1. The SA node is located in the right atrium just inferior and posterior to the entrance of the superior vena cava.

2. It is commonly referred to as the pacemaker of the heart, its primary function.

3. The SA node has an intrinsic rate of depolarization of 60 to 100/min.

4. The rate of depolarization ordinarily is under the control of the sympathetic and parasympathetic nervous systems.

5. The wave of depolarization initiated from the SA node travels outwardly through the atrial musculature in concentric circles, thus depolarizing the atria and ultimately the atrioventricular (AV) node.

6. For all practical purposes, the left and right atria depolarize simultaneously.

1. The AV node is located in the right atrium between the opening of the coronary sinus and interatrial septum.

2. Histologically it comprises cells identical to those of the SA node.

3. The function of the AV node is threefold:

4. The AV node becomes continuous with the common bundle branch (bundle of His) and is the only normal pathway for electric conduction between atria and ventricles. The fibroskeleton of the heart electrically separates atrial from ventricular muscle. The AV node and common bundle penetrate the cardiac fibroskeleton.

5. The tissue of the AV node slows the rate of electrical conduction and accounts for the time delay between atrial and ventricular depolarization. This time delay also allows optimal ventricular filling before ventricular contraction (systole).

6. The rate of conduction of electrical impulses through the AV node is under the control of the sympathetic and parasympathetic nervous systems.

1. It is also known as the AV bundle or bundle of His.

2. It is located on the right side of the interventricular septum and penetrates the right fibrous trigone.

3. Its function is twofold:

4. The common bundle branch travels inferiorly in the interventricular septum for approximately 10 to 12 mm and then divides into one right and two left bundle branches.

1. The right bundle branch appears simply as a continuation of the common bundle branch and follows an inferior course toward the apex of the heart.

2. The left bundle branch penetrates the interventricular septum and divides into an anterior and posterior left bundle branch. Both bundle branches course inferiorly on the left side of the interventricular septum.

3. The function of the three major bundle branches is to conduct electric impulses from the common bundle branch to the Purkinje fibers.

1. The Purkinje fibers are fine ramifications of the bundle branches, which terminate on the endocardial layer of the heart.

2. They are located throughout the entire endocardial layer of both ventricles and conduct the electrical impulses from the bundle branches to the ventricles. Depolarization of the ventricles begins when impulses leaving the Purkinje fibers invade the endocardial layer.

3. The wave of depolarization travels from the endocardial layer outward toward the epicardium and also from the apex toward the base of the ventricles.

1. Impulses originate in the right atrium by spontaneous depolarization of the SA node.

2. Impulses are conducted through atrial muscle, which results in depolarization of right and left atria and AV node.

3. The impulse is delayed at the AV node and then conducted through the cardiac fibroskeleton by the AV node and common bundle branch.

4. The impulse is then conducted from the common bundle branch through the right and left bundle branches to the Purkinje fibers.

5. The impulses exit the Purkinje fibers and cause depolarization of the ventricle from inside out and from apex to base.

6. Design of the electrical conduction system of the heart allows simultaneous depolarization of the right and left atria totally separate from simultaneous depolarization of the right and left ventricles. This fact has important implications for the mechanical function of the heart (Figure 11-2).

1. The polarized state is the normal resting state of cardiac muscle fiber. The extracellular charge is positive with respect to the intracellular charge (Figure 11-3, A).

2. Depolarization is the process of reversing the normal state of polarity. Thus depolarization causes the extracellular charge to be negative with respect to the intracellular charge. This is largely because inflow of extracellular sodium ions is faster than outflow of intracellular potassium ions. Reversal of the cellular membrane charge is transmitted along cardiac muscle fiber, depolarizing subsequent fibers (see Figure 11-3, B). If the muscle fibers are normal, this electrochemical stimulation results in mechanical activity (e.g., shortening of muscle fibers and cardiac contraction).

3. Repolarization is the process of reestablishing the normal state of polarity (i.e., reestablishing a positive extracellular charge with respect to the intracellular charge). The reestablishment of the resting cellular membrane charge is transmitted along the cardiac muscle fiber, repolarizing subsequent fibers (see Figure 11-3, C). If muscle fibers are normal, this electrochemical stimulation results in lengthening of muscle fibers and cardiac relaxation. (Note: It is a mistake to assume that the mechanical activity of the heart is normal simply because the electric activity [ECG] is normal.)

1. The P wave is produced by atrial depolarization and normally is 0.06 to 0.11 second in duration.

2. The QRS complex is produced by ventricular depolarization and normally is 0.03 to 0.12 second in duration. Repolarization of the atria occurs simultaneously with ventricular depolarization and is masked by the overwhelming electric event of the QRS complex.

3. The T wave is produced by ventricular repolarization and normally is 0.14 to 0.26 second in duration.

1. PR interval: Time from the beginning of atrial depolarization to the beginning of ventricular depolarization. The PR interval normally is 0.12 to 0.20 second in duration.

2. RR interval: Time from the peak of one QRS complex to the next QRS complex. It is used to measure the total cardiac cycle and normally is 0.6 to 1.0 second in duration.

3. PP interval: Time from the beginning of one P wave to the beginning of the next P wave. It can be used to measure the total cardiac cycle time and hence the cardiac rate. It is normally equal to the RR interval (0.6 to 1.0 second in duration).

1. The three standard limb leads are lead I, lead II, and lead III (Figure 11-5).

2. The six standard unipolar precordial leads are V₁, V₂, V₃, V₄, V₅, and V₆, where the V represents voltage and each of the numbers 1 through 6 designates a standard position on the chest of the positive (+) electrode (Figure 11-6).

3. Therefore the same consistently generated electrical activity of the heart will be depicted graphically differently for each lead (Figure 11-7).

4. The standard monitoring lead for acutely ill patients is typically a modified lead II position with the electrodes moved onto the chest and somewhat arbitrarily placed to generate the clearest, consistent representation of ECG events (see Figure 11-5, insert).

5. There are three more unipolar extremity leads, V_R,, V_L, and V_F, which along with the aforementioned nine standard leads comprise the diagnostic 12-lead ECG. Each of the V_R, V_L