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28 Cards in this Set
- Front
- Back
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What is the definition of automaticity?
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The ability to spontaneously depolarize above threshold voltage in rhythmic fashion.
Occurs in the SA node, AV node and to a lesser extent the purkinje system and bundle of His |
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What is the native pacemaker?
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SA node
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What are the latent pacemakers?
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AV= Bundle of His > Purkinjue
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What cells provide the majority of cardiac contraction?
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Non-pacemaker cells
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What ions are needed for depolarization in pacemaker cells/non-pacemaker cells?
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Na, Ca, K (re-polarization)
Na, K (re-polarization) In non-pacemaker cells, Ca is important for conduction-contraction coupling |
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Describe the action potential in SA and AV nodes.
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It is about 150 ms in duration
Consists of phases 4, 0, 3 4 = slow depolarization (primarily inward Na via nonselective cation channels) 0 = AP upstroke (Inward Ca via voltage gated Ca channels) 3 = Outward K via K channels |
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How is the upstroke (0) initiated?
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Once enough Na has leaked in down its gradient, Calcium enters and you get a depolarization
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Describe the 3 ways you can alter the SA/AV firing rate.
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1) Alter rate of spontaneous depolarization in phase 4 (Increase rate --> Increase firing)
2) Alter threshold potential (Increase threshold --> Decrease firing) 3) Alter Maximum Diastolic Potential aka RMP (Increase RMP --> Increase firing) |
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What is the SNS's effect on conduction?
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It increases open probability of cation channels leading to faster depolarization
It increases the open probability of Ca channels which increases the threshold potential (increases strenght of ap) Overall this increases heart rate |
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What is the PSNS's effect on conduction?
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It decreases the open probability of cation channels leading to slower depolarization
It increases the open probability of Calcium channels which increases threshold potential. The resting membrane potential is decreased which induces a slower reset Overall this decreases heart rate |
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Describe the phases of the Ventricular Action potential.
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APD = 300 ms
Gives SA node time to reset 4 = RMP (equal currents) 0 = Rapid Depolarization (Inward Na via voltage gated channels) 1 = Early repolarization (Less inward Na, transient K outward) 2 = Plateau (Inward Ca via T and L type Ca channels, outward K via K channels) 3 = Late/rapid deplorization (Less inward Ca, more outward K) |
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Phase 4 is flat. What is the significance of this?
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In the SA/AV node AP it is upward sloping signifying its internal automaticity. The non-pacemaker cells require an input to fire.
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Which phase is the rate-limiting refractory period?
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Phase 0 rapid depolarization
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In an ECG, how does the P wave relate to ion flux?
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P wave = Atrial Depolarization
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QRS complex?
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Ventricular Depolarization
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T wave?
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Ventricular repolarization
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PR interval?
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Time between the beginning of atrial depolarization and the beginning of ventricular depolarization
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QT interval?
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Time between the beginning of ventricular depolarization and the end of ventricular repolarization.
This is the ventricular action potential duration. |
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ST segment?
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Time between ventricular depolarization and ventricular repolarization.
This is the plateau in phase 2 |
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What are the 4 main ways an arrhythmia can be generated?
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Defects in impulse formation:
Altered automaticity in SA node Triggered activity (afterdepolarization) Defects in impulse conduction: Re-entry Conduction Block |
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What is happening in when there is a defect in impulse formation due to altered automaticity?
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The SA nodal firing rate is slowed or the latent pacemakers are firing faster than the SA node.
The latent pacemakers are setting the firing rate. |
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If the SA node is firing too slowly what is the consequence?
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An escape beat/rhythm
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If the latent pacemakers are firing faster than the SA node, what is the consequence?
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An ectopic beat/rhythm
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What can cause altered automaticity?
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SA node dysfunction
Ischemia Electrolyte imbalance Sympathetic stimulation Tissue Damage Disrupted membrane Decreased cell connectivity |
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What is an afterdepolarization?
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The Na channels reset (open back up) too early during repolarization and another depolarization occurs. This can cause arrhythmia.
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What causes afterdepolarizations?
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Na channels reset too quickly or APD is lengthened to the point where the Na channels can reset.
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Pertaining to the defect of impulse Conduction, re-entry and conduction block are two main problems. What is happening during re-entry?
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Normally, the refractory period prevents retrograde depolarization. Cell damage or long refractory state causes low conduction through a branch. When the signal finally gets through the damaged region, the healthy tissue has repolarized and fires again. This causes tachycardia
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What is the consequence of conduction block?
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There is a region of non-excitable myocardium.
This blocks conduction from the SA node and thus blocks its overdrive suppression. Non-pacemaker cells are now free to beat more slowly and non-uniformly. This causes bradycardia. |
