Learning Modules

• The Conduction System• ECG Basics• How to Analyze a Rhythm• Normal Sinus Rhythm• Heart Arrhythmias• Advanced 12-Lead Interpretation

(1) Generating rhythmical electrical impulses to cause rhythmical contraction of the heart muscle

(2) Conducting these impulses rapidly through the heart

The Conduction System

Depolarization and Impulse Conduction

• Heart is autorhythmic

• Depolarization begins in sinoatrial (SA) node

• Spread through atrial myocardium

• Delay in atrioventricular (AV) node

Depolarization and Impulse Conduction

• Spread from atrioventricular (AV) node– AV bundle– Bundle branches– Purkinje fibers

Specialized Excitatory and Conductive System of the Heart

Sinoatrial node

Internodal pathways

AV node

A-V Bundle (Bundle of His)

Bundle Branches

Purkinje fibers

• Sinoatrial (also called sinus or S-A node, in which the normal rhythmical impulse is generated

• Internodal pathways that conduct the impulse from the sinus node to the atrioventricular (A-V) node

• A-V node, in which the impulse from the atria is delayed before passing into the ventricles

• A-V bundle, which conducts the impulse from the atria into the ventricles

• Left and right bundle branches of Purkinje fibers, which conduct the cardiac impulse to all parts of the ventricles

The impulse reaches the A-V node about 0.03 second after its origin in the sinus node

Then there is a delay of another 0.09 second in the A-V node

A final delay of another 0.04 second occurs mainly in penetrating of the A-V bundle

The total delay in the A-V nodal and A-V bundle system is about 0.13 second. This, in addition to the initial conduction delay of 0.03 second from the sinus node to the A-V node

This makes a total delay of 0.16 second before the excitatory signal finally reaches the contracting muscle of the ventricles

The total time for transmission of the cardiac impulse from the initial bundle branches to the last of the ventricular muscle fibers in the normal heart is about 0.06 second.

• The impulse spreads through the atria but is delayed more than 0.1 second in the A-V nodal region before appearing in the ventricular septal A-V bundle

• Once it has entered this bundle, it spreads very rapidly through the Purkinje fibers to the entire endocardial surfaces of the ventricles

• Then the impulse spreads through the ventricular muscle to the epicardial surfaces

The Spread of the Cardiac Impulse Through the Heart

Pacemakers of the Heart

• SA Node - Dominant pacemaker with an intrinsic rate of 60 - 100 beats/minute.

• AV Node - Back-up pacemaker with an intrinsic rate of 40 - 60 beats/minute.

• Purkinje fibers - Back-up pacemaker with an intrinsic rate of 20 - 45 bpm.

Slow spontaneous

Inward Na+ ions

Outward K+

current (repolarization)

Inward Na+ and Ca++ ions

(depolarization)

Threshold

Atrial Muscle (Nodal) Action Potential

Automaticity - a pacemaker cell’s ability to spontaneously depolarize, reach threshold, and propagate an AP

Self-Excitation of Sinus Nodal Fibers

Sodium ions from outside the fibers tend to leak to the inside Influx of positively charged sodium ions causes a slow rise in the resting membrane potential in the positive direction

Sodium- Calcium channels become activated and causing the action potential

Influx of sodium and calcium ions then ceases while large quantities of positive potassium ions diffuse out of the fiber

Normal Electrocardiogram

The electrocardiogram (EKG) is a representation of the electrical events of the cardiac cycle

The electrical potentials generated by the current can be recorded; the recording is known as an electrocardiogram

It is most valuable diagnostic tools for the recognition of large variety of cardiac disorders