Cardiac Conduction

Question 1

1. Describe the relationship between the ventricular action potentials of individual cardiac myocytes and the surface electrocardiogram (ECG).

Answer 1

See board
R = Phase 0 (Na rush in)
S-T = Phase 2 (K out, Ca in)
T = Phase 3 (same direction as R because endo depolarizes faster than epi, transmural gradient, epi REpolarize earlier)

Question 2

Know the components of the cardiac electrical conduction system and the sequence of its activation.

Answer 2

SA –> AV via atrial muscle

AV to heart through bundle of his in septum

Question 3

Describe the P wave, QRS complex, T wave, PR interval and the QT interval.

Answer 3

P wave = atrial depolarization
QRS =ventricular DEpolarization
T wave = ventricular REpolarization
PR interval = index of conduction time across the AV node
QT interval = total duration of depolarization and repolarization

Question 4

Know the three types of atrioventricular block.

Answer 4

1: P always conducts, it’s just late
2: Some P conducts
- Mobitz 1: DJ lengthen and beat is dropped
- Mobitz 2: QRS with like every other P or every third
3: P doesn’t conduct, it’s just doing it’s own thing

Question 5

Understand the effects of left or right bundle branch block on cardiac conduction.

Answer 5

LBBB: wide QRS w/ delayed conduction to LV
RBBB: wide QRS w/ delayed conduction to RV
Fasicle: normal QRS, shift in depolarization

Question 6

Know the three major mechanisms by which disturbances in cardiac conduction cause tachyarrhythmias.

Answer 6

Abnormal reentry pathways: THINK loop going around triangle instead of splitting and meeting up on the other side. There’s a unidirectional block and slowed conduction through the reentry pathway. After the slow reentry the previously depolarized tissue has recovered and reentry into it will occur. The most common mechanism of serious tachycardias.

Ectopic foci: a focus of myocardium outside the conduction system acquires automaticity and if the rate of depolarization exceeds that of the sinus node an abnormal rhythm occurs. These can be isolated ectopic beats or sustained tachyarrhythmias.

Triggered activity: abnormal “afterpolarizations” may be triggered by the preceding action potential.