Electrocardiogram (ECG) is commonly used in cardiology to evaluate both the conduction system of the heart and the status of the myocardium (cardiac muscle). An ECG provides a graphic representation of the heart’s electrical activity; this is initiated by the atria (in the sinoatrial node) and spreads to the ventricles. It is this electrical activity which causes the heart to beat.
An ECG is obtained by applying electrodes to the patient’s left arm, right arm and left leg. The right hindleg connects the animal to the ground. The machine records the difference in electrical activity between the electrodes used to create the various lead systems.
A normal ECG consists of:
- a “P wave”: represents depolarization of the right and left atria.
- a “QRS complex”: depolarization of the ventricles.
- a “T wave”: repolarization of the ventricles.
Irregularities in the shape (morphology) of these P-QRS-T complexes or the timing of the wave forms (rhythm) can signal heart muscle abnormalities.
An ECG may be indicated in any of the following situations:
- Arrhythmia or murmur ausculted on physical exam
- Acute onset of dyspnea (difficulty breathing)
- Tachycardia (increased heart rate), or bradycardia (decreased heart rate)
- Cardiomegaly (enlarged heart) viewed on chest radiographs
- Electrolyte disturbances (especially potassium)
There are a number of other instances in which the ECG can be a useful diagnostic tool. However, it is important to note that the ECG also has its limitations. For example, it cannot tell us about the mechanical status of the heart, and it is not always a reliable prognostic indicator. As a result an ECG should always be evaluated in combination with a complete history, physical examination, and laboratory data ( typically includes bloodwork, but may also include analysis of urine and extravascular fluids).