7/12/2015 ECG tutorial: Physiology of the conduction system

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AuthorJordan M Prutkin, MD,MHS, FHRS

Section EditorAry L Goldberger, MD

Deputy EditorGordon M Saperia, MD,FACC

ECG tutorial: Physiology of the conduction system

All topics are updated as new evidence becomes available and our peer review process is complete.Literature review current through: Jun 2015. | This topic last updated: Dec 24, 2013.

INTRODUCTION — The cardiac conduction system is designed for electrical impulse creation andpropagation. It allows for initiation of impulses in the atrium, slowed conduction in the atrioventricular (AV)node, and rapid propagation through the His­Purkinje system to allow synchronous contraction in theventricles. Layers of redundancy occur, so that if one portion is damaged, there may be other areas that cancompensate for the loss of function.

Cardiac cells have the inherent property of spontaneous depolarization, which creates the cardiac impulse.Cells within the sinus node have the fastest rate of spontaneous depolarization, and, therefore, the sinusnode is the main pacemaker region of the heart. The AV node has the second fastest rate of spontaneousdepolarization, which allows it to create an escape rhythm if the sinus node is diseased.

ATRIAL ACTIVATION — The sinus node (the most proximal part of the conduction system) exhibits themost automaticity and functions as the dominant pacemaker in normal circumstances. This structuregenerates a slow action potential, mediated by calcium currents, that exits the node and activates the atrialmyocardium. (See "Myocardial action potential and action of antiarrhythmic drugs".) The atrial myocardiumaction potential has a rapid upstroke, mediated by sodium ions (figure 1) that help to quickly transmit thesignal.

Several preferential tracts exist in the atria to more quickly spread electrical signals [1]. In the right atrium,these include the crista terminalis and pectinate muscles. The Bachman bundle begins anterior to thesuperior vena cava and crosses the superior interatrial septum to facilitate right to left atrial conduction. Asuperior pulmonary bundle and septo­atrial bundle speed conduction in the left atrium.

As the atrium is depolarized, a "P" wave is transcribed on the surface electrocardiogram (figure 2). Since thesinus node is in the superior right atrium, the signal goes from superior to inferior, anterior to posterior, andright to left. The P wave is upright and slightly notched in all of the limb leads, with the exception of aVRwhich has a negative P wave. The precordial leads also show an upright P wave, although in leads V1 andV2 the P wave is usually biphasic; an initial positive followed by a negative deflection reflects depolarizationof the right atrium (which is anterior) and then the left atrium (which is posterior).

ATRIOVENTRICULAR NODE AND BUNDLE OF HIS ACTIVATION — After atrial activation, the impulsereaches the atrioventricular (AV) node. This structure generates a slow calcium mediated action potential(figure 1). Thus, there is a delay in impulse transmission through this structure.

Once the action potential traverses the AV node, it activates the proximal portion of the bundle of His, aspecialized conducting tissue that generates a fast action potential. Thus, impulse conduction through thisstructure is rapid. This period of time from the end of the P wave to the beginning of the QRS, during whichthere is activation of both the AV node and bundle of His, is termed the PR segment. The PR interval, incontrast, includes the P wave (atrial activation) as well as the PR segment (figure 2). Because the AV nodeand bundle of His are small, there is no electrical activity manifest on the surface electrocardiogram (ECG)from their conduction. The electrical activity that is occurring during the PR interval can be measured,however, using intracardiac electrodes during electrophysiologic testing. (See "Invasive cardiacelectrophysiology studies".) Abnormalities of conduction in the AV node and bundle of His are manifest onthe surface ECG as first, second, or third degree AV block. (See "ECG tutorial: Atrioventricular block".)

VENTRICULAR ACTIVATION — After impulse transmission through the bundle of His, the impulse isconducted to the right and left bundle branches; these are extensions of the His bundle that generate a fast

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7/12/2015 ECG tutorial: Physiology of the conduction system

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action potential (figure 1). The impulse travels through the bundles and the Purkinje fiber network,generating a fast action potential and resulting in rapid activation and depolarization of the myocardium ofthe right and left ventricles. The impulse enters the ventricular myocardium first from a septal portion of theleft bundle. Thus, the left intraventricular septum is the first part of the ventricle to become depolarized,resulting in the following electrocardiogram (ECG) appearance of the QRS (figure 2):

Since the mass of the left ventricle is much greater than that of the right, the normal ECG primarily reflectsleft ventricular depolarization (figure 2). The impulse spreads from the septum to the lateral wall in a right toleft and superior to inferior direction, thereby generating the following ECG appearance:

The last part of the left ventricle to become depolarized is the high lateral region. Thus, the last part of theQRS may be reflected on the surface ECG as a small terminal S wave in leads I, II, aVF and V4 to V6, and asmall r' in lead aVR and occasionally V1 and V2.

