Lecture 8: Introduction to Cardiac Electrophysiology

Question 1

What are the key characteristics of the SA node?

Answer 1

1. Its function is to initiate a normal sequence of activation in heart and to modulate heart rate
2. activated first because of sinus node automaticity
3. SA nodal cells have slow response action potentials (aka calcium)
4. automaticity modulated by sympathetic and parasympathetic nerves

Question 2

What are the key characteristics of the atria?

Answer 2

1. electrical function is to trigger synchronous atrial contraction to fill the ventricles
2. cells have fast response action potentials (Na channels) to carry the conduction from right to left rapidly
3. short AP duration and refractory periods make atria capable of supporting abnormally fast heart rhythms

Question 3

What are the key characteristics of the AV node?

Answer 3

1. Functions to produce a 120-200ms delay between atrial and ventricular contraction to protect ventricles from fast atrial arrhythmias
2. only electrical connection between atria and ventricles
3. slow response action potentials (Ca channels)
4. Uses long refractory period to protect the ventricles from rapid atrial rates
5. vulnerable to conduction block which prevents normal sinus impulses from getting to the ventricle
6. some AV nodal cells have automaticity

Question 4

What are the key characteristics of the His Purkinje System?

Answer 4

1. starts with Common His Bundle (connected to AV node) then bifurcates into left and right bundle branches
2. left bundle branch has left anterior and left posterior fascicles
3. very fast conduction because of specialized fast response AP
4. Conduction block in one bundle branch only DELAYS ventricular activation but doesn’t prevent it due to alternate pathway
5. fastest cells lie in common His Bundle
6. Has long action potentials and long refractory periods

Question 5

What are the key characteristics of the ventricular electrical conduction system?

Answer 5

1. impulses spread from endocardium to epicardium via fast response AP)
2. Repolarization spreads from epicardium to endocardium because epicardial AP < endocardial AP
3. AP shorten at fast heart rates to preserve time for ventricular filling
4. Interventricular septum is activated by connections from the LEFT bundle branch

Question 6

What is automaticity?

Answer 6

Property of some specialized cells to spontaneously depolarize and reach threshold to trigger an action potential

Question 7

What is a sinus rhythm?

Answer 7

When SA node is pacemaker

Question 8

What is junctional rhythm?

Answer 8

When the AV junction (between AV node and common bundle of His) is the pacemaker

Question 9

What is phase 4 depolarization?

Answer 9

The LACK of a flat line in the electrical wave of conducting cells
Where there should be a flat line (normal phase 4) there is instead a gradual depolarization
This gradual depolarization is known as diastole
Due to an increasing inward current or a decreasing outward current

Question 10

What is the dominant current of the Purkinje automaticity?

Answer 10

I_f activates on repolarization (known as the pacemaker current)
Allows both Na and K to pass through
Much more important in purkinje fibers because I_f is activated at much LOWER membrane potentials
I_k and I_ca play no role in Purkinje Fiber automaticity

Question 11

What are the dominant currents in the SA node?

Answer 11

1. I_k (the deactivation of them)
2. I_f
3. I_Ca (T-type carry the last part of depolarization

Question 12

What are fast response APs?

Answer 12

Ones that use the sodium current as the excitatory current

Located in atria, ventricles and His-purkinje system

Question 13

What are slow response APs?

Answer 13

Ones that use the calcium current as the excitatory current

Located in the SA and AV nodes

Question 14

What is excitability?

Answer 14

The ability of a depolarizing current to trigger the excitatory current and initiate an AP upstroke
Degree of excitability depends on resting membrane potential

Question 15

How does the extracellular potassium concentration increase in MI?

Answer 15

The damaged cells of the infarc will spill their intracellular potassium out to the ECM, thereby making nearby cells inexcitable

Question 16

What are key differences between fast and slow response AP?

Answer 16

Slow response AP have longer duration of current flow and a more positive threshold for activation, thereby ensuring that areas like the AV node conducts electricity to slowly to ensure diastole

Question 17

What are the factors that maintain the normal hierarchy of pacemakers?

Answer 17

1. Difference in intrinsic rates between SA and Purkinje fibers
2. Overdrive suppression of Purkinje fiber pacemakers

Question 18

What is the difference between SA and purkinje fiber beats?

Answer 18

1. SA node = 60-80 bpm

2. Purkinje fibers = 30-50 bpm

Question 19

What is overdrive suppression?

Answer 19

Refers to a reduction in the rate of firing of pacemaker cells after they are driven at a faster rate than intrinsic firing
In order to suppress pacemaker activity, the Na/K pump is activated
Na/K pump actually HYPERPOLARIZES the membrane potential
Hyperpolarization makes sure the firing is not too rapid

Question 20

What does digoxin do? Significance?

Answer 20

Digoxin blocks Na/K pump
Removal of the Na/K pump removes safety factor
Therefore, you get mini pacemakers that occur outside of SA node
Because Na/K pump is necessary for overdrive suppression (safety against unnecessary pacemakers)

Question 21

What are the three important factors that determine conduction velocity?

Answer 21

1. magnitude of the excitatory current
2. the speed with which it activates
3. The resistance to axial current flow through gap junctions

Question 22

When do gap junctions close and cut off conduction velocity?

Answer 22

1. high intracellular concentration of calcium

2. intracellular acidosis

Question 23

What are two ways there can be increased extracellular K+?

Answer 23

1. K+ leaks from MI cells

2. Elevated K from renal failure

Question 24

What is the safety factor for propagation?

Answer 24

The ratio of the axial current generated during propagation to the minimum amount needed to sustain propagation
Safety factor will be lower if membrane excitatory current is reduced or resistance to flow is increased
Significance: that is why slow response AP have low safety factors (because they have slow conduction and more resistance in terms of number of gap junctions one has to pass through)
The LOWER the safety factor, the EASIER it is to have a conduction block

Question 25

What are causes of slow conduction?

Answer 25

1. When gap junctions close due to higher intracellular calcium or acid levels
2. elevation of extracellular potassium
3. greater resistance to axial current flow between cells if electricity has to travel over infarcted plane
4. blockade of sodium channels by drugs

Question 26

What happens if the SA node is too slow?

Answer 26

The junctional pacemaker takes over

Question 27

What happens if there is an AV block?

Answer 27

The junctional pacemaker takes over

Question 28

What is an effective refractory period (ERP)?

Answer 28

The period of time following an AP upstroke that a stimulus is unable to elicit an action potential strong enough to propagate
This is due to the closure of the sodium/calcium inactivation gates
Divided into two components
i. Action potential duration (the time until the cell repolarizes)
ii. The time after repolarization needed for Na/Ca channels to recover to resting state

Question 29

What is the difference between fast and slow response tissue in terms of ERP?

Answer 29

Fast response tissue = ERP ends before full repolarization

Slow response tissue = ERP ends until AFTER full repolarization