Veterinary Research Communications, ]1 (1987) 281-291 281 Geo Abstracts Ltd, Norwich - pr in ted in Eng]and

APNEIC OXYGENATION IN ANESTHETIZED PONIES AND HORSES

C. A. BLAZE 1 and N. E. ROBINSON 2

1Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan 48824, U.S.A.

2Departments of Physiology and Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan 48824

ABSTRACT

Blaze, C .A. and Rob inson , N . E . , 1987. Apneic o x y g e n a t i o n in a n e s t h e t i z e d ponies and ho r se s . Ve t e r i na ry Research Communications, 11(3) , 281-291

Apneic oxygenation was studied in six ponies for 30 minutes, and six horses for 10 minutes. Arterial blood was sampled at regular intervals for measure- ment of oxygen and carbon dioxide tensions (PaO 2 and PaCO 2) and calculation of alveolar-arterial oxygen tension difference (PAO2--PaO2). In both groups of animals, PaO 2 decreased rapidly during the first 3 minutes of apnea, then more slowly. Although the mean value was above 100 mmHg at 10 minutes, there was considerable inter-animal variability. Before apnea, PAO2--PaO 2 was slightly, but not significantly, larger in horses than in ponies and increased in both groups during the first 3 minutes of apnea, after which the increase was slower. There was no significant difference between ponies and horses up to 10 minutes, suggesting that PAO2--PaO 2 is independent of body size. In ponies, the PAO 2- PaO 2 did not change significantly between 10 and 30 minutes. Final PaO 2 could not be correlated with initial PaO 2 or initial PAO2--PaO 2. The rate of rise of PAO2--PaO 2 could not be predicted from baseline values. The rate of rise of PaCO 2 was similar and fairly constant in ponies and horses, and did not con- tribute to the rapid initial decrease in PaO 2. It appears that apneic oxygena- tion should not be used in the equine species, since it is impossible to predict in which animals the technique is safe for more than a few minutes.

INTRODUCTION

Apneic oxygenation means the oxygenation of arterial blood by flow of

oxygen down the trachea. Prerequisites for oxygenation of greater than a few

minutes duration are replacement of alveolar nitrogen with oxygen before apnea,

an unobstructed airway, oxygen as the ambient gas rather than room air, and

an adequate circulation.

Apneic oxygenation is not a new concept, having been studied extensively

by Draper and Whitehead (1944, 1949) and Draper et ai. (1949) in dogs, and by

Holmdahl (1956) in dogs and rabbits. Other workers studied the technique in

cats (Tenney, 1956), and in man (Enghoff et al., 1951; Frumin et al., 1959).

In humans, the technique has been used for procedures such as bronchoscopy,

but no reports could be found of its use in the equine species.

0165-7380/87/S03.50 (~) 1987 Geo Abstracts Ltd

282

A common su rg i ca l p r o c e d u r e in the horse is l a ryngotomy, usual ly with v e n -

t r i cu l ec tomy . D u r i n g th i s p r o c e d u r e , which is done in dorsal r e c u m b e n c y , the

e n d o t r a c h e a l t u b e is u s u a l l y removed to allow maximum su rg ica l access to the

l a r y n x . The most i m p o r t a n t problem caused by th i s is a dramatic decrease in a r -

t e r i a l oxygen t e n s i o n , to va lues as low as 35 mmHg (Blaze, pe r sona l obse r va t i on ) .

This s t u d y was d e s i g n e d to assess whether apne ic o x y g e n a t i o n can be used

in the equ ine species to p rov ide adequa te oxyge na t i on of a r t e r i a l blood.

MATERIALS AND METHODS

Six adu l t ponies and six adul t ho r se s , of both sexes and in good h e a l t h ,

were u s e d in th is s t u d y . Ponies weighed from 145 to 186 kg (mean 169 k g ) ,

and ho r se s weighed from 409 to 586 kg (mean 497 k g ) .

Af te r o v e r n i g h t f a s t i n g , ponies were p remedica ted with xy laz ine (0.5 mg / kg

i n t r a v e n o u s l y ) (Rompun , Bayve t Labora to r i e s , Shawnee, K a n s a s ) , the j u g u l a r

ve in was c a t h e t e r i z e d , and anes the s i a was i n d u c e d with a g iyce ry l gua iaco la te -

(Gecola te , Summit Hill Labora to r ies , Avalon, New Je r sey ) th iamyl (Biotal , Bio-

Ceut ic Labora to r i e s , St . Joseph , Missouri) m i x t u r e g iven i n t r a v e n o u s l y to effect .

