repolarize (T-wave). This is an important clue in recognizing arte facts and differentiating them from premature QRS-complexes.

The first step in the ECG-interpretation is to determine the heart rate on the recording by counting the QRS-complexes in a certain time interval. The differential diagnosis list can be narrowed depending whether the frequency is within the physiological range of that species, below (bradycardia) or above it (tachycardia).

The next step is to determine the regularity of the intervals between the QRS-complexes. Certain arrhythmias are typically irregular (atrial fibrillation), whereas others are regular (supraventricular tachycardia). [...]