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Electrical Activity

of the Human HeartOguz Poroy, PhD

Assistant ProfessorDepartment of Biomedical Engineering

The University of Iowa

Outline

Basic Facts about the Heart

Heart Chambers and Heart Valves

The Pulmonary Circulation

The Systemic Circulation

The Cardiac Cycle

Muscle Types

The Electric Cardiac Cycle

The Electrocardiograph (ECG)

ECG measurements

Summary

What is the Heart?

The heart is a very specializedmuscle that pumps blood throughthe body, transporting oxygen,carbon dioxide, nutrients andwaste.

The heart is located in the middleof the chest, between the lungs.Its bottom is tipped to the left.

The Heart as a Pump

The heart is like two pumps: onepumping blood into the body andone pumping blood out of thebody.

The heart is about as big as twoclenched fists put together.

The heart pumps blood in beats.

The Hard Work

184 millionliters

2.5billion

70 years

2.6 millionliters

38million

1 year

7200 liters1000001 day

5 liters~ 701 minute

70 milliliters1< 1 second

Amount ofblood pumped

# ofbeats

Time

Anatomy of the Heart

Copyright 2000 HealthEon/WebMD Corp.

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Heart Chambers -1-

Atrium is a chamber thatpumps blood into the heart.

Ventricle is a chamber thatpumps blood out of the heart.

The atria and the ventriclesregulate blood flow bypumping blood in and out ofthe heart.

Heart Chambers -2-

Right

Atrium

Left

Atrium

Right

VentricleLeft

Ventricle

Septum

Heart Valves -1-

There are four valves in theheart.

These are unidirectionalvalves that allow blood flow inonly one direction.

They prevent blood fromflowing back to the chamberthat it has just left.

Heart Valves -2-

RA LA

RV LVTricuspid

Valve

Pulmonary

Valve

Mitral

Valve

Aortic

Valve

Heart Valves -3-

The tricuspid valve and the mitralvalve are also called A-V valves,

because they separate an atriumfrom a ventricle.

The pulmonary valve and theaortic valve are also called arterialvalves, because they separate aventricle from an artery.

Arteries and Veins -1-

Artery is a blood vessel thatdelivers blood out of the heart.

The two arteries of the heartare connected to ventricles.

Vein is a blood vessel thatdelivers blood into the heart.The two veins of the heart are

connected to atria.

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Arteries and Veins -2-

RA LA

RV LV

Pulmonary

Artery Aorta

Pulmonary

VeinVena Cava

Pulmonary Circulation -1-

Right

Ventricle

Deoxygenated (used)

blood is pumped out

of the right ventricle.

It travels through the

pulmonary valve into

the pulmonary artery,

leaving the heart.

Pulmonary

Valve

PulmonaryArtery


Right

Ventricle

Deoxygenated (used)

blood reaches the

lungs. Here, carbon

dioxide is removed

from the blood and

oxygen is added to it.

The fresh bloodleaves the lungs.

Pulmonary

Valve

Pulmonary

Artery

Lungs

O2

CO2


Right

Ventricle

Fresh blood enters the

left atrium through the

pulmonary vein.

Then, it is pumped

through the mitral

valve into the left

ventricle. This endsthe pulmonary

circulation.

Pulmonary

Valve

Pulmonary

Artery

Lungs

O2

CO2

Pulmonary

Vein

Mitral

ValveLeft

Ventricle

LA

Systemic Circulation -1-

Oxygenated (fresh)

blood is pumped out

of the left ventricle. It

travels through the

aortic valve into the

aorta, leaving the

heart.

Left

Ventricle

Aortic

Valve

Aorta

Systemic Circulation -2-

Oxygenated (fresh)

blood reaches the

head and the body

(gut, kidney,

muscles), where the

oxygen in it is used

and replaced by

carbon dioxide.

Left

Ventricle

Aortic

Valve

Aorta

Body & Head

CO2

O2

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The Cardiac Cycle Phase 4

Reduced Ejection:1. Ventricles start torelax.

2. Ventricular pressurestarts to drop.

3. Blood is slowlyejected from theventricles.

4. Atria continue to fill.

RV LV

P P


IsovolumetricRelaxation:

1. Ventricles continue torelax.

2. Ventricular pressurecontinues to drop.

3. Arterial valves close(Second heartsound).


RV LV

P P


Rapid Filling:

1. Atrial pressureexceeds ventricularpressure.

2. A-V valves open.

3. Blood flows rapidlyfrom the atria to theventricles.


RV LV

RA LA


Reduced Filling:

1. Atrial pressure drops.

2. Ventricular pressureincreases.

3. Blood flow from theatria to the ventriclesslows down.

4. Atria continue to fill.RV LV

RA LA

P P

P P

Muscle Types -1-

Skeletal Muscle:

Fast-twitching

Voluntary control

Gets tired

Arms, legs etc.

Muscle Types -2-

Smooth Muscle:

Slow-twitching

Involuntary control

Does not get tired

Stomach, bladder, bloodvessels etc.

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Muscle Types -3-

Cardiac Muscle: Built more like skeletal muscle,

but works more like smoothmuscle.

Involuntary control

Does not get tired

The heart!

Questions

The heart continuously goesthrough the cardiac cycle.

The cardiac cycle is rathercomplex and consists ofseven phases.

How is each cardiac cyclestarted and controlled?

