Mechanisms of Cardiac Arrhythmias - Roth

Question 1

Give the conventional definitions of each:

1. Bradycardia
2. Tachycardia

Answer 1

1. Slow heart rate (<60 bpm)
2. Fast heart rate (>100 bpm)

Question 2

What is a slow physiologic rhythm called?

Answer 2

sinus bradycardia

Question 3

What is a fast physiologic rhythm called?

Answer 3

sinus tachycardia

Question 4

Define bradyarrhythmia

Answer 4

abnormal bradycardic rhythm (<60bpm)

excludes phsyiologic bradycardia such as sleep or rest

Question 5

Define tachyarrhythmia

Answer 5

abnormal tachycardic rhythm (>100bpm)

excludes physiologic sinus rhythms, such as exercise or stress

Question 6

Define supraventricular tachycardia (SVT)

Answer 6

abnormal tachycardia requiring participation of either atrial or AV nodal tissue

Chaotic: atrial fibrillation

Question 7

Define ventricular tachycardia (VT)

Answer 7

abnormal tachycardia originating in the His bundle or Purkinje system. Does not require involvement of the atrium or AV node.

Chaotic: vetricular fibrillation

Question 8

All disorders of heart rhythm arise as a consequence of what (2) effects?

Answer 8

1. alterations in impulse function
2. alterations in impulse conduction

Question 9

Explain overdrive suppression

Answer 9

The hyperpolarizing current increases when a cell is forced to fire faster than its intrinsic pacemaker rate. The more frequently the cell is depolarized, the greater the quantity of Na+ ions that enter the cell per unit time. As a result of the increased intracellular Na+, the Na+K+-ATPase pump becomes more active, thereby tending to restore the normal transmembrane Na+ gradient. This increased pump activity provides a larger hyperpolarizing current, opposing the depolarizing current If, and further decreases the rate of spontaneous depolarization. Thus, overdrive suppression decreases a cell’s automaticity when that cell is driven to depolarize faster than its intrinsic discharge rate.

Question 10

Give the normal intrinsic rates of the following:

SA Node

AV Node

His-Purkinje System

Answer 10

60-100bpm

50-60bpm

30-40bpm

Question 11

Where does normal rhythm usually originate?

Answer 11

SA Node

Question 12

What is the appriximate normal resting potential of sinud node and AV nodal pacemaker cells?

What about working myocytes?

Answer 12

~-60mV

~-90mV

Question 14

Normal automaticity is regulated by what?

What factors determine automaticity?

Answer 14

autonomic tone (sympathetic and parasympathetic)

• Rate of diastolic depolarization (mostly If)
• Maximum negative diastolic potential
• Threshold potential

Question 15

The occasional long pause in the attached image is indicative of what? Is this pathological? What are the symptoms?

Answer 15

Near syncope (increased parasympathetic tone)

Not pathological

Sx: nausea, sweating, ‘not feeling well’

Question 16

What is a ventricular escape?

Describe it

Answer 16

An escape arising from infranodal tissue

Wide QRS

Question 17

What dominates at rest (sympathetic or parasympathetic tone)?

Answer 17

parasympathetic

Question 18

Describe briefly the sympathetic and parasympathetic effects on automaticity

Answer 18

Sympathetic

• Rate augmented (increased) by sympathetic tone
• Beta-andrenergic stimulation increases the ‘open’ probability of the pacemaker current (increases If), increasing the rate of diastolic depolarization
• Lowers the threshold potential (makes more negative)

Parasympathetic

• Rate depressed (decreased) by parasympathetic tone
• Decreases the open probability of the pacemaker current (decreases If)
• Increases the threshold potential (makes more positive)
• Increases probability of ACh-sensitive K+ channels (IKACh) being open at rest, leading to a more negative membrane diastolic potential

Question 19

Why might automaticity be augmented under pathologic fibrosis?

Answer 19

Pacemaker cells are no longer inhibited by neighboring working myocardium.

Normally, intracellular current between more negative working cells and less-negative pacemaker cells actually makes the pacemaker cell more negative, decreasing automaticity.

Question 20

What is a Junctional escape?

Describe it.

Answer 20

An escape rhythm arising in the AV node.

Narrow QRS without a preceding P wave, typically 50-60bpm

Question 21

What is abnormal automaticity?

Answer 21

Abnormal impulse or automaticity arising from any tissue not normally capable of pacemaker activity. Often a consequence of cellular injury, coronary ischemia, myocardial disease, etc

Question 22

What is triggered activity?

Answer 22

Single or repetitive cellular activity following a prior action potential due to oscillations in membrane potential. Can be triggered by tachycardia or bradycardia

Question 23

Define EAD (early afterdepolarization)

Define DAD (delayed afterdepolarization)

Answer 23

EAD

• membrane oscillations which occur within the action potential
• usually occur during phase 2 (Ca current) or phase 3 (reactivation of fast Na channels)
• Promoted by conditions which prolong the action potential (QT prolongation)
• mechanism underlying Tosades de Pointes VT
• Often precipitated by QT-prolonging drugs

DAD

• membrane oscillations occurring after completion of full repolarization (phase 4)
• promoted by conditions that lead to higher than normal intracellular Ca
• promoted by catecholamines, inhibited by Ca channel blockers
• mechanism underlying Idiopathic VT and digitalis-induced PVC or VT

Question 24

Name the three key requirements for reentry:

Answer 24

1. Two distinct paths
2. slowed conduction
3. unidiretional block - tissue capable of conduction in one direction but not the other. Often ‘functional’ - that is, depending on timing errors elsewhere

Question 25

What condition often shows delta waves?

Answer 25

Wolf-Parkinson-White Syndrome (WPW)

Question 26

Functional conduction (slight delays, not complete blocks) obstacles lead to waves and ‘whorls’ or re-entry.

What does this lead to (what is it called)?

Answer 26

fibrillation

Question 27

What is an ectopic beat?

Answer 27

Abnormal automaticity leading to beats arising from unusual locations

may contribute to tachycardia