HRR: Pacemakers And Electrical Conduction Flashcards
What is the electrical connection between the atria and ventricles under normal conditions?
AV node
The AV node serves as a critical pathway for electrical signals between the atria and ventricles.
Describe the electrical flow through the heart.
SA node > AV node > bundle of His > right/left bundle branch > His-Purkinje system
This sequence outlines the conduction pathway that regulates heartbeats.
What are intermodal conduction pathways?
They are specialized conduction pathways in the atrium that connect the SA node to areas of the atrium
These pathways facilitate rapid electrical conduction across the atrial tissue.
Are gap junctions unidirectional or bidirectional?
Bidirectional!
This allows electrical signals to pass freely between adjacent cardiac cells.
What are the elements of intercalated discs?
Gap junctions and cadherins/connexins
These structures connect adjacent cells and allow for synchronized contraction.
What allows for unidirectional conduction?
Cells becoming refractory
This refractory period prevents re-excitation and ensures organized electrical activity.
What kind of cells make up the SA node?
Specialized myocyte with high automaticity
These cells are responsible for initiating the electrical impulses of the heart.
Describe the distribution of Purkinje cells.
They branch into multiple places in the endocardium of ventricles
This extensive branching allows for rapid conduction of impulses throughout the ventricles.
Compare phase 4 in working myocyte and nodal myocyte.
Phase 4 in working myocyte is flat at around -89/-90 mV; in nodal myocyte, it is sloped and around -60/-50 mV
This difference reflects the distinct functions of these myocytes in electrical conduction.
What makes phase 4 more unsteady in nodal myocyte?
Funny currents, acetylcholine-dependent potassium channel, and T-type calcium channels
These factors contribute to the automaticity of nodal myocytes.
What is necessary for funny currents to activate?
Hyperpolarization
This condition allows for the influx of sodium ions, initiating the depolarization phase.
Compare phase 0 in working myocytes and nodal myocytes.
Phase 0 in working myocytes shows rapid depolarization; in nodal myocytes, it is slower due to L-type calcium current
This difference is crucial for the timing of heartbeats.
What occurs during phase 1 in working myocytes?
Early repolarization occurs
Phase 1 is absent in nodal myocytes.
What characterizes phase 2 in working myocytes?
A plateau due to L-type calcium channels
Phase 2 is absent in nodal myocytes.
How do phase 4 characteristics differ between working myocytes and nodal myocytes?
Both show repolarization, but nodal myocytes experience a period of hyperpolarization
Working myocytes do not have this hyperpolarization.
What is the major repolarizing current in cardiac myocytes?
Potassium
What does automaticity in cardiac myocytes depend on?
Threshold rate, hyperpolarization degree, duration of depolarization
It also includes the corresponding refractory period.
How does the PSNS impact SA nodal action potential?
M2 stimulation increases outward potassium, leading to hyperpolarization
This results in a slower firing rate.
What effect does the SNS have on SA nodal action potential?
Increases sodium and calcium channel activity, resulting in a quicker threshold reach
This increases the firing rate.
What are some factors that cause bradycardia?
- Hypothyroidism
- Hyperkalemia
- Hypoxia
- Calcium channel blockers
- Muscarinic agonists
- Beta-blockers
- Digitalis in therapeutic doses
What are some factors that cause tachycardia?
- Hyperthyroidism
- Hypokalemia
- Muscarinic antagonists
- Epinephrine
- Digitalis in toxic doses
Where is conduction velocity at its slowest?
The AV node
Where is conduction velocity at its fastest?
In the ventricles
What is the secondary pacemaker if the SA node fails?
The AV node
This results in a slower heart rate due to reduced conduction velocity.
How long does it take for the signal to travel from the SA node to the AV node?
30 ms
How long does the signal stay at the AV node?
90 ms
How long does it take for the signal to travel from the AV node to the bundle of His?
40 ms
How long does it take for the signal to reach Purkinje fibers?
40 ms
How long does it take for the signal to reach the apex?
60 ms
How long does ventricular repolarization last?
300 ms
Where does SA node firing show up on EKG?
It doesn’t show up
What does the P wave on an EKG represent?
Atrial depolarization
What is the PR interval?
Impulse traveling from SA node to AV node and bundle of His
What does the QRS complex represent?
Ventricular depolarization
In what order do the layers of the heart reach action potential?
Endocardium > mid-myocardium > epicardium
Why does an EKG show a 0 voltage after the QRS complex?
Each layer is in the plateau stage of action potential; this is called the ST segment
What is the QT interval?
Ventricular repolarization
In what order do the layers of the heart repolarize?
Epicardium > mid-myocardium > endocardium
What does the T wave represent?
Repolarization of the ventricle
Why do we not see atrial repolarization on an EKG?
It occurs during the QRS complex and is masked by ventricular depolarization
How long is the PR interval?
.12-.20 sec
How long is the QRS interval?
.06-.12 sec
How long is the QT interval?
.4-.43 sec
How long is the RR interval?
0.6-1.0 sec
What is the heart rate (HR) when the AV node is in charge?
40-60 bpm
What is the heart rate (HR) when the His-Purkinje system is in charge?
15-40 bpm
What determines which cells become the primary pacemaker?
Whichever is fastest takes over
What are some issues that can occur with pacemaker cells?
- Failure to generate impulse
- Conduction delay or interruption
- Reentry
Describe the process of reentry in pacemaker cells.
Impulse comes to a cell, passes to a cell below it, moves downstream, and then moves backwards to the cell that did not get depolarized.
What is needed for reentry to occur?
- An alternate pathway
- Slow anterograde conduction
- Cells proximal to the slow pathway cannot be refractory
