Physio - Cardio Electrophysiology

Question 1

What is the difference btw Chronotrophy, Dromotropy, Inotropy?

Answer 1

Chronotropy:

• Effects on the heart rate
• SA node

Dromotropy:

• Effects on AP conduction velocity
• AV node

Inotropy:

• Contractility
• Purkinje Fibers

Question 2

Cardiac Action Potentials

Answer 2

Basics:

• SA & AV nodal cells = 1 type of AP
  
  • slow response type AP
  • lacks a stable RMP
• Atrial myocytes/ventricular myocytes/Purkinje fibers = other type of AP
  
  • fast response type AP
  • includes plateau phase + long refractry period

Question 3

Explain the Action Potentials of ventricles, atria and the Purkinje system

Answer 3

Phase 0 = Upstroke, depolarization

• Na+ influx (via voltage gates Na channels)
  
  • Inside becomes more (+)
  • Threshold open = -65mV
• Slope = upstroke = velocity of movement

Phase 1 = Initial repolarization

• Fast Na+ channels close
• Outwark K+ current (via K+ channels)
  
  • I_to = transient outward current

Phase 2 = Plateau Phase

• Inward Ca2+ current (slow)
  
  • mostly L-type Ca channels
  • long lasting
  • excitation-contraction coupling
    
    • inward Ca2+ –> triggers release of Ca2+ from SR –> muscle contraction
  • Blocked by:
    
    • verapamil, nifedipine, diltiazem
  • Increased by:
    
    • catecholamines via b-adrenergic receptors (rise of cAMP)
• Outward K+ current (I_kr, I _ks)
• Ca2+ in and K+ out balance each other
  
  • creates plateau

Phase 3 = Repolarization

• Slow Ca channels close
• Outward K+ currents (I_kr, I_k1)
  
  • inside gets more (-)

Phase 4 = RMP

• Na+-K+ ATPase activity
  
  • always there in excitable cells
• Na+-Ca2+ exchanger
• Ca2+ ATPase activity
  
  • both help Ca removal from cytoplasm
• K+ current (I_K1)
  
  • Functionally similar to K+ leak channels

Question 4

What are the Implications of lower dV/dT?

Answer 4

Lower conduction speed (dromotropy)

• size of inward current = reduced
  
  • less current spreads to adjacent sites to depolarize

Weaker force of contraction (inotropy)

• less time for calcium inward current during phase 2
  
  • smaller plateau

Question 5

Explain the Cardiac Action Potential of Conducting Nodal Cells

Answer 5

Basics:

• 3 phases
• present in SA & AV node only

Phase 0 = Depolarization

• Inward Ca++
  
  • T-type (first)
  • L-type (second)

Phase 3 = Repolarization

• Outward K+ current

Phase 4 = Spontaneous Depolarization

• In SA node, no stable RMP
  
  • Max diastolic potential = ~65mV
• Pacemaker potential
  
  • induce by the balance of 3 varying currents:
    
    1. i_f = funny current
       
       • inward Na++ –> hyperpolarization current
       • activated at -50mV or below
    2. i_ca = inward Ca++ current
       
       • activated at -55mV
    3. i_k = outward K++ current
       
       • opposing i_f & i_ca
       • diminishes throughout Phase 4
• Activity of Na-K-ATPase, Na-Ca exchanger, Ca-ATPase

Note:

• Phase 4 determines heart beat rhythem

Question 6

What are Latent Pacemakers?

Answer 6

Basics:

• Normal pacemaker cells = SA node
  
  • sinus rhythm w/ auto-rhythmicity of 70bpm

When things go wrong…

• Abnormal (latent) pacemaker cells:
  
  1. AV node (50 bpm)
  2. Bundle of His
  3. Purkinje Fibers (30 bpm)
     
     • automaticity = suppressed by stimulating them at a higher frew than intrinsic rhythm
• Can take over if SA node is messed up

Complete heart block:

• AV node = messed up
  
  • Atria = SA node (70 bpm)
  • Ventricles = Purkinje fibers (30 bpm)
    
    • much slower than normal (AV node 50 bpm)

Premature Ventricular Contraction (PVC):

• Ectopic focus = faster than SA node
  
  • abnormal pacemaker
  • ie: Purkinje fibers = 140 bpm

Question 7

What is the Conduction Velocity at different parts of the heart?

