ECG interpretation and management of arrhythmias (Proceedings)
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.
• Where do the cardiac impulses originate (site of origin)? The four possible choices include:
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.