Cardiac Conduction System and EKG

Question 1

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

Answer 1

• The initial rapid upward deflection of the R wave corresponds to phase 0 of the action potential, which is due to the fast sodium current.
• The isoelectric ST segment on the ECG which links the QRS to the T wave is isoelectric normally and corresponds to phase 2 of the action potential—in which there is a long plateau with little change in voltage (calcium influx and potassium efflux are balanced).
• The T wave of the ECG (in which repolarization is occurring) corresponds to phase 3 of the action potential in which there is a rapid decrease in voltage as potassium efflux continues
• The isoelectric segment after the T wave corresponds to phase 4 of the action potential.

Question 2

Electrical impulses are initiated by ______ and spread via ______

Answer 2

• pacemaker cells in the SA node (which is high in the right atrium)
• cell to cell through gap junctions

Question 3

Wave of depolarization from the SA node spreads through ______ generating a ___ wave

Answer 3

• right and then left atrium

- P

Question 4

After generating a P wave, the wave of depol arrives at ________. What happens here?

Answer 4

• the AV node
• there is a delay before the depolarization wave enters the ventricles—ensures that contraction of the atria ends before depolarization of the ventricles occurs

Question 5

After leaving the AV node, what happens?

Answer 5

Depolarization wave then proceeds through the bundle of His into the left and right bundle branches. The bundles then divide into fibers made up of Purkinje cells. These Purkinje fibers radiate toward the contractile cardiac myocytes that induce contraction

Question 6

what bundles supply the RV and LV?

Answer 6

• R: right bundle is a single entity

- L: divides into anterior and posterior branches

Question 7

what kinds of speeds to the bundle of his, R and L branches and Purkinje fibers have?

Answer 7

very rapid (compared to the vast majority of cardiac cells)

Question 8

P wave:

Answer 8

small depolarization (reflecting the depolarization of the atria) prior to the larger depolarization of the ventricles (QRS complex).

Question 9

QRS complex:

Answer 9

a large wave reflecting the depolarization of the ventricles after the P wave (depolarization of the atria) and before the T wave (repolarization of the ventricles).

Question 10

T wave:

Answer 10

a small wave after the QRS complex that reflects the repolarization of the ventricles.
o The T wave and the QRS complex should always be in the same direction.

Question 11

PR interval:

Answer 11

the plateau between the P wave and the initiation of the QRS complex, where depolarization pauses at the bundle of His after depolarization of the atria and before depolarization of the ventricles.
o The PR interval is also the index of conduction time across the AV node.

Question 12

QT interval:

Answer 12

The plateau after the QRS complex and the T wave, reflecting the period of time between depolarization and repolarization of the ventricles.
o The QT time: total duration of depolarization and repolarization