Electrical Circuitof the Heart

RA

LA

RV LV

Sino-atrial

(S-A) Node

Atrioventricular

(A-V) Node

Right BranchBundle

Left Branch

BundlePurkinje Fibers

A-V Bundle

Junctional

Fibers

The S-A Node

The S-A Node is the mostimportant element in the electricalcircuit of the heart.

It starts the cardiac cycle byperiodically generating actionpotentials without any externalstimulation. (Therefore, it is said tobe autorhythmic.)

It is also known as the pacemaker

of the heart.

The A-V Node

The atrioventricular nodeperiodically receives actionpotentials via the junctional fibers.

The most important function of theA-V node is to regulate the timingof the ventricular contraction bydelaying the action potentials.

The delayed action potentials arespread over the ventricles tocause a contraction.

The Electrical Cycle1.The S-A Node

generates an actionpotential.

2. The action potential

propagates in theatria and causes acontraction. It is alsotransmitted to the A-V Node.

3.The action potentialis delayed at the A-VNode.

4.The action potentialis transmitted to theventricles and causesa contraction.

RA

LA

RV LV

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The Electrocardiogram

The electrocardiogram (ECG) is astandardized way to measure anddisplay the electrical activity of theheart.

Physicians can diagnose problemswith the heart by analyzing itsECG and comparing it to the ECGof a healthy heart.

ECG Waves

The P Wave: Depolarization of theatria

The QRS Complex: Depolarizationof the ventricles

The T Wave: Repolarization of theventricles

P

QRS

T P

QRS

T

ECG Intervals PQ: Time delay

between atrial andventriculardepolarization

QS: Duration of theventriculardepolarization

QT: Durationventriculardepolarization-repolarization cycle

PP: Duration of the

cardiac cycle

P

QRS

T

PQ

QS

QT

PP

A-V Blocks

From J. G. Webster (ed.),Medical instrumentation: application

and design. 3rd ed. New York: John Wiley & Sons, 1998.

Complete

Heart Block

A-V Node not

conducting at

all, ventricles

depolarizing

independently

First-degree

Heart Block

A-V Node

introducing too

much delay

Heart Flutter



Paroxysmal

Tachycardia

Ventricles

depolarizing

irregularly at a

high rate

Atrial Flutter

Atria

depolarizing

irregularly at a

high rate

Extra V-Contraction



Extrasystole: An accidental pacemaker node

causes an extra beat

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Fibrillation



Atrial

FibrillationAtria twitch

irregularly

Ventricular

Fibrillation

Ventricles

twitch

irregularly

Dipole Model of the Heart



The Dipole Model

The electric field

generated by this

dipole is equivalent

to the electric field

generated by the

heart at the peak of

the QRS complex.

Lead Position

Mava=

11

1a

M



2a

The location of electrodes relative to the dipoleM is very important in ECG measurements.

Limb Leads



120lll: LA-LL

60ll: RA-LL

0l: RA-LA

Relativeangle

Lead

Augmented Leads -1-



Augmented Leads -2-



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Chest Leads



12-lead ECG

Simultaneous measurementsfrom:

Three limb leads

Three augmented limb leads

Six chest leads

Process data from all leads toobtain ECG waveform

Noise in ECG



60 Hz Noise

EMG Interference

(noise caused by movement)

ElectromagneticInterference



Electromagnetic

Interference

Twisted wires

No interference

An ECG Amplifier


and design. 3rd ed. New York: John Wiley & S ons, 1998.

The Electrocardiograph -1-

1. Protect ion Circuit Protects electrocardiograph from high voltages

2. Lead Selecto r Selects one or more leads to be recorded

Computer-controlled or manual operation3. Calibration Signal

1 mV precision signal to calibrate channels

4. Preamplifie r Multi-OpAmp differential amplifier stage

Gain control (automatic or manual)

5. Isolation Circuit Protects patient against shock hazards Reduces noise caused by leakage currents

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The Electrocardiograph -2-

6. Driven Right Leg Circuit

Provides a reference at ground potential Protects patient (The patient is notgrounded.)

7. Driver Amplifier AC coupled amplifier stage Bandpass filtering to optimize signal-to-noise ratio

8. Display and Memory System Prints electrocardiogram on chart paper Samples, digitizes and stores ECG in memory

9. Microcomputer Automatic control of the electrocardiograph via

user interface

10. Recorder-Printer Records ECG along with patient information etc.

Common Problems1. Frequency Distortion

High-frequency distortion: Sharp corners arerounded off

Low-frequency distortion: Baseline is not horizontal

2. Saturation or Cutoff Distortion Peaks are cut off because of improper internal

voltage levels

3. Ground Loops Potential difference between grounded points

causes errors

4. Open Lead Wires Disconnection of lead from electrode or of electrode

from patient Lead acts as an antenna and picks up electrical

noise

5. Artifact from Large Electric Transients

6. Interference from Electric Devices

Summary

The heart consists of four chambers. There are two arteries exiting and two veins

entering the heart. Blood flow in and out of the heart is controlled by

four unidirectional valves. The heart pumps blood through the lungs for

oxygenation (pulmonary circulation) and thebody (systemic circulation).

The heart works in cycles (cardiac cycle).

The cardiac cycle is controlled by electricsignals, which are generated in the S-A Nodeand propagate through the atria and ventricles.

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