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THE PROBLEMS OF CONTROLLED VENTILATION IN ANIMALS
147.
THE PROBLEMS OF CONTROLLED VENTILATION IN ANIMALS
B. M.
Q. Weaver
Wellcome Comparative A n a e s t h e t i c Laboratory, Department of V e t e r i n a r y Surgery, U n i v e r s i t y of B r i s t o l
From t h e e a r l i e s t t i m e s , c o n d i t i o n s which l e d t o a d i s r u p t i o n o f t h e p l e u r a l c a v i t y , n o t a b l y wounds of t h e chest w a l l , were regarded as extremely s e r i o u s .
Further-
more, i t was r e a l i s e d t h a t t h e harmful e f f e c t s were, i n g e n e r a l , r e l a t e d t o t h e s i z e o f t h e opening into t h e p l e u r a l cavity. The problem of pneumothorax, however, w a s only beginning t o be overcome i n human medical p r a c t i c e towards t h e end o f t h e l a s t c e n t u r y .
This i s s o , i n s p i t e o f t h e f a c t
t h a t i n essence i t had been solved i n animals s e v e r a l c e n t u r i e s e a r l i e r when e n d o t r a c h e a l i n t u b a t i o n was c a r r i e d out t o m a i n t a i n l i f e by i n f l a t i n g t h e l u n g s when t h e c h e s t was opened f o r anatomy demonstrations.
This w a s done
by V e r s a l i u s who used a sow i n t h e 1640's and by Robert Hooke i n 1667 u s i n g a dog. I n 1914 Charles Waterton s u c c e s s f u l l y r e s u s c i t a t e d an ass which had r e c e i v e d t h e poison Wourali
-
by opening i t s
windpipe and i n f l a t i n g t h e lungs with a p a i r o f bellows f o r f o u r hours. I n 1915 Dennis E. JacksOD, a pharmacologist, developed t h e r e b r e a t h i n g method w i t h a b s o r p t i o n of e x p i r e d carbon
148.
dioxide. His medical colleagues, however, were reluctant to make use of his laboratory experiments on animals.
Thus,
it was the establishment of endotracheal intubation using a wide-bore tube in human anaesthetic practice by Magill and Rowbothom in 1921 which paved the way for controlled ventilation in humans. If, however, the development of controlled ventilation in medical practice suffered from some 'starting inertia', this was also the case in veterinary practice in spite of the early work on laboratory animals. In London in the 1940's,
Gordon Knight pioneered
open chest surgery in small animals controlling ventilation by means of a Starling pump borrowed from our late Honorary member Professor E. C. Amoroso who was then Professor of Veterinary Physiology at the Royal Veterinary College.
Mr. Knight realised that intermittent positive pressure ventilation (1.P.p.V.) was essential for any operation involving pneumothorax.
In 1950 he demonstrated a machine
he had devised with a colleague.
This was the Knight-Wood
pump and it delivered a positive flow of air to the lungs which could be enriched with oxygen.
The flow inflated
the lungs and could be interrupted from 3
-
80 times per
minute to enable the lungs to deflate, expired air then being dispelled to the atmosphere. At about this time, Gordon Knight made the following observation: ''Due to the frail mediastinal septum in small animals artificial respiration is essential for any surgical operation involving pneumothorax'.
149.
Today the need to control ventilation in both small and large animals on occasions is well recognised.
It is
essential for: .I.
2.
The repair of large wounds to the chest wall,
All surgical operations involving the creation of an open pneumothorax.
3.
Whenever muscle relaxant drugs axe used as part of a balanced anaesthetic regime.
4.
For the correction of ventilatory inefficiency.
In spite of the very clear background that life can
be sustained by intermittent inflation of the lungs, problems can arise due to distTnbance of the normal physiological processes associated with breathing.
Thus, if intermittent
positive pressure ventilation is not correctly carried out
or is used on animals which cannot compensate for the resulting disturbances however minimal, undesirable consequences can result.
The possible harmful effects of controlled
ventilation axe as foilows: 1.
Interference with cardiovascular function. During spontaneous breathing, ventilation is achieved by intermittent negative intrapulmonary and increased negative intrapleural pressures which, in addition
to ventilating the lungs, serve to assist venous return to the heart.
With positive pressure in-
flation, the venous return to the heart and hence the cardiac output are reduced.
In addition, dur-
ing positive pressure inflation of the lungs, there is a tamponade of the heart which becomes
150.
compressed between t h e expanding lungs.
I n add-
i t i o n , during i n f l a t i o n , t h e pulmonary c a p i l l a r y c i r c u l a t i o n i s decreased p u t t i n g a f u r t h e r s t r a i n on t h e r i g h t v e n t r i c l e of t h e h e a r t . 2.
The lungs may be damaged.
With c a r e t h i s i s un-
l i k e l y t o occur s i n c e a s a f e maximum i n t r a p u l monary p r e s s u r e w i t h t h e c h e s t i n t a c t i s considered t o be 70 cmH20.
I t should be remembered, however,
t h a t with t h e c h e s t open and t h e l u n g s unsupported, r q t u r e , which i n m a m m a l s i s usually i n t o t h e mediastinum, can occur with pressures as low as 40 cmH2C.
