The electrocardiogram (ECG) is a commonly employed diagnostic tool to help in the evaluation of cardiac arrhythmias, to help
detect cardiac chamber enlargement, and to identify electrolyte abnormalities. The ECG may also detect alterations of sympathetic
and parasympathetic tone, identify myocardial inflammation, ischemia and necrosis, evaluate drug effects and reflect the presence
of pericardial disease. The key to understanding electrocardiograms is to develop a systematic approach for the determination
of heart rate, rhythm, and to recognize patterns of chamber enlargement. This handout and the associated lecture will focus
on the recognition of common physiologic and pathologic cardiac arrhythmias.
Cardiac conduction system
The cardiac conduction system is a group of specialized cells that exhibit the property of automaticity. These cells are able
to depolarize spontaneously and with regularity, and hence serve as the physiologic pacemaker. The normal cardiac pacemaker
is a group of specialized cells located near the junction of the cranial vena cava and right atrium termed the Sinoatrial
(SA) Node. The remainder of the conduction system is comprised of atrial internodal tracts, the atrioventricular (AV) node,
the His bundle, the right and left bundle branches, and the Purkinje fiber network. The SA node displays the most rapid rate
of spontaneous depolarization (70-140 beats/minute in dogs) and hence controls the heart rate. Junctional cells within the
region of the AV node spontaneously depolarize between 40-60 beats/minute while ventricular cells with automaticity depolarize
between 20-40 beats/minute in dogs.
Normal propagation of the electrical impulse
The specialized cells within the SA node initiate the cardiac impulse, which travels through internodal tracts to depolarize
the right and left atria. The AV node is then depolarized and carries the impulse to the ventricles, but it conducts electrical
activity more slowly to allow proper atrioventricular synchrony. The His bundle, right and left bundle branches and the Purkinje
fibers depolarize finally followed by cell to cell conduction through the ventricular muscle.
Electrical activity and the electrocardiogram
Individual electrical events within the heart correspond to an electrical potential that may be measured by the surface ECG.
The P wave represents depolarization of the atria while the QRS complex represents depolarization of the ventricles. The PR
interval is the time from atrial depolarization to ventricular depolarization. The T wave represents ventricular repolarization.
Ventricular depolarization must be followed by repolarization, therefore every QRS complex must have a T wave.
Interpretation of the ECG
A systematic approach to the evaluation of the ECG will ensure against overlooking important abnormalities. The following
characteristics should be evaluated in every ECG. Familiarity with the normal parameters for the ECGs of the various species
is, of course, essential for accurate interpretation.
Determine the heart rate
If the heart rate is regular, the number of small boxes (mm) between QRS complexes can be divided into 3,000 (at 50 mm/sec)
or 1,500 (at 25 mm/sec) to find the instantaneous heart rate. The heart rhythm in animals, especially in dogs, is frequently
irregular. In this circumstance the more accurate average heart rate is found by counting the number of beats in a known time
interval and multiplying appropriately. Single channel ECG paper on analog recorders is usually marked by a vertical line
at the top of the paper at 75 mm (1 mm = 1 small box) intervals. At a paper speed of 50 mm/sec, 75 small boxes (equivalent
to 15 large boxes) represent 1.5 seconds so the heart rate per minute can be calculated by counting the number of QRS complexes
in 1.5 seconds and multiplying by 40. At a paper speed of 25 mm/sec, 75 small boxes (15 large boxes) represent 3.0 seconds
and the number of QRS complexes in 3.0 seconds is multiplied by 20. Many of the newer digital ECG machines calculate heart