Cardiology Logo
Cardiovascular Physiology
The Electrical Side of the Heart
1. What is the role of the electrical system of the heart?
  • To maintain an adequate heart rate (proportional to the energy expenditure of the body)
  • To coordinate the contraction of the atria and the ventricles in series
  • To coordinate the contraction of each chamber, especially the ventricles
  • 2. What are the components of the electrical system in the heart?
  • The Sino-Atrial (SA) Node:
    • It is the command center for the coordination of all electrical activity
    • The cells of the SA node have the property of automaticity
    • These cells are under the influence of the autonomic nervous system.
      • The sympathetic system (Beta adrenergic stimulation) increases the firing rate of the SA node.
      • The parasympathetic system (Acetylcholine stimulation) decreases the firing rate of the SA node.
  • The Atrio-Ventricular (AV) Node:
    • It serves as the gateway for electrical transmission between the atria and the ventricles.
    • It serves to insert an appropriate delay between atrial and ventricular contraction.
    • It prevents the ventricles from being activated too rapidly in cases of excessively rapid atrial activation.
  • Bundle of HIS:
    • A short segment of "electrical highway" connecting the AV node and the Bundle Branches.
  • Bundle Branches:
    • Left and right bundle branches deliver the electrical current from the bundle of HIS to the left and right ventricle respectively.
  • Purkinje Fibers:
    • The bundle branches terminate in the myocardial tissue along the endocardium in purkinje fibers.
    3. How does the horse and cow differ from the dog and cat with respect to the anatomy of their conduction system?
    Based on the distribution of the purkinje fibers, animals can be divided into two groups:
  • Class I: Hoofed mammals (horses, cows, sheep, pigs) and dolphins have a purkinje fiber distribution that penetrates extensively from the endocardium to epicardium and also contains extensive cross fiber anastomotic bridges. The result is that the majority of the ventricular mass is activated simultaneously. The resultant QRS represents primarily the activation of the base of the ventricular muscle. In general one cannot make a statement about ventricular mass based on the QRS morphology in these animals.
  • Class II: Dogs, cats, rats and people have a purkinje fiber distribution that penetrates minimally from the endocardium into the myocardium. The purkinje fibers do not penetrate more than 1/3rd of the distance from the endocardium to the epicardium. The result is that a wave of depolarization washes through the ventricular mass from the endocardium to the epicardium. The resultant QRS has a morphology that is determined by the mass of the ventricles in these animals. Hence, we can make a statement about ventricular enlargement.
  • Refer to electrocardiographic evaluation for more information on this topic.
  • 4. What is occurring during the components of the ECG?
  • P wave: Electrical activation (depolarization) of the atrial myocardium.
  • PR segment: This is a time of electrical quiescence during which the wave of electrical excitation (depolarization) passes through mainly the AV node. In addition the wave of depolarization moves through the bundle of HIS, bundle branches and purkinje fibers. Since the wave of depolarization moves through the AV node at a speed of about 1/100th the speed the wave moves through the bundle of HIS, bundle branches and purkinje fibers, most of the PR segment is associated with the passage of the wave of depolarization through the AV node.
  • PR interval: Represents the combination of P wave and PR segment activity.
  • QRS wave: Depolarization of the ventricular myocardium.
  • T wave: Repolarization of the ventricular myocardium.
  • 5. Identify the lead systems used to obtain an ECG;
  • The frontal plane leads: I, II, III, aVR, aVL, and aVF.
  • The horizontal plane leads: V1, V2, V3, etc
  • Refer to electrocardiology for more information.
  • 6. What is a lead?
    Consists of one negative pole and one positive pole.

    It provides "one line of sight", in a direct line from the negative pole to positive pole of that lead, to "view" the wave of depolarization as it passes into that line of sight.

    7. In what capacity is the ECG a valuable tool in the assessment of cardiovascular disease in domestic animals?
  • The main purpose of the electrocardiogram is to identify rhythm abnormalities.
  • In animals with a purkinje fiber distribution as represented by the dog, the ECG can also be used to make a statement about ventricular chamber enlargement.
  • 8. What is an action potential?
    It is a graphic representation of the change in the cellular membrane potential as the cell depolarizes and repolarizes as the wave of excitation passes over the cell.
    9. How does a myocardial action potential differ from a skeletal muscle action potential?
  • The duration of the action potential:
    • In skeletal muscle: very short; order of milliseconds
    • In myocardial muscle: very long; order of 100's of milliseconds
  • Length of the refractory period:
    • In skeletal muscle: shorter than the length of the action potential
    • In myocardial muscle: lasts almost the length of the action potential
  • Main impact:
    • In skeletal muscle: repetitive stimulation can induce tetany
    • In myocardial muscle: repetitive stimulation cannot induce tetany. Tetany for the heart would cause death. The heart needs a pause between each contraction to allow for filling. The long duration of the action potential and refractory period allows the contraction of the heart to end before the next excitation can induce a second contraction. Hence between two repetitive beats filling of the heart can occur.
    10. What are the components of the myocardial action potential?

  • Phase 0: phase of rapid depolarization; due mainly to the influx of Na into the cell. The slope of Phase 0 determines the speed of conduction of the wave of excitation. The greater the slope, the greater the speed of conduction.
  • Phase 1: short phase of repolarization; due mainly to the loss of K from the cell (Ito).
  • Phase 2: the plateau phase; due mainly to the influx of Ca into the cell (Long acting type Ca channel involved).
  • Phase 3: the repolarization phase; due mainly to the outward movement of K from the cell (Ik).
  • Phase 4: the resting membrane phase for working myocardial cells: no net gain or loss of ions. In pacemaker cells phase 4 involves a process of gradual depolarization due to the influx of mainly Na and K ions (If).
  • 11. What is the refractory period?
    The refractory period (RP) represents a time period after the onset of phase 0 of the action potential during which another stimulus, no matter how strong, fails to induce another depolarization within that cell.

  • Why is it important?
    • The RP prevents a cell from being depolarized at an extremely high rate. For myocardial muscle cells in prevents the heart muscle from experiencing tetany.

  • When does it occur relative to the action potential?
    • For all cardiac cells except the SA and AV nodal cells, the RP ends during Phase 3 of the action potential. For cells of the SA and AV node, the RP ends after the end of Phase 3 (during phase 4) of the action potential.

  • When does it occur relative to the surface ECG?
    • The RP ends during the first half of the T wave. It should be finished by the middle of the T wave.
    12. Myocardial cells can be divided into automatic and non-automatic cells.
    a. Identify the members of each group:
    • Automatic cells are the cells of the SA node, some cells in the leaflets of the AV valves, some cells around the coronary sinus, cells of the distal AV node, cells of the HIS, Bundle Branches and Purkinje system.

    b. What determines automaticity?

    • Automaticity is determined by the ability of cells to spontaneously depolarize during Phase 4 of the action potential.
    • Cells that are not normally automatic may gain automaticity a result of disease or electrolyte disturbance. Thus these cells with normally a flat Phase 4 develop a Phase 4 that gradually depolarizes.

    c. What affects automaticity?

    • Disease can induce automaticity in cells that are normally devoid of this property.
    • The autonomic nervous system can alter automaticity:
      • The sympathetic nervous system increases automaticity.
      • The parasympathetic nervous system reduces automaticity.