The Heart & Circulation - PowerPoint Presentation

Slide1

The Heart & Circulation

Asst. Prof. Dr.

Ghaith Ali Jasim Al

Zubaidy

PhD Pharmacology

MSc

Physiology & Pharmacology

dr.ghaithali@yahoo.com

Slide2

Cardiovascular System Function

Functional components of the cardiovascular system:

Heart

Blood Vessels

Blood

General functions these provide:

Transportation

Everything transported by the blood

Regulation

Of the cardiovascular system

Intrinsic v extrinsic

Protection

Against blood loss

Production/Synthesis.

Slide3

To create the “pump” we have to examine the Functional Anatomy of

:

Cardiac muscle

Chambers

Valves

Intrinsic Conduction System

Slide4

Functional Anatomy of the Heart

Cardiac Muscle

Characteristics

Striated

Short branched cells

Uni

-nucleate

Intercalated discs

T-tubules larger and

over z-discs

Slide5

Functional Anatomy of the Heart

(Continued)

Chambers

4 chambers

2 Atria

2 Ventricles

2 systems

Pulmonary

Systemic

Slide6

Functional Anatomy of the Heart

(Continued)

Valves

Function is to prevent backflow

Atrioventricular

Valves

Prevent backflow to the atria

Prolapse

is prevented by the

chordae

tendinae

Tensioned by the papillary musclesSemilunar ValvesPrevent backflow into ventricles

Slide7

Functional Anatomy of the Heart

(Continued)

Intrinsic Conduction System

Consists of “pacemaker” cells and conduction pathways

Coordinate the contraction of the atria and ventricles

Slide8

The heart circulates the blood round the body.

The left side of the heart transfers blood from the pulmonary veins to the aorta.

Slide9

whilst the right side of the heart transfers blood from the great veins, the superior and inferior venae cavae, to the pulmonary artery.

Slide10

Each side of the heart consists of two chambers, an atrium, previously known as the auricle, and a ventricle.

Slide11

Valves exist between the atria and the ventricles, and between the ventricles and the aorta and pulmonary artery.

The

left

atrioventricular

valve has

two cusps

and is termed the

mitral

valve.

The

right

atrioventricular valve known as the tricuspid valve has

three

cusps;

so also

have the

aortic

and

pulmonary

valves.

Slide12

Slide13

These valves

enabl

e

the alternate

contraction

and

relaxation

of the ventricles to

pump blood round

the body.

If a valve becomes

faulty, the efficiency of the heart as a pump is greatly impaired.

Slide14

The Structure of the Heart:

The heart lying in the thorax resembles an

inverted cone

.

The

superior

aspect of the heart, where the vessels enter, is called its

base

.

The

extremity

of the ventricles is termed the

apex.

Slide15

The two atria and the two ventricles of the heart lie side-by­ side.

Slide16

When the heart is beating the

atria contract simultaneously

, then, after a short pause,

both the ventricles contract

.

There

is then a

longer pause

during which time the whole heart is in a

state of relaxation.

Slide17

The inlet and outlet valves of each ventricle lie alongside one another. All the four valves lie in the same plane, in the fibrous septum or ring which separates the atria from the ventricles.

Slide18

This

property is developed to the greatest

extent

in a region known as the

sino-atrial

node (S.A. node)

which is

situated

in the wall of the right atrium

near the entrance of the superior vena cava.

Slide19

The heart is composed of

cardiac muscle

, and

such muscle has

the inherent

property

of

rhythmicity

, that is, the muscle fibres contract and relax alternately in a rhythmical manner

.

Slide20

This property is developed to the greatest

extent

in a region known as the

sino-atrial

node (S.A. node)

which is

situated

in the wall of the right atrium

near the entrance of the superior vena cava.

Slide21

Slide22

The

S.A

-node

originates

each heart beat

...It

is known as the

pacemaker

.

Cardiac muscle cells, known as fibres, are

cylindrical in shape with central nuclei and faint cross-striations.

The fibres

branch to form a network or sheet of muscle

in which it is difficult to see

where one cell ends and another begins.

Slide23

Slide24

It behaves like a syncytium. (If it were a true syncytium, there would be no cell boundaries at all.) This

syncytial

-like arrangement

enables a contraction wave to spread rapidly from cell to cell

until the whole muscle mass is in a state of contraction.

