Cardiac Conduction System & ECG Flashcards
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
- pacemaker cells @ SA node generate electrical impulse –> spread via gap jxns
- depolarization –> R&L atria = P wave
- depolarization –> AV node –> delay: ensures end of atrial contraction before ventricle depolarization
- AV node –> R&L bundle of His branches –> fibers of Purkinje cells
- 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)
- abnormal reentry pathways
- ectopic foci
- 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
- giant T wave
- t-wave inversion (ischemia)
- ST elevation (current of injury)
- 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
