Understanding Cardiac Electrophysiology

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Understanding Cardiac Electrophysiology

• BY NICOLE BROOKER

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TABLE OF CONTENTS

• Electrical conduction system
• Electrical conduction pathway
• Conduction components
• The sinoatrial node (SA)
• Bachmanns bundle
• Internodal tracts
• Atrioventricular node (AV)
• Right and Left Bundle branches
• Perkinje fibers
• Ventricles
• Basic electrocardiograph (EKG) waveform
• P wave
• PR interval
• QRS complex
• ST segment
• T wave
• QT interval

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Electrical Conduction System of the Heart

• The heart is supplied with an electrical
  conduction system that generates and conducts
  electrical impulses along specialized pathways
• Contraction is sequence specific so the atria
  contracts before the ventricle and the ventricle
  contracts from apex to base for efficient
  ejection of blood.

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ELECTRICAL CONDUCTION PATHWAY
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CONDUCTION COMPONENTS

• Sinoatrial (SA) node
• Interatrial tract (Bachmanns bundle)
• Internodal tracts, the atrioventricular (AV) node
• Bundle of His
• Right and left bundle branches
• Purkinje fibers.

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THE SINOATRIAL NODE (SA)

• The SA node, located in the upper right atrium,
  is the primary pacemaker of the heart (60 to 100
  beats).
• If the SA node fails to generate impulses or if
  those impulses are blocked, pacemaker cells in
  other sites can assume control, but at a slower
  rate.

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BACHMANNS BUNDLEINTERNODAL TRACTS

• Bachmanns bundle conducts impulses to the left
  atrium, while the internodal tracts conduct
  impulses to the AV node, located in the lower R
  atrium near the interatrial septum.

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ATRIOVENTRICULAR NODE

• Primary function-slow conduction of electrical
  impulses coming from the atria, allowing time for
  the atria to contract and empty their contents
  into the ventricles.
• It can functions as a backup pacemaker (40 to 60
  beats). When the atrial rate is rapid, the AV
  node blocks some of the impulses being conducted
  to the ventricles, protecting them from
  dangerously fast rates.

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RT AND LT BUNDLE BRANCHES

• From the AV node, the electrical impulse moves
  rapidly through the bundle of His and to the R
  bundle branch (R ventricle) and L bundle branch (
  L ventricle).

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PURKINJE FIBERS

• The impulse then enters the Purkinje system where
  Purkinje fibers conduct the impulse to myocardial
  cells of the ventricle, causing ventricular
  depolarization and contraction.

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THE VENTRICLES

• The ventricles also have pacemaker cells (30 to
  40 beats) that take over if impulses are not
  being transmitted by the SA or AV nodes.
• Repolarization follows.

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Basic ECG Waveform

• The hearts electrical activity is represented by
  an ECG tracing by three basic waveforms the P
  wave, the QRS complex, and the T wave.

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BASIC ECG WAVEFORM

• Between the waveforms are the following segments
  and intervals the PR interval, the ST segment,
  and the QT interval. A U wave is sometimes
  present.

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THE P WAVE

• P wave - atrial depolarization, or the spread of
  the impulse from the SA node throughout the
  atria.

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THE PR INTERVAL

• PR interval - the time required for the impulse
  to leave the SA node, travel through the atria,
  AV node, bundle of His, bundle branches, and
  Purkinje fibers.

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THE QRS COMPLEX

• QRS Complex-represents ventricular depolarization
  and is represented by three waves Q, R and S.

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THE T WAVE

• T wave - the latter phase of ventricular
  repolarization, and the vulnerable period of
  repolarization (R on T phenomenon).

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THE ST SEGMENT

• ST segment - represents the end of ventricular
  depolarization and the beginning of ventricular
  repolarization.

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THE QT INTERVAL

• QT interval - the time between the onset of
  ventricular depolarization and the end of
  ventricular repolarization
• Includes the QRS complex, ST segment, and T wave