Slide25

No nerves are involved in the spread of a contraction wave

through cardiac muscle. This may be

confirmed

by making a series of

interdigitating

cuts

in a piece of atrial muscle.

Such cuts

would

sever any nerves

running in the muscle,

yet

when

one part of the muscle is made to

contract,

the

contraction wave spreads

through the

intact parts

of the muscle to

reach the whole muscle mass.

Slide26

Slide27

Furthermore,

in the embryo

the heart

beats

before the nerves have developed.

Although the

two atria are separate chambers,

the

muscle fibres are arranged in rings

round both atria.

Slide28

The heartbeat, originating

as a contraction wave

at the

S.A. node

,

spreads rapidly through the atrial

muscle causing both atria

to contract simultaneously

.

Slide29

The blood in the atria is

forced through

the

atrio­ventricular valves

into the ventricles.

Rings

of cardiac muscle

around the entry

of the

superior and inferior

venae

cavae

, and the

pulmonary veins

,

close off

the veins

with a sphincter-like action

so that

blood is not regurgitated back

into the

veins

when the

atria contract.

Slide30

The

spread

of the contraction

wave

through the cardiac muscle

ceases

at the fibrous septum

between

the atria and the ventricles

which contains the four heart valves [plane A-B].

Slide31

The

only pathway

through this non-conducting septum is

from

the atrioventricular node

(A.V. node)

down the atrio­ventricular bundle

(bundle of His)

named after the German physiologist Wilhelm His.

This bundle runs into the ventricles in the septum between the right and left ventricles.

Slide32

Slide33

The

passage of the contraction

wave down this bundle of modified cardiac muscle is

not visible

from the

surface of the heart

, and there

appears

to be a

slight pause

following the

atrial

contraction

,

The contraction

wave enters

the

ventricles near the apex

and

spreads upwards

towards the base. The

blood

in the ventricles is

forced upwards

towards the base of the heart

and out

through the

aortic and pulmonary valves.

Slide34

Slide35

Pericardium:

The heart lies in a

conical sac

known as the

pericardium

. This

consists

of an

inner serous

pericardium and

an outer

fibrous

pericardium. The inner serous pericardium is composed of

two smooth layers, the visceral and parietal layers,

one attached

to the

heart

and the

other

to the

fibrous sac

. These two smooth layers

allow

the heart

to beat

in the

mediastinum

of the thorax with

the minimum of friction

.

Slide36

Slide37

The pericardium:

1 -

Sets a limit

to the

maximum size

of the

chambers

of the heart.

2 -

Prevents

excessive stretching

of the

cardiac muscle fibres due to overfilling with blood. The pericardium is

attached

to the

diaphragm

, and when the

heart beats

, it behaves

as if the apex were relatively fixed.

Slide38

Slide39

Thus

when the ventricles contract

,

instead

of the

apex moving upwards

towards the base,

the base

, and

particularly the

atrio­ventricular

ring

, descends

towards the apex.

This

has the effect

of

increasing the size

of

the atria at the same time as blood is ejected from the ventricles.

Slide40

Cardiac Cycle-The Heart as a Pump:

The sequence of events may, therefore, be summarized as follows:

The heart beat originates in the S.A. node, and shortly afterwards the atria contract.

This is followed by a short pause whilst the contraction wave is moving down the bundle of His.

Then the ventricles contract, the atrioventricular ring moves downwards and blood is ejected into the arteries.

The ventricular muscle relaxes and the atrioventricular ring returns to its initial position.

There is then a long pause, when all chambers are relaxed before the next beat occurs.

Slide41

The ventricular contraction phase is termed systole (pro­nounced

sis'to.lee

). It lasts for 0·3 seconds.

The ventricular relaxation phase is termed diastole (pronounced

dye.ess'to.lee

) and this lasts for 0.5 seconds.

The complete sequence of events, the cardiac cycle, lasts for 0.8 seconds, so that there are:

60~70

cycles/minute. This is the heart rate.

The heart beats continuously for the whole of a person's life and its only period of rest is after each contraction, during diastole which becomes shorter.

Slide42

The Heart Sounds

The

valves

in the heart

close passively

whenever there is a tendency for the

blood to flow in the reverse direction

.

Since

blood flows

from a

region of high Pressure

to a region of low pressure, it is the

relative pressures in the atria, ventricles and arteries that will determine the

opening and shutting

of the valves.

