Figure 4-27:

In these two examples of left ventricular enlargement, the QRS complexes have normal configuration but are too wide. A, This QRS complex from a miniature poodle is 0.07 second (three boxes) wide. B, This QRS complex from a Doberman Pinscher is 0.09 second (four boxes) wide. A small dog such as the Poodle should not have a QRS complex wider than 0.05 second (wider), and the larger dog's QRS complex should not exceed 0.06 second (three boxes). Because each dog's QRS complex is too wide, left ventricular enlargement is diagnosed in both cases. The Doberman Pinscher has no P waves because it is in atrial fibrillation. (Paper speed is 50 mm/sec; 1 cm equals 1 mV.)

(From Edwards NF: Bolton's handbook of canine and feline electrocardiography, ed 2, Philadelphia, 1987, WB Saunders.)