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
rate automatically.
Determine the cardiac rhythm.
The heart's rhythm is evaluated by inspection of the ECG and the findings are correlated with the physical findings. Analysis
of the heart's underlying rhythm should include the following steps.
• What is the rhythm (including the regularity and the relationship among complexes)?
- Regular?
- Regularly irregular with a consistent and repeating pattern to the variation in the rate?
- Irregularly irregular where the rhythm is chaotic and there is no pattern to the irregular nature of the rhythm?
- Paroxysmal (which is defined as a sudden outburst)? When applied to the ECG, a paroxysm refers to a series of
rapid ectopic beats, which begins and ends abruptly. The series may be as short as 3 beats or may last for minutes to hours.
- What is the relationship between the P and QRS complex? Is there a P wave for every QRS complex? Is there a QRS
complex for every P wave? Is the duration of time between the various components (P-R interval, Q-T interval) normal? Is the
duration of time between the various complexes consistent?
• Where do the cardiac impulses originate (site of origin)? The four possible choices include:
- The sinoatrial (SA) node
- The atria
- The atrioventricular (AV) node/junctional
- The ventricles and His-Purkinje system
Impulses originating from the SA node, atria or AV node are grouped together under the heading supraventricular while impulses
from the ventricles or His-Purkinje system are termed ventricular. Supraventricular beats should maintain a relatively tall,
upright and narrow QRS complex because the impulse must utilize the His-Purkinje system to transmit the impulse to the ventricles.
Therefore the ventricular muscle depolarizes uniformly with a set activation sequence. But when impulses arise from the ventricles
or terminal branches of the His-Purkinje system they are slowly transmitted from individual myocardial cell to myocardial
cell. This produces a relatively wide and bizarre QRS-T complex.