Slide43

Since the valves are

passive structures

and contain

no contracting muscle

, diseased valves may be

replaced

by mechanical valves (prosthetic valves) which have been

specially designed

with

smooth surfaces to minimize the risk of blood clotting and

red cell haemolysis

.

Slide44

With the

onset of ventricular systole

, the

pressure

in the ventricles

starts to rise

. As soon as it

exceeds

that in the corresponding

atrium, the A-V valves will shut. This simultaneous

closure

of the mitral and tricuspid valves

with the onset of systole can be heard by applying the ear to the chest wall of a subject (or by stethoscope). The sound

heard may be likened to the word 'lub

'

spoken very softly. It is termed the

first heart

sound

Slide45

The

sound

of the

valvular closure

may be

augmented

by the

impact

of the

heart against the chest wall

and

by the

noise produced by the contraction of the ventricular muscle fibres.

It lasts for 0.15 seconds and the principal

frequencies

of the sound produced are in the range of

25-45 cycles/second

.

Slide46

Slide47

The

impact of the apex

of the heart

against

the

chest wall

with each

systo1e

can be

felt

, and frequently

seen

, in the 5

th left intercostal space. It is termed the apex beat.

The

intercostal spaces

are named

according

to the

rib that lies immediately above

. Thus the

5

th

interspace

lies between

the 5

th

and 6

th

ribs.

Slide48

Slide49

The apex beat lies about 3-3.5 in. from the midline. A line through the apex beat to the midline passes through the middle of the clavicle (mid-clavicular line).

As soon as the

ventricular pressures exceed

those

in the aorta

and the

pulmonary artery

, the

aortic and pulmonary valves will open

.

Slide50

The

opening

of the valves

does not

produce any

detectable sound

. (A clapping sound is produced when we bring our bands sharply together, but there is no sound when we take them apart again).

During the

short interval

of time,

between the closure

of the

mitral

valve and the

opening of the aortic

valve, the

left ventricle is a closed chamber

.

Slide51

At

the same time

the

right

ventricle will be

closed off

by the

closure of the tricuspid and pulmonary valves

.

Blood is incompressible and although the contraction

of the ventricular muscle is

increasing the pressure

in the ventricles there is no actual change in volume during this phase. It is known as the isometric contraction phase

.

Slide52

As soon as the

aortic and pulmonary valves open

, the

ventricles decrease

in

size

as the blood is ejected into the aorta and pulmonary artery.

At

the end

of

ventricular systole

the

pressure in the ventricles drops

and the

aortic and pulmonary valves close

since the

pressure

in these

vessels

now

exceeds

that in

the ventricles

.

Slide53

There is a

short isometric relaxation phase

during which time the

ventricles are once again closed

chambers,

but

as soon as the ventricu1ar

pressure

has

fall

to

below that in the atria

the mitral and tricuspid valves will open.

Slide54

The closure of the aortic and pulmonary valves

gives rise to

the second heart sound

. This is a

shorter and sharper

sound and has been likened to the word

'dup'.

It

lasts for 0.1

seconds. The principal

frequency

is of the order of

50 cycles/second.

Slide55

Since

systole is shorter than diastole

the

rhythm when listening

to the heart is:

'LUB' 'DUP' 'LUB' 'DUP'

I - II Pause I - II Pause

with a

shorter interval

between the

first and next second heart sound

than

between

the second and next first sound.

Slide56

Under

suitable conditions

a

3

rd

heart sound

can be heard. This is

caused

by the

blood rushing

into the ventricles

during diastole

.

The third heart sound is probably

due to vibrations

of the

mitral valve cusps

since it is

no longer

heard in patients with

prosthetic mitral valves.

Slide57

The

sound of atrial contraction

is

sometimes audible

and

when present

is

termed the A sound

or

the 4

th

heart

sound.

The

contribution of each valve to the heart sounds

is usually

heard best at

the sites shown in the Figure, known as the

valve areas

.

Slide58

It follows that the

1

st

heart sound will usually be

heard loudest at the mitral and tricuspid areas

, whilst the

2

nd

heart sound will be

heard loudest at the pulmonary and aortic areas

.

Slide59

Slide60

Blood Flow Round the Circulation

* With each

systole 70 ml

. of blood are ejected from each ventricle.

* This quantity is termed the

stroke volume

.

