for Normal Electrocardiogram

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© Springer Nature Singapore Pte Ltd. 2020
J. VarshneyElectrocardiography in Veterinary

5. Benchmarks for Normal Electrocardiogram

J. P. Varshney1 

Veterinary Medicine, Shri Surat Panjarapole Prerit Nandini Veterinary Hospital, Surat, Gujarat, India


To differentiate an abnormal electrocardiogram from the normal one is the most important step in interpreting the electrocardiographic findings. The criteria for normal electrocardiogram provide basis for comparing electrocardiograms of ailing dogs. Much information is available on electrocardiograms of the diseased dogs rather than that of healthy dogs. Large studies are also not available on ECG patterns of healthy dogs in India. ECG of a healthy male Labrador with detailed analysis of measurements of the complexes and intervals is illustrated in Fig. 5.1. Like hemogram and biochemical values, electrocardiogram is not an absolute indicator of health and diseases. Departure from criteria for normal electrocardiogram suggests abnormality but fails to identify structural involvement of the heart or valvular defects. Electrocardiogram is undoubtedly a valuable diagnostic modality in the diagnosis of arrhythmias in dogs and cats and chamber enlargement in dogs.


Fig. 5.1

Electrocardiogram of healthy dogs (sensitivity 1, speed 25 mm/s) of medium and small breeds. (a) Adult male Labrador showing heart rate, complexes, and intervals (HR 100 bpm, P 0.2 mV, 0.04 s, P-R interval 0.08 s, q 0.2 mV, R 1.2 mV, QRS 0.04 s, S-T 0.08 s, T −ve 0.2 mV, 0.06 s). (b) Adult male Pomeranian showing heart rate, complexes, and intervals (HR 160 bpm, P 0.15 mV, 0.03 s, P-R interval 0.09 s, q 0.1 mV, R 0.6 mV, QRS 0.03–0.04 s, S-T 0.14 s, T +ve, very small)

5.1 Benchmarks for Normal Canine Electrocardiogram

Heart rate varies in dogs. Dogs of small breeds have higher heart rate as compared to dogs of medium and large breeds. In adults heart rate varies from 70 to 160 bpm. In toy breeds heart rate may vary from 80 to 180 bpm. Puppies have the highest heart rate, and it may go up to 220 bpm. Normal heart rhythm is called sinus rhythm where R-R interval is almost constant. When R-R interval is varying and heart rate is within normal range, it is designated as sinus arrhythmia. Varying amplitude of P wave is called wandering pace maker and is generally considered as innocuous. Electrocardiographic measurements are generally taken in lead II. Values of electrocardiographic indices observed in healthy dogs at the hospital are given in Table 5.1.

Table 5.1

Values of electrocardiographic parameters in healthy dogs

Electrocardiographic parameters

Range values

Heart rate

 Small/toy breeds

 Large breeds


70–170 bpm

80–170 bpm

70–160 bpm

200–220 bpm

“P” amplitude

0.15–0.40 mV

“P” duration

0.02–0.04 s

“R” amplitude

 Small/toy breeds

 Large breeds

0.90–2.8 mV

0.9–2.4 mV

1.20–2.8 mV

“QRS” duration

 Small breeds

 Large breeds

0.03–0.05 s

0.03–0.04 s

0.04–0.05 s

“T” amplitude

0.15–0.50 mV

“T” duration

0.04–0.08 s

“P-R” interval

0.08–0.12 s

“S-T” segment


Depression 0.05 mV

Elevation 0.05 mV

S-T segment duration

0.04–0.1 s

“Q-T” interval

0.11–0.23 s depending on heart rate

“R-R” interval

0.35–0.8 s depending on heart rate


+40° to +100°

Electrocardiograms of a healthy Labrador (medium breed) and Pomeranian (small breed) are given in to compare their waves, segments, and intervals Fig. 5.1. These electrocardiograms of healthy adult male dogs show that the heart rate is more and amplitude of R wave is less in Pomeranian as compared to Labrador.

5.2 Effect of Breeds, Age, and Sex on ECG Indices

Heart rate in small and toy breeds is higher than that of large breeds. However, heart rate varies from 70 to 170 bpm in healthy dogs. Heart rate is also influenced by age as puppies and younger dogs have higher heart rate as compared to adults. Sex does not seem to significantly influence the heart rate in dogs. P duration in small breeds is slightly short than that of large breeds. Breeds of dog seem to influence the amplitude of R wave. It is comparatively tall in large breeds. Dalmatian and Pomeranians seem to have slightly low amplitude of “T” wave as compared to Boxers and Cocker Spaniels. P-R interval increases with advancing age. S-T segment duration also varies among different breeds. ST segment is longest in Dalmatians and shortest in Cocker Spaniels. S-T segment also increases with advancing age (Changkija 2007) (Table 5.2).
Jul 17, 2021 | Posted by in INTERNAL MEDICINE | Comments Off on for Normal Electrocardiogram
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