Heart : ECG

• Ionic permeabilities in human cardiac muscle cells (ventricular myocytes)


  Vm, K+, Na+, Ca2+
• Sinoatrial node (SAN)


  • Atrioventricular node (AVN)
  • Purkinje fibers
  • Left and right bundle branches
  • Bundle of His
  • Bachmann's bundle
• Coordinated flow of electrical activity controls contraction
• Action Potential


  • Annulus fibrosus
  • Cardiac conduction system
• Cell to cell activation- wave of depolarisation


  • Functional syncytium-
  • Electrical coupling
  • Gap junctions
  • Large pores permeable to ions and small molecules
• Ventricular versus sinoatrial node cell APs

  • Pacemaker potential
  • IK1
  • If
  • INa
  • ICa
• Skeletal vs cardiac muscle APs- Refractory Period
• Electrocardiogram (ECG)


  ECG amplitude ~ 1 mV

  
• ECG: dipoles and vectors

  Signal recorded from these electrodes depends on: a) strength of dipole, b) orientation of dipole
• ECG placements: Small animal: Dog
• ECG placements: Large animal: Horse
• ECG from your phone…
• Interpreting the ECG:


  • 25 mm/s (standard output)
  • 50 mm/s (better for HR)
  • 1 mV
  • 200 ms
• Calculating heart rate:
• Cardiac Cycle:

  • Atrial systole (contraction)
  • Ventricular systole (contraction)
  • Ventricular diastole
  • Blood flow
• Cardiac Cycle Simulation
• Conduction pathway and cardiac cycle:

  • Electrical impulse originates from SAN located in wall of right atrium (diastole)
  • causes contractions in atria/atrial depolarisation (60ms) (diastole)
  • delay at AVN provides time required for RV/LV filling (100 ms) (diastole)
  • impulses then travels down the bundle of His and Purkinje fibres
  • rapid conduction through RV/LV ensures coordinated contraction, followed by repolarisation (systole)
  • ventricular relaxation (diastole)
• Cardiac Cycle- Electrical Activity of Heart

  • P: Atrial depolarisation in response to signalling from SAN
  • PQ: Atrial contraction and delay at AV node
  • QRS: Ventricular depolarisation, triggered by signals from AVN (ventricular contraction)
  • T: Ventricular repolarisation (ventricular relaxation)
• Summary

  • The cardiac action potential – sinus node versus ventricular APs and the threshold potential
  • Function of the cardiac conduction system
  • The ECG – principals, lead positioning and interpretation
  • Cardiac cycle- electrical activity precedes and drives mechanical events
• Cardiac Action Potential (Ventricular)

  
• During which phase of a ventricular cardiomyocyte AP will sodium channels and calcium channels be closed but potassium channels open ?

  
  

  Phase 4