Af te r i n t u b a t i o n with a cuf fed endo t r achea l t u b e , ponies b r e a t h e d 100% O 2

t h r o u g h a demand va lve (Hudson demand t u b e va lve , Model 5040, Hudson Oxygen

T h e r a p y Sales, Wadsworth, Ohio) , and a n e s t h e s i a was main ta ined with i n c r e m e n -

tal bo lus i n j ec t ions of the g l y c e r y l gu ia iaco la te - th iamyla l mix tu re . A p e r i p h e r a l

a r t e r y was c a t h e t e r i z e d and the c a t h e t e r c o n n e c t e d to an ane ro id manometer via

an e x t e n s i o n se t , and ponies were placed in dorsa l r e c u m b e n c y in a padded

V - t r o u g h .

A small a rea o f the v e n t r a l neck , j u s t a n t e r i o r to the thorac ic in le t , was

c l ipped and s c r u b b e d . A 10 gauge , 4 - inch needle was i n s e r t e d t h r o u g h the

sk in , be tween t r achea l r i n g s and into the t r achea l lumen. The e n d o t r a e h e a l

t u b e was s l i gh t l y moved a n t e r i o r l y so tha t the needle lumen was not o b s t r u c t e d .

The e x t e r n a l end of the needle was sealed with a 3-way s topcock and c onne c t e d

to r u b b e r t u b i n g which would d e l i v e r o x y g e n to the t r achea once apnea was i n -

duced . This s t age of p r e p a r a t i o n was u sua l l y completed within 45 minu tes of

i n d u c t i o n . Heart and r e s p i r a t o r y ra tes were r e c o r d e d , as well as mean a r t e r i a l

blood p r e s s u r e i n d i c a t e d by the ane ro id manometer . Body t e m p e r a t u r e was

measu red rec ta l ly with a cl inical m e r c u r y the rmomete r . When a n e s t h e t i c dep th

was j u d g e d to be adequa t e acco rd ing to accep ted methods of c h e c k i n g pa l pe b r a l

r e f l exes a n d v i ta l s i g n s , manual ven t i l a t ion was b e g u n by p r e s s i n g the b u t t o n

on the demand va lve to in f la te the l u n g s . The ponies were g iven at leas t I0

l a rge b r e a t h s , at approx imate ly 4 - second i n t e r v a l s , and a base l ine a r t e r i a l blood

sample was anaerob ica l ly d rawn from the a r t e r i a l c a t he t e r , sealed, and placed on

an ice s l u s h .

283

Succinylcholine chloride (Succostrin HP, E.R. Squibb and Sons, Inc.,

Princeton, New Jersey) (0.1 mg/kg intravenously) was injected and an infusion

of succinyleholine chloride begun immediately at 2.2 mg/kg/hr while manual

ventilation was continued. When muscle fasciculation ceased, manual ventilation

was stopped, an arterial blood sample was drawn, and simultaneously the 3-way

stopcock on the tracheal needle was turned to allow oxygen to flow into the

trachea at 2 I/min. The demand valve was removed from the endotracheal tube

to p r o v i d e an o p e n a i r w a y for t h e e s c a p e of e x c e s s g a s .

A r t e r i a l b lood s amp le s for b lood gas a n a l y s i s were d r a w n at 1," 2, 3, 4, a n d

5 m i n u t e s a f t e r t h e o n s e t of a p n e a , a n d t h e n e v e r y 5 m i n u t e s up to a n d i n -

c l u d i n g 30 m i n u t e s . The a n a e r o b i c a l l y d r a w n s amp le s were s e a l e d a n d p l a c e d on

ice u n t i l a n a l y z e d . D u r i n g a p n e a , i n c r e m e n t a l d o s e s of t h e a n e s t h e t i c m i x t u r e

were g i v e n , c o r r e s p o n d i n g to t h e r e q u i r e m e n t s of t h e p o n y b e f o r e a p n e a , a n d

v i t a l s i g n s were m o n i t o r e d . The s u c c i n y l c h o l i n e i n f u s i o n was t e r m i n a t e d at

a b o u t t h e 28 th m i n u t e . At 30 m i n u t e s , t h e d e m a n d v a l v e was r e p l a c e d on t h e

e n d of t h e e n d o t r a c h e a l t u b e , a n d t h e l u n g s were i n f l a t e d s e v e r a l t imes wi th

o x y g e n . S i m u l t a n e o u s l y , t h e 3 -way s t o p c o c k was t u r n e d to seal t h e e x t e r n a l

e n d of t h e t r a c h e a l n e e d l e . Mechan ica l v e n t i l a t i o n c o n t i n u e d u n t i l s p o n t a n e o u s

v e n t i l a t i o n r e s u m e d , u s u a l l y w i th in 4 to 5 m i n u t e s of t e r m i n a t i o n of t h e s u c c i n y l -

cho l ine i n f u s i o n .

