CVPR 03-25-14 09-10am Cardiac Conduction System - Horwitz

Question 1

Spontaneous depolarization in SA and AV nodes

Answer 1

Phase 4 (repolarization) is not as static as in contracile myocytes; there is a slow depolarizing due to the “funny current” or “pacemaker current” until a critical voltage is reached where a more rapid Phase 0 occurs entirely due to Ca2+ current (there is no fast Na+ current).

Question 2

SA vs AV nodes in pacemaking

Answer 2

The SA node is the usual cardiac pacemaker b/c it has the fastest spontaneous rate…….the AV node has a slower spontaneous rate but it delays conduction at the junction between the atria & ventricles.

Question 3

Phases 4 & 0 in Contractile Myocytes

Answer 3

Contractile myocytes have a stable baseline during repolarization in stage 4, but exhibit rapid robust increase during phase 0 due to the fast Na+ current; they are fairly good at conducting from cell to cell, enabling fairly rapid electrical excitation of the atria & ventricles

Question 4

Purkinje cell action potentials

Answer 4

Have similar shape to contractiel myocytes w/ a slightly higher voltage during phase 0 and a longer total duration; Conduct faster than contractile myocytes (probably due to more fast Na+ channels); Allow extremely fast conduction from AV node to the ventricles through the left and right bundle & their extension into the myocardium.

Question 5

Spread of electrical impulse, after initiation by pacemaker

Answer 5

Spreads rapidly through gap junctions

Question 6

Gap junctions

Answer 6

Cylindrical structures formed by connexins that allow ions to pass from cell to cell

Question 7

SA node - path of AP

Answer 7

SA located high in right atrium –> depolarization wave goes through rt then lt atrium, generating the P wave –> arrives at AV node btwn tricuspid & mitral valves separating atriua & ventricles (“junction”) –> Delay (allowing contraction of atria to end before ventricular contraction starts) –> enters ventricles –> throug bundle of His into Lt & Rt Bundle branches –> Purkinje cell fibers –> through contractile cardiac myocytes

Question 8

Purkinje fibers

Answer 8

radiate toward contractile cardiac myocytes that induce contraction

Question 9

Right bundle branch

Answer 9

A single entity primarily supplying the right ventricle

Question 10

Left bundle branch

Answer 10

Divides into anterior & posterior branches (or fascicles) that supply corresponding regions of the left ventricle.

Question 11

Bundle of His, Lt & Rt Bundle branches, & Purkinje fibers all contain cells that…

Answer 11

conduct depolarization very rapidly

Question 12

Contractile myocytes

Answer 12

The majority of ventricular cardiac myocytes are primarily specialized to contract and conduct depolarization waves much more slowly than the specialized cells in the bundle of His, bundle branches & Purkinje fibers

Question 13

Electrocardiogram (ECG)

Answer 13

Invented by Einthoven ~100 years about; Electrical signals of the heart can be recorded onthe skin surface, using electrodes

Question 14

P wave of ECG

Answer 14

1st hump, due to depolarization of the atria

Question 15

QRS on ECG

Answer 15

after P wave and before T wave; due to depolarization in the ventricles; The Q is an initial downward deflection, the R is the upward deflction, and the S is a terminal negative deflection.

Question 16

T wave of ECG

Answer 16

last hump; due to repolarization of the ventricles

Question 17

Repolarization of the atria on the ECG

Answer 17

Not seen, b/c it normally occurs at then same time as ventricular depolarization, and is buried in the much larger signal from the ventricles

Question 18

Direction of repolarization/depolarization on ECG

Answer 18

On ECG, the T wave of ventricular repolarization is in the same direction as the QRS depolarization signal, whereas in individual myocytes, depolarization & repolarization are in opposite directions

Question 19

Discordance

Answer 19

In each cardiac myocytes, repolarization is in the same direction as depolarization but b/c of different polarity, the two waves are in opposite directions.

Question 20

Concordance

Answer 20

The QRS of depolarization and the T wave of repolarization are in the same direction on ECG. The endocardium depolarizes earlier than the epicardium; however, there is a transmural repolarization gradient and epicardial cells repolarize earlier than endocardial cells b/c they have shorter action potential duration….i.e., endocardial cells have a longer AP than epicardial cells

Question 21

If QRS is positive, T wave should be…? If QRS is negative, T wave should be…?

Answer 21

If QRS +, T+. If QRS -, T -.

Question 22

Discordance between QRS & T waves

Answer 22

In any lead, this is pathological, reflecting abnormalities such as ischemia or ventricular hypertrophy.

