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Flashcards in Cardiac Conduction System & ECG Deck (33):

Action potentials of myocytes --> surface ECG (summary)

  • R wave = phase 0 
  • isoelectric ST segment = phase 2 
  • T wave = phase 3 
  • isoelectric segment (after T wave) = phase 4


R wave = 

The initial rapid upward deflection of the R wave corresponds to phase 0 (fast sodium current)


ST segment = 

isoelectric ST segment (links the QRS to the T wave) = phase 2 (plateau=calcium influx and potassium efflux are balanced).


T wave = 

T wave (in which repolarization is occurring) = phase 3 (rapid decrease in voltage as potassium efflux continues).


Isoelectric segment 

isoelectric segment after T wave = phase 4


Sequence of cardiac electrical system acrtivation

1. pacemaker cells @ SA node generate electrical impulse --> spread via gap jxns

2. depolarization --> R&L atria = P wave

3. depolarization --> AV node --> delay: ensures end of atrial contraction before ventricle depolarization

4. AV node --> R&L bundle of His branches --> fibers of Purkinje cells

5. Purkinje fibers --> ventricular cardiac myocytes --> contraction


Right bundle of His --> ______

single entity that primarily supplies electrical stimulation to R ventricle


Left bundle of His --> ____

divides into anterior and posterior branches and supplies those regions of the L ventricle w/electrical stimulation


P wave = 

small depolarization (reflecting the depolarization of the atria) prior to the larger depolarization of the ventricles (QRS complex).


QRS complex=

a large wave reflecting the depolarization of the ventricles after the P wave (depolarization of the atria) and before the T wave (repolarization of the ventricles).


T wave = 

 a small wave after the QRS complex that reflects the repolarization of the ventricles (should always be in same direction as QRS complex). 


PR interval = 

  •  the plateau between the P wave and the initiation of the QRS complex, where depolarization pauses at the bundle of His after depolarization of the atria and before depolarization of the ventricles.
  • index of conduction time across the AV node


QT interval = 

  • The plateau after the QRS complex and the T wave, reflecting the period of time between depolarization and repolarization of the ventricles.
  • QT time: total duration of depolarization and repolarization


First degree atrioventricular block = 

conduction delayed but all P waves conduct to the ventricles.


Second degree atrioventricular block = 

some P waves conduct, others do not.


Third degree atrioventricular block = 

 none of the P waves conduct and a ventricular pacemaker takes over.


Major mechanisms of disturbance in cardiac conduction --> tachyarrhythmias (3)

1. abnormal reentry pathways

2. ectopic foci

3. triggered activity


Characteristics of abnormal reentry pathways tachyarrhythmias

  • present in the atria, ventricles or the junctional tissue.
  • occurs when there is a unidirectional block and slowed conduction through the reentry pathway.
  • After the slow reentry the previously depolarized tissue has recovered and reentry into it will occur.
  • most common mechanism of serious tachycardias



Characteristics of ectopic foci tachyarrhythmias

  • myocardium outside the conduction system acquires automaticity
  • rate of depolarization exceeds SA node --> abnormal rhythm occurs.
  • can be isolated ectopic beats or sustained tachyarrhythmias.


Characteristics of triggered activity tachyarrhythmias

  •  abnormal “afterpolarizations” may be triggered by the preceding action potential.
  • early afterpolarization before the action potential has fully repolarized triggers tachyarrhythmia
  • Delayed afterpolarizations appearing after an action potential is complete can also trigger arrhythmias.


Effects of left bundle branch block

  • QRS widening on ECG
  • delayed conduction to left ventricle


Effects of right bundle branch block

  • QRS widening on ECG
  • delayed conduction to right ventricle


Effects of block of left bundle fascicles


Sizable Q wave =

transmural infarction/necrosis


Q waves @ inferior leads (II, III, aVF) = 

inferior infarts


Q waves @ V1-V4 = 

anterior wall infarcts


Q waves @ I, aVL, V5, V6 =

lateral wall infarcts


Evolving EKG of transmural myocardial infarct

  1. giant T wave
  2. t-wave inversion (ischemia)
  3. ST elevation (current of injury)
  4. Q waves + T inversion



Causes of prolonged QT interval

  • congenital long-QT syndrome
  • hypocalcemia
  • Class 1A or 3 anti-arrhytmic drugs
  • hypothermia
  • hypokalemia


EKG findings in hypokalemia

  • QT generally prolonged
  • prominent U waves
  • T waves may be inverted
  • hypokalemia = common cause of arrhythmias


EKG findings in hyperkalemia

  • mild --> peaked high T voltages
  • moderate --> P waves flatten, QRS & T broaden
  • very high --> sinusoidal w/out P or R


EKG changes in L ventricular hypertrophy

  • increased voltage of R peak @ L-sided leads
  • L leads = I, aVL, V5, V6


EKG changes in right ventricular hypertrophy

  • increased voltage peak in R wave @ R-sided leads
  • R leads = V1, V2