Cardiac Physiology: Junctional Tissue Function

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Questions and Answers

What is the rate at which junctional tissue generates impulses?

40-60 beats/minute

What are the characteristics of a junctional rhythm?

P wave is inverted or misplaced (correct)

P wave is visible

Heart rate between 40-60 beats/minute (correct)

Regular rhythm (correct)

Why might the P wave be inverted in a junctional rhythm?

Impulses are generated below the SA node.

In sinus bradycardia, the P wave is typically inverted.

False Signup and view all the answers

What is sinus bradycardia?

A heart rate slower than 60 beats/minute. Signup and view all the answers

Which of the following statements about escape rhythms is true?

They indicate that the higher pacemaker is failing. Signup and view all the answers

What is indicated by a junctional tachycardia?

A junctional rhythm greater than 100 beats/minute. Signup and view all the answers

In accelerated junctional rhythm, the heart rate is between _ and _ beats per minute.

60, 100 Signup and view all the answers

The P wave can be absent in junctional rhythms.

True Signup and view all the answers

What role does junctional tissue play when the sinus node fails?

Acts as an alternative cardiac pacemaker Signup and view all the answers

What are junctional rhythms also referred to as?

Junctional escape rhythms Signup and view all the answers

What are premature junctional contractions (PJCs)?

Extra heartbeats originating from the junctional tissue Signup and view all the answers

What is junctional tachycardia?

A rapid heart rate originating from the junctional tissue Signup and view all the answers

AV junctional tissue generates impulses when the SA node is functioning properly.

False Signup and view all the answers

How long are the P waves created by the SA node impulse?

< 0.12 seconds long Signup and view all the answers

What changes occur in the appearance of P waves during junctional escape rhythms?

All of the above Signup and view all the answers

What is the primary cause of SA node failure?

Myocardial infarction Signup and view all the answers

What might cause increased excitability of AV junctional tissue?

Myocardial ischaemia Signup and view all the answers

Secondary pacemakers can function if the primary pacemaker fails.

True Signup and view all the answers

What is the heart rate at which AV junctional tissue can generate impulses?

40-60 beats per minute Signup and view all the answers

What can cause the heart rate to exceed 100 beats per minute in a healthy person?

Fever Signup and view all the answers

Study Notes

Junctional Tissue as a Secondary Pacemaker

SA Node Function: The SA node (sinus node) is the primary cardiac pacemaker, generating impulses at a rate of 60-100 beats/minute. Impulses spread from the SA node through the atria, AV node, Bundle of His, Bundle branches, and Purkinje fibers, ultimately causing ventricular depolarization and contraction.
Alternative Pacemaker: If the SA node fails to generate an impulse, junctional tissue located in the AV node can act as a secondary pacemaker.
Junctional Tissue: The AV junctional tissue, located between the lower atria and the upper ventricles, is capable of generating impulses at a rate of 40-60 beats/minute.
Junctional Escape Rhythms: Junctional escape rhythms occur when the SA node fails, and the junctional tissue takes over as the pacemaker. These rhythms are characterized by a missing, inverted (upside down), or misplaced (after the QRS complex) P wave.

Causes of SA Node Failure

Digoxin Toxicity: High digoxin levels can suppress the SA node, allowing the AV junctional tissue to become the pacemaker.
Myocardial Infarction: Infarcted tissue cannot transmit electrical impulses, potentially causing SA node failure.
Drugs: Drugs such as beta-blockers, calcium channel blockers, other anti-arrhythmic medications, digoxin, and acetylcholinesterase inhibitors can suppress SA node function.
Ischemia & Hypoxia: Reduced blood flow or oxygen to the heart can also lead to SA node dysfunction.
Vagal Stimulation: Stimulation of the vagus nerve, which slows heart rate, can temporarily suppress SA node function.
Degeneration of SA Node Cells: Sick Sinus Syndrome, a disease where the SA node cells deteriorate, can result in SA node failure.
Hypothyroidism & Hypothermia: Low thyroid hormone levels or abnormally low body temperature can also influence SA node function.

Identifying Junctional Rhythms

Regularity: Junctional rhythms are regular.
Heart Rate: Junctional rhythms have a heart rate between 40-60 beats/minute.
P Wave Characteristics: The identifying feature of a junctional rhythm is the missing, inverted, or misplaced P wave. The P wave may be:
- Not visible: Completely absent.
- Inverted: Upside down from its normal position.
- Misplaced: Following the QRS complex.
P Wave Placement: The P wave may appear before, during, or after the QRS complex.