3.
The v e n t i l e t i n g g z s may be unevenly d i s t r i b u t e d . If t h i s happens t h e normal veiitj l a t i o n / p e r f u s i o n
ratio
i n t h e lungs i s d i s t u r b e d and a venous shunt
from t h e r i g h t t o t h e l e f t of t h e h e a r t can occur. Uneven v e n t i l a t i o n mag r e s u l t from t h e presence of some p a t h o l o g i c a l c o n d i t i o n b u t i t i s perhaps b e s t known i n l a r g e animals such as t h e horse where t h e dependenl l u n g o r a r e a s o f l u n g tend t o c o l l a p s e w i t h reduced v e n t i l a t i o n and where compans a t o r y r e d i s t r i b u t i o n of t h e pulmonary perfusion
i s not l i k e l y t o
l a s t more than h a l f an hour.
1. Acid-base d i s t u r b a n c e s may occur.
Over v e n t i l a t i o n
w i l l l e a d t o r e s p i r a t o r y a l k a l o s i s and c e r e b r a l
151.
vasoconstriction.
Under v e n t i l a t i o n on t h e o t h e r
hand may r e s u l t i n hypoxaemia and w i l l cause r e s piratory acidosis. The harmful e f f e c t s o f c o n t r o l l e d v e n t i l a t i o n a r e minimised by keeping t h e mean p r e s s u r e i n the lungs as l o w
as p o s s i b l e .
This i s done by using a small i n s p i r a t o r y :
e x p i r a t o r y r a t i o (1:E) and ensuring that t h e e x p i r a t o r y flow i s unimpeded o r , p e r h a p s , a s s i s t e d by a negative phase. Although h e l p f u l i n speeding up e x p i r a t i o n , however, a negative phase i n some c a s e s can i t s e l f be harmful.
This
i s e s p e c i a l l y s o i n emphysema where compliance i s high and t h e airway r e s i s t a n c e i s a l s o high, s o t n a t during t h e negative phase t h e small airways c o l l a p s e and gas i s t r z p ped i n t h e a l v e o l i .
Furthermore, a s h o r t i n s p i r a t o r y time
i n e v i t a b l y means t h a t t h e gas flow r a t e has t o be
high
and while t h i s may reduce t h e mean p r e s s u r e , any uneven d i s t r i b u t i o n o f gas within t h e lungs i s l i k e l y t o be exacerbated. For c o n t r o l l e d v e n t i l a t i o n t o be s u c c e s s f u l considerat i o n must be given t o t h e i n d i v i d u a l , i t s v e n t i l a t o r y r e quirement, compliance and airway r e s i s t a n c e on t h e one hand, and t h e v e n t i l a t i n g mechanism, s e l e c t i o n o f t h e optimum i n s p i r a t 0 r y : e x p i r a t o r y r a t i o , s u i t a b l e i n s p i r a t o r y flow r a t e and s u i t a b l e peak p r e s s u r e a t t h e mouth, on t h e o t h e r . The e a r l y use of automatic v e n t i l a t o r s i n London mentioned above became r e p l a c e d by t h e use of manual c o n t r o l
o f v e n t i l a t i o n i n t h e 1950's.
This means o f v e n t i l a t i n g
152.
t h e lungs allowed f o r g r e a t e r f l e x i b i l i t y i n t h e p r e s s u r e s and volumes a p p l i e d t o t h e l u n g s and a n a e s t h e t i s t s l e a r n e d how t o i n t e r p r e t t h e f e e l of t h e l u n g s through t h e r e s e r v o i r bag g i v i n g r i s e t o t h e term 'educated hand'.
Neasure-
ments made of t h e e f f e c t i v e n e s s o f manual c o n t r o l o f v e n t i l a t i o n , however, have shown t h a t s t e a d y s t a t e s of c o n t r o l i n t h i s way are d i f f i c u l t t o achieve.
Nevertheless, t h i s
means o f c o n t r o l l i n g v e n t i l a t i o n remains an important and p r a c t i c a l method. Uhen, however, v e n t i l a t i o n i s t o be c o n t r o l l e d for prolonged p e r i o d s o r i t i s t o i3e done i n l a r g e animals which requires considerable physical e f f o r t , th er e i s
2
need f o r
t h e job t o be done by mechanical means which, i n a d d i t i o n , has t h e advantage o f f r e e i n g t h e a n a e s t h e t i s t ' s hands.
Most
work on this t o p i c has been done s m c e t h e 1950's with t h e a d d t human i n mind and s o m o s t i n f o r m a t i o n i s a v a i l a b l e and t h e m o s t s u i t a b l e v e n t i l a t o r s e x i s t f o r t h i s type of i n d i v i d u a l who weiqha approximately 66 kg (Mushin e t a l . , 1980).
I t i s convenient t h e r e f o r e t o c o n s i d e r t h i s 66 kg
i n d i v i d u a l as a ' s t a d a r d one' f o r whom t h e problems have been l a r g e l y overcome and s e e how t h e s e problems v a r y w i t h size.
I n comparing t h e s t a n d a r d i n d i v i d u a l with t h e small
3 kg one, f o r e x m p l e c a t s and small dogs, compliance i s low, t i d a l volume i s small, frequency o f b r e a t h i n g i s h i g h and i n s p i r a t o r y flow r a t e s a r e low.