H o r s e s were f a s t e d o v e r n i g h t , p r e m e d i c a t e d wi th x y l a z i n e (0.44 m g / k g i n t r a -

v e n o u s l y ) , a n d a n e s t h e s i a was i n d u c e d i n t r a v e n o u s l y wi th a g l y c e r y l g u a i a c o l a t e -

t h i amy la l m i x t u r e to e f f e c t . A c u f f e d e n d o t r a c h e a l t u b e was p l a c e d in t h e

t r a c h e a , a n d m e c h a n i c a l v e n t i l a t i o n was b e g u n wi th a h a l o t h a n e - ( H a l o t h a n e USP,

H a l o c a r b o n L a b o r a t o r i e s , H a c k e n s a c k , New J e r s e y ) o x y g e n m i x t u r e . A p e r i p h -

e r a l a r t e r y was c a t h e t e r i z e d a n d v i t a l s i g n s were m o n i t o r e d . H o r s e s we re p l a c e d

in d o r s a l r e c u m b e n c y a n d were v e n t i l a t e d for a minimum of 48 m i n u t e s b e f o r e

a p n e a was i n d u c e d .

A 1.5 cm d i a m e t e r p l a s t i c t u b e was p l a c e d a g a i n s t t h e n e c k , wi th one e n d at

t h e l eve l of t h e t h o r a c i c i n l e t , a n d a m a r k was made on t h e t u b e to c o r r e s p o n d

wi th t h e l eve l of t h e i n c i s o r t e e t h . Th i s t u b e would be u s e d l a t e r to d e l i v e r

o x y g e n to t h e h o r s e d u r i n g a p n e a . When a n e s t h e t i c d e p t h was s t a b i l i z e d , a

b a s e l i n e a r t e r i a l b lood sample was d r a w n .

S u c c i n y l c h o l i n e c h l o r i d e (0 .1 m g / k g i n t r a v e n o u s l y ) was i n j e c t e d a n d an i n -

f u s i o n of t h i s d r u g b e g a n at 2.2 m g / k g / h r . Mechan ica l v e n t i l a t i o n c o n t i n u e d ,

a n d at t h e time when musc le f a s c i c u l a t i o n s c e a s e d , an a r t e r i a l b lood sample was

d r a w n , v e n t i l a t i o n was s t o p p e d , a n d t h e p r e m a r k e d p l a s t i c t u b e wi th o x y g e n

f lowing t h r o u g h i t a t 15 I / ra in , was q u i c k l y i n s e r t e d i n t o t h e e n d o t r a c h e a l t u b e .

A r t e r i a l b lood samples were d r a w n at t h e same t ime i n t e r v a l s as in t h e p o n i e s .

The s u c c i n y l c h o l i n e i n f u s i o n was s t o p p e d a t l0 m i n u t e s , t h e p l a s t i c t u b e

284

c a r r y i n g 0 2 was removed from the e n d o t r a e h e a l t u b e , and manual ven t i l a t i on

was r e s u m e d . All samples were ana lyzed wi th in an hour . Blood gas va lues

were c o r r e c t e d to body t e m p e r a t u r e . Alveolar o x y g e n t ens ion (PAO 2) was ca l -

c u l a t e d from the formula . PAO 2 = [(PB--PH2 O) x FIO2--PaCO2], where PB is ba r o -

metr ic p r e s s u r e , PH20 is s a tu r a t ed vapor p r e s s u r e of water, FIO 2 is f rac t iona l con-

e e n t r a t i o n of i n s p i r e d o x y g e n and PaCO 2 is a r t e r i a l ca rbon dioxide t e n s i o n .

A l v e o l a r - a r t e r i a l o x y g e n t ens ion d i f fe rence (PAO2--PaO 2) was t hen de t e r mi ne d .

Data were ana lyzed by factor ia l ana ly s i s of va r i a nc e . When s i gn i f i c a n t F

va lues were o b t a i n e d , factor means were compared u s i n g T u k e y ' s ~ s t a t i s t i c .

