© 2021 MJH Life Sciences and dvm360 | Veterinary News, Veterinary Insights, Medicine, Pet Care. all rights reserved.

© 2021 MJH Life Sciences™ and dvm360 | Veterinary News, Veterinary Insights, Medicine, Pet Care. all rights reserved.

Understanding the basic electrical principles of the heart is essential to explain this valuable diagnostic test.

The electrocardiogram is to record the electrical activity of the heart on the body surface. It is a record of the average electrical potential generated in the heart plotted with voltage and time. The specific waveform represents the stages of myocardial depolarization and repolarization.

The electrocardiogram (ECG) is a valuable diagnostic test in veterinary medicine, and it is easily available. This is the most important test performed on animals that can hear arrhythmia (except for sinus arrhythmia in dogs). The electrocardiogram can also produce useful information about ventricular dilation and hypertrophy. However, the electrocardiogram does not record the mechanical activity of the heart, so it does not produce information about the contractility of the heart. It is also important to remember that even in the face of advanced cardiovascular disease, the ECG may be normal.

The bipolar triaxial lead system we use today was developed by Dutch physiologist Willem Einthoven in the early 20th century with the P-QRS-T term describing the ECG waveform complex. The lead consists of the electrical activity measured between the positive and negative electrodes. The direction of the lead relative to the heart is called the lead axis. Electrical pulses with a net direction toward the positive electrode will produce a positive waveform or deflection, while electrical pulses far away from the positive electrode will produce a negative waveform or deflection.

As the angle between the lead axis and the direction of the activation wave increases, the ECG deflection of the lead becomes smaller. The electric pulse whose net direction is perpendicular to the positive electrode does not produce a waveform or deflection at all, and is called an isoelectric pulse.

Standard ECG leads are used to create multiple angles to evaluate the waveform through the three-dimensional heart. A wire can only provide information about one dimension of current. For the purpose of this overview, we will focus on lead II, a bipolar lead in which the right arm (RA-the right front leg of a veterinarian patient) is negative and the left leg (LL-or left hind leg) is positive (figure 1). We will mainly discuss rhythm diagnosis.

Figure 1. Three bipolar frontal plane leads (I, II, and III) and the Einthoven triangle (red). In lead I, the right front leg (RA) is negative and the left front leg (LA) is positive. In lead II, the right front leg (RA) is negative, and the left hind leg (LL) is positive. In lead III, the left front leg (LA) is negative and the left hind leg (LL) is positive. In dogs and cats, the net depolarization through the ventricle (green arrow) is usually towards the left hind leg (the positive pole of lead II), so the QRS complex is mainly positive in lead II. For an ECG machine using four electrodes, the electrode placed on the right hind leg is the ground (it is not part of any lead).

The ECG should be recorded in an area that is as quiet and undisturbed as possible. Noise from clinical activities and other animals may significantly affect the patient's heart rate and rhythm. In quadrupeds, the size and direction of the electrocardiogram vector determined by the limb leads will change greatly due to changes in the attachment position of the shoulder straps to the thoracic muscles.

Each pair of limbs should be kept parallel and not allowed to touch each other. During the ECG examination, the animal should remain as still as possible, and if possible, prevent the dog from panting. In some cases, gently closing the animal's mouth or placing a hand on the chest (if tremor is present) may help. Alligator clips or adhesive electrodes can be used, but the teeth of the alligator clip should be dull and the spring relaxed to reduce discomfort.

Limb electrodes are placed on the distal end of the elbow and knee joints and are moistened with 70% isopropanol or ECG paste to ensure good electrical contact. If the ECG complex is too large to fit completely within the grid of paper, the calibration should be changed from standard (1 cm = 1 mV) to one-half standard (0.5 cm = 1 mV). The voltage and paper speed calibration used for recording must be written during the recording process so that this information becomes part of the permanent record.

The artifact area should be identified when ECG is recorded so that it can be dealt with if possible. Electrical (60-cycle) noise may be caused by poor electrical grounding (object, electrocardiograph, or workbench where ECG is performed) or adjacent equipment (such as lights or other electrical equipment). Electrical noise appears on the ECG as regular fine and sharp vertical oscillations. As mentioned earlier, placing a hand on the animal’s chest may help reduce shaking or breathing artifacts.

It is important not to misunderstand artifacts during the ECG evaluation. The heart rate (atria and ventricles) should be calculated, and the waveform amplitude and interval duration should be measured on the part of the ECG (lead II) running at a paper speed of 50 mm/s. On a normal ECG, each P wave is followed by a QRS complex at regular intervals for a particular species. Specific measurements to obtain a thorough ECG interpretation include:

In addition, the average electrical axis and heart rhythm should be determined. The rhythm analysis will be discussed below.

There are several important principles to keep in mind when performing ECG rhythm assessment. All normal heart cells can be depolarized (excited) when stimulated by neighboring cells, and then stimulate the neighboring cells to discharge. This ability is called conductivity. However, only certain cells can automatically (beat by themselves). These pacemaker cells spontaneously depolarize during diastole (the resting potential becomes less negative) until the threshold potential is reached. Changes in the spontaneous depolarization rate occur gradually over several cardiac cycles. Therefore, tachycardia associated with pacemaker cells (for example, sinus tachycardia) gradually accelerates within a few seconds, which is different from tachycardia originating from ectopic lesions, which usually accelerates suddenly.

Specialized cells in the sinoatrial node, atrioventricular (AV) node, and His-Purkinje system are autonomous. However, under normal circumstances, pacemaker cells outside the sinus node will not reach the threshold because the depolarization wavefront of the sinus node discharges them before they automatically depolarize. This is because the pacemaker cells located at the distal end of the sinus node are called auxiliary pacemaker cells, and their depolarization rate is slower than that of the sinus node. Immediately after depolarization, cardiomyocytes are refractory. Once the cell returns to its resting potential, it will be excited again (it can be activated again).

