Cardiac conduction system and electrocardiogram

Question 1

What occurs during phase 4 of SA/AV node AP?

Answer 1

Slow depolarization due to a funny current (pacemaker current) until it reaches a critical voltage

Question 2

Describe AP of purkinje fibers

Answer 2

Similar shape as contractile myocytes but higher voltage during phase 0 and longer total duration (Conduct faster probably due to more Na channels)

Question 3

The left bundle branch divides into __ and ____ before becoming purkinje fibers

Answer 3

Anterior and posterior branches (fasicles)

Question 4

What produces each of the components of an ECG?

Answer 4

P wave: depolarization of atria
QRS: depolarization in ventricles (+ atrial repolarization)
T wave: repolarization of ventricles

Question 5

compare T wave in ventricles vs. individual myocyte

Answer 5

In ventricles, T wave is in the same direction as QRS, whereas in individual myocytes depolarization and repolarization are in opposite directions.

Question 6

Which parts of the ventricular AP correlate with the ECG components?

Answer 6

phase 0- R wave (Na influx)
phase 2- ST segment (Ca influx and K efflux are balanced)
phase 3- T wave (K efflux)

Question 7

Why are the QRS and T wave in same direction? What is the term for this?

Answer 7

concordance- due to differences in the speed of depolarization and repolarization of the endo and epicardium. There should be concordance of QRS and T wave in every lead. I

Question 8

What does discordance in QRS and T wave indicate?

Answer 8

pathology- ischemia, ventricular hypertrophy

Question 9

Why is QRS voltage greater than P voltage?

Answer 9

ventricular mass exceeds atrial mass

Question 10

Why is T wave wider than QRS?

Answer 10

Ventricular repolarization takes much longer than depolarization

Question 11

What is the PR interval?

Answer 11

From beginning of p wave to beginning of R wave- index of conduction time across AV node

Question 12

What is the QT interval?

Answer 12

From beginning of Q to end of T wave= total duration of depolarization and repolarization

Question 13

If the activation wave is toward a sensing electrode, a ___________deflection will be recorded

Answer 13

Positive (upward)

Question 14

How does muscle mass affect voltage in EKG?

Answer 14

increased muscle mass increases voltage

Question 15

A lead with positive electrode near right arm will have a positive or negative QRS?

Answer 15

negative

Question 16

Describe sequence of ventricle activation

Answer 16

1. upper septum depolarized from left to right
2. depolarization down septum to apex
3. depolarization from endocardium to epicardium
4. Depol moves from apex upward in walls of ventricles
5. base of ventricles

Question 17

Name 3 sites where conduction can be delayed

Answer 17

SA node abnormalities (sick sinus syndrome) , AV node, bundle branches

Question 18

Name the types of AV block

Answer 18

first degree- conduction delayed but all P waves conduct to ventricles.
Second degree- some P waves conduct, others do not.
Third degree- None of P waves conduct, and ventricular pacemaker takes over

Question 19

Describe bundle branch blocks

Answer 19

Block right BB- QRS widening, delayed conduction to right ventricle.
Block left BB- QRS widening, delayed conduction to left ventricle.
Block left bundle fascicles- shifts direction of depolarization but NO QRS widening

Question 20

Describe 3 common mechanisms that lead to arrhythmia

Answer 20

1. Abnormal re-entry pathways (most common)-a unidirectional block in atria, ventricles or junctions slows conduction and results in the circuit looping back over the same spot
2. Ectopic foci- a focus outside of the normal conduction system acquires automaticity and rate of depolarization exceeds SA node. Abnormal rhythm occurs.
3. Triggered activity- abnormal afterpolarization is tiggered by the the preceding AP. Seen as a long QT interval representing a delay in repolarization