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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