Cardiac Conduction System and ECG

Question 1

A patient comes into the ED and you have to give them a shock. During what point of the ECG should you NOT shock them?

Answer 1

You shouldn’t shock them during the relative refractory period. If you do that, you could cause an arrhythmia. You should shock them during the absolute refractory period (the early part of the T wave)

Question 2

What are the 3 types of atrioventricular blocks?

Answer 2

1st degree: delayed conduction but all P waves conduct to the ventricles

2nd degree: Some p waves conduct but others do not

3rd degree: no P waves conduct to the ventricles, so a ventricular pacemaker takes over.

Question 3

T or F?

When there is a left bundle branch block, there is QRS widening with delayed conduction to the right ventricle

Answer 3

False.

There will be QRS widening in either left or right bundle branch blocks, but since this is the left one there will only be delayed conduction the left ventricle

Question 4

An elderly lady comes in to your office and you discover she has a SA node block. What are potential findings?

Answer 4

“Sick sinus syndrome” resulting in slow sinus rates or takeover by other pacemakers which may be too slow or too fast.

Question 5

T or F?

The greater the muscle mass, the less voltage produced by that muscle.

Answer 5

False.

The greater the muscle mass, the greater the voltage.

Question 6

You perform an ECG on someone and notice that the QRS and T waves are in the same direction. Is this normal?

Answer 6

Si señor. The QRS and T waves should always be in the same direction (positive and positive, negative and negative).

Question 7

What does a QRS and a T wave facing opposite directions indicate?

Answer 7

Discordance between the two is pathological and could indicate abnormalities such as ischemia or ventricular hypertrophy.

Question 8

In what 2 locations does the action potential tend to be slower in the heart?

Answer 8

The SA node and the AV node.

Question 9

What is atrial repolarization typically not seen in the ECG?

Answer 9

It occurs around the same time as ventricular depolarization, which pretty much drowns out that repolarization signal on the ECG.

Question 10

What does the P wave represent in cardiac conduction?

Answer 10

Depolarization of the atria

Question 11

What does the QRS wave represent in cardiac conduction?

Answer 11

Depolarization of the ventricles. Phase 0, which is due to the fast sodium current.

Question 12

What does the isoelectric segment between S and T on the ECG represent?

Answer 12

This corresponds to phase 2 of the cardiac AP, when there is a long plateau with little change in voltage when calcium influx and potassium efflux is balanced.

Question 13

What does the T wave represent?

Answer 13

Ventricular repolarization, Phase 3 of the AP when there is still a great amount of potassium efflux occuring.

Question 14

What does the isoelectric segment after the T wave represent?

Answer 14

This represents Phase 4 of the AP.

Question 15

What does the PR interval represent?

Answer 15

The index of conduction time across the AV node.

Question 16

What does the QT interval represent?

Answer 16

The total time of ventricular depolarization and repolarization.

Question 17

T or F?

The QRS is positive on right sided leads.

Answer 17

False

Depolarization moving TOWARDS a positive electrode produces a positive deflection.

Since conduction moves from right downwards to left, the right sided leads are negative (downward peaks) and the left sided leads are positive (upward peaks)

Question 18

T or F?

In the activation of the ventricles, the upward portion of the septum is depolarized from left to right

Answer 18

True.

This is the first step.

Question 19

T or F?

Depolarization is from epicardium to endocardium?

Answer 19

False.

Depolariation is from endocardium to epicardium.

That electric feeling starts inside and then moves outwards.

Question 20

T or F?

The apices of the ventricles are depolarized before the bases of the ventricles?

Answer 20

True.

It starts in the septum, moves down to the apices, then moves back up towards the bases. This makes it so that the ventricles contract and force the blood upwards and out through the different arteries (pulmonary, aorta).

Question 21

What are the three major mechanisms whereby abnormal tachyarrhythmias may arise?

Answer 21

1) Re-entry: Usually caused by a uni-directional block, this is the most common cause.
2) Abnormal automaticity (Ectopic foci): a focus of myocardium outside of the normal conduction pathways gains automaticity and its rate exceeds that of the normal pathway.
3) Triggered Activity: Afterpolarizations (EADs DADs) are triggered by the precding action potential.