EKG: Cardiac Electrical Conduction System

Cardiac Electrical Conduction System: Atrioventricular Node

Focus topic: Cardiac Electrical Conduction System

The atrioventricular (AV) node is located in the inferior right atrium. It is an essential piece of the picture as the fibrous skeleton that is present between the atria and ventricles prevents electrical currents from crossing this barrier otherwise. It sits in close proximity to the opening of the coronary sinus and behind the tricuspid valve. The metrics of this node include a length of 22 mm, a width of 10 mm, and a thickness of 3 mm in the normal adult. This node is also supplied by both sympathetic and parasympathetic nervous system fibers. The AV node is a pathway for the conduction of the impulse and actually has no pacemaker cells. The surrounding junctional tissue does contain pacemaker cells that have the ability to fire at a backup rate of 40 to 60 times per minute if necessary. This is known as an escape pacemaker. For the pediatric patient under the age of three, this AV nodal back up can fire at a rate of 50 to 80 times per minute. The impulse travels from the atria to the AV node, but its course is delayed 0.04 seconds in order to allow the ventricles to complete their filling and not contract too quickly. This delay also allows the atria to empty completely and provide the atrial kick discussed in Chapter 2. If this delay by the AV node did not occur, the atria and ventricles would contract at the same time thus decreasing stroke volume (SV) and compromising cardiac output. The short isoelectric line on the EKG after the P wave is the indication of the delay that takes place in this node (Conduction pause at the AV node).

Clinical Alert

If the AV node were not present when fast atrial rates are present, such as with atrial fibrillation, these dangerously high rates of impulses would feed into the ventricles. This then is a protective section of the whole impulse conducting system.

Cardiac Electrical Conduction System: Conduction pause at the AV node

Focus topic: Cardiac Electrical Conduction System

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Cardiac Electrical Conduction System: Bundle of His

Focus topic: Cardiac Electrical Conduction System

The impulse then travels from the AV node, through the AV junction, to the bundle of His. The AV junction is the AV node and the solid or nonbranching portion of the bundle of His that is made up of specialized conduction tissue. The function of the bundle of His is to conduct impulses to the ventricles as they come through the AV node.

Clinical Alert

The bundle of His receives a dual blood supply from the left anterior and posterior descending coronary arteries that makes it less vulnerable to ischemia.

Cardiac Electrical Conduction System: Bundle Branches

Focus topic:Cardiac Electrical Conduction System

The bundle of His divides into right and left bundle branches with the right bundle branch continuing on the right side of the interventricular septum, carrying the impulse through the right ventricle. The left bundle branch stretches down the left side of the interventricular septum and is responsible for the impulse to be carried through the left ventricle. The left bundle branch splits even further into three separate branches or subdivisions known as fasciculi (anterior, posterior, and septal). This allows the left bundle branch to deliver impulses more quickly to the thicker, muscular left ventricle than the right bundle branch which feeds the thinner, less muscular walls of the right ventricle. This enables both ventricles to contract at the same time.

Cardiac Electrical Conduction System: Heart Pacemaker Sites

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Cardiac Electrical Conduction System: Purkinje Fibers

Focus topic: Cardiac Electrical Conduction System

The right and left bundle branches divide into smaller branches which end in a network of filaments known as Purkinje fibers. These fibers spread into the endocardium and the papillary muscles to assist in the transference of the impulse. This causes the ventricles to depolarize and contract in a rotating or twisting type of motion that helps to squeeze blood out of the ventricles, forcing it into the arteries to feed the body. When the SA node is damaged and can’t send impulses, as in the case of particular myocardial infarctions, the Purkinje fibers will initiate an impulse on their own. These fibers also have pacemaker cells which can fire at a rate of 20 to 40 beats per minute (Heart Pacemaker Sites). Children under the age of three have an inherent rate of 40 to 50 beats per minute in the Purkinje fibers. These activities of the bundle of His, the separate bundle branches, and the Purkinje fibers (known collectively as the His-Purkinje system) creates the QRS pattern on the EKG tracing (Movement of impulse through His-Purkinje system).

