Cardiac Conduction System & ECG

Question 1

Action potentials of myocytes –> surface ECG (summary)

Answer 1

• R wave = phase 0
• isoelectric ST segment = phase 2
• T wave = phase 3
• isoelectric segment (after T wave) = phase 4

Question 2

R wave =

Answer 2

The initial rapid upward deflection of the R wave corresponds to phase 0 (fast sodium current)

Question 3

ST segment =

Answer 3

isoelectric ST segment (links the QRS to the T wave) = phase 2 (plateau=calcium influx and potassium efflux are balanced).

Question 4

T wave =

Answer 4

T wave (in which repolarization is occurring) = phase 3 (rapid decrease in voltage as potassium efflux continues).

Question 5

Isoelectric segment

Answer 5

isoelectric segment after T wave = phase 4

Question 6

Sequence of cardiac electrical system acrtivation

Answer 6

1. pacemaker cells @ SA node generate electrical impulse –> spread via gap jxns
2. depolarization –> R&L atria = P wave
3. depolarization –> AV node –> delay: ensures end of atrial contraction before ventricle depolarization
4. AV node –> R&L bundle of His branches –> fibers of Purkinje cells
5. Purkinje fibers –> ventricular cardiac myocytes –> contraction

Question 7

Right bundle of His –> ______

Answer 7

single entity that primarily supplies electrical stimulation to R ventricle

Question 8

Left bundle of His –> ____

Answer 8

divides into anterior and posterior branches and supplies those regions of the L ventricle w/electrical stimulation

Question 9

P wave =

Answer 9

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

Question 10

QRS complex=

Answer 10

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 11

T wave =

Answer 11

a small wave after the QRS complex that reflects the repolarization of the ventricles (should always be in same direction as QRS complex).

Question 12

PR interval =

Answer 12

• 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.
• index of conduction time across the AV node

Question 13

QT interval =

Answer 13

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

Question 14

First degree atrioventricular block =

Answer 14

conduction delayed but all P waves conduct to the ventricles.

Question 15

Second degree atrioventricular block =

Answer 15

some P waves conduct, others do not.

Question 16

Third degree atrioventricular block =

Answer 16

none of the P waves conduct and a ventricular pacemaker takes over.

Question 17

Major mechanisms of disturbance in cardiac conduction –> tachyarrhythmias (3)

Answer 17

1. abnormal reentry pathways
2. ectopic foci
3. triggered activity

Question 18

Characteristics of abnormal reentry pathways tachyarrhythmias

Answer 18

• present in the atria, ventricles or the junctional tissue.
• occurs when there is 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.
• most common mechanism of serious tachycardias

Question 19

Characteristics of ectopic foci tachyarrhythmias

Answer 19

• myocardium outside the conduction system acquires automaticity
• rate of depolarization exceeds SA node –> abnormal rhythm occurs.
• can be isolated ectopic beats or sustained tachyarrhythmias.

Question 20

Characteristics of triggered activity tachyarrhythmias

Answer 20

• abnormal “afterpolarizations” may be triggered by the preceding action potential.
• early afterpolarization before the action potential has fully repolarized triggers tachyarrhythmia
• Delayed afterpolarizations appearing after an action potential is complete can also trigger arrhythmias.

Question 21

Effects of left bundle branch block

Answer 21

• QRS widening on ECG
• delayed conduction to left ventricle

Question 22

Effects of right bundle branch block

Answer 22

• QRS widening on ECG
• delayed conduction to right ventricle

Question 23

Effects of block of left bundle fascicles

Question 24

Sizable Q wave =

Answer 24

transmural infarction/necrosis

Question 25

Q waves @ inferior leads (II, III, aVF) =

Answer 25

inferior infarts

Question 26

Q waves @ V1-V4 =

Answer 26

anterior wall infarcts

Question 27

Q waves @ I, aVL, V5, V6 =

Answer 27

lateral wall infarcts

Question 28

Evolving EKG of transmural myocardial infarct

Answer 28

1. giant T wave
2. t-wave inversion (ischemia)
3. ST elevation (current of injury)
4. Q waves + T inversion

Question 29

Causes of prolonged QT interval

Answer 29

• congenital long-QT syndrome
• hypocalcemia
• Class 1A or 3 anti-arrhytmic drugs
• hypothermia
• hypokalemia

Question 30

EKG findings in hypokalemia

Answer 30

• QT generally prolonged
• prominent U waves
• T waves may be inverted
• hypokalemia = common cause of arrhythmias

Question 31

EKG findings in hyperkalemia

Answer 31

• mild –> peaked high T voltages
• moderate –> P waves flatten, QRS & T broaden
• very high –> sinusoidal w/out P or R

Question 32

EKG changes in L ventricular hypertrophy

Answer 32

• increased voltage of R peak @ L-sided leads
• L leads = I, aVL, V5, V6

Question 33

EKG changes in right ventricular hypertrophy

Answer 33

• increased voltage peak in R wave @ R-sided leads
• R leads = V1, V2