Page 66 - Small Animal Internal Medicine, 6th Edition

38 PART I Cardiovascular System Disorders

TABLE 2.4
VetBooks.ir Normal Electrocardiographic Reference Ranges for Dogs and Cats

DOGS
CATS
Heart Rate
60-160 beats/min (adults) to 220 beats/min (puppies) (120-)140-240 beats/min
Mean Electrical Axis (Frontal Plane)
+40 to +100 degrees 0 to +160 degrees
Measurements (Lead II)
P-wave duration (maximum)
0.04 sec (0.05 sec, giant breeds) 0.035-0.04 sec

P-wave height (maximum)
0.4 mV 0.2 mV

PR interval
0.06-0.13 sec 0.05-0.09 sec

QRS complex duration (maximum)
0.05 sec (small breeds) 0.04 sec
0.06 sec (large breeds)
R-wave height (maximum)
2.5 mV (small breeds) 0.9 mV in any lead; QRS total in any lead <1.2 mV
3 mV (large breeds)*

ST segment deviation
<0.2 mV depression <0.1 mV deviation
<0.15 mV elevation
T wave
Usually < 25% of R wave height; can be positive, negative, Maximum 0.3 mV; can be positive (most common),
or biphasic negative, or biphasic

QT interval duration
0.15-0.25 (to 0.27) sec; varies inversely with heart rate 0.12-0.18 (range 0.07-0.2) sec; varies inversely with heart
rate
Chest Leads
V 1 ; rV 2 : positive T wave R wave 1.0 mV maximum in chest leads
V 2-3 : S wave 0.8 mV maximum; R wave 2.5 mV maximum*
V 4-6 : S wave 0.7 mV maximum; R wave 3 mV maximum*
V 10: negative QRS; negative T wave R/Q < 1.0; negative T wave

Each small box on the ECG paper grid is 0.02 second wide at 50 mm/sec paper speed, 0.04 second wide at 25 mm/sec, and 0.1 mV
high at a calibration of 1 cm = 1 mV.
*May be greater in thin, deep-chested dogs younger than 2 years old.

ECTOPIC RHYTHMS a longer pause (considered late or escape). Escape com-
Impulses originating from outside the sinus node (known plexes represent activation of a subsidiary pacemaker and
as ectopic impulses) are abnormal and create an arrhythmia function as a rescue mechanism for the heart. Premature
(dysrhythmia). Ectopic impulses are described on the basis ectopic impulses (complexes) occur singly or in multiples;
of their general site of origin (atrial, junctional, supraven- groups of three or more constitute an episode of tachycar-
tricular, ventricular) and their timing (Fig. 2.28). Timing dia. Episodes of tachycardia can consist of a brief run of
refers to whether the impulse occurs earlier than the next ventricular premature complexes (VPCs; paroxysmal tachy-
expected sinus impulse (and therefore is premature) or after cardia) or can be quite prolonged (sustained tachycardia).