Updated: 3/14/2019

Spread of Excitation

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https://upload.medbullets.com/topic/108069/images/080817vldcardiacheartexcitation.jpg
Snapshot
  • A 30-year-old man presents to the emergency room for palpitations and a syncopal episode. His blood pressure is 140/95 mmHg, pulse is 180/min, and respirations are 20/min. An electrocardiogram is ordered and reveals a supraventricular tachycardia. Vagal maneuvers are attempted with no effect. An intravenous medication that slows conduction through the atrioventricular node is started. (Adenosine)
Introduction
  • Two types of muscle cells exist in the heart
    • contractile cells
      • working cells of the heart that contract
    • conducting cells
      • tissues
        • sinoatrial (SA) node
          • generate action potentials spontaneously
        • atrioventricular (AV) node
        • atrial intermodal tracts
        • bundle of His
        • Purkinje system
      • rapidly spread action potentials
      • except for the SA node, all other tissues’ ability to generate action potentials are suppressed
Spread of Excitation
  • SA node → atrial intermodal tracts → left and right atria → AV node → bundle of His → Purkinje system → ventricles
  • SA node
    • serves as the pacemaker, generating the action potential
  • AV node
    • slows conduction velocity
      • ensures ventricles have enough time to relax and fill with blood before contraction
      • rapid conductions through AV node can lead to decreased stroke volume and cardiac output
  • His-Purkinje system is very rapid
    • low-resistance pathways
    • essential for efficient contraction
  • Relative conduction velocities 
    • Purkinje fibers > atria > ventricles > AV node 
 

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Questions (3)
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(M1.CV.12.13) A researcher measures action potential propagation velocity in various regions of the heart in a 42-year-old Caucasian female. Which of the following set of measurements corresponds to the velocities found in the atrial muscle, AV Node, Purkinje system, and venticular muscle, respectively?

Tested Concept

QID: 100529
1

2.2 m/s, 0.3 m/s, 0.05 m/s, 1.1 m/s

11%

(9/85)

2

1.1 m/s, 0.05 m/s, 2.2 m/s, 0.3 m/s

54%

(46/85)

3

0.5 m/s, 1.1 m/s, 2.2 m/s, 3 m/s

11%

(9/85)

4

0.3 m/s, 2.2 m/s, 0.05 m/s, 1.1 m/s

14%

(12/85)

5

0.05 m/s, 1.1 m/s, 2.2 m/s, 3.3 m/s

4%

(3/85)

M 1 E

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