The P wave is produced by the depolarization of the atria. Several types of arrhythmias may not have P waves, including atrial fibrillation and atrial arrest. Alternatively, the P wave may be hidden in other waveforms (and therefore not visible), which usually occurs in supraventricular tachycardia (Figure 2). P wave enlargement (higher or wider than normal) is considered an indicator of atrial enlargement.

Figure 2. Lead II ECG from a dog (25 mm/sec; 10 mm/mV).

Heart rate: 150 bpm in the early and late part of the strip; 300 bpm in the middle part; tachycardia.

RR regularity: the early and late band regularity is irregular; the regularity is in the middle part.

QRS wave shape: narrow, normal-looking QRS wave shape; supraventricular origin.

P wave: Except for the tachycardia zone, each QRS complex has a P wave, and each P wave has a QRS complex.

Rhythm diagnosis: sinus rhythm with supraventricular tachycardia or atrial tachycardia. (Due to the sudden change and rapid speed of sinus rhythm, sinus tachycardia can be ruled out.)

The PR interval represents atrial depolarization and conduction through the AV node. Prolonged PR interval is called first-degree atrioventricular block. Depolarization originating from the ventricle (premature ventricular beats [VPC; Figure 3] or escape beats) will not spread through the fast conduction pathway, but will spread between cells in a slower way, resulting in an abnormally complicated QRS wave widening . The resulting T waves can also be abnormal and often inconsistent (in the opposite direction of the QRS complex).

Figure 3. Lead II ECG from a dog (25 mm/sec; 5 mm/mV).

Heart rate: 170 bpm; normal to slightly elevated.

RR regularity: irregular regularity (the heartbeat is regularly fast and then slow); one complex is earlier than the others.

QRS complex morphology: Except for one complex that is earlier than the others, the QRS complex has a narrow shape and normal appearance; it is mainly supraventricular rhythm, accompanied by a premature wide QRS complex.

P wave: Each QRS complex has a P wave. Except for the wide wave group, each P wave has a QRS complex.

Rhythm diagnosis: mild sinus tachycardia with univentricular premature beats.

Enlargement of the ventricle may change the QRS wave deflection, duration, or amplitude. For similar reasons, abnormal conduction, such as through the right or left bundle branch, can also change the QRS wave duration and shape (shape). If conduction through the bundle branches of the ventricle is blocked, the depolarization wavefront cannot propagate along the fast conduction path of the affected ventricle. Conduction will still travel from cell to cell, but at a much slower speed (so the QRS wave becomes wider). Other causes of wide QRS complexes include electrolyte abnormalities, such as hyperkalemia and certain drugs (ie, some antiarrhythmic drugs).

When evaluating heart rhythm on ECG, using a stepwise method can simplify the process.

1. Calculate the heart rate and determine whether it is normal or abnormal (bradycardia or tachycardia). In the case of AV separation, there may be different atrial and ventricular rates.

2. Look at the law of RR. Rhythms originating from a single part of the ventricle or atrium are usually regular, while rhythms originating from the sinus node are usually irregular due to changes in adrenergic activity (Figure 4).

Figure 4. Lead II ECG from a dog (25 mm/sec; 10 mm/mV).

RR regularity: irregular regularity (the heart accelerates regularly and then gradually slows down).

QRS wave shape: narrow, normal-looking QRS wave shape; supraventricular origin.

P wave: Each QRS complex has a P wave, and each P wave has a QRS complex; the shape of the P wave looks normal.

3. Evaluate the shape or morphology of the QRS complex. Does it look normal or wide? Reasons for QRS complexes wider than normal include ventricular origin (Figure 5), electrolyte abnormalities (hyperkalemia), abnormal conduction (bundle branch block), ventricular hypertrophy, or certain drugs. Does each QRS complex have a P wave, and each P wave has a QRS complex? If so, it is probably sinus rhythm. If there is a QRS complex P wave, there is AV block.

Figure 5. Lead II ECG from a dog (25 mm/sec; 5 mm/mV).

QRS wave form: broad wave group; ventricle (always keep in mind other possible causes of wide compound tachycardia, especially if the heart rhythm does not respond as expected to treatment).

Rhythm diagnosis: ventricular tachycardia, persistent.

4. Determine the extent of the AV block. In the first-level atrioventricular block, each P wave produces QRS complexes, but the atrioventricular conduction is slow, so the PR interval is prolonged. If there is only partial P wave block (that is, no QRS complexes are produced), the heart rhythm is a second-degree AV block (Figure 6). Second-degree atrioventricular block is further divided into Mobitz type 1 (Wenkebach type), in which the PR interval is gradually extended until P wave block occurs, or Mobitz type 2, in which the PR interval is constant. Complete separation (there are P waves and QRS complexes but there is no connection between them) there is a third degree or complete heart block

Figure 6. Lead II ECG from a dog (25 mm/sec; 5 mm/mV).

RR regularity: irregular regularity (the heart accelerates regularly and then gradually slows down).

QRS wave shape: narrow, normal-looking QRS wave shape; supraventricular origin.

P wave: Several P waves appear without following the QRS complex (after the second, sixth, and ninth QRS complex).

Rhythm diagnosis: second-degree atrioventricular block (PR interval is fixed; therefore, second-degree atrioventricular block type 2).

As with any clinical skill, proficiency in ECG interpretation requires practice. Understanding the basic electrical principles of the heart helps to read all electrocardiograms and is essential for explaining more complex arrhythmias.

Dr. Sleeper is a clinical professor of cardiology at the University of Florida School of Veterinary Medicine. He has published more than 75 peer-reviewed original articles, 50 review articles or case reports, and four books.