Abnormal Wave Forms, Segments, and Intervals in Electrocardiogram

Abstract

When the values of waves (P, QRS, T), segments (P-R segment, S-T segment), and intervals (P-R interval, Q-T interval, R-R interval) do not conform to normal values for the species of the animal under examination, it is said to be an abnormal electrocardiogram. Tall P wave (>0.4 mV), broad P wave (>0.04 s), tall R wave (>2.5 mV in dogs of small breeds or >3.0 mV in dogs of large breeds), and broad QRS (>0.06 s), deep Q, or S wave are indicative of right atrium, left atrium, left ventricle, and right ventricle enlargement, respectively. R wave of <0.5 mV is generally referred as low-voltage complex. J wave has been observed in hypothermic or normal dogs. Elevation (>0.15 mV) or depression (>0.2 mV) of S-T segment has been considered pathological. P-R interval of >0.13 s suggests first-degree heart block. Changes in T wave are generally considered nonspecific in dogs; nevertheless sudden polarity change, biphasic T, and sharp heightened T waves are of significance. Variations in R-R interval are suggestive of arrhythmia, sinus block, or sinus arrest. Short or prolonged Q-T intervals are associated with calcium or potassium imbalances. The abnormal wave forms and different intervals have been illustrated through electrocardiograms. Indications of these electrocardiographic changes have also been given in this chapter.