div epub:type=”chapter” role=”doc-chapter”>
2. Electrocardiography: Its Uses and Limitations
2.1 Electrocardiograph, Electrocardiography and Electrocardiogram
Electrocardiograph is a device/machine used to diagnose heart diseases by measuring electrical potentials on the surface of the body. Electrocardiography is the process of producing an electrocardiogram using an electrocardiographic machine. ECG stands for electrocardiogram, or electrocardiograph. In some countries, “EKG” is abbreviated for electrocardiogram. An electrocardiogram is a graphic record of the voltage produced by cardiac muscle cells during atrial and ventricular depolarization and repolarization plotted against time. In other words it is a graphic representation of electrical activities going on in the heart during depolarization and repolarization. It can provide useful information even about congenital heart diseases. A normal electrocardiogram does not rule out cardiac malformation or other cardiac changes. An abnormal ECG may suggest side of the heart affected or about some other abnormalities related to rate and rhythm of the heart.
2.2 Landmarks in the Development of Electrocardiography
In 1616, Harvey discovered that blood circulation is due to heartbeat.
Later it was recognized that heartbeat was an electrical process.
In 1887, Augustus Desire Waller recorded the electrical changes of the human heart for the first time using the capillary electrometer.
In 1895, Einthoven coined the terms P, Q, R, S and T for the electrocardiographic deflections.
Around 1901, Einthoven invented string galvanometer-based 3-lead EKG machine for accurate tracings of cardiac electrical activity.
In 1908, EKG was introduced in the United States.
In 1909 Waller reported electrical activity of canine heart with Einthoven’s string galvanometer. He used the term electrocardiogram for the first time.
Clinical application of electrocardiography in dogs was initiated by Norr in 1922.
In 1924 Einthoven won Nobel Prize for his extraordinary contribution.
During 1934–1938, Wilson invented the central terminal precordial leads.
In 1954 American Heart Association standardized 12 lead EKG.
Electrocardiogram of dogs received major attention in 1949, in decades of the 1960s and 1970s, and now again during recent years.
2.3 Application of Electrocardiography in Canine Medicine
Differential diagnosis of arrhythmias.
Cardiac monitoring during anesthesia and surgery.
Evaluation of pets with gasping/panting/dyspnea/seizures/syncope.
Evaluation for cardiac enlargement.
Prognostic monitoring of hypothermic/hyperthermic dogs.
Cardiac evaluation in diseases associated with multi organ failure.
Evaluation of animals in acute emergencies and in intensive care units.
Evaluation of geriatric pets.
Evaluation of pets with peritoneal effusions.
Evaluation of sports/snuffer animals.
Routine health checkup.
Evaluation of dogs with trauma/accidents.
Evaluation of cardiac diseases with respect to anatomical changes particularly heart enlargements in dogs.
Evaluation of electrolyte disorders.
Assessing the efficacy of drugs (digitalis, quinidine) used for the treatment of heart diseases.
Evaluation of disease progression.
Database to differentiate nonspecific causes of weakness, fatigue, lethargy, syncope, collapse, seizure and fever.
Evaluation of metabolic problems (eclampsia, ketoacidosis).
Evaluation of patients with epilepsy.
Routine presurgical examination.
Routine medical checkup.
2.4 Utility of Electrocardiography
It provides additional information to facilitate diagnosis and treatment of diseased conditions.
It is noninvasive and quick tool.
Electrocardiogram is immediately available.
It is easy to read and results are immediately available.
Evaluation of arrhythmic animals in a better way.
Evaluation of cardiac compromised animals in a better way.
ECG provides visual records of patient’s cardiac status.
2.5 Limitations of Electrocardiogram
Despite a very useful technique, it has its limitation as with any other technique. It cannot provide definite diagnosis of congestive heart failure. Prognosis during anesthesia or surgical procedures is unpredictable. Heart chamber wall thickness cannot be measured. Diseases of heart valves, endocardium, or pericardium cannot be diagnosed. Obstructive blood flow diseases also remain undiagnosed. Wide variations among breeds and dogs of different body conformations make the interpretation of ECG difficult. Normal electrocardiogram may not always rule out cardiac disease.
2.6 The Electrocardiograph and Recording
2.6.3 Lead Systems
Bipolar standard lead system—This consists of three leads designated as lead I, II, and III. These leads are placed as follows and compares potential difference of two limbs. It evaluates activation of the heart in the frontal plane.
Lead I—Lead I is formed by a pair of electrode applied on right forelimb and left forelimb and compares the electric potential of right forelimb (−) with that of left forelimb (+).
Lead II—Lead II is formed by a pair of electrode applied on right forelimb and left hind limb and compares the electric potential of right forelimb (−) with that of left hind limb (+).
Lead III—Lead III is formed by a pair of electrode applied on left forelimb and left hind limb and compares the electric potential of left forelimb (+) with that of left hind limb (+).
Augmented unipolar limb lead system—The augmented unipolar limb leads (aVR, aVL, and aVF) are the other leads which provide additional information about cardiac vector. An augmented lead compares the electrical potential at the reference limb to the sum of electrical activity at the other two limbs. The word “a” stands for augmented, “V” stands for vector and words R, L, and F stand for right forelimb, left forelimb, and frontal limb (represents left hind limb), respectively. Therefore there are three leads in this system called aVR (augmented vector lead of the right arm or right forelimb), aVL (augmented vector lead of left arm or left forelimb), and aVF (augmented vector lead of frontal plane, i.e., left hind limb).
aVR—Electrical potential of right forelimb (+) is compared to the sum of electrical potential of left forelimb and left hind limb (−).
aVL—Electrical potential of left forelimb (+) is compared to the sum of electrical potential of right forelimb and left hind limb (−).
You may also need