Cardiac conduction system and electrocardiogram Flashcards
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?
negative
Describe sequence of ventricle activation
- upper septum depolarized from left to right
- depolarization down septum to apex
- depolarization from endocardium to epicardium
- Depol moves from apex upward in walls of ventricles
- 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
- 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
- Ectopic foci- a focus outside of the normal conduction system acquires automaticity and rate of depolarization exceeds SA node. Abnormal rhythm occurs.
- Triggered activity- abnormal afterpolarization is tiggered by the the preceding AP. Seen as a long QT interval representing a delay in repolarization
