Cells in the sinoatrial node and AV node are capable of _____.
How are SA and AV node cells able to spontaneously depolarize?
their phase 4 periods have slow depolarization due to the funny current and are thereby unstable
What kind of channels are absent in the SA and AV node cells?
Why is the SA node the usual cardiac pacemaker?
its cells have the fastest spontaneous depolarization rate, and the AV node cells' delay allows the atrium to contract first
Why do Purkinje cells have such fast APs?
they have a large # of Na channels
What makes up a gap junction?
What is automaticity?
the ability to spontaneously depolarize
What is the P wave?
depolarization of the R and then L atrium
Where is the AV node located?
between the fibrous tricuspid and mitral valve rings that separate the atria from the ventricles
What is found between the fibrous tricuspid and mitral valve rings that separate the atria from the ventricles?
the AV node
Where does the depolarization wave go after the AV node?
through the bundle of His into the left and right bundle branches
What do the bundle branches divide into?
What does the right Purkinje bundle divide into?
it doesn't- it's a single entity supplying the R ventricle
What does the left Purkinje bundle divide into?
anterior and posterior branches or fascicles
What does the QRS complex represent?
depolarization of the ventricles
What is the T wave?
repolarization of the ventricles
What is phase zero of the ventricular AP?
the fast sodium current causes rapid depolarization
What does phase two of the ventricular AP look like?
a long plateau and little change in voltage
What does the T wave on the EKG correspond to in the ventricular AP?
phase 3- a rapid decrease in voltage as potassium efflux continues
If the _____ is positive the T wave should be positive.
If the QRS is positive the _____ should be positive.
If the QRS is negative the _____ should be negative as well.
if the _____ is negative the T wave should be negative as well
Discordance between the ___ and ____ in any lead is pathological, reflecting abnormalities such as ischemia or ventricular hypertrophy.
QRS and T waves
Discordance between the QRS and T waves in any lead is pathological, reflecting abnormalities such as _____ or _____.
ischemia; ventricular hypertrophy
What is the PR interval?
index of conduction time across the AV node
What is the index of conduction time across the AV node?
the PR interval
What is the QT interval?
total duration of depolarization and repolarization
What is total duration of depolarization and repolarization called?
the QT interval
How long is a normal P wave?
How long is a normal QRS interval?
How long is a normal PR interval?
How long is a normal Q-Tc interval?
less than 0.44 s
Where is atrial depolarization in the EKG?
it's buried in the QRS (the larger signal)
Where can conduction be delayed or blocked with clinical consequences?
1. SA node block 2. AV node block (3 types) 3. bundle branch blocks (R, L, L bundle fascicles)
What does SA node block commonly cause?
sick sinus syndrome- slow sinus rates or takeover by other pacemakers
What are the 3 types of AV block?
1. first degree- conduction delayed but all P waves conduct 2. 2nd degree- some P waves conduct but others not 3. 3rd degree- none of the P waves conduct and a ventricular pacemaker takes over
What is a first degree AV block?
conduction delayed but all P waves conduct to the ventricles
What is a second degree AV block?
some P waves conduct but others do not
What is a third degree AV block?
none of the P waves conduct & a ventricular pacemaker takes over
What are the different types of bundle branch blocks?
1. right bundle branch block 2. left bundle branch block 3. left bundle fascicles block
What is a right bundle branch block?
QRS widening with delayed conduction to the right ventricle
What is a left bundle branch block?
QRS widening with delayed conduction to the left ventricle
What is a left bundle fascicles block?
shifts in direction of depolarization but no QRS widening.
What are 3 common mechanisms leading to arrhythmia?
1. abnormal re-entry 2. ectopic foci 3. triggered activity
Where can abnormal re-entry occur?
in the atria, ventricles, or junctional tissue
Reentry occurs when there is a _____ and slowed conduction through the reentry pathway.
What is the most common cause of serious tachycardias?
abnormal re-entry pathways
____ occur when a focus of myocardium outside the conduction system acquires automaticity.
Ectopic foci occur when a focus of myocardium outside the conduction system acquires _____.
When does an abnormal rhythm occur?
when the rate of depolarization exceeds that of the SA node
What is triggered activity?
abnormal “afterpolarizations” are triggered by the preceding action potential
What are abnormal “afterpolarizations” triggered by the preceding action potential called?
How does a triggered activity look on an EKG?
a long QT interval