Physiology - heart cardiac cycle, conduction

Question 1

Describe the electrophysiological basis of the ECG. include pqrst intervals

Answer 1

Body fluid is a good conductor, and thus fluctuations in in potential due to the sum of action potentions in the myocardium can be recorded extracellularly in the form of an ECG

Depolarisation moving towards a lead will result in a positive deflection and vice versa for a negative deflection

PR = atrioventricular conduction. 0.12-0.2s
QRS = ventricular depolarisation. 0.08-0.1s
QT = ventricular action potential. 440-460 (females)
ST = plateau of ventricular action potential

Question 2

Draw an action potential of a pacemaker vs myocyte

Answer 2

Refer to one note

Myocyte
- No prepotential
Phase 0 - rapid depolarisation
-> Na influx through rapidly opening Na channels (Ina - Na current)
Phase 1 - Initial rapid repolarisation
-> Inactivation of Na channels
Phase 2 - Plateau
-> Ca influx through slowly opening Ca channels (Ica - Ca current)
Phase 3 - Slow repolarisation
-> Net K efflux through multiple types of K channels (Ik - K current)
Phase 4 - return to resting membrane potential

Pacemaker

• After each impulse membrane potential declines to the firing level
• > this prepotential/pacemaker potential triggers the next impulse
• At impulse peak, Ik starts repolarisation (K efflux)
• Eventually membrane negatively charged and close K channel
• Funny current dominating (Ih) causing Na efflux, increasing membrane potential
• At voltage threshold, Ca channel opens
  
  • 2 types of calcium channels
  • Transient: causes prepotential
  • Long lasting (L type) - produces impulse
• At certain positive threshold Ca channels close and voltage gated K channels open and result in K efflux and repolarisation

Latent pacemakers are present in other portions of conduction system that can take over when SA/AV node are blocked/depressed

Question 3

Describe sympathetic/vagal effects on pacemaker nodes

Answer 3

Cholinergic vagal fibers
ACh -> M2 musc -> G protein -> K channel opening
-> slows depolarizing effect of Ih
ACh -> M2 musc -> decrease cAMP -> slow opening of Ca channel -> slower firing rate

Sympathetic activation
-> Noradrenaline binds to B1 receptors -> increased cAMP -> type L Ca channels open -> more rapid depolarisation

Also affected by drugs and temperature
- Increased temperature increases rate of discharge of SA Node

Question 4

Which cardiac fibres have fast conduction and which have slow? what are the speeds?

Answer 4

Pacemaker fibres - slow
-> allow atrial contraction and filling of ventricles
Myocytes - fast

SA 0.05 m/s
Atrial pathways 1
AV  0.05
Bundle of His 1
Purkinje fibres 4
ventricular muscle 1

Question 5

Draw the cardiac cycle with pressure volume loops and label it

Answer 5

refer to one note