REPOLARIZATION — After the entire myocardium of the left and right ventricles completes depolarization,there is a period of time immediately prior to the final phase of repolarization or recovery during which thereis no apparent electrical activity on the surface electrocardiogram (ECG), the isoelectric ST segment phase.During this period, corresponding to phase 2 of the action potential, the ventricular myocytes are at aboutthe same potential, so no net current flow occurs. Thereafter, the ventricular myocardium undergoes the finalphase of repolarization, generating a T wave on the surface ECG (figure 2). Since the wave of repolarizationoccurs from the epicardium to the endocardium and is of opposite electrical charge as depolarization, it isusually positive on the ECG. The last wave form in this cycle, usually most apparent in the mid­precordialleads, is the U wave, which is of uncertain origin but may represent repolarization of the His­Purkinje systemor of the mid­myocardial M cells.

SUMMARY

An initial small septal Q wave on the surface ECG leads that measure electrical activity toward the leftside of the heart (leads I, aVL, and V4 to V6) since the impulse is moving away from these leads.

A small initial R wave in the leads pointed toward the right side of the heart (aVR, V1, and V2) since theimpulse is moving in the direction of these leads (figure 1).

A tall upright R wave in the limb leads I, II, aVF and the precordial leads V3 to V6 since the impulse ismoving towards these leads.

A deep negative deflection or S wave in the leads that are directed to the septum and right side of theheart (aVR, V1, and V2) since the impulse is moving away from these leads.

Cardiac cells possess inherent spontaneous automaticity. The tissue that possesses the greatestdegree of automaticity (eg, has the fastest rate of spontaneous depolarization) functions as thedominant pacemaker; it generates a spontaneous action potential that is conducted along the rest ofthe conduction system, activating the myocardium in a uniform fashion. (See 'Introduction' above.)

The sinus node exhibits the fastest automaticity and initiates each beat in most cases. Electrical activityspreads from the sinus node to the atrium, causing a P wave on the electrocardiogram (ECG). (See'Atrial activation' above.)

After atrial activation, the impulse reaches the atrioventricular (AV) node. There is relatively slowtransmission of the electrical current here. Once the action potential traverses the AV node, it activatesthe proximal portion of the bundle of His. Although there is no manifestation of AV nodal or bundle ofHis activation on the surface ECG, conduction through these structures occurs during the PR segment.The PR interval, which also includes atrial conduction, is comprised of the P wave as well as the PRsegment. (See 'Atrioventricular node and bundle of His activation' above.)

Sequential activation of different parts of the left and right ventricles follows and causes thecharacteristic QRS complex. As the left ventricular muscle mass is far greater than that of the rightventricle, the QRS complex primarily represents left ventricular depolarization.

After the QRS, the ventricular myocytes normally remain at about the same potential, so the surface

7/12/2015 ECG tutorial: Physiology of the conduction system

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REFERENCES

1. Zhao J, Butters TD, Zhang H, et al. An image­based model of atrial muscular architecture: effects ofstructural anisotropy on electrical activation. Circ Arrhythm Electrophysiol 2012; 5:361.

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ECG returns to baseline­ST segment phase. Thereafter, the ventricular myocardium undergoes thefinal phases repolarization, generating a T wave and sometimes a small U wave. (See 'Repolarization'above.)

7/12/2015 ECG tutorial: Physiology of the conduction system

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GRAPHICS

Action potentials generated by different parts ofconduction system

The sinoatrial (SA) and atrioventricular (AV) nodes generate a slowaction potential, mediated by calcium ions. In comparison, the tissues ofthe atria, ventricles, and the His­Purkinje system generate a fast actionpotential mediated by sodium ions. Sequential activation of thesestructures results in the characteristic waveforms visible on the surfaceECG. The AV node and bundle of His are small structures; as a result,no electrical activity is recorded on the surface ECG during theiractivation.

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7/12/2015 ECG tutorial: Physiology of the conduction system

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Formation of the major deflections on the ECG

The electrical impulse is initiated in the sinus node and then activates the rightand left atria, generating the P wave. Impulse conduction through theatrioventricular (AV) node and bundle of His, which are small structures, doesnot generate any ECG activity; this period of "electrical silence" is the PRinterval. The first part of the ventricle to be depolarized is the left side of theinterventricular septum, producing a small septal Q wave, followed bydepolarization of the remainder of the ventricular myocardium, generating thefull QRS complex. The T wave represents ventricular repolarization. A U wavemay be present, representing repolarization of the His Purkinje system.

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7/12/2015 ECG tutorial: Physiology of the conduction system

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Disclosures: Jordan M Prutkin, MD, MHS, FHRS Grant/Research/Clinical Trial Support: BostonScientific [Heart block (Pacemakers and ICDs)]; St. Jude Medical [Sudden death (Pacemakers andICDs)]. Ary L Goldberger, MD Nothing to disclose. Gordon M Saperia, MD, FACC Nothing to disclose.Contributor disclosures are reviewed for conflicts of interest by the editorial group. When found, these areaddressed by vetting through a multi­level review process, and through requirements for references to beprovided to support the content. Appropriately referenced content is required of all authors and mustconform to UpToDate standards of evidence.Conflict of interest policy

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