Junctional Premature Complexes (PJCs)

Appearance: PJCs can have an inverted P wave, a P wave buried within the QRS complex, or a P wave appearing immediately after the QRS complex.
Mechanism: PJCs occur when the junctional tissue discharges prematurely, interrupting normal sinus rhythm.
Compensation: The junctional tissue typically takes over as the pacemaker, but may fail to depolarize the SA node.
Post-Ectopic Pause: This can result in a fully compensatory pause, where the next beat occurs after a long pause to maintain the normal beat frequency.

Understanding Automaticity

Automaticity: This is the ability of heart cells to initiate their own electrical impulses.
Pacemakers: The SA node is the primary pacemaker, but the heart has backup pacemakers:
- AV Junctional Tissue: Impulse rate of 40-60 beats/minute.
- Bundle of His: Impulse rate of 40 beats/minute.
- Purkinje Fibers: Impulse rate of 20 beats/minute.
Backup Pacemaker Function: If the SA node fails, the next fastest pacemaker will take over.
Cardiac Output: A heart rate of 20 beats/minute is insufficient for adequate cardiac output and oxygenation to the body, necessitating a temporary pacemaker.

Sinus Pause and Escape Complexes

Sinus pause occurs when the sinus node fails to generate an impulse, causing a brief pause in the heart rhythm
An escape complex occurs when a subsidiary pacemaker takes over to maintain heart rhythm
In sinus pause with AV junctional escape complex, the AV junction takes over as the subsidiary pacemaker.
In sinus pause with ventricular escape complex, a ventricular pacemaker takes over, suggesting significant disease in the AV junction.
In sinus pause with atrial escape complex, the atria take over as the subsidiary pacemaker.

Sinus Bradycardia

Sinus bradycardia is a heart rate less than 60 beats per minute with normal PQRST waves.
The p wave is small, upright, rounded, and precedes each QRS complex.
The p wave duration is less than 0.12 seconds.
The p-r interval is less than 0.20 seconds.
The QRS complex is less than 0.12 seconds long.

P wave in Sinus Bradycardia

The p wave is upright in sinus bradycardia because it originates from the atria and is recorded in Lead II.
Sinus bradycardia is a normal heart rhythm at a slower rate, but it can also be caused by illness.
The rhythm is regular in sinus bradycardia and the impulse travels normally through the heart.

Junctional Rhythm

The p wave is inverted in a junctional rhythm because the impulse is generated below the SA node in the AV junction.
The p wave can be absent, appear before or after the QRS complex, or be buried inside the QRS complex.
A missing p wave is a strong indication of a junctional rhythm.

Premature Junctional Contraction (PJC)

A PJC is a single junctional ectopic beat that occurs during normal sinus rhythm (NSR).
The QRS complex may occur without a p wave, or the p wave may appear before or after the QRS complex.
If a patient is on digoxin, suspect digitalis toxicity and check their digoxin level.

Other Junctional Rhythms

Accelerated Junctional Rhythm: The AV junction becomes the main pacemaker, with a rate exceeding 60 beats per minute and less than 100 beats per minute. This is often associated with digitalis toxicity.
Junctional Tachycardia: The junctional rate is greater than 100 beats per minute.

Criteria for Junctional Rhythms

The rhythm is usually regular.
The intrinsic rate of the AV junction is 40-60 beats per minute.
An accelerated junctional rhythm occurs when the rate is between 60 and 100 beats per minute.
A junctional tachycardia occurs when the rate is greater than 100 beats per minute.
The P waves may be absent, appear immediately before or after the QRS complex, or have a shortened P-R interval.

EKG Lead Placement and Implications

Live EKG monitoring is usually performed in Lead II.
The p wave appears upright in Lead II because the impulse is recorded from the top of the atria.
The p wave may be inverted in other leads, but it is important to maintain Lead II for live monitoring.
The lower in the heart the impulse is generated, the more likely the p wave will appear inverted.
Changing leads to keep the p wave upright can compromise patient care.

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Description

This quiz explores the role of junctional tissue as a secondary pacemaker in cardiac physiology. It covers the functions of the SA node, alternative pacemaker mechanisms, and characteristics of junctional escape rhythms. Test your knowledge of these essential concepts in heart function.