On t h e o t h e r hand,
expectedly, i f we l o o k a t a l a r g e i n d i v i d u a l of 500 kg, f o r example ax? a d u l t h o r s e , t h e r e v e r s e i s t h e c a s e for each of t h e s e parameters.
153.
Geometric s c a l i n g for t h e p r e d i c t i o n of v e n t i l a t i o n parameters has been confirmed by measurement on t h e basis t h a t volume i s p r o p o r t i o n a l t o body mass, l e n g t h ( h e i g h t ) i s p r o p o r t i o n a l t o t h e cube r o o t o f mass and pressure i s
independent of body mass.
Thus, together with measurement
where t h i s has been done, t h e parameters of v e n t i l a t i o n can be t h e o r e t i c a l l y considered as i l l u s t r a t e d i n Figure 1 . It can be seen that t h e time taken f o r t h e t i d a l volume
(V,)
t o e n t e r t h e lungs can be worked o u t and hence t h e
average i n s p i r a t o r y f l o w r a t e (av. V i ) .
Then, Cram a
knowledge o f t o t a l compliance ( C ) , and t h e t i d a l volume, t h e peak a l v e o l a r p r e s s u r e (PA) can be derived which can then be used with t h e a d d i t i o n a l knowledge o f airway r e s i s tance (airway S ) t o work out t h e p r e s s u r e which must be generated a t t h e mouth (Y mouth).
This has been done f o r
t h e 3 kg i n d i v i d u a l s i n c e i n medical p r a c t i c e i t i s now a common procedure for neonate b a b i e s weighing around 3 kg t o be v e n t i l a t e d o f t e n f o r long periods.
Figure 2 shows how
t h e parameters work out f o r t h e l a r g e i n d i v i d u a l weighing 500 kg.
By f i l l i n g i n some reasonable f i g u r e s based on
observation and measurement i t can be seen t h a t t h e average i n s p i r a t o r y f l o w r a t e (av. ;i)
i s 180 L min-'
, the
peak
a l v e o l a r p r e s s u r e (PA) i s 1 7 - 4 5 m H 2 0 and the p r e s s u r e t o be generated a t t h e mouth (I? mouth) i s 22-55 cmH20.
Figure
3 shows how t h e parameters o f v e n t i l a t i o n worked o u t i n t h i s
way vary between ' s m a l l ' ,
' s t a n d a r d ' and ' l a r g e ' i n d i v i d u a l s .
It i s important t o n o t e , however, t h a t t h e s e f i g u r e s are
only a p p l i c a b l e where t h e lungs a r e normal w i t h normal
..
v e n t i l a t i o n p e r f u s i o n r a t i o s (V/Q)
and a r e based on an
assumption t h a t t h e i n s p i r e d / e x p i r e d r a t i o (1:E) i s 1:2.
..
The a n a e s t h e t i s e d h o r s e ' s (V/Q)
r a t i o s are often
s e r i o u s l y d i s t u r b e d and during i n t e r m i t t e n t p o s i t i v e press u r e v e n t i l a t i o n t h e d i s t r i b u t i o n of t h e i n s p i r e d g a s i s u n l i k e l y t o be even.
Furthermore t h e 1:E r a t i o i s n o t
normally 1:2 s i n z e t h e i n s p i r e d volume i s u s u a l l y d e l i v e r e d , even when t h e animals a r e b r e a t h i n g spontaneously, i n about one second and n o t 1 - 6 7 seconds.
Thus i n s p i r a t o r y
f l o w r a t e s a r e u s u a l l y g r e a t e r than 200 L min-l
arid t h e
p r e s s u r e drop a c r o s s t h e airway r i s e s t o more t h a n 5.1
cmH20.
These f a c t o r s e x p l a i n t h e f a c t t h a t i n f l a t i o n
pressures required t o v e n t i l a t e horses a r e usually nearer
30 cmH20 than 2 2 cm H20.
A l a r g e number o f v e n t i l a t d r s a r e a v a i l a b l e f o r t h e tystanda:dlt
and ttsmalltti n d i v i d u a l s (Mushin e t a l . , 1980)
and many a r e e l a b o r a t e and capable of meeting s p e c i a l p a t i e n t needs. B v e n t i l a t o r designed f o r t h e ' s t a n d a r d '
66 kg i n d i v i d u a l
c a n be adapted f o r a s n a l l e r one and most o f t e n t h i s i s done by d i s p o s i n g o f t h e excess flow v i a an i n t e n t i o n a l leak.
However, c a r e f u l monitoring i s needed i f t h i s i s
done t o be c e r t a i n t h a t t h e p a t i e n t is g e t t i n g t h e r i g h t amour,t of flow.
The Flomasta (Weaver and Jones, 1981)
i s an example of an inexpensive v e n t i l a t o r which can be
used on individuals varying considerably in size.
In
inflating the lungs, the Flomasta acts as a discharging compliance. Reducing this compliance thus increases the cycling frequency and decreases the tidal volume.
It
should be noted, however, that increasing the resistance with this ventilator will decrease the frequency and increase the tidal volume. For the large individual some home-made systems have been developed and can work satisfactorily but they axe usually cumbersome and impractical.
