Learn it well Demo Chapter 3: The Course of Electricity through the Normal Heart

Follow the course of electricity (red arrows), the wave of depolarization through the normal heart in Fig. 3, Fig. 4, and Fig 5.

1. The dominant pacemaker of the heart is the sino-atrial (SA) node. It has a normal rate of 60-100/min. Depolarization of the SA node and its conduction to the atria inscribes a P wave on the EKG and causes the atria to contract.



2. The wave of depolarization next passes to the Atrio-Ventricular node (AV node) which is at the junction of the atria and the ventricles (AV junction). In the AV node there is a pause (2 vertical red lines) that allows each atrium to contract and empty its contents into its respective ventricle before the depolarization passes to the ventricles, inscribing an R wave on the EKG and causing the ventricles to contract. During this pause in the AV node the inscription on the EKG is at baseline; there is no wave.

   The interval from the onset of the P wave to the onset of the R wave, known as the PR interval, includes the P wave and the pause and normally measures 0.12 to 0.20 seconds (s) (120-200 milliseconds [ms]) in an adult.

3. The wave of electricity moves from the AV node to the atrioventricular bundle (A-V Bundle or bundle of His), then to the left bundle branch (LBB) and right bundle branch (RBB) nearly simultaneously.

   This starts the electrical activation of the ventricles, passing anatomically within the interventricular septum. The bundle branches are specialized electrical conduction pathways in the ventricles that conduct more quickly than conduction from myocardial cell to myocardial cell. Purkinje fibers branch from these specialized electrical conduction bundles to deliver the impulse to the ventricular myocardium. On the EKG this results in inscription of the QRS complex. The QRS complex is initially negative (downward) inscribed as a Q wave (if it is present), then is positive (upward) inscribed as an R wave and ends with a negative (downward) S wave. The ventricles contract after electrical activation and inscription of the QRS on the EKG.

4. The next wave on the EKG is called the T wave which is inscribed during ventricular repolarization; Phases 2 and 3 on the Cardiac Action Potential when the myocardial cells regain negative polarity and become ready for the next depolarization. The normal isoelectric baseline between the QRS and T wave is called the ST segment and is considered a part of normal repolarization (see Fig. 6).

   Why are ventricular depolarization and ventricular repolarization both upright deflections on the normal EKG?

   Ventricular depolarization starts in the subendocardium and travels outward through the myocardium, subepicardially (depolarization is endo to epi). This outward movement of positive (+) charges inscribes an upright R wave on the EKG in leads toward which this wave is traveling.

   The direction of the (+) charges reverses during ventricular repolarization and one would expect the T wave on the EKG to have a downward deflection. But, since repolarization occurs from subepicardium to subendocardium (repolarization is epi to endo), this reversal cancels the reversal in direction of charges during repolarization and the T wave on the EKG has an upright inscription. Thus, in a normal ventricle, depolarization (R wave) and repolarization (T wave) are both upright (see Fig. 6).

5. Although the atria repolarize like the ventricles, the atria only have a visible wave on the EKG to represent this when the atria are particularly large. Visible waves of atrial repolarization are called Ta waves (the T wave of the atrium). Ta waves are demonstrated in Chapter 13. Hypertrophy and Enlargement.

Fig. 6: PQRST and Intervals; labeling waves and intervals on the EKG