I
HEART, ARTIFICIAL
HEARING TESTS
Tests
Function
Tuning
fork tests
These basic tests are used to
determine whether hearing loss is
conductive or sensorineural. In
Rinne’s test, the patient is asked
whether the sound is louder with the
vibrating tines held near the opening
ofthe ear canal (air conduction) or
with the base ofthe fork held
againstthe mastoid bone (bone
conduction). In a normal ear or one
with sensorineural loss, air
conduction is greater than bone
conduction. In conductive hearing
loss, bone conduction is greater
than or equal to air conduction.
Weber’s test, in which the base of
the fork is placed on the forehead,
is useful for diagnosing unilateral
hearing loss. Ifthe hearing loss is
conductive, the patient hears the
tuning fork better in the ear with the
poorer hearing.
Pure-tone
audiometry
This is a test in which an audiometer
is used to generate sounds of
different frequencies and intensities
and a person’s ability to hear these
sounds is measured. Hearing is first
assessed by transmitting the sounds
through one earphone while the
other ear is prevented from hearing
them. The sound frequencies range
from 250 to 8,000 Hertz; for each
frequency, the sound is increased in
intensity until it can be heard. The
patient gives a signal at the moment
when he or she detects each sound
and the results are recorded on a
graph called an audiogram. Bone
conduction is then assessed in the
same way but the signal is
transmitted through a rubber pad
held againstthe bone behind the ear.
Auditory
evoked
response
This test may be used to evaluate the
presence of hearing in someone who
cannot cooperate with other tests
(such as a very young baby). The
brain’s response to sound stimulation
produced by an audiometer is
analysed by means of electrodes
placed on the scalp. Auditory evoked
response can also help to rule out
acoustic neuroma (a benign tumour
affecting the auditory nerve, part of
the vestibulocochlear nerve).
Impedance
audiometry
Also known as tympanometry, this
test is used to assess middle-ear
damage occurring in cases of
conductive deafness or to detect the
effects of fluid within the middle ear.
A probe is fitted tightly into the
entrance ofthe ear canal, thereby
sealing off the outside air pressure
and sound. The probe emits a
continuous sound. Air is pumped
through the probe at varying
pressures and, at the same time, a
microphone in the probe registers the
differing reflections of sounds from
the eardrum as pressure changes in
the ear canal. The reflections are
recorded on a graph known as a
tympanogram. The pattern of differing
reflections relates to the type of
disease that is causing the deafness.
l e v e l a t w h i c h a p e r s o n c a n h e a r a n d
r e p e a t
w o r d s
( t h e
s p e e c h
r e c e p t i o n
t h r e s h o l d ) i s t e s t e d i n e a c h e a r , a s i s t h e
a b i l i t y
t o
h e a r
w o r d s
c l e a r l y
( s p e e c h
d i s c r i m i n a t i o n ) . T u n i n g f o r k t e s t s
( s e e
R i n n e ’s t e s t
)
c a n b e u s e d t o
d e t e r m i n e
w h e t h e r h e a r i n g l o s s i s c o n d u c t i v e o r
s e n s o r i n e u r a l ( s e e
d e a f n e s s
).
heart
T h e m u s c u l a r p u m p i n t h e c e n t r e o f t h e
c h e s t t h a t b e a t s c o n t i n u o u s l y a n d r h y t h -
m i c a l l y t o s e n d
b l o o d
t o t h e l u n g s a n d
t h e r e s t o f t h e b o d y D u r i n g a n a v e r a g e
l i f e t i m e , t h e h e a r t c o n t r a c t s m
o r e t h a n
2 , 5 0 0 m
i l l i o n t i m e s .
STRUCTURE
M
u c h
o f
t h e
h e a r t
c o n s i s t s
o f m y o -
c a r d i u m , a t y p e o f m u s c l e , w h i c h , w h e n
g i v e n
o x y g e n
a n d n u t r i e n t s ,
c o n t r a c t s
r h y t h m i c a l l y a n d a u t o m a t i c a l l y w i t h o u t
a n y o t h e r s t i m u l u s . T h e i n t e r n a l s u r f a c e
o f t h e
h e a r t
i s
l i n e d
w i t h
a
s m o o t h
m e m b r a n e c a l l e d e n d o c a r d i u m , a n d t h e
e n t i r e
h e a r t
i s
e n c l o s e d
i n
a
t o u g h ,
m e m b r a n o u s b a g , t h e
p e r i c a r d i u m
.
I n s i d e t h e h e a r t t h e r e a r e f o u r c h a m
-
b e r s . A t h i c k c e n t r a l m u s c u l a r w a l l , t h e
s e p t u m ,
d i v i d e s
t h e
h e a r t
c a v i t y
i n t o
r i g h t a n d l e f t h a l v e s . E a c h h a l f c o n s i s t s
o f a n u p p e r c h a m b e r , c a l l e d a n
a t r i u m
( p l u r a l : a t r i a ) , a n d a l a r g e r l o w e r c h a m
-
b e r , c a l l e d a
v e n t r i c l e
.
