Electrocardiography 101

Electrocardiogram (ECG) is quick, safe, painless, and inexpensive test that detects and records the changing electrical activity in the heart over time. In terms of general structure, dogs and cats are practically alike to human with 4-chamber heart, thus the ECG application is relatively similar. The principle lies in the measurement of heart depolarizing and repolarizing activities recorded by positive and negative electrodes on two sides of the heart. As electrical impulse travels through the heart during each cardiac cycle, cardiac muscle cells of atria and ventricles contract (depolarize) and relax (repolarize), results in the chambers pumping blood throughout the body. 

A typical ECG tracing of the cardiac cycle (heartbeat) consists of a P wave (atrial depolarization ), a QRS complex (ventricular depolarization), and a T wave (ventricular repolarization), representing electrical activity in specific regions of the heart.

Photo Courtesy: (Left) Texas Health Institute, (Right) EpoMedicine

The breakdown of an ECG cycle:

P Wave: represents the depolarization of atria preceded by an action potential created by the sinoatrial (SA) node.

P-R Segment: indicates a delay in electrical activity at the atrioventricular (AV) node after the depolarization of the whole atria to allow blood flowing from atria to ventricles prior to ventricular contraction. P-R segment marks the end of P wave to the beginning of QRS complex.

P-R Interval: the time between the beginning of atrial depolarization and the beginning of ventricular depolarization.

QRS Complex: represents the complete depolarization of the ventricles which takes up from the beginning of the Q wave to the end of S wave. The contraction of ventricles leads to the ejection of blood into the aorta and pulmonary arteries.

S-T Segment: indicate a delay in electrical activity after the complete depolarization of the ventricles to allow blood flowing out of the ventricles prior to ventricular repolarization.

S-T Interval: the time between the end of ventricular depolarization and the end of ventricular repolarization.

Q-T Interval: the time between the beginning of the ventricular depolarization and the end of repolarization.

T Wave: represents the repolarization of the ventricles. Unlike in humans whose T wave is always upright in most leads, T wave in dogs and cats is rather uncertain and it can be positive, negative or combination of both (biphasic). Therefore, the diagnostic value correlated to the abnormalities of T wave is less significant compared to humans.