* The aorta and large arteries are

elastic vessels

and they accommodate this blood with a relatively small rise in pressure. During the ensuing

diastole

, there

is no output from the heart, and the pressure in the arteries falls. The blood now in the arteries is pulsatile, and if an artery is cut, the blood shoots out in spurts.

Slide61

*

The blood flow

to the tissues

is maintained by the

elastic recoil

of the arterial walls, and by the time the

capillaries

have been

reached

, the flow has

ceased to be pulsatile

; the

steady flow

of blood through these vessels shows no variation

in

systole or diastole

.

Slide62

The blood

returns

to the heart along

the veins

, and the venous

flow

is a

steady

one. If a

vein is cut

the blood

oozes out

from the distal end

without any pulsation.

On

approaching

the heart, the flow

again becomes pulsatile

as the ventricles are

unable

to receive the

venous blood during systole

Slide63

Function of the Atria:

*The blood

returning

to the heart during ventricular

diastole

passes through the atria and enters the ventricles

.

*The blood

returning during systole is

unable to do this

as the

A-V valves are closed. *The atria act as a storage reservoir for this blood until the end of systole when the A-V valves open. With the opening of these valves the

ventricles fill rapidly with the waiting blood.

Slide64

*

Further filling

occurs as

blood returns during diastole

.

One-tenth

of a second

before

the onset of

systole

, when the

ventricles

are already

70% full of blood, the

atria contract and complete the filling of the ventricles

by

adding

the remaining

30%.

*Atrial contraction is

not essential

for life, but the

heart

is a much more

efficient pump

when the atria are contracting.

*The

flow

of blood through the

lungs and tissues

is

continued

but the flow is

intermittent through the heart

.

Slide65

Pressure Changes In The Heart:

Although the

pressures

could be

expressed

in dyne/cm.

2

or lb. per in.

2

, it is usual to express blood pressure

in terms of

the height of a corresponding column

of mercury measured in millimeters. The units are mm. Hg. These pressures are measured with

reference to the atmospheric pressure.

Slide66

The

pressure

in the

intra-thoracic part

of the circulation

may be less than

the

atmospheric

pressure. This is due to the

elastic recoil

of the lungs.

The pressure in the atria,

for example

, may be

755 mm. Hg

(absolute) whilst the

outside

barometric pressure is

760 mm. Hg.

Such a pressure

is denoted as -5 mm. Hg

meaning

5 mm.

Hg

less than

atmospheric.

Such a pressure

is referred to as a

negative pressure

.

Slide67

The

atria are thin-walled vessels

and the

pressure of blood

in them is

never very far

from the

pressure outside

in the surrounding

mediastinum

.

This pressure is slightly negative (-2 mm. Hg) and it becomes more negative when breathing in (-8 mm. Hg).

Slide68

Respiratory variations

in atrial pressure are

seen

in records of

atrial pressure

which are

obtained

with the

chest closed

by passing

a catheter

along the veins

into the heart

and

into the right atrium

. They

disappear

when the chest is

opened

as, for example, in thoracic surgery.

Slide69

When the

A-V valves

(mitral and tricuspid) are

open

during diastole

(up to 0.1 and from 0.45 to 0.9 seconds), the

atrial pressure

is very

slightly greater

than the

ventricular

pressure

,

otherwise

the blood

would not enter

the

ventricles

. But to all intents and purposes the atrial and ventricular pressures are identical during this phase.

Slide70

Similarly during

ventricular systole

, when the

aortic and pulmonary valves are open

, the

pressures

in the

ventricles

will be

equal

to those in the

aorta and pulmonary artery

(

from 0.15 to 0.4 and from 0.95 seconds onwards).

Slide71

Ventricular Pressure Changes:

The left ventricle

develops a

maximum pressure of 120 mm. Hg

during

systole

and

therefore

the

aortic pressure reaches the same peak value

.

During

diastole the ventricular pressure falls to that in the thorax

which is approximately 0 mm. Hg (atmospheric pressure). The ventricular changes are thus

120/0 mm. Hg

.

Slide72

The

pressure in the aorta

, however, is

maintained

by the

elastic recoil

of the

arterial walls

. Due to

this elastic

recoil the

pressure in the aorta has only fallen to 80 mm. Hg

by the

time the next systole occurs.

Thus

the pressure

in

the aorta fluctuates

between

120 mm. Hg and 80 mm. Hg.

These

changes are denoted as 120/80 mm. Hg.