Question 23

PR interval on ECG

Answer 23

index of conduction time across the AV node

Question 24

QT interval on ECG

Answer 24

total duration of depolarization & repolarization

Question 25

Sinus node depolarization on ECG

Answer 25

This initiates the beat but is too small to see on ECG…so, the first signal is the P wave generated when the atria depolarize

Question 26

ECG leads & shape of QRS

Answer 26

There are 12 different leads in the ECG and the shape of QRS depends on where the positive electrode is placed.

Question 27

Why QRS voltage is much greater P voltage

Answer 27

b/c ventricular mass exceeds atrial mass

Question 28

Why T wave is wider than QRS

Answer 28

b/c ventricular repolarization take considerable longer than depolarization

Question 29

Direction of Activation wave relative to sensing electrode - effect on deflection direction recorded

Answer 29

If activation wave is toward a sensing electrode, a positive (upward) direction will be recorded

Question 30

Muscle mass & voltage

Answer 30

The greater the muscle mass, the greater the voltage recorded

Question 31

Direction of depolarization wave and placement of electrodes

Answer 31

SA node is high in right atrium, and depolarization wave sweeps downward & leftward. Thus, a lead with a positive electrode near the right arm normally has a predominantly negative QRS (i.e., a “QS” wave…not much of the R), while a lead w/positive electrode near the left leg has a positive QRS (pretty much just an R wave)

Question 32

Movement of depolarization through ventricle

Answer 32

Upper portion of septum is depolarizated from Lt to Rt. Then, there is depolarization downward in septum to the apex. From the apex, depolarization moves upward in the free walls of both ventricles and finally into the base of the ventricles.

Question 33

Direction of depolarization in layers of the cardiac wall

Answer 33

Depolarization goes from endocardium to epicardium

Question 34

Sites at which Conduction can be Delayed/Blocked w/Clinical Consequences

Answer 34

SA node, AV block, Bundle branch blocks

Question 35

SA node abnormalities

Answer 35

Commonly cause “sick sinus syndrome” resulting in slow sinus rates or takeover by other pacemakers which may be either too fast or too slow….Common in elderly & common reason to put in a pacemaker

Question 36

AV block - 3 types

Answer 36

1st degree,
2nd degree,
3rd degree

Question 37

1st degree AV block

Answer 37

Conduction delayed but all P waves conduct to ventricles…Fairly harmless…Often from drugs (beta or Ca2+ channel blockers)

Question 38

2nd degree AV block

Answer 38

some P waves conduct but others do not…Can be problematic; typically slows HR significantly, associated w/MIs,etc; Sometimes requires pacemaker

Question 39

3rd degree AV block

Answer 39

none of the P waves conduct & a ventricular pacemaker (purkinje cells, contractile myocytes) takes over…. very dire situation; Ectopic pacemakers = generally very & discharge the ventricles completely independently from SA/AV nodes; Usually requires pacemaker

Question 40

Right bundle branch blocks

Answer 40

–> QRS widening w/delayed conduction to the right ventricle

Question 41

Left bundle branch blocks

Answer 41

–> QRS widening w/delayed conduction to left ventricle

Question 42

Blocking of left bundle fascicles

Answer 42

–> shifts in direction of depolarization but no QRS widening….e.g., frontal plane mean axis is altered (depolarization of septum in opposite direction)

Question 43

3 common mechanisms leading to arrhythmia

Answer 43

Abnormal reentry pathways, Ectopic foci, Triggered activity

Question 44

Abnormal reentry pathways

Answer 44

Can be present in atria, ventricles, or the junction tissue; Reentry occurs when there is a unidirectional block and slow conduction through the reentry pathway; After the slow reentry, the previously depolarized tissue has recovered and reentry into it will occur; Probably the most common mechanism of serious tachycardias

Question 45

Ectopic foci

Answer 45

Occur when a focus of myocardium outside the conduction system acquires automaticity; if rate of depolarization exceeds that of the sinus node, an abnormal rhythm occurs….Can be isolated “ectopic beats” or sustain tachyarrhythmias.

Question 46

Triggered activity

Answer 46

Abnormal “afterpolarizations” may be triggered by the preceding AP…… Both early afterpolarization (when AP has only partially repolarized) and delayed afterpolarization (after AP is complete) can trigger arrhythmias.

Question 47

Arrhythmias due to triggered activity mechanism, on ECG

Answer 47

Usually associated with a delay in repolarization seen in the ECG as a “long QT interval”

Question 48

Time period (on ECG) of absolute refractory period & meaning for shocks

Answer 48

Absolute refractory period occurs right before Q and lasts til middle of T (why you give shock during QRS…you know the shock won’t set off another beat at that time).

Question 49

Time period (on ECG) of relative refractory period & meaning for shocks

Answer 49

Relative refractory period lasts during the later half of the T wave –> a shock could set off new irregular beats at this time…the longer the T wave (lengthier repolarization), the more chance of this there is