Chapter 144 Electrocardiography
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
• To diagnose an arrhythmia detected on physical examination (auscultation, arterial pulse deficits, palpable precordial heart beat irregularities, or abnormal jugular venous pulsations).
• 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.
• 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.
Technique for Recording an Electrocardiogram
• 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.
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
Components of the Electrocardiographic Tracing (Fig. 144-2)
• 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.
How to Evaluate the Electrocardiogram
• 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).
• 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.
ELECTROCARDIOGRAPHIC ABNORMALITIES
Intraventricular Conduction Disturbances
• The right bundle branch is thinner and therefore more susceptible to injury than the left bundle branch.
• 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)
• ECG characteristics of right bundle branch block (RBBB) include the following:
• Wide S waves in leads I, II, III, and aVF and lower left precordial leads, such as CV6LL and CV6LU
• 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.
