Wave Forms, Segments, and Intervals in Electrocardiogram

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© Springer Nature Singapore Pte Ltd. 2020
J. VarshneyElectrocardiography in Veterinary Medicinedoi.org/10.1007/978-981-15-3699-1_6

6. Abnormal Wave Forms, Segments, and Intervals in Electrocardiogram

J. P. Varshney1 

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


When the measurements of different wave forms, their intervals, and/or duration of segments are not in line with the values detailed for normal electrocardiogram, it is said to be an abnormal electrocardiogram. Changes in the measurements of the waves, intervals, or segments are indicative of specific cardiac abnormalities. The details are illustrated below:

Abnormal wave forms


“P” wave (Fig. 6.1)

Broad (duration >0.04 s and/or notched)

Left atrium enlargement (P-mitrale)

Tall (amplitude >0.4 mV)

Right atrium enlargement (P-pulmonale)

Tall and broad (>0.4 mV and >0.04 s)

Biatrial enlargement

Variable amplitude of P wave

Wandering pace maker

Absence of P wave

Atrial standstill or silent atrium

Ta wave (increased height of descending arm of “P”)

Right atrium enlargement

“QRS” complex (Fig. 6.2)

Tall “R” (amplitude >2.5 mV in small breeds >3.0 mV in large breeds)

Left ventricular enlargement (LVE) or left bundle branch block (LBBB)

Wide “QRS” (duration >0.05 s small breeds, >0.06 s large breeds)


Deep “S” (amplitude >0.35 mV in leads II, III, aVF; >0.8 mV in lead CV5RL)

Right ventricular enlargement (RVE) or right bundle branch block (RBBB)

Deep “Q” wave (amplitude >0.5 mV in leads II, aVF)


Low-voltage “QRS” complexes (amplitude <0.5 mV in leads I, II, III, aVF)

Pericardial effusions

Pleural effusions





Loose lead contact with skin

Notched “QRS” with normal duration

Normal or minor atrioventricular defect

“R”-alternans (varying amplitude)

Pericardial effusion

Alternating bundle branch block

Supraventricular tachycardia

“J” wave (Fig. 6.3)

Deflection at “R” ST junction

Hypothermic dogs

Normal dogs

S-T segment/J point (Fig. 6.4)

Elevation (>0.15 mV), i.e., above the baseline




Secondary change (ventricular hypertrophy, VCPs, conduction disturbance)

Left ventricular epicardial injury.

Transmural myocardial infarction

Digoxin toxicity

Depression (>0.2 mV), i.e., below the baseline

Infarction, ischemia

Conduction disturbance

Cardiac trauma

Hyper- or hypokalemia

Secondary change (VCPs, ventricular hypertrophy, conduction disturbance)


Myocardial infarction/injury

False depression

Slurring or coving

Left ventricular hypertrophy

P-R interval (Fig. 6.5)

P-R interval is inversely proportional to heart rate

Increase (>0.13 s)

First-degree heart block

Q-T interval (Fig. 6.6)

Increased (>0.25 s)




CNS disorders

Ventricular hypertrophy

Conduction disorders

Strenuous exercise

Quinidine toxicity

Ethylene glycol poisoning

Secondary to prolonged QRS duration

Short (<0.15 s)



Digitalis toxicity

“T” wave (Fig. 6.7)

“T” wave changes are mostly nonspecific

Sudden change in polarity


Abnormal conduction

Ventricular enlargement

Metabolic disorders

Height >25% of “R”


Small biphasic


Large T wave

Myocardial hypoxia

Ventricular enlargement

Intraventricular conduction abnormalities


Metabolic diseases

Respiratory diseases

Normal variation

Tented T wave


T-alternans (amplitude varying)

Pericardial effusion

Alternating bundle branch block

Supraventricular tachycardia

“U” wave (Fig. 6.8)

Small rounded deflection after T wave.

It was first described by Einthoven (1906).

Represents last phase of ventricular repolarization.

Usually monophasic, positive, or negative.

Characteristic of hypokalemia in dogs (Tai Fu et al. 1984).

U wave with same polarity of T has also been seen in normal human beings (Goesing et al. 2009).

“R-R” interval (Figs. 6.9, 6.10, and 6.11)



Pause between R-R <twice of normal R-R interval

Sinus block

Pause between R-R >twice of normal R-R interval

Sinus arrest

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Jul 17, 2021 | Posted by in INTERNAL MEDICINE | Comments Off on Wave Forms, Segments, and Intervals in Electrocardiogram

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