Answer 7

SA node/Artial muscle = ~1 m/sec

AV node = ~0.01 - 0.05 m/sec (SLOWEST)

Bundle of His = ~2 m/sec

Purkinje fibers = ~ 4 m/sec (FASTEST)

Question 8

What are the different Excitability and Refractory Periods?

Answer 8

Absolute refractory period (ARP)

• time second AP absolutely cannot be initiated
  
  • In ventricle 0.25-0.3 sec, in atrium 0.15 sec

Effective refractory period (ERP)

• time normally no AP occurs
  
  • slightly longer than absolute refractory period

Relative refractory period (RRP)

• time second AP is inhibited, but not impossible
  
  • In ventricle 0.05 sec, in atrium 0.03 sec

Supranormal period (SNP)

• time where s_lightly smaller than normal stimulus elicits response_
• Typically, the amplitude of the new AP is reduced

Vulnernable period

• Time during serious arrythmias are likely if a stimulus occurs
  
  • late ERP & SNP

Question 9

Explain the Autonomic Control of the Heart

Answer 9

Sympathetic:

• Preganglia:
  
  • Spinal cord
  • lower 1-2 cervical to upper 5-6 thoracic segments
    
    • C6-7 - T1-6
• Efferent nerve:
  
  • paravertebral chains
• Postganglia:
  
  • in the stellate & middle cervical ganglia
• Effector organs:
  
  • SA node, AV node, Atrial/Ventricular myocardial cells, blood vessels

Parasympathetic:

• Preganglia:
  
  • dorsal motor nucleus of vagus
  • nucleus ambiguus in medulla oblongata
• Efferent nerve:
  
  • Vagus nerves
• Postganglia:
  
  • epicardial surface
  • walls of heart near SA node & AV node
• Effector organs:
  
  • SA/AV nodes & only few blood vessels

Question 10

Autonomic Receptors of the Cardiovascular System

Answer 10

Sympathetic:

• NE = beta₁ = Heart
• NE = alpha₁ + beta₂ = skeletal muscle

Parasympathetic:

• Ach = M₂ = Heart
• Ach = M₃ = skeletal muscle
  
  • indirect vasodilation due to NO

Question 11

What are the Autonomic Control Centers?

Answer 11

Hypothalamus, Thalamus, Cerebral cortex

• Cardiac response to:
  
  • environmental temp changes
  • exercise
  • excitement, anxiety, and other emotional states

Sympathetic - Medulla Oblongata:

• Rostral Ventrolateral nucleus (RVL)
  
  • Distinct accelerator & augmentor cells
  • Provide tonic excitatory activity to heart & blood vessels

Parasympathetic - Medulla Oblongata:

• Nucleus vagus & Nucleus ambiguus
  
  • triggered by reflexes & respiratory center

Question 12

Effect of Parasympathetic on the Heart

Answer 12

Basics:

• Muscarinic acetylcholine receptors (M₂ type)
  
  • slows down HR
• Blocked by atropine
• Beat by beat control

Effects:

• (-) chronotropic effects on SA node
  
  • Effects Phase 4
    
    • Increase in K+ permeability (I_k-ach)
    • Decrease in Na+ permeability (I_f)
• (-) dromotropic effects on AV conduction velocity
  
  • Effects Phase 0
    
    • decrease in I_ca
    • increase in I_k-ach
• Only minor inotropic effects on myocardial cell
  
  • mainly on atria
• Some fibers end on sympathetic fibers
  
  • downregulate their activity

Note:

• Effects appear & disappear quickly due to high efficiency of acetylcholinesterase

Question 13

What is the Effect of Sympathetic on the Heart?

Answer 13

Basics:

• beta₁-adrenergic receptors
• (+) Agonist: Isoproterenol
  
  • ​speed up HR
• (-) Antagonist: Beta blockers e.g. propranolol
  
  • slow down HR

Effects:

• NE has (+) chronotropic effects
  
  • Effects Phase 4
  • Increase in Na+ permeability (I_f)
    
    • Enhances ventricular contractility
    • Enhances “atrial kick”
• ​NE has (+) dromotropic & inotropy
  
  • Increase in I_ca

Note:

• Effect of stimulation decays slowly
  
  • does NOT allow beat by beat regulation