They have, however,
served to study the ventilatory requirements (Weaver and Walley, 1975) and, in recent years, a few purpose-built coumercial large animal ventilators have become available:1.
The Narkovet-E Large Animal Anesthesia Control Center made by North American Drager.
This vent-
ilator has a pneumatically driven bellows and fine fluidic control.
It is time-cycled allowing the
number o f breaths per minute (B.P.M.) to be preset from 6 to 18.
In use, the maximum minute
volume has been found to be about 70 litres with a maximum tidal volume between 10 and 11 lites (Webb, 1982, personal communication).
In addition,
Dr. Webb has encountered difficulty in weaning patients off the ventilator since the lowest cycling rate is 6 B.P.M.
This feature has been improved
by the Narkovet E-2 model which incorporates electronic circuitry and the frequency cam be reduced to nearly zero if required.
156.
2.
The American Bird v e n t i l a t o r has been modified f o r use i n l a r g e animals.
I t i s a p r e s s u r e cycled
machine w i t h an o p t i o n a l p a t i e n t t r i g g e r f a c i l i t y which has been found u s e f u l i n weaning p a t i e n t s from t h e automatic v e n t i l a t i o n (Webb, 1982, p e r s o n a l communication).
I t does n o t , however, have a
s e p a r a t e r e s e r v o i r bag f o r spontaneous v e n t i l a t i o n .
3.
The Cambridge R e s p i r a t o r i s made by Sowring Engineering.
This v e n t i l a t o r u t i l i z e s a h y d r a u l i -
c a l l y a c t i v a t e d bellows t o compress gas i n a c y l i n d e r which ir. t u r n d i s p l a c e s a v a r i a b l e volume of gas f r o m t h e r e s e r v o i r / b r e a t h i n g bag contained i n the cylinder.
By means o f t h e a d j u s t a b l e
c o n t r o l s , c y c l i n g can be made t o occur according
t o time, p r e s s u r e , volume o r a combination o f t h e settings.
D i f f i c u l t y can a r i s e i n t h a t t h e s e t t i n g
o f one o o n t r o l may over-ride t h e s e t t i n g o f another and i t i s d i f f i c u l t t o a s s e s s t h e a c t u a l volume
.
d e l i v e r e d t o t h e animal.
The v e n t i l a t o r does,
however, have ample c a p a c i t y t o v e n t i l a t e l a r g e h o r s e s and c a t t l e .
I n conclusion, v e n t i l a t i o n can be c o n t r o l l e d i n a n i m a l s varying i n s i z e from l e s s t h a n 3 kg t o more
t h a n 500 kg
and when c o r r e c t l y c a r r i e d o u t , harmful e f f e c t s a r e minimized.
It can be done by i n t e r m i t t e n t manual compression
o f t h e r e s e r v o i r bag i n a s u i t a b l e b r e a t h i n g system o r by
157-
t h e use o f an automatic v e n t i l a t o r .
The c o s t of purpose
b u i l t v e n t i l a t o r s can be h i g h e s p e c i a l l y f o r t h e l a r g e i n d i v i d u a l s but t h e y are e s s e n t i a l i f c o n t r o l l e d v e n t i l a t i o n
i s t o be c a r r i e d o u t for long p e r i o d s . The v e n t i l a t i n g requirements f n r a n i m a l s with normal h e a l t h y lungs can be a s s e s s e d according t o s i z e but modi f i c a t i o n s need t o be made i n t h e presence of pathop h y s i o l o g i c a l cardio-pulmonary d i s t u r b a n c e s i n o r d e r t h a t t h e r i g h t type of v e n t i l a t i o n i s a p p l i e d t o each p a t i e n t .
REFWENCES
Jackson, D.E. (1 927). J o u r n a l of L a b o r a t o r y and C l i n i c a l Medicine l2, 998. Mushin, W.W., Rendell-Baker, L., Thompson, P.W. & Xapleson, U.W. (1980). Automatic V e n t i l a t i o n o f t h e Lungs, 3 r d e d i t i o n . B l a c k w e l l S c i e n t i f i c P u b l i c a t i o n s , Oxford. Weaver, B.M.Q. & Walley, R.V. J o u r n a l 1,No. 1 . Weaver, B.M.Q.
-
1 21 1 22.
a(
Jones, W.
(1975).
(19el).
Equine V e t e r i n a r y
V e t e r i n a r y F.ecord
108,
159-
Flg 2
Parameiers of Ventllaiim la MO r9 Animal I 12min-I
60
:.cycle
lasts
If I:E
.
1:2
I
f
x 5
-
12
-
1.67sec
vr Saa ml and enters lungs In 1.67 IIC
Pmouth
'* I'
-
--
PA + Pdrwrrossalrway PA + Vi x airway
17.45 +
X
17.45 + 5.1 22.55 cmH@
Fig 3
Parameters d Ventilatian for Small. Standard and Lam sized individuals W lkgl
small 3
asr
Stanhrd
66
8.
