Page 53 - Basic Monitoring in Canine and Feline Emergency Patients

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VetBooks.ir Electrocardiography

Jessica L. Ward*

Iowa State University, Ames, Iowa, USA

3.1 Physiology: How Does Normal locations other than the AV node. From the AV
Cardiac Conduction Occur? node, the wave of depolarization crosses from atria
to ventricles through the bundle of His, and splits
Before cardiac myocytes can contract (and thus before into left and right bundle branches that subdivide
the heart can beat), myocytes must first receive further before terminating as Purkinje fibers (see Fig.
electrical signals that cause cellular depolarization. 3.1). Depolarization of ventricular myocytes follows
The goal of an electrocardiogram (ECG) is to as the QRS complex, normally the tallest and most
depict these electrical events involved in cardiac distinctive waveform on the ECG. The specialized
conduction. Waves of depolarization and repolari- ventricular conduction pathway (His bundle, bundle
zation are sensed using metal electrodes placed around branches, and Purkinje fibers) quickly spreads the
the body, and electrical activity is represented as electrical impulse throughout the ventricles, allow-
deflections on graph paper. In order to understand ing rapid ventricular depolarization and synchro-
ECGs, one must first understand how electricity nous ventricular contraction. This pathway allows
normally moves through the heart (see Fig. 3.1). the normal QRS complex to be relatively narrow
Normal cardiac conduction begins with depolari- (approximately the same width as the P wave),
zation of the sinoatrial (SA) node, a small group of despite representing electrical activity for a much
cells located high in the right atrium that has the larger mass of tissue (ventricles versus atria). If the
fastest rate of spontaneous diastolic depolarization specialized conduction pathway cannot be utilized
of any cardiac tissue. In other words, the SA node for some reason, the ventricles must depolarize cell-
‘fires’ fastest; therefore, in normal animals, the SA to-cell, resulting in QRS complexes that are wide
node sets the pace of the heart. From the SA node, and bizarre. After ventricular depolarization has
the wave of depolarization travels cell-to-cell completed, ventricular repolarization follows, repre-
throughout the atria, creating the P wave on ECG sented by the T wave on ECG.
(see Table 3.1 and Fig. 3.2). The impulse also travels Knowing how normal cardiac conduction is
along specialized interatrial tracts to the atrioven- depicted on an ECG allows identification of abnor-
tricular (AV) node, located low in the right atrium mal electrical activity within the heart. If P waves are
near the tricuspid valve and interventricular septum. absent, then the atria did not depolarize. If a P wave
Impulse conduction slows in the AV node during the is present with no QRS complex following, then elec-
PR interval (also called the PQ interval), giving time trical conduction was ‘blocked’ at the level of the AV
for active atrial contraction to occur before the ven- node. If QRS complexes are wide and bizarre, then
tricles depolarize. AV nodal delay allows extra blood the ventricles depolarized cell-by-cell rather than
from atrial contraction (the ‘atrial kick’) to augment using the specialized ventricular conduction pathway;
passive ventricular filling, thus increasing stroke this can occur if the impulse originated within the
volume. In the normal heart, the AV node is the only ventricles (as with ventricular premature complexes)
pathway by which electrical stimulation can travel or if delay occurred within the specialized conduction
from the atria to the ventricles; the top and bottom system (as with bundle branch block).
of the heart are electrically ‘insulated’ from one The rate of SA node depolarization, as well as
another (no electrical conduction tissue present) at all the rate of AV nodal conduction, are regulated by

* Corresponding author: jward@iastate.edu

© CAB International, 2020. Basic Monitoring in Canine and Feline Emergency Patients 45
(eds E.J. Thomovsky, P.A. Johnson and A.C. Brooks)