Page 21 - Rapid Review of ECG Interpretation in Small Animal Practice, 2nd Edition
P. 21
Principles of Electrocardiography
VetBooks.ir The cardiac conduction system and components of the P–QRS–T complex
Through a complex change of ionic concentrations across the cell membrane, an extracellular potential
field is established which then excites neighboring cells, and a cell-to-cell propagation of electrical events
occurs. Because the body acts as a purely resistive medium, these potential fields extend to the body
surface. The character of the body surface waves as seen on the ECG depends on the amount of tissue
activated at one time and the relative speed and direction of the activation wavefront. The 12 ECG leads
provide information about the magnitude of the electrical activity of the heart and the direction of the
moving depolarization wavefront in multiple orientations. A wavefront traveling toward the positive
terminal of a lead results in a positive deflection of the ECG in that lead. When a wavefront travels
away from the positive electrode, a negative deflection occurs. A lead axis in parallel to the direction
a wavefront is moving results in a large deflection, while a lead axis perpendicular to the direction of a
moving wavefront results in a small (or no) deflection on the ECG.
An early pioneer of the ECG, Einthoven chose the letters of the alphabet “PQRST” to avoid conflict
with other physiological waves being studied at about the same time. The wavefront initiated by the sinus
node, located at the junction of the cranial vena cava and the right atrium, depolarizes the atria from right
to left and cranial to caudal. This results in a small, upright P wave in the caudal leads I, II, III, and aVF, while
it appears as a negative or isoelectric deflection in aVR and aVL. The P wave amplitude is most prominent
in lead II, because the average vector of the atrial depolarization is traveling toward the positive terminal
of lead II, while the P is almost invisible in leads which are oriented perpendicular to lead II (aVL). While
the impulse conducts slowly through the AV node and enters the bundle of His and spreads down into
the bundle branches and to the Purkinje fibers, the ECG displays an isoelectric (baseline) line in all leads.
These conduction tissues are “insulated” so that the action potential travels mostly intracellular and does
not spread into the muscle as a measureable signal on the surface ECG.
Once the impulse reaches the ventricular myocardium, it spreads into the interventricular septum
and then across both ventricles, producing the QRS complex on the surface ECG. The initial vector of
depolarization into the septum can point cranially and to the right, that is, away from the positive pole
of lead II, producing the small negative deflection of the Q wave. Because the muscle mass of the left
ventricle exceeds that of the right ventricle, the summation of all vectors of ventricular depolarization
point toward the left side and caudally, that is, the positive terminal of lead II, resulting in the large,
positive deflection of the R wave. Lead II typically shows the largest R wave amplitude, but normally all
caudal leads (I, II, III, and aVF) will depict a positive R wave. The final phase of ventricular depolarization
includes the heart base, such that the sum of the vectors is pointed again cranially, producing a small
negative deflection, the S wave. Not all dogs and cats display Q waves or S waves as the presence of
these waves depends somewhat on the horizontal position of the heart in the chest.
Ventricular repolarization does not occur in the inverse direction as the depolarization, but is a slow
process that starts at the epicardium and the ventricular apex, and ends at the endocardium and heart
base. The resulting various vectors of repolarization nearly cancel each other out, thus generating the low
amplitude T wave in lead II. It can be a positive or negative deflection in dogs, or nearly isoelectric in cats.
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