Large 500
60
40
16
12
16
500
uxa
2.5
50
3
2
0.5
1.25
286.5
1.7 ILarer ainkayl 1.67
im
1.9
24
6.4
10
11.45
7.4
10.8
a55
R
1.7
THE PROBLEMS OF CONTROLLED VENTILATION IN ANIMALS
B. M.
Q. Weaver
Wellcome Comparative A n a e s t h e t i c Laboratory, Department of V e t e r i n a r y Surgery, U n i v e r s i t y of B r i s t o l
From t h e e a r l i e s t t i m e s , c o n d i t i o n s which l e d t o a d i s r u p t i o n o f t h e p l e u r a l c a v i t y , n o t a b l y wounds of t h e chest w a l l , were regarded as extremely s e r i o u s .
Further-
more, i t was r e a l i s e d t h a t t h e harmful e f f e c t s were, i n g e n e r a l , r e l a t e d t o t h e s i z e o f t h e opening into t h e p l e u r a l cavity. The problem of pneumothorax, however, w a s only beginning t o be overcome i n human medical p r a c t i c e towards t h e end o f t h e l a s t c e n t u r y .
This i s s o , i n s p i t e o f t h e f a c t
t h a t i n essence i t had been solved i n animals s e v e r a l c e n t u r i e s e a r l i e r when e n d o t r a c h e a l i n t u b a t i o n was c a r r i e d out t o m a i n t a i n l i f e by i n f l a t i n g t h e l u n g s when t h e c h e s t was opened f o r anatomy demonstrations.
This w a s done
by V e r s a l i u s who used a sow i n t h e 1640's and by Robert Hooke i n 1667 u s i n g a dog. I n 1914 Charles Waterton s u c c e s s f u l l y r e s u s c i t a t e d an ass which had r e c e i v e d t h e poison Wourali
-
by opening i t s
windpipe and i n f l a t i n g t h e lungs with a p a i r o f bellows f o r f o u r hours. I n 1915 Dennis E. JacksOD, a pharmacologist, developed t h e r e b r e a t h i n g method w i t h a b s o r p t i o n of e x p i r e d carbon
148.
dioxide. His medical colleagues, however, were reluctant to make use of his laboratory experiments on animals.
Thus,
it was the establishment of endotracheal intubation using a wide-bore tube in human anaesthetic practice by Magill and Rowbothom in 1921 which paved the way for controlled ventilation in humans. If, however, the development of controlled ventilation in medical practice suffered from some 'starting inertia', this was also the case in veterinary practice in spite of the early work on laboratory animals. In London in the 1940's,
Gordon Knight pioneered
open chest surgery in small animals controlling ventilation by means of a Starling pump borrowed from our late Honorary member Professor E. C. Amoroso who was then Professor of Veterinary Physiology at the Royal Veterinary College.
Mr. Knight realised that intermittent positive pressure ventilation (1.P.p.V.) was essential for any operation involving pneumothorax.
In 1950 he demonstrated a machine
he had devised with a colleague.
This was the Knight-Wood
pump and it delivered a positive flow of air to the lungs which could be enriched with oxygen.
The flow inflated
the lungs and could be interrupted from 3
-
80 times per
minute to enable the lungs to deflate, expired air then being dispelled to the atmosphere. At about this time, Gordon Knight made the following observation: ''Due to the frail mediastinal septum in small animals artificial respiration is essential for any surgical operation involving pneumothorax'.
149.
Today the need to control ventilation in both small and large animals on occasions is well recognised.
It is
essential for: .I.
2.
The repair of large wounds to the chest wall,
All surgical operations involving the creation of an open pneumothorax.
3.
Whenever muscle relaxant drugs axe used as part of a balanced anaesthetic regime.
4.
For the correction of ventilatory inefficiency.
In spite of the very clear background that life can
be sustained by intermittent inflation of the lungs, problems can arise due to distTnbance of the normal physiological processes associated with breathing.
Thus, if intermittent
positive pressure ventilation is not correctly carried out
or is used on animals which cannot compensate for the resulting disturbances however minimal, undesirable consequences can result.
The possible harmful effects of controlled
ventilation axe as foilows: 1.
Interference with cardiovascular function. During spontaneous breathing, ventilation is achieved by intermittent negative intrapulmonary and increased negative intrapleural pressures which, in addition
to ventilating the lungs, serve to assist venous return to the heart.
With positive pressure in-
flation, the venous return to the heart and hence the cardiac output are reduced.
In addition, dur-
ing positive pressure inflation of the lungs, there is a tamponade of the heart which becomes
150.
compressed between t h e expanding lungs.
I n add-
i t i o n , during i n f l a t i o n , t h e pulmonary c a p i l l a r y c i r c u l a t i o n i s decreased p u t t i n g a f u r t h e r s t r a i n on t h e r i g h t v e n t r i c l e of t h e h e a r t . 2.
The lungs may be damaged.
With c a r e t h i s i s un-
l i k e l y t o occur s i n c e a s a f e maximum i n t r a p u l monary p r e s s u r e w i t h t h e c h e s t i n t a c t i s considered t o be 70 cmH20.
I t should be remembered, however,
t h a t with t h e c h e s t open and t h e l u n g s unsupported, r q t u r e , which i n m a m m a l s i s usually i n t o t h e mediastinum, can occur with pressures as low as 40 cmH2C.
3.
