Electrical Activity and EKG

The Cardiac Action Potential Has a Prolonged Refractory Period

  • Heart action potential has a prolonged spike (depolarized)
  • Membrane is refractory for a long time
  • This prevents summation and gives the heart time to fill

All Parts of the Heart Beat Spontaneously

  • Heart muscle does not require stimulation by a nerve
  • Nerves usually inhibit the heart beat; cutting the nerves -> heart speeds up
  • Beat originates as a depolarization in the heart muscle cell itself (self stimulation)
  • All parts of the heart can beat spontaneously
  • Advantage: if one part of the heart is damaged another part can still produce a beat
  • Risk: beats originating outside of the pacemaker can produce life-threatening arrhythmias

Normally the Heart Beat Originates in the SA Node

  • The heart beat originates from the part of the heart with the fastest beat
  • Normally this is the SA (sinoatrial) node of the right atrium
  • The SA node is called the pacemaker
  • Ectopic beats are those originating outside of the normal pacemaker

The Atria Are Electrically Insulated From the Ventricles

  • The upper part of the heart (the 2 atria) is insulated from the lower part
  • Electrical excitation can pass from the atria to the ventricles only at the AV node

The Heart Has Special Electrical Conducting Tissue

  • Electrical excitation is passed through special conducting tissue from the AV node to the ventricles
  • Route: bundle of His -> bundle branches -> Purkinje fibers

Excitation is Delayed in the AV Node

  • The excitation starts in the SA node and spreads across the atria
  • When the excitation reaches the AV node there is a delay of about 0.1 seconds before it passes into the bundle of His
  • The delay allows the atria to contract before the ventricles are stimulated
  • This results in better filling of the ventricles
  • From the AV node impulses enter the Bundle of His and then travel along the right and left bundle branches in the septum between the right and left ventricles
  • In the ventricles the impulse spreads through the Purkinje fibers

Gap Junctions in the Intercalated Discs Spread Electrical Excitation Between Heart Cells

  • Gap junctions are low resistance connections between 2 cells
  • When the impulse reaches cardiac muscle cells it is rapidly passed from one muscle cell to the next because of gap junctions in the intercalated discs

Excitation of the Heart Can be Followed From the Body Surface With the EKG

  • EKGs are not measured across the membranes of cardiac cells
  • EKGs are measured by electrodes attached to the skin of the body surface
  • These electrodes record the average activity of millions of heart cells
  • EKG voltages are very small because the electrodes are far from the heart and most of the electrical activity cancels out
  • The most common set of connections is with electrodes connected to both arms and the left leg (Einthoven’s triangle).
  • This produces the 3 limb leads (I, II, III).
  • Alternate connections give 3 more limb leads (AVL, AVR, AVF)- the 6 limb leads give electrical views of the heart in the frontal plane
  • 6 chest leads are also used (V1, V2, V3, V4, V5, V6)- these give views of the heart in a transverse plane

A Typical EKG Record Contains P, QRS and T Waves

  • The P wave is caused by depolarization (excitation) of the atria
  • The QRS is produced by depolarization (excitation) of the ventricles
  • The T wave represents repolarization (recovery) of the ventricles
  • The small atrial recovery wave cannot be seen- it is swamped out by the large QRS wave
  • Different electrodes give different views of these waves- the wave below is typical for lead II (lead II has the same direction as the axis of the normal heart)

The EKG Gives Information on Heart Rate, Rhythm, Orientation and Pathology

  • If 2 QRS waves are close together the heart is beating at a fast rate; if they are far apart the heart has a slow rate
  • If the heart is functioning properly each P wave is followed by a QRS wave
  • If electrical conduction between the atria and ventricles is partially or completely blocked there will be a disturbance of the heart rhythm- the atria and ventricles may beat independently of each other
  • The orientation of the heart can be determined from the sizes of the waves coming from different electrodes
  • Damage to the heart caused by poor coronary circulation can cause the waves to widen (slower conduction). There are also other changes in wave shape, such as depression of the ST segment

More Information

Several websites have collections of abnormal EKGs with good explanations:

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