Introduction to Cardiac Conduction

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Cardiovascular

Title

Lecture

Cardiovascular Physiology

Structures and Function

One electrical system sets the beat for the whole heart. We trace it, name every part, and watch what happens when a piece of it fails.

Dr. Sharilyn Rennie

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01

Where we are going

Learning Outcomes

Roadmap

By the end you can

Locate the heart chambers, valves, and great vessels relevant to electrophysiology
Describe the conduction components (SA node, AV node, His bundle, Purkinje fibers) and their roles
Explain, in concept, how the electrical impulse spreads through the heart and triggers contraction
Identify the cardiac cell types and what each does
Relate the electrical activity to the mechanical cardiac cycle
Read basic ECG components (P, QRS, T) and what they represent
Explain the fibrous skeleton as structural support and electrical insulation
Apply the anatomy to common arrhythmias and their treatment

04

Bridge

First, Review Normal Conduction

Set the stage

First, we review how the heart normally conducts an impulse.

Build the normal system, structure by structure, top to apex
See what each part does and how fast it can fire
Then see exactly where a signal can break down

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Why it exists

Purpose of the Conduction System

Big picture

The job, top to bottom

Ultimate goal: generate the blood pressure and cardiac output that deliver oxygen-rich blood to the organs.

The chambers of the heart are pumps
The conduction system generates the electrical impulses that make myocardial cells contract, and so the chambers pump
Right timing matters: the atria must finish filling the ventricles before the ventricles fire

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Physiology

Two Types of Cardiac Cells

Cells

Contractile cells (cardiac muscle)

Do the pumping when the conduction system stimulates them
Propagate the action potential along the sarcolemma
Each heartbeat, muscle contracts twice: once in the atria, once in the ventricles

Non-contractile cells (nodal)

Specialized cells that fire an action potential on their own: automaticity
They do not contract

Key idea: nodal cells set the rhythm, contractile cells do the work. Every level can fire on its own, but the fastest pacemaker wins: the SA node keeps resetting the slower ones before they fire. A slower one only takes over, an escape rhythm, if the faster signal fails or cannot reach it, which is exactly what rescues the ventricles when the main signal cannot reach them.

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Pacemaker 1

Sinoatrial (SA) Node

Primary

The natural pacemaker of the heart

Location: junction of the right atrium and the superior vena cava
Intrinsic rate: 60 to 100 bpm
Blood supply: SA nodal artery, a branch of the right coronary artery (about 68 percent); left circumflex in about 24 percent

Clinical relevance: because the SA node is usually fed by the right coronary artery, an inferior (right coronary) heart attack can disturb the pacemaker and the rhythm.

Labeled cardiac conduction system, anterior view: SA node, AV node, bundle of His, bundle branches, Purkinje fibers

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Pacemaker 2

Atrioventricular (AV) Node

The gate

The secondary pacemaker and the delay

Location: base of the interatrial septum, just above the tricuspid valve, near the opening of the coronary sinus
Function: passes the impulse from the atria to the ventricles, and holds it briefly (the AV delay) so the atria finish filling the ventricles before they fire
Intrinsic rate: 40 to 55 bpm
Blood supply: AV nodal artery, most commonly a branch of the right coronary artery
The only route: the fibrous skeleton (dense connective-tissue rings that also anchor the valves) insulates everything else, so the AV node is the sole electrical path from the atria to the ventricles

Backup pacemakers

Pacemaker Hierarchy and Escape Rhythms

Rates

Every part of the conduction pathway can fire on its own, but the intrinsic rate gets slower the farther down you go.
SA node: about 60 to 100 per minute, and normally it sets the beat.
AV node / junction: about 40 to 60 per minute.
Bundle branches and Purkinje (ventricular): about 20 to 40 per minute.
The fastest pacemaker runs the heart; the slower ones are reset before they can fire, so they stay quiet.
If the route is interrupted, the structure below the break takes over as an escape pacemaker at its own slower rate.

The escape rate localizes the break: a slow escape near 30 to 40 points low, to the His-Purkinje system; a faster escape near 40 to 60 points to the AV junction.

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The wiring

Bundle of His and Bundle Branches

Staircase

From the gate down the septum

Location: the bundle of His leaves the AV node and runs down the interventricular septum, dividing into the right and left bundle branches toward the apex
Intrinsic rate: 40 to 55 bpm

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The wiring

Purkinje Fibers

Last mile

Subendocardial fibers

Location: arise from the bundle branches at the apex and spread up the walls of the ventricles (subendocardial)
Intrinsic rate: 25 to 40 bpm, the slowest backup in the system

When it breaks

Where the Route Can Break: AV Blocks

Localize it

First-degree block: every atrial impulse still reaches the ventricles, just slowly. The route is intact but delayed at the AV node.
Second-degree block: some atrial impulses get through and some do not.
Third-degree (complete) block: nothing crosses. The atria and ventricles beat independently, each on its own pacemaker.
Anatomically the lesion sits either at the AV node or lower in the His-Purkinje system (infranodal); the escape rate helps place it.
The moderator band (septomarginal trabecula) carries the right bundle branch across the right ventricle to the anterior papillary muscle, the last stretch of the route on the right side.

Locating a block is anatomy: trace the route from the SA node down and find the segment the signal cannot cross.

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Read the rhythm

Normal Rhythm and Heart Block

Compare

Normal sinus rhythm

Lead IIEach P is followed by one QRS, in step

Complete heart block, live

JACKcomplete heart blockBrady

P waves near 80 and QRS near 38, dissociated.

HR

38bpm

SpO2

95%

NIBP

86/54

MAP 65

RR

18/min

P and QRS never line up

Normal: every P drags a QRS with it. Complete block: the P waves (about 80) and QRS complexes (about 38) run on separate clocks. That dissociation is the diagnosis.

Team review · tRAT

Go Over Your Answers

Teams

As a team, compare your individual answers and agree on one answer per question. Be ready to defend where each structure sits.

Question 1

The heart's natural pacemaker is the SA node. Where is it located?

Interventricular septum
Wall of the right atrium, near the opening of the superior vena cava
Apex of the left ventricle
Base of the aorta

Question 2

Put the conduction pathway in order, from the pacemaker to the ventricular muscle.

SA node, AV node, bundle of His, bundle branches, Purkinje fibers
AV node, SA node, Purkinje, bundle branches
Bundle of His, SA node, AV node, Purkinje
SA node, Purkinje, AV node, bundle branches

Question 3

The fibrous skeleton insulates the atria from the ventricles. Which structure is the only electrical bridge across it?

The coronary sinus
The moderator band
The AV bundle (bundle of His)
The interatrial septum

Question 4

The moderator band carries the right bundle branch across the right ventricle to which structure?

The SA node
The anterior papillary muscle
The aortic valve
The left atrium

Question 5

An acute MI: the SA node stops and the heart rate is 48. Which artery is most likely occluded?

Right coronary artery
Left anterior descending artery
Left circumflex artery
Coronary sinus

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