The v e n t i l e t i n g g z s may be unevenly d i s t r i b u t e d . If t h i s happens t h e normal veiitj l a t i o n / p e r f u s i o n
ratio
i n t h e lungs i s d i s t u r b e d and a venous shunt
from t h e r i g h t t o t h e l e f t of t h e h e a r t can occur. Uneven v e n t i l a t i o n mag r e s u l t from t h e presence of some p a t h o l o g i c a l c o n d i t i o n b u t i t i s perhaps b e s t known i n l a r g e animals such as t h e horse where t h e dependenl l u n g o r a r e a s o f l u n g tend t o c o l l a p s e w i t h reduced v e n t i l a t i o n and where compans a t o r y r e d i s t r i b u t i o n of t h e pulmonary perfusion
i s not l i k e l y t o
l a s t more than h a l f an hour.
1. Acid-base d i s t u r b a n c e s may occur.
Over v e n t i l a t i o n
w i l l l e a d t o r e s p i r a t o r y a l k a l o s i s and c e r e b r a l
151.
vasoconstriction.
Under v e n t i l a t i o n on t h e o t h e r
hand may r e s u l t i n hypoxaemia and w i l l cause r e s piratory acidosis. The harmful e f f e c t s o f c o n t r o l l e d v e n t i l a t i o n a r e minimised by keeping t h e mean p r e s s u r e i n the lungs as l o w
as p o s s i b l e .
This i s done by using a small i n s p i r a t o r y :
e x p i r a t o r y r a t i o (1:E) and ensuring that t h e e x p i r a t o r y flow i s unimpeded o r , p e r h a p s , a s s i s t e d by a negative phase. Although h e l p f u l i n speeding up e x p i r a t i o n , however, a negative phase i n some c a s e s can i t s e l f be harmful.
This
i s e s p e c i a l l y s o i n emphysema where compliance i s high and t h e airway r e s i s t a n c e i s a l s o high, s o t n a t during t h e negative phase t h e small airways c o l l a p s e and gas i s t r z p ped i n t h e a l v e o l i .
Furthermore, a s h o r t i n s p i r a t o r y time
i n e v i t a b l y means t h a t t h e gas flow r a t e has t o be
high
and while t h i s may reduce t h e mean p r e s s u r e , any uneven d i s t r i b u t i o n o f gas within t h e lungs i s l i k e l y t o be exacerbated. For c o n t r o l l e d v e n t i l a t i o n t o be s u c c e s s f u l considerat i o n must be given t o t h e i n d i v i d u a l , i t s v e n t i l a t o r y r e quirement, compliance and airway r e s i s t a n c e on t h e one hand, and t h e v e n t i l a t i n g mechanism, s e l e c t i o n o f t h e optimum i n s p i r a t 0 r y : e x p i r a t o r y r a t i o , s u i t a b l e i n s p i r a t o r y flow r a t e and s u i t a b l e peak p r e s s u r e a t t h e mouth, on t h e o t h e r . The e a r l y use of automatic v e n t i l a t o r s i n London mentioned above became r e p l a c e d by t h e use of manual c o n t r o l
o f v e n t i l a t i o n i n t h e 1950's.
This means o f v e n t i l a t i n g
152.
t h e lungs allowed f o r g r e a t e r f l e x i b i l i t y i n t h e p r e s s u r e s and volumes a p p l i e d t o t h e l u n g s and a n a e s t h e t i s t s l e a r n e d how t o i n t e r p r e t t h e f e e l of t h e l u n g s through t h e r e s e r v o i r bag g i v i n g r i s e t o t h e term 'educated hand'.
Neasure-
ments made of t h e e f f e c t i v e n e s s o f manual c o n t r o l o f v e n t i l a t i o n , however, have shown t h a t s t e a d y s t a t e s of c o n t r o l i n t h i s way are d i f f i c u l t t o achieve.
Nevertheless, t h i s
means o f c o n t r o l l i n g v e n t i l a t i o n remains an important and p r a c t i c a l method. Uhen, however, v e n t i l a t i o n i s t o be c o n t r o l l e d for prolonged p e r i o d s o r i t i s t o i3e done i n l a r g e animals which requires considerable physical e f f o r t , th er e i s
2
need f o r
t h e job t o be done by mechanical means which, i n a d d i t i o n , has t h e advantage o f f r e e i n g t h e a n a e s t h e t i s t ' s hands.
Most
work on this t o p i c has been done s m c e t h e 1950's with t h e a d d t human i n mind and s o m o s t i n f o r m a t i o n i s a v a i l a b l e and t h e m o s t s u i t a b l e v e n t i l a t o r s e x i s t f o r t h i s type of i n d i v i d u a l who weiqha approximately 66 kg (Mushin e t a l . , 1980).
I t i s convenient t h e r e f o r e t o c o n s i d e r t h i s 66 kg
i n d i v i d u a l as a ' s t a d a r d one' f o r whom t h e problems have been l a r g e l y overcome and s e e how t h e s e problems v a r y w i t h size.
I n comparing t h e s t a n d a r d i n d i v i d u a l with t h e small
3 kg one, f o r e x m p l e c a t s and small dogs, compliance i s low, t i d a l volume i s small, frequency o f b r e a t h i n g i s h i g h and i n s p i r a t o r y flow r a t e s a r e low.
