Cardiac conduction system and electrocardiogram Flashcards Preview

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Flashcards in Cardiac conduction system and electrocardiogram Deck (20):

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

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


Describe AP of purkinje fibers

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


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

Anterior and posterior branches (fasicles)


What produces each of the components of an ECG?

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


compare T wave in ventricles vs. individual myocyte

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


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

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


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

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


What does discordance in QRS and T wave indicate?

pathology- ischemia, ventricular hypertrophy


Why is QRS voltage greater than P voltage?

ventricular mass exceeds atrial mass


Why is T wave wider than QRS?

Ventricular repolarization takes much longer than depolarization


What is the PR interval?

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


What is the QT interval?

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


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

Positive (upward)


How does muscle mass affect voltage in EKG?

increased muscle mass increases voltage


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



Describe sequence of ventricle activation

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


Name 3 sites where conduction can be delayed

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


Name the types of AV block

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


Describe bundle branch blocks

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


Describe 3 common mechanisms that lead to arrhythmia

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