Cardiac Electrical Conduction System: Movement of impulse through His-Purkinje system

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Cardiac Electrical Conduction System: ABNORMAL HEART IMPULSES

Focus topic: Cardiac Electrical Conduction System

Abnormal impulses can occur in the cardiac conduction system resulting in aberrant heart rhythms. An EKG or a cardiac monitor strip can electronically demonstrate these dysrhythmias. Causes of abnormal impulses include enhanced automaticity, triggered activity, reentry, backward conduction, escape rhythms, and conduction disturbances. These disruptions in normal heart rhythm can be caused by trauma, drug toxicity, electrolyte disturbances, myocardial ischemia, and myocardial infarction.

  • Enhanced Automaticity: In normal automaticity, pacemaker cells generate impulses automatically without the need for stimulation. The SA node is the preferred pacemaker and its automaticity and faster firing rate inherently supersedes the others. The normal PQRST pattern is derived from this generated impulse (Normal PQRST). Additional areas in the heart that have the capability to produce impulses are located in other areas of the atria, the AV nodal junction, and the ventricles. In enhanced automaticity, cardiac cells that do not typically act as pacemakers depolarize spontaneously, or one of these alternate pacemaker sites increases its firing rate through acceleration of its inherent automaticity to beyond normal and therefore, takes over control of the heart rate. A decrease in the automaticity and firing of the SA node can also allow these substitute pacemakers to seize control. Some disease processes that allow this enhanced automaticity to occur include ischemia (lack of oxygen) of the myocardium, injuries to the heart muscle, increased sympathetic tone, digitalis overdose, low potassium (hypokalemia), high potassium (hyperkalemia), and low calcium (hypocalcemia). This altered automaticity can cause atrial flutter; atrial fibrillation; ventricular fibrillation (Ventricular fibrillation); ventricular tachycardia (Ventricular tachycardia); supraventricular tachycardia (Supraventricular tachycardia); junctional tachycardia; accelerated idioventricular rhythm; accelerated junctional rhythm; or premature atrial, junctional or ventricular complexes (Premature ventricular contraction).

Cardiac Electrical Conduction System: Normal PQRST

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Cardiac Electrical Conduction System: Ventricular fibrillation

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Cardiac Electrical Conduction System: Ventricular tachycardia

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Cardiac Electrical Conduction System: Supraventricular tachycardia

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Cardiac Electrical Conduction System: Premature ventricular contraction

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  • Triggered Activity: In triggered activity, abnormal electrical impulses (also called afterdepolarizations) are conducted during the normal resting period of repolarization. This occurs when pacemaker cells from an alternative site (from a location other than the SA node) receive a single stimulus, but then can respond by depolarizing more than once. This can occur as both an early afterdepolarization when the cell has not had time to fully repolarize or as a delayed afterdepolarization when it occurs after the cell has had time to completely repolarize. These impulses are known as ectopic and can arise from either the atria or ventricles. These beats can occur as a single beat or in pairs (coupling). “Runs” can also be experienced when three or more beats occur in succession and longer “runs” can take place as a sustained rhythm. These individual beats are known as premature atrial, junctional, or ventricular beats (Premature atrial and premature junctional beats). This is another potential cause of ventricular tachycardia. Causes of triggered activity include: hypokalemia, hypercalcemia, slow pacing rates, drug toxicity (especially digoxin), hypomagnesemia, myocardial ischemia, myocardial injury, hypoxia, and increased catecholamine release.

Clinical Alert

Some medications can cause a prolongation of the repolarization process that can produce triggered activity. When this occurs, the potential for a particular rhythm, known as torsades de pointes (twisting of the points) can take place. This is a type of ventricular tachycardia that has a twisting type of baseline. Some of these medications involve antiarrhythmics such as: quinidine (Quinaglute), sotalol (Betapace), and amiodarone (Pacerone), antibiotics such as: erythromycin (E.E.S.), azithromycin (Zithromax), and clarithromycin (Biaxin), and antipsychotics such as: haloperidol (Haldol) and ziprasidone (Geodon).