On t h e o t h e r hand,
expectedly, i f we l o o k a t a l a r g e i n d i v i d u a l of 500 kg, f o r example ax? a d u l t h o r s e , t h e r e v e r s e i s t h e c a s e for each of t h e s e parameters.
153.
Geometric s c a l i n g for t h e p r e d i c t i o n of v e n t i l a t i o n parameters has been confirmed by measurement on t h e basis t h a t volume i s p r o p o r t i o n a l t o body mass, l e n g t h ( h e i g h t ) i s p r o p o r t i o n a l t o t h e cube r o o t o f mass and pressure i s
independent of body mass.
Thus, together with measurement
where t h i s has been done, t h e parameters of v e n t i l a t i o n can be t h e o r e t i c a l l y considered as i l l u s t r a t e d i n Figure 1 . It can be seen that t h e time taken f o r t h e t i d a l volume
(V,)
t o e n t e r t h e lungs can be worked o u t and hence t h e
average i n s p i r a t o r y f l o w r a t e (av. V i ) .
Then, Cram a
knowledge o f t o t a l compliance ( C ) , and t h e t i d a l volume, t h e peak a l v e o l a r p r e s s u r e (PA) can be derived which can then be used with t h e a d d i t i o n a l knowledge o f airway r e s i s tance (airway S ) t o work out t h e p r e s s u r e which must be generated a t t h e mouth (Y mouth).
This has been done f o r
t h e 3 kg i n d i v i d u a l s i n c e i n medical p r a c t i c e i t i s now a common procedure for neonate b a b i e s weighing around 3 kg t o be v e n t i l a t e d o f t e n f o r long periods.
Figure 2 shows how
t h e parameters work out f o r t h e l a r g e i n d i v i d u a l weighing 500 kg.
By f i l l i n g i n some reasonable f i g u r e s based on
observation and measurement i t can be seen t h a t t h e average i n s p i r a t o r y f l o w r a t e (av. ;i)
i s 180 L min-'
, the
peak
a l v e o l a r p r e s s u r e (PA) i s 1 7 - 4 5 m H 2 0 and the p r e s s u r e t o be generated a t t h e mouth (I? mouth) i s 22-55 cmH20.
Figure
3 shows how t h e parameters o f v e n t i l a t i o n worked o u t i n t h i s
way vary between ' s m a l l ' ,
' s t a n d a r d ' and ' l a r g e ' i n d i v i d u a l s .
It i s important t o n o t e , however, t h a t t h e s e f i g u r e s are
only a p p l i c a b l e where t h e lungs a r e normal w i t h normal
..
v e n t i l a t i o n p e r f u s i o n r a t i o s (V/Q)
and a r e based on an
assumption t h a t t h e i n s p i r e d / e x p i r e d r a t i o (1:E) i s 1:2.
..
The a n a e s t h e t i s e d h o r s e ' s (V/Q)
r a t i o s are often
s e r i o u s l y d i s t u r b e d and during i n t e r m i t t e n t p o s i t i v e press u r e v e n t i l a t i o n t h e d i s t r i b u t i o n of t h e i n s p i r e d g a s i s u n l i k e l y t o be even.
Furthermore t h e 1:E r a t i o i s n o t
normally 1:2 s i n z e t h e i n s p i r e d volume i s u s u a l l y d e l i v e r e d , even when t h e animals a r e b r e a t h i n g spontaneously, i n about one second and n o t 1 - 6 7 seconds.
Thus i n s p i r a t o r y
f l o w r a t e s a r e u s u a l l y g r e a t e r than 200 L min-l
arid t h e
p r e s s u r e drop a c r o s s t h e airway r i s e s t o more t h a n 5.1
cmH20.
These f a c t o r s e x p l a i n t h e f a c t t h a t i n f l a t i o n
pressures required t o v e n t i l a t e horses a r e usually nearer
30 cmH20 than 2 2 cm H20.
A l a r g e number o f v e n t i l a t d r s a r e a v a i l a b l e f o r t h e tystanda:dlt
and ttsmalltti n d i v i d u a l s (Mushin e t a l . , 1980)
and many a r e e l a b o r a t e and capable of meeting s p e c i a l p a t i e n t needs. B v e n t i l a t o r designed f o r t h e ' s t a n d a r d '
66 kg i n d i v i d u a l
c a n be adapted f o r a s n a l l e r one and most o f t e n t h i s i s done by d i s p o s i n g o f t h e excess flow v i a an i n t e n t i o n a l leak.
However, c a r e f u l monitoring i s needed i f t h i s i s
done t o be c e r t a i n t h a t t h e p a t i e n t is g e t t i n g t h e r i g h t amour,t of flow.
The Flomasta (Weaver and Jones, 1981)
i s an example of an inexpensive v e n t i l a t o r which can be
used on individuals varying considerably in size.
In
inflating the lungs, the Flomasta acts as a discharging compliance. Reducing this compliance thus increases the cycling frequency and decreases the tidal volume.
It
should be noted, however, that increasing the resistance with this ventilator will decrease the frequency and increase the tidal volume. For the large individual some home-made systems have been developed and can work satisfactorily but they axe usually cumbersome and impractical.
