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. 2011 Oct 6:442–550. doi: 10.1016/B978-1-4377-0798-4.00004-9

Figure 4-28:

Figure 4-28:

A, In this tracing the R wave averages 3.8 mV (38 boxes). The R wave should not be taller than 3.0 mV (30 boxes) in any dog. A tall R wave indicates left ventricular enlargement. The measurement is made from the baseline (not from the bottom of the Q wave) to the top of the R wave. Two other criteria that indicate left ventricular enlargement are present. The QRS complex is 0.07 second (three boxes) wide and ST segment slurring is present, because the ST segment moves into the T wave without straightening out along the baseline. (Paper speed is 50 mm/second; 1 cm equals 1 mV.) B, Left ventricular enlargement in a cat. This lead II electrocardiogram was recorded from an aged cat with hyperthyroidism. Thyroxine levels were 9.9 mg/dL. Note the tall R waves (>.9 mV). (Paper speed is 50 mm/sec; 1 cm equals 1 mV.) ST segment slurring is characterized by the slurring of the downstroke of the R wave into the T wave, with no discernible ST segment. This occurs because of ischemia resulting from wall strain in cardiac enlargement.

(Courtesy NS Moise, New York State College of Veterinary Medicine, Cornell University, Ithaca, New York. From Edwards NF: Bolton's handbook of canine and feline electrocardiography, ed 2, Philadelphia, 1987, WB Saunders.)