Electrocardiography

Chapter 144 Electrocardiography

Francis W.K. Smith, Jr. , Larry Patrick Tilley, Michael S. Miller

Electrocardiography is the graphic representation of the electrical activity of the heart with time displayed on the x-axis and voltage displayed on the y-axis. Electrocardiograms (ECGs) are easy to perform and readily available to practicing veterinarians. There are numerous indications for performing ECGs, and the examination provides information that is useful and often pivotal to the diagnosis and management of cardiac and systemic disturbances. Veterinarians or their technicians can perform and interpret their own ECGs or utilize readily available fax, transtelephonic, or computer consultation services.

GENERAL PRINCIPLES OF ELECTROCARDIOGRAPHY

Indications for Performing Electrocardiography

Electrocardiography is the examination of choice for diagnosis of a normal cardiac rhythm and arrhythmias and for monitoring the effect of antiarrhythmic therapy. There are a number of clinical indications for recording and interpreting an ECG:

• To diagnose an arrhythmia detected on physical examination (auscultation, arterial pulse deficits, palpable precordial heart beat irregularities, or abnormal jugular venous pulsations).

Perform an ECG on all animals with tachycardia or bradycardia and on all cats with any irregularity in rhythm. Perform an ECG on any dog with an irregularity in rhythm that is related to a pulse deficit or is not related to phases of respiration.

• To identify arrhythmias or conduction disturbances in patients with a history of syncope, seizures, or exercise intolerance.

• To monitor the effectiveness of antiarrhythmic therapy. In this regard, the ambulatory (Holter) ECG is the best approach for monitoring effectiveness of antiarrhythmic therapy for ventricular arrhythmias.

• To assess cardiac size in patients with known or suspected cardiac disease. While the ECG is not a very sensitive indicator of heart size, a cardiac enlargement pattern usually correlates with cardiac chamber enlargement or hypertrophy. The more criteria for heart enlargement present in a patient, the more likely the heart is enlarged. Most dogs with normal hearts will not exhibit heart enlargement patterns on ECGs.

A normal ECG does not rule out a diagnosis of heart enlargement. Thoracic radiographs and echocardiography are more sensitive tests for evaluating heart size.

• To help individualize and monitor therapy in patients with heart failure by accurately assessing cardiac rhythm. It is important to understand that a diagnosis of congestive heart failure (CHF) cannot be made based on an ECG alone.

An arrhythmia or a heart enlargement pattern supports the diagnosis of heart disease but does not confirm CHF. Obtain thoracic radiographs, along with an ECG, in a patient suspected of having CHF. The ECG is useful in patients with heart failure for the diagnosis and treatment of associated arrhythmias.

• To evaluate patients with suspected digoxin or other cardiac drug toxicity.

• To screen for electrolyte disturbances, especially hyperkalemia, hypercalcemia, and hypocalcemia.

• To look for evidence that may support the diagnosis of pericardial effusion, hypothyroidism, hyperthyroidism, or hypoadrenocorticism (Addison’s disease).

Technique for Recording an Electrocardiogram

• For accurate measurements, place the patient in right lateral recumbency.

If the animal is dyspneic or if restraint would be dangerous to the patient, obtain a rhythm strip with the animal supported in any comfortable position.

• Wet the skin with alcohol or ECG electrode gel and attach the electrodes just above the elbows and stifles.

• Hold the upper limbs perpendicular to the long axis of the patient and parallel to the floor. If the thoracic limbs are not parallel and perpendicular to the long axis of the animal, the mean electrical axis will be altered.

• Record approximately three to four complexes in each of the six limb leads, and then record a long lead II strip for rhythm evaluation. Push the standard calibration button at the beginning and end of each recording.

• Chest leads can be obtained using the lead V electrode while keeping the limb leads attached. See Table 144-1 for electrode placement.

• Chest leads are not necessary for all patients. However, they may be quite useful when the ECG complexes in the limb leads are small and difficult to evaluate. Often, P waves that cannot be seen in the limb leads become apparent on a chest lead.

• Evaluation of chest leads may also be helpful in evaluating heart enlargement patterns.

• The chest leads most commonly used in veterinary patients are CV₅RL (rV₂), CV₆LL (V₂), CV₆LU (V₄), and V₁₀.

Table 144-1 ELECTRODE PLACEMENT

Chest Lead	Placement of the V Lead
CV₅RL (rV₂)	Fifth intercostal space on the right side near the sternum
CV₆LL (V₂)	Sixth intercostal space on the left side near the sternum
CV₆LU (V₄)	Sixth intercostal space on the left side at the costochondral junction
V₁₀	Over the dorsal spine of the seventh thoracic vertebra

Normal Cardiac Conduction

The ECG records the electrical activity of the heart. Electrical impulses originate in specialized pacemaker tissue in the sinus node of the right atrium. The impulse rapidly traverses the atrium, causing atrial contraction, and then slows as it passes through the atrioventricular (AV) node located at the proximal portion of the interventricular septum. Electrical activity then rapidly passes through the bundle of His, the anterior and posterior branches of the left bundle branch, the right bundle branch, and the terminal Purkinje fibers. This causes activation of the interventricular septum and the left and right ventricular myocardium (Fig. 144-1).

Figure 144-1 Normal conduction pathway in the heart.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(From De Sanctis RW: Disturbances in cardiac rhythm and conduction. In Rubenstein R [ed]: Scientific American Medicine, section 1, subsection VI. © 1991 Scientific American, Inc. All rights reserved.)