RESULTS

F i g u r e s 1 and 2 i l l u s t r a t e the changes in a r t e r i a l oxygen and c a r b o n dioxide

t e n s i o n s in ponies and h o r s e s , r e s p e c t i v e l y . Ar t e r i a l oxygen t ens ion dec reased

PONIES

• P o 0 2 30O

,oor l . . . * * "10 so~'w~PaCOz I I i i 0 5 I0

Minutes

40

HORSES 400 '1

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~50

40

O

ZO

Io

Fig. I . Changes in a r t e r i a l o x y g e n a n d c a r b o n dioxide t e n s i o n s d u r i n g 10 minu tes of apne ic o x y g e n a t i o n in pon ies .

Fig. 2. Changes in a r t e r i a l o x y g e n and c a r b o n dioxide t e n s i o n s d u r i n g I0 minu tes of apne ic oxyge na t i on in ho r se s .

r a p i d l y in bo th pon ies and hor ses d u r i n g the f i r s t 3 minu tes , t h e n dec reased

more slowly up to l0 minu t e s . Al though the mean value was above 100 mmHg in

both g r o u p s of animals at l0 minu tes of apnea , t he r e was c o n s i d e r a b l e i n t e r -

animal v a r i a b i l i t y . The lowest va lues of PaO 2 were 76.1 mmHg and 53.7 mmHg

i n pon ies and in h o r s e s , r e s p e c t i v e l y . In pon ies , c a rbon dioxide t e n s i o n i n -

c r e a s e d at a r a t e of 5.31_+0.31 mmHg/min d u r i n g the f i r s t l0 minu tes of apnea .

I n h o r s e s , the r a t e of i n c r e a s e was 5.46+_0.64 mmHg/min. The h i g h e s t value of

PaCO 2 a f t e r l0 minu tes was 98.5 mmHg in a p o n y a nd 106.4 mmHg in a ho r se .

Ca rbon dioxide t e n s i o n i n c r e a s e d more slowly d u r i n g the r e m a i n i n g 20 minutes

(3.67_+0.25 mmHg/min) . The mean va lue of PaCO 2 at 30 minutes was 146.6-+7.4 mmHg,

285

The decrease in pH was similar in both groups of animals; pH averaging 7.15_ +

0.01 and 7.19+0.03 at 10 minutes in ponies and horses, respectively. Mean pH

of ponies at 30 minutes of apnea was 6.99_+0.02. Table 1 shows blood gas values

and alveolar-arterial oxygen tension difference in ponies during 30 minutes of

apnea. Table 2 shows acid base status, and oxyhemoglobin saturation in horses

during 10 minutes of apneic oxygenation.

F igu re s 3 and 4 i l l u s t r a t e the a l v e o l a r - a r t e r i a l o x y g e n t e n s i on d i f f e r ences

d u r i n g 10 minu te s of apnea in ponies and h o r s e s . A l though the PAO2--PaO 2

before apnea was s l i gh t l y l a r g e r in ho r ses t han in pon ies , the d i f f e r ence was not

s i g n i f i c a n t . The re was a r ap id i n c r e a s e in PAO2-PaO 2 d u r i n g the f i r s t 3 minu tes

in both g r o u p s , and the PAO2-PaO 2 t hen i n c r e a s e d more slowly a f t e r 3 minu t e s .

At no time up to 10 minu tes was t he r e a s i gn i f i c a n t d i f f e rence in PAO2-PaO 2 be -

tween ponies and h o r s e s . PAO2-PaO 2 in ponies r emained c o n s t a n t be tween 10

and 30 minu t e s .

DISCUSSION

This study suggests that it is not possible to maintain acceptable blood

gases for more than a few minutes in anesthetized ponies and horses using the

technique of apneic oxygenation. The mean arterial oxygen tension remained

above 100 mmHg for 10 minutes in both ponies and horses. There was, however,

a great deal of inter-animal variability in the PaO 2 after 10 minutes of apnea. In

3 of 6 horses, PaO 2 was less than 70 mmHg at I0 minutes. This variability was

also apparent in ponies, one having a PaO 2 of 107.8 mmHg at 10 minutes, from a

baseline value of 443.2 mmHg, and another having a PaO 2 of 188.7 mmHg at 10

minutes, from a baseline value of 332.7 mmHg, PaO 2 values lower than 70 mmHg

were seen in one horse at 3 minutes of apnea, and in one pony at 15 minutes of

apnea, The lowest value of PaO 2 in a pony was 66.6 mmHg at 20 minutes of

apnea, suggesting that apneic oxygenation may be useful for periods up to 15

minutes in some ponies. However, pH at this time was 7.039 and PaCO 2 was

122.6 mmHg. This great variability in PaO 2 between animals makes lhe technique

of apneic oxygenation unsuitable for routine use.

