Electrophysiology Lecture 3 -- Clinical Arrhythmia 1

Question 1

Significance of a wide QRS complex

Answer 1

There is either a block somewhere in the ventricular conducting system, or the rhythm is originating somewhere in the ventricles (“ventricular source”) = activation pattern is different from when the impulse traverses the conducting system from the AV node or above

Question 2

What is the usual cause of premature beats with narrow QRS complexes

Answer 2

Early atrial firing (atrial premature beats, APBs; atrial premature complexes, APCs; or premature atrial complexes, PACs – all mean the same thing)

Question 3

Usual origin of premature extra beats with wide QRS complexes

Answer 3

In the ventricles (ventricular premature beats, VPBs; or ventricular premature complexes, VPCs; or premature ventricular complexes, PVCs – all the same)

Question 4

Define atrial (supraventricular) tachycardia

Answer 4

Fast rhythms (>100 bpm) that generally have narrow QRS complexes

Question 5

Define ventricular tachycardias

Answer 5

Fast rhythms (>100 bpm) that generally have wide QRS complexes

Question 6

Potential consequence of pathological bradyarrhythmia

Answer 6

Clinically significant = syncope Severe = cardiac arrest

Question 7

Define sick sinus syndrome

Answer 7

Sinus node intermittently fails to fire –> sinus pauses or sinus arrest (stops altogether)

Question 8

Typical consequence of sick sinus syndrome

Answer 8

Syncope (rarely sudden death)

Question 9

Population most affected by sick sinus syndrome

Answer 9

Tends to occur in the elderly, more rarely with ion channel gene mutations

Question 10

ECG of sick sinus syndrome

Answer 10

No P wave Abnormal pause Abnormal atrial complex

Question 11

Define first degree atrioventricular block

Answer 11

Slowing of atrial-ventricular conduction, no blocked beats

Question 12

ECG of first degree atrioventricular block

Answer 12

Long PR interval

Question 13

Define second degree AV block

Answer 13

Some beats fail to conduct. Has 2 subtypes:

1. Mobitz Type 1 (also called Wenkebach block)
2. Mobitz Type 2

Question 14

Define Mobitz Type I

Answer 14

Progressive PR lengthening on a beat-to-beat basis until conductoin to the ventricles fails (“blocked P wave”). Cycle begins again.

Problem is almost always in the AV node and rarely progresses to third degree block

Question 15

Define Mobitz Type 2

Answer 15

Blocked P waves occur without gradual PR lengthening

QRS duration usually long because of disease ni the His-Purkinje system, commongly progresses to third degree block and an indication for pacemaker

Question 16

Define third degree AV block

Answer 16

Also called “complete AV block”; no sinus impulses get to ventricles, almost always requires pacemaker

Question 17

Define paroxysmal supreventricular tachcardia

Answer 17

Reentrant arrhythmias involving the AV node that occur randomly

Question 18

What is the most common reentrant arrhythmia in individuals with otherwise normal hearts

Answer 18

AV node reentry

Question 19

Why is AV node reentry so common in individuals with otherwise normal hearts?

Answer 19

Slow confuction in the AV node makes stable reentry much more likely

Question 20

Most common form of AV node reentry

Answer 20

AV node reentrant tachycardia

Question 21

Describe the direction of electrical activity in AV node reentry

Answer 21

• Atria are fired whenever im[ulse goes up to the top of the AV node (retrograde direction)
• Ventricles are fired whenever impulse goes dow nthe AV node (antegrade direction)
• Maintained by reentry in AV node fast-channel tissue (note that the tissue is still slow-channel, just one side is faster than the other)

Question 22

Length of normal fast channel tissue

Answer 22

L = CT x V

L = 0.3 s x 1 m/s = 0.3 m

Question 23

Length of normal slow channel tissue

Answer 23

L = CT x V

L = 0.3 s x 0.3 m/s = 0.009 m = 9 mm

Question 24

How to stop AV node reentry?

Answer 24

Depress AV nodal (slow channel) conduction to make the reentering impulse block (die out)

Question 25

How to depress AV nodal conduction (2 methods)

Answer 25

1. Block calcium channels mediating AV node conduction (intravenous calcium channel blocker)
2. Hyperpolarize AV node cells by enhancing K+ current so that their action potentials are further away from threshold and calcium current is insufficient to allow continued firing

Question 26

2 ways to hyperpolarize AV node cells

Answer 26

1. Pharmacologically enhancing K+ current (intravenous adenosine, I_Kado; or increasing I_KAch)
2. Enhancing vagal tone

Question 27

2 ways to enhance vagal tone

Answer 27

1. Via a physiological maneuver like Valsalva or carotid sinus massage
2. Digitalis administration (not very effective acutely)

Question 28

Define Wolff-Parkinson-White

Answer 28

AV reentry going in one direction, the other which goes through a bypass tract (abnormal congenital fast-tissue connection between atria and ventricles, through fibrous tissue)

Question 29

Most common pathway of conduction in Wolff-Parkinson-White

Answer 29

Goes down (antegradely) through AV node

Back (retrogradely) through bypass tract

Question 30

Parts of the Wolff-Parksinson-White macroreentrant circuit

Answer 30

Atrial tissue between bypass and AV node

Ventricular tissue between AV node and bypass

Question 31

How to terminate Wolff-Parkinson-White

Answer 31

Attack the weakest link of circuit, usually AV node (slow channel tissue)

Question 32

How to pharmacologically prevent recurrences of AV node reentry or reentry involving bypass

Answer 32

Pharmacological therapy directed against AV node (i.e. calcium channel blockers) or for WPW, fast-channel tissue in the bypass tract (Na+ channel blockers or class III agents)

Question 33

How to prevent AV node reentry or reentry involving bypass recurrences non-pharmacologically

Answer 33

Treatment of choice for patients not easily controlled with drugs =

• AV node reentry = interventional catheter-based procedure to ablate one of the AV node pathways
• Bypass = Ablation has 95% cure rate with rare complications