Slide73

The

right ventricle

pumps out

exactly

the

same quantity

of blood

as the lef

t ventricle

but

at a

much lower pressure

.

The

right ventricle

develops a

maximum pressure of 25 mm. Hg

. The

pressure falls to 0 mm. Hg

in

diastole

and the right ventricle

pressure changes are 25/0 mm. Hg.

The pressure in the

pulmonary artery falls to 8 mm. Hg

during

diastole

and the

pressure changes

in this vessel are, therefore

25/8 mm. Hg

.

Slide74

Atrial Pressure Changes

The atrial pressure tracing follows a complex pattern due to the interplay of several factors.

Atrial contraction (atrial systole) is associated with a pressure rise in the atria. It lasts for only 0.1 seconds (time 0 to o.1 and 0.8 to 0.9 seconds).

For the reminder of the cardiac cycle the atrial muscle is in u state or relaxation (atrial diastole).

Slide75

The isometric contraction of the ventricles causes the mitral and tricuspid valves to bulge into the atria causing a further increase in atrial pressure (time 0.1 to 0.15 and 0.9 to 0.95 seconds).

However, as soon as the aortic and pulmonary valves open, the ventricular volumes decrease rapidly.

Slide76

The A-V ring descends increasing the volume of the atria. The atrial pressure falls sharply.

Blood returning to the heart cannot now enter the ventricles and the pressure in the atria builds up (time 0.2 to 0.45 seconds).

Slide77

With the relaxation of the ventricles the atrioventricular ring moves upwards towards the base of the heart decreasing the volume of the atria and causing the atrial pressure to rise further (just before 0.45 seconds).

Slide78

With the opening of the mitral and tricuspid valves at time 0.45 seconds the pressures in the atria and ventricles fall.

Blood returning to the heart enters the atria and ventricles and their pressures build up together till time 0.8 seconds is reached when another atrial contraction occurs.

Slide79

Venous Pressure Changes:

The

veins near the heart

show

similar pressure

waves

to

those in the

right atrium

.

Since

the

veins and atria are in communication at all times

except during the brief atrial systole, the

atrial pressure changes

will be

transmitted back

to the

veins

.

Slide80

There are

three

maxima in each

cardiac cycle lettered

a, c and v

.

The [a]

wave is

caused by

atrial systole

. The sphincter-like

action

of the

atrial muscle

round the

entry

of the veins

prevents this being

due simply to a

transmission of atrial systole to the veins

.

The

pressure builds up

because the

veins are unable to empty

their blood

into the atria.

Slide81

The [c]

wave is

synchronous

with the

pulse wave

in

carotid artery

hence the letter c.

It

corresponds

to the atrial

pressure peaks

occurring

at 0.15 and 0.95 seconds

and

is duo to

the

bulging

of the

A-V valves

during the

isometric contraction phase

.

Slide82

The [v]

wave corresponds

to the peak

in the

atrial pressure

, which

occurs

just

before 0.45 seconds

and is

due to

the

filling of the atria

whilst the

A-V valves are shut

and the upward

movemen

t of the

A-V ring

at the

end of ventricular systole.

These pressure changes

in the

veins

produce

the

venous pulse

which is

visible

in the

neck veins

when the subject is

lying down

.

Slide83

All veins

above

the sternal angle are usually

collapsed

because their

pressure is sub-atmospheric

.

No pulsations

are therefore

visible in the neck veins

in the

upright posture

unless

venous congestion is present.

Slide84

The

venous pulsations

are

seen

best in the

right external jugular

vein which is in direct

continuity

with the

superior vena cava and the right atrium. The venous pressure changes

are

too small

to be palpable.

Slide85

Ventricular Volume Changes

The

ventricular volume

is

constant

during

both

the isometric

contraction

phase and the isometric

relaxation

phase.

Thus with the

onset of systole

, for the

first 0.05 second

there is

reduction in volume.

Slide86

Slide87

As soon as the

aortic valve

has

opened

the

ventricular volume decreases

as blood is ejected

into the aorta

. The ejection is

rapid at first

. Late in systole the ejection rate

is reduced

.

For the

first 0.05 second

of

diastole

there is

again no ventricular volume change

as the ventricular muscle fibers

relax

isometrically

.

Slide88

As soon as the

mitral valve opens

the blood

rushes into the ventricle

, the ventricular

volume increases rapidly

at

first

and

later more slowly

.