Cardiac Electrical Conduction System: Premature atrial and premature junctional beats

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  • Reentry: In reentry, the impulse spreads through the same tissue again at a faster-than-normal rate that it has previously stimulated. Normally, an impulse spreads through the conduction system and is followed by subsequent impulses. In a reentry variant, this impulse follows its intended route, but then, reenters and begins the depolarization process again in the same area. The original impulse is delayed long enough to allow cells to repolarize, thus creating a situation in which they are available for another impulse. As long as contractile cells of the cardiac muscle continue to accept this impulse, it will follow a circular pathway. One of three factors can be present to activate this process: 1. a potential or accessory pathway; 2. some type of block within the circuit; 3. a delay in the conduction of the impulse. This can cause premature atrial, junctional, and ventricular beats as seen in the figures previously and can precipitate short bursts of fast heart rates such as paroxysmal supraventricular tachycardia and ventricular tachycardia. Causes of reentry are hyperkalemia, myocardial ischemia/injury, an accessory conduction pathway between the atria and ventricles, and some antiarrhythmic medications.

Clinical Alert

When an extra conduction pathway exists, a special form of tachycardia can arise called Wolff-Parkinson-White Syndrome (WPW). In these individuals, the length of the PR interval is very short. Usually the AV node is the only pathway, but, with this abnormality other impulses are directed to the ventricles and then reentered into the circuit again. This extra pathway is found in the bundle of Kent (The bundle of Kent).

  • Retrograde: In backward (retrograde) conduction, impulses below the AV node are transmitted in a backward motion so that they then enter the atria in a reverse fashion. This may cause the atria and ventricles to beat asynchronously and an increase in length of time for conduction can occur. This can cause junctional tachycardias. Several changes may take place on the EKG including P wave location changes and a short interval between the P wave and the QRS complex (Junctional rhythm).
  • Escape Rhythms: Escape rhythms are produced when the SA node slows or fails to initiate the depolarization process and a lower site assumes the task of creating impulses to activate heart function. The AV junction and ventricles act as escape pacemakers to ensure cardiac output is maintained. Escape rhythms include junctional rhythm and idioventricular rhythm (Idioventricular rhythm). Escape beats can also occur to protect the cardiac output. These would include both junctional and ventricular escape beats that would come into play with very slow rates in an attempt to preserve cardiac output (Ventricular escape beat).
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Cardiac Electrical Conduction System: The bundle of Kent

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Cardiac Electrical Conduction System: Junctional rhythm

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Cardiac Electrical Conduction System: Idioventricular rhythm

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Cardiac Electrical Conduction System: Ventricular escape beat

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Cardiac Electrical Conduction System: Complete heart block

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  • Conduction Disturbances: Different etiologies such as trauma, electrolyte disturbances, myocardial ischemia, myocardial injury, and myocardial infarction as well as certain drug toxicities can cause disturbances through the conduction system. This can cause both rapid and slow conduction. One example of this problem in conduction is atrio-ventricular blocks (Complete heart block).

Cardiac Electrical Conduction System: Conclusion

Focus topic: Cardiac Electrical Conduction System

The cardiac conduction system is comprised of an electrical system of pathways among the sinus node, atrial tissue, and the AV junction. The EKG is used by health care professionals to monitor phases of the cardiac conduction cycle and to identify rhythm and conduction disturbances. To see this conduction system in action, the electrocardiogram or the rhythm strip is utilized. Note these key points about the cardiac conduction system:

  • The heart’s cardiac cells have one of two functions: mechanical (contractile) or electrical (pacemaker). The two types of cardiac cells are the myocardial cells and the pacemaker cells.
  • An inotropic response relates to the contractility of the heart muscle.
  • A chronotropic response relates to the heart rate.
  • Action potential describes the electrolyte exchanges that occur across the cardiac cell membranes during depolarization. The resting potential occurs when a fully depolarized cell returns to its resting state and restores its electrical charges to normal in a process called repolarization.
  • There are five stages in the depolarization process: rapid depolarization, initial repolarization, plateau, final rapid depolarization, and diastolic depolarization.
  • The electrolytes of major importance in the depolarization process are sodium, potassium, and calcium.
  • Refractoriness is the ability of cardiac cells to remain unresponsive to stimuli or to reject an impulse. The three refractory stages are: absolute refractory period, relative refractory period, and super-normal period.
  • Cardiac cells are especially vulnerable during the relative refractory period.
  • The cardiac conduction system ensures the chambers of the heart contract in a coordinated fashion.
  • The electrical conduction system normally passes from the SA node to the AV node into the bundle of HIS and the bundle branches ending in the Purkinje fibers.
  • Causes of abnormal heart impulses include enhanced automaticity, triggered activity, reentry, backward conduction, escape rhythms, and conduction disturbances.
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