They have, however,
served to study the ventilatory requirements (Weaver and Walley, 1975) and, in recent years, a few purpose-built coumercial large animal ventilators have become available:1.
The Narkovet-E Large Animal Anesthesia Control Center made by North American Drager.
This vent-
ilator has a pneumatically driven bellows and fine fluidic control.
It is time-cycled allowing the
number o f breaths per minute (B.P.M.) to be preset from 6 to 18.
In use, the maximum minute
volume has been found to be about 70 litres with a maximum tidal volume between 10 and 11 lites (Webb, 1982, personal communication).
In addition,
Dr. Webb has encountered difficulty in weaning patients off the ventilator since the lowest cycling rate is 6 B.P.M.
This feature has been improved
by the Narkovet E-2 model which incorporates electronic circuitry and the frequency cam be reduced to nearly zero if required.
156.
2.
The American Bird v e n t i l a t o r has been modified f o r use i n l a r g e animals.
I t i s a p r e s s u r e cycled
machine w i t h an o p t i o n a l p a t i e n t t r i g g e r f a c i l i t y which has been found u s e f u l i n weaning p a t i e n t s from t h e automatic v e n t i l a t i o n (Webb, 1982, p e r s o n a l communication).
I t does n o t , however, have a
s e p a r a t e r e s e r v o i r bag f o r spontaneous v e n t i l a t i o n .
3.
The Cambridge R e s p i r a t o r i s made by Sowring Engineering.
This v e n t i l a t o r u t i l i z e s a h y d r a u l i -
c a l l y a c t i v a t e d bellows t o compress gas i n a c y l i n d e r which ir. t u r n d i s p l a c e s a v a r i a b l e volume of gas f r o m t h e r e s e r v o i r / b r e a t h i n g bag contained i n the cylinder.
By means o f t h e a d j u s t a b l e
c o n t r o l s , c y c l i n g can be made t o occur according
t o time, p r e s s u r e , volume o r a combination o f t h e settings.
D i f f i c u l t y can a r i s e i n t h a t t h e s e t t i n g
o f one o o n t r o l may over-ride t h e s e t t i n g o f another and i t i s d i f f i c u l t t o a s s e s s t h e a c t u a l volume
.
d e l i v e r e d t o t h e animal.
The v e n t i l a t o r does,
however, have ample c a p a c i t y t o v e n t i l a t e l a r g e h o r s e s and c a t t l e .
I n conclusion, v e n t i l a t i o n can be c o n t r o l l e d i n a n i m a l s varying i n s i z e from l e s s t h a n 3 kg t o more
t h a n 500 kg
and when c o r r e c t l y c a r r i e d o u t , harmful e f f e c t s a r e minimized.
It can be done by i n t e r m i t t e n t manual compression
o f t h e r e s e r v o i r bag i n a s u i t a b l e b r e a t h i n g system o r by
157-
t h e use o f an automatic v e n t i l a t o r .
The c o s t of purpose
b u i l t v e n t i l a t o r s can be h i g h e s p e c i a l l y f o r t h e l a r g e i n d i v i d u a l s but t h e y are e s s e n t i a l i f c o n t r o l l e d v e n t i l a t i o n
i s t o be c a r r i e d o u t for long p e r i o d s . The v e n t i l a t i n g requirements f n r a n i m a l s with normal h e a l t h y lungs can be a s s e s s e d according t o s i z e but modi f i c a t i o n s need t o be made i n t h e presence of pathop h y s i o l o g i c a l cardio-pulmonary d i s t u r b a n c e s i n o r d e r t h a t t h e r i g h t type of v e n t i l a t i o n i s a p p l i e d t o each p a t i e n t .
REFWENCES
Jackson, D.E. (1 927). J o u r n a l of L a b o r a t o r y and C l i n i c a l Medicine l2, 998. Mushin, W.W., Rendell-Baker, L., Thompson, P.W. & Xapleson, U.W. (1980). Automatic V e n t i l a t i o n o f t h e Lungs, 3 r d e d i t i o n . B l a c k w e l l S c i e n t i f i c P u b l i c a t i o n s , Oxford. Weaver, B.M.Q. & Walley, R.V. J o u r n a l 1,No. 1 . Weaver, B.M.Q.
-
1 21 1 22.
a(
Jones, W.
(1975).
(19el).
Equine V e t e r i n a r y
V e t e r i n a r y F.ecord
108,
159-
Flg 2
Parameiers of Ventllaiim la MO r9 Animal I 12min-I
60
:.cycle
lasts
If I:E
.
1:2
I
f
x 5
-
12
-
1.67sec
vr Saa ml and enters lungs In 1.67 IIC
Pmouth
'* I'
-
--
PA + Pdrwrrossalrway PA + Vi x airway
17.45 +
X
17.45 + 5.1 22.55 cmH@
Fig 3
Parameters d Ventilatian for Small. Standard and Lam sized individuals W lkgl
small 3
asr
Stanhrd
66
8.
Large 500
60
40
16
12
16
500
uxa
2.5
50
3
2
0.5
1.25
286.5
1.7 ILarer ainkayl 1.67
im
1.9
24
6.4
10
11.45
7.4
10.8
a55
R
1.7