By the time

0.4 of the 0.5 second

of

diastole

has elapsed

the filling has been such that the

ventricular volume

has

increased by 50 m1= 70%

of the

next stroke volume

.

Atrial contraction

in the

last 0.1 second

of

ventricular diastole completes the filling by the addition of a further 20 ml

.

Slide89

Murmurs

Murmurs

are

sounds

heard

during the cardiac cycle

in addition to the normal

heart sounds

.

Those

occurring

during systole

are known as systolic murmurs. Those occurring during diastole

are termed diastolic murmurs. They are

caused by turbulence

in the

blood

as it flows through the

valve

.

Slide90

Valvular disease

may

prevent adequate closure

of the

valves

, so that blood

regurgitates

back

through the partially closed valves. This will

occur during systole

in a case of

mitral or tricuspid incompetence, giving rise to a systolic murmur. It will occur during diastole if the aortic or pulmonary valve is

faulty-giving a diastolic murmur.

Slide91

Murmurs are

also caused

by a

narrowing

of the valve

orifice

which is known as a

stenosis

. The murmur will occur during the

rapid passage

of

blood

through

the valve

.

Thus,

mitral and tricuspid stenosis

will give rise to a

diastolic murmur

which will

reach

a

maximum during late diastole

when atrial

contraction forces

blood through the valve into the ventricle.

Aortic and pulmonary stenosis

will give rise to a

systolic murmur

.

Slide92

If the

turbulence produces vibrations

of the

chest wall

with

low frequency components

, the vibrations

can be felt

when the

flat of the hand

(or the ulnar border of the hand) is

applied to the chest

wall. Such a

palpable murmur

is termed

a thrill

.

In general,

low frequencies can be felt rather than heard

, whilst

high frequencies can be heard rather than felt.

Slide93

Heart Block

The

duration of the P-Q

, or as it is often called

the P-R, interval

is

a measure of the conduction time in the bundle

. It is normally

(0.15- 0.2)

seconds.

Any

longer time

denotes delay in the transmission along the

bundle and maybe the forerunner of complete failure of conduction

along the bundle

known as heart block.

Slide94

Complete

heart block occurs

when the ventricles

will be

cut off

from the

pacemaker

and will

stop beating

. However, they may restart at a

slow independent rate

due to the

inherent rhythmicity of the ventricular muscle. The rate (about 30 beats/minute) is often

too slow for an efficient circulation.

Slide95

In all probability

the S.A. node and atria

will

continue

at their

original rate of 70 beats/minute

.

Atrial contractions will cease

to be

effective

in

aiding the circulation

as many will occur

whilst the A-V valves are shut.

The

ECG will show numerous P waves

but

few abnormal QRS complexes

and there will be

no time correlation between them

.

Slide96

In such a case

an electrical stimulator

may be

implanted

in the axilla or abdominal wall and

connected

to electrodes in the

ventricle

.

Such a device

acts as an artificial pacemaker

and the ventricular contractions follow at the rate set by the electronic circuits. About 20 x 10

-6

joules of energy are needed for each pulse.

Slide97

If a

ventricular beat originates

at some

ectopic focus,

an

abnormal QRST

complex will

appear

on the

E.C.G

.

Slide98

This is a

common

occurrence in

young adults

and is known as an

extra-systole

or

dropped beat

.

The term dropped beat is used

because the

ectopic beat

may prevent the next pacemaker beat from contracting the ventricular muscle.

Slide99

There is therefore

a pause

until the next successive beat arrives

[Fig-34B].

Slide100

Atrial Fibrillation

Atrial

fibrillation occurs

when the

increased excitability of the atrial

muscle

leads

to

abnormal beats

arising from

ectopic loci

in the

atria. As a result, different parts of the atria are

contracting and relaxing out of phase with one another.

Slide101

The

general appearance

of the

atria during fibrillation

has been

described

as

resembling a bag of wriggling worms

.

The atria never empty and the atrial wall

becomes a

quivering

sheet of muscle.

Slide102

In

atrial fibrillation

the

atria cease to pump blood into the ventricles

.

In addition,

the beat originated

by the

pacemaker

is not transmitted to the ventricles

.

Instead,

impulses down the bundle of His in a haphazard fashion at a rate too fast

for the ventricle to respond to each impulse.

Slide103

The

ventricular contractions

which do occur

are irregular

both

in rate and amplitude.

A

proportion

of these beats

fail to develop sufficient pressure

to

open the aortic valve

. As a result the radial pulse is "irregularly irregular"

and the

heart rate

measured

at the

heart

may be

higher than

if it is

measured by the pulse at the wrist

. This is known as a

pulse deficit

.

Slide104

Ventricular Fibrillation

Should the

ventricular muscle fibrillate

, the

circulation stops immediately

. Unless immediate

cardiac massage

is

carried out

,

life will cease

.

Under suitable conditions ventricular fibrillation may be stopped by passing an electric current

through the heart. The apparatus is termed a cardiac defibrillator.

Slide105

The

E

lectro

C

ardio

G

ram (

E.C.G

)

The

invasion

of the cardiac muscle

by the contraction wave is associated with

electrical changes. These potentials may be

recorded

at

points remote from the heart

and

after electronic amplification

may be

displayed

on a

pen recorder

or a cathode ray oscilloscope.

The

wave form

obtained is termed

ECG

.

The

various waves

present in the

waveform

are

lettered

starting with the

letter P

.

Slide106

Each small box = 0.04 sec

Each large box = 0.2 sec

Each 5 large box = 1 sec

Slide107

Slide108

The

P

wave corresponds to the

spread

of

excitation

from the

S.A. node

over the

atrial muscle

and

therefore

represents

atrial systole.

Slide109

The

propagation

of the contraction

wave down

the bundle

of His

does not produce any

detectable

external electrical change. It is denoted on the E.C.G. by the iso-electric

P-Q

interval where there

is no deflection. The QRS complex at the commencement of the ventricular excitation,

and the T wave at the end are all that remain of this algebraic summation. The relaxation phase

T-P

is

iso-electric

.

Slide110

The detailed

waveform depends

on the

site

of the

recording electrodes

.

To record

the standard limb leads

, electrodes are applied to the left arm,

right arm

, and

left leg. These electrodes usually consist of a metal plate which is strapped over the flat part of the limb.Electrode

fluid or jelly is used between the electrode and the skin to ensure a good electrical connexion.

Leads I, II, Ill, aVR, aVL and aVF

Slide111

The

right leg

is

not used

for

recording

the ECG, but a further electrode

is often applied to

this limb

to 'earth' the subject, and thus minimize interference from the mains and electrical apparatus.

In order to make a recording,

two input connexions

must be made to the amplifier and there is usually a switch on the

electrocardiograph apparatus which makes the appropriate connexions internally when a particular lead is chosen.

Slide112

When the

switch is set at

Lead I

,

the

electrodes

on

the

right and left arms are

connected

to the amplifier. The

amplifier then records the potential difference between the right arm and the left arm.

When the switch is set to Lead II

,

the

potential difference

between the

right arm and left leg is recorded

.

When

set

to

Lead III

, the potential

difference

between

the left arm and left leg is recorded

.

Slide113

An

alternative way

of

recording

the ECG is to

connect

one input

of the

amplifier to a limb electrode, and to

connect

the

other input of the amplifier to the other two limbs through two resistances. Such an

arrangement records the difference between the potential in one limb

and the

mean potential

in the other two.

If the

single limb

is the

right arm,

the

lead is termed 'aVR'

, if the

left arm,

it is

termed 'aVL'

, and if the

left leg

(or foot), it is

termed 'aVF

'.

Leads I, II III, aVR, aVL and aVF are the

six standard limb leads

which

are recorded clinically

.

Slide114

Cardiac Vector

The figure shows the ECG recorded

using six limb leads.

They have been

arranged

to correspond to the figure below.

Slide115

Chest Leads

The

six limb

leads (so far discussed) give

information

about the

'electrical axis of the heart'

in the

frontal plane.

To study the

activity

in a horizontal plane, chest leads are used.

These usually take the form of a single chest electrode which will

adhere

to the

chest wall by suction

, and which is

connected

to

one input

of the amplifier.

The

other input

of the amplifier

is connected

to an

electrically neutral point V

which is

obtained

by

joining

the

three limb leads together through resistances

.

Slide116

It is found that

the point V

does not change

its

potential

during the

cardiac cycle

and this

potential may thus be used as a reference.

As the search

electrode is moved across

the chest, the ECG shows a dominant S-wave in chest positions 1 and 2, and a dominant

R-wave in chest positions 5 and 6.

Slide117