Components of the Electrocardiographic Tracing (Fig. 144-2)

• P wave indicates atrial depolarization.

• P-R interval indicates time for conduction of the impulse from the sinoatrial (SA) node to the AV node and delay of the impulse in the AV node, His bundle, bundle branches, and Purkinje system.

• QRS complex indicates ventricular myocardium depolarization.

• Q wave (lead II) is associated with depolarization of the interventricular septum.

• R wave (lead II) is associated with depolarization of the left ventricle.

• S wave (lead II), when present, is associated with depolarization of the right ventricle.

• T wave indicates ventricular repolarization.

• Q-T interval indicates an approximate interval of ventricular systole.

Figure 144-2 Normal electrocardiogram with components labeled.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(From Tilley LP: Essentials of Canine and Feline Electrocardiography, 3rd ed. Philadelphia: Lea & Febiger, 1992.)

How to Evaluate the Electrocardiogram

• Always evaluate the ECG from left to right.

• Identify and label the ECG waveforms (P-QRS-T).

• Calculate the approximate heart rate (HR) by counting the number of R-R intervals in 3 seconds (two sets of time markers at 50 mm/second) and multiplying by 20.

• Determine the mean electrical axis.

• One of the easiest ways to approximate the axis is to identify the isoelectric lead (sum of the positive and negative deflections of the QRS complex closest to zero).

• Determine the perpendicular lead, and evaluate that lead to see if the complexes are positive or negative.

• The axis is in the direction of the main deflection of the perpendicular lead (see Fig. 144-3 for an example).

• If all leads are isoelectric, an axis cannot be determined in the frontal plane.

• An alternative approach is to identify the frontal plane lead with the largest net positive QRS complex. In dogs, the frontal axis is normal if lead II or aVF is largest, the axis is deviated to the left if lead I or lead aVL is most positive, and the axis is deviated to the right if lead III or lead aVR is most positive.

• A normal axis does not exclude the possibility of a cardiomegaly pattern or conduction disturbance.

• Determine the rhythm.

• A sinus rhythm has nearly constant P-R and R-R intervals (usually varying by <10%).

• A sinus arrhythmia has a nearly constant P-R interval, but the R-R interval varies, usually with the phase of respiration.

• Measure the height and width of the complexes.

• Determine the P-R and Q-T intervals and evaluate the S-T segment.

• Compare the heart rate, rhythm, and sizes of the complexes with the normal values (Table 144-2).

Figure 144-3 Electrocardiogram from a cat. Lead aVR is isoelectric. Lead III is perpendicular to aVR. The QRS deflection in lead III is positive, making the axis +120 degrees. If lead III had been negative, the axis would have been −60 degrees.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(From Tilley LP: Essentials of Canine and Feline Electrocardiography, 3rd ed. Philadelphia: Lea & Febiger, 1992.)

Table 144-2 NORMAL CANINE AND FELINE ELECTROCARDIOGRAM VALUES^*

ELECTROCARDIOGRAPHIC ABNORMALITIES

Electrocardiographic abnormalities can be divided into those involving heart rate and rhythm and those involving the configuration of the complexes. This chapter discusses abnormalities involving the ECG complexes. Arrhythmias are discussed in more detail in Chapter 145.

Intraventricular Conduction Disturbances

• The right bundle branch is thinner and therefore more susceptible to injury than the left bundle branch.

• Injury to more than one bundle branch may result in first-, second-, or third-degree AV block.

• A bundle branch block does not cause significant hemodynamic changes and therefore does not warrant therapy.

• Rate-related bundle branch blocks can occur intermittently in association with either bradycardia or tachycardia. The right bundle branch cells have a longer refractory period than the left bundle branch cells. This makes the right bundle more sensitive to abrupt changes in the heart rate.

Right Bundle Branch Block (Fig. 144-4)

• Incomplete RBBB may be present if the aforementioned criteria are present but complexes are of normal width. Incomplete RBBB can be a normal variant in beagles. The pattern may be confused with a left posterior fascicular block (for which criteria are not firmly established).

• Differentiate RBBB from right ventricular enlargement by radiographs or echocardiography. Right ventricular enlargement and RBBB can occur together.

• RBBB has been associated with the following conditions: congenital malformations such as pulmonic stenosis, cardiac surgery, cardiac needle puncture or cardiac arrest, cardiac neoplasia, trauma, Trypanosoma cruzi infection (dog), cardiomyopathy, hyperkalemia (cat), acute ventricular dilation, balloon valvuloplasty, and doxorubicin cardiotoxicity. RBBB may be an incidental finding in patients without apparent cardiac disease.

Figure 144-4 Example of right bundle branch block in the dog. Note the right axis (−110); the wide S wave in leads I, II, III, and aVF; and the M-shaped CV₅RL and W-shaped V₁₀.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(From Tilley LP: Essentials of Canine and Feline Electrocardiography, 3rd ed. Philadelphia: Lea & Febiger, 1992.)

Left Bundle Branch Block (Fig. 144-5)

• LBBB indicates greater cardiac damage than RBBB—often cardiomyopathy.

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Tags: Saunders Manual of Small Animal Practice

Aug 27, 2016 | Posted by admin in SMALL ANIMAL | Comments Off

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