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
- sinoatrial (SA) node
- rapidly spread action potentials
- tissues
- contractile cells
- 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
- slows conduction velocity
- His-Purkinje system is very rapid
- low-resistance pathways
- essential for efficient contraction
- Relative conduction velocities
- Purkinje fibers > atria > ventricles > AV node