Because the increase in PaCO 2 was similar and fairly constant in both ponies

and horses, the magnitude of hypoxemia was not determined primarily by the rate

of carbon dioxide accumulation in the alveoli. The primary determinant of the

degree of hypoxemia was the initial PAO2-PaO 2, and its rate of increase. In

both ponies and horses, the rate of increase was rapid during the first 3 minutes,

then became slower after that time. The PAO2-PaO 2 actually decreased slowly

from 15 to 30 minutes, although this decrease was not significant. The initial

PA02-PaO2 was slightly, but insignificantly, larger in, horses than in ponies,

but the rate of increase was similar in both groups, suggesting that this

286

The dec rease in pH was similar in both g r o u p s of animals ; pH a v e r a g i n g 7 .15- +

0.01 and 7.19-+0.03 at 10 minutes in ponies and ho r se s , r e s p e c t i v e l y . Mean pH

of pon ies at 30 minu tes of apnea was 6.99-+0.02. Table 1 shows blood gas va lues

a n d a l v e o l a r - a r t e r i a l o x y g e n t e n s i o n d i f fe rence in ponies d u r i n g 30 minu tes of

apnea . Table 2 shows acid base s t a t u s , and oxyhemoglobin s a tu r a t i on in horses

d u r i n g 10 minu tes of apne ic o x y g e n a t i o n .

F i g u r e s 3 and 4 i l l u s t r a t e the a l v e o l a r - a r t e r i a l o x y g e n t ens ion d i f f e rences

d u r i n g 10 minu tes of apnea in ponies and h o r s e s . Al though the PAO2-PaO 2

before apnea was s l i gh t l y l a r g e r in horses t h a n in pon ies , the d i f f e rence was

not s i g n i f i c a n t . The re was a r ap id i nc r ea se in PAO2-PaO 2 d u r i n g the f i r s t 3

minu tes in both g r o u p s , and the PAO2-PaO 2 t h e n i n c r e a s e d more slowly a f te r 3

m i n u t e s . At no time up to 10 minutes was the re a s ign i f i can t d i f fe rence in

PAO2-PaO 2 be tween pon ies and ho r se s . PAO2-PaO 2 in ponies remained c o n s t a n t

between i0 and 30 minutes.

DISCUSSION

This s t u d y s u g g e s t s tha t it is not poss ib le to mainta in acceptab le blood

gases for more t han a few minutes in a n e s t h e t i z e d ponies and horses u s i n g the

t e c h n i q u e of apne ic o x y g e n a t i o n . The mean a r t e r i a l oxygen t ens ion remained

above 100 mmHg for 10 minu tes in both ponies a nd ho r se s . The r e was, how-

e v e r , a g rea t deal of i n t e r - a n i m a l va r i ab i l i t y in the PaO 2 a f te r 10 minu tes of

apnea . In 3 of 6 h o r s e s , PaO 2 was less t h a n 70 mmHg at 10 minu te s . This

v a r i a b i l i t y was also a p p a r e n t in ponies , one h a v i n g a PaO 2 of 107.8 mmHg at 10

m i n u t e s , from a base l ine va lue of 443.2 mmHg, and a n o t h e r h a v i n g a PaO 2 of

188.7 mmHg at 10 m i n u t e s , from a base l ine va lue of 332.7 mmHg. PaO 2 va lues

lower t h a n 70 mmHg were seen in one horse at 3 minu tes of apnea , a nd in one

pony a t 15 min tues of apnea . The lowest va lue of PaO 2 in a pony was 66.6

mmHg at 20 minu tes of apnea , s u g g e s t i n g tha t apneic oxyge na t i on may be use fu l

for pe r iods up to 15 minu tes i n some p ° n i e s . However, pH at th is time was

7.039 and PaCO 2 was 122.6 mmHg. This g rea t va r i ab i l i t y in PaO 2 be tween

animals makes the t e c h n i q u e of apneic o x y g e n a t i o n u n s u i t a b l e for r o u t i n e use .

Because the i n c r e a s e in PaCO 2 was similar and fa i r ly c o n s t a n t in both

pon ies a n d h o r s e s , the magn i tude of hypoxemia was de te rmined by the

ra t e of i n