( S e e
S t r u c t u r e o f t h e
h e a r t
b o x , o v e r l e a f . )
T h e
b o d y ’s
p r i n c i p a l
b l o o d
v e s s e l s
e m e r g e f r o m t h e t o p a n d s i d e s o f t h e
h e a r t . T h e s u p e r i o r a n d i n f e r i o r v e n a e
c a v a e ( s e e
v e n a c a v a
),
t h e l a r g e s t v e i n s ,
d e l i v e r
d e o x y g e n a t e d
b l o o d
f r o m
t h e
b o d y t o t h e h e a r t . T h e
a o r t a
,
w h i c h i s
t h e
m a i n
a r t e r y ,
c a r r i e s
o x y g e n - r i c h
b l o o d t o b e c i r c u l a t e d a r o u n d t h e b o d y
T h e p u l m o n a r y
v e s s e l s c a r r y b l o o d t o
a n d f r o m
t h e l u n g s , w h e r e t h e
b l o o d
a b s o r b s o x y g e n a n d r e l e a s e s t h e w a s t e
p r o d u c t c a r b o n d i o x i d e .
FUNCTION
T h e t w o s i d e s o f t h e h e a r t h a v e d i s t i n c t ,
t h o u g h i n t e r d e p e n d e n t , f u n c t i o n s . T h e
r i g h t
a t r i u m
r e c e i v e s
d e o x y g e n a t e d
b l o o d f r o m t h e e n t i r e b o d y v i a t h e t w o
v e n a e
c a v a e . T h i s
b l o o d i s t r a n s f e r r e d
t o
t h e
r i g h t
v e n t r i c l e ,
t h e n
p u m p e d
t o t h e l u n g s v i a t h e p u l m o n a r y a r t e r y
t o b e o x y g e n a t e d a n d l o s e c a r b o n d i o x -
i d e . T h e l e f t a t r i u m o f t h e h e a r t r e c e i v e s
o x y g e n a t e d b l o o d f r o m t h e l u n g s
( v i a
t h e
p u l m o n a r y
v e i n s ) ;
t h i s
i s
t r a n s -
f e r r e d t o t h e l e f t v e n t r i c l e a n d p u m p e d
t o a l l b o d y t i s s u e s .
O n e - w a y v a l v e s a t
t h e
c h a m b e r
e x i t s
e n s u r e
t h a t
b l o o d
f l o w s i n o n e d i r e c t i o n ( s e e
h e a r t v a l v e s
).
THE CARDIAC CYCLE
T h e p u m p i n g a c t i o n o f t h e h e a r t c o n -
s i s t s
o f t h r e e
p h a s e s ,
w h i c h
t o g e t h e r
m a k e u p a c y c l e c o r r e s p o n d i n g t o o n e
h e a r t b e a t ( s e e
H e a r t c y c l e
b o x , o v e r l e a f ) .
T h e s e p h a s e s a r e
c a l l e d d i a s t o l e ,
a t r i a l
s y s t o l e , a n d v e n t r i c u l a r s y s t o l e . I n d i a s -
t o l e , t h e h e a r t m u s c l e
r e l a x e s a n d t h e
h e a r t f i l l s w i t h b l o o d . I n a t r i a l s y s t o l e ,
t h e a t r i a
c o n t r a c t ,
s e n d i n g
b l o o d
i n t o
t h e v e n t r i c l e s . I n v e n t r i c u l a r s y s t o l e , t h e
v e n t r i c l e s c o n t r a c t , p u m p i n g b l o o d i n t o
t h e b l o o d v e s s e l s .
F o r t h e h e a r t t o f u n c t i o n e f f i c i e n t l y ,
t h e m u s c u l a r
c o n t r a c t i o n s
m u s t
o c c u r
i n a p r e c i s e s e q u e n c e . T h i s s e q u e n c e i s
r e g u l a t e d
b y
e l e c t r i c a l
i m p u l s e s
t h a t
e m a n a t e
f r o m
t h e
s i n o a t r i a l n o d e
( a n
a r e a o f s p e c i a l i z e d m u s c l e a t t h e t o p o f
t h e
r i g h t
a t r i u m
) ,
t h e
h e a r t ’s
n a t u r a l
p a c e m a k e r . T h e
e l e c t r i c a l i m p u l s e s
a r e
c a r r i e d t o t h e a t r i a a n d v e n t r i c l e s p a r t l y
b y
t h e
h e a r t
m u s c l e
i t s e l f a n d
p a r t l y
b y n e r v e f i b r e s .
I n o r d e r t o p r e v e n t b o t t l e n e c k s f r o m
d e v e l o p i n g i n t h e b l o o d c i r c u l a t i o n , t h e
v o l u m e
t h a t
i s
p u m p e d
a t
e a c h
b e a t
b y t h e t w o s i d e s o f t h e h e a r t m u s t b e
b a l a n c e d
e x a c t l y
R e s i s t a n c e
t o
b l o o d
f l o w t h r o u g h t h e g e n e r a l c i r c u l a t i o n i s
m
u c h
g r e a t e r
t h a n
r e s i s t a n c e
t h r o u g h
t h e l u n g s . T h e r e f o r e , t h e l e f t s i d e o f t h e
h e a r t m u s t c o n t r a c t m
o r e f o r c i b l y t h a n
t h e r i g h t ; f o r t h i s r e a s o n , t h e l e f t s i d e
h a s g r e a t e r m u s c u l a r b u l k .
heart, artificial
A n i m p l a n t a b l e m e c h a n i c a l d e v i c e t h a t
t a k e s o v e r t h e a c t i o n o f t h e
h e a r t
.
H o w -
e v e r , t h e t e r m i s a l s o u s e d t o d e s c r i b e
d e v i c e s t h a t a s s i s t t h e h e a r t i n m a i n t a i n -
i n g t h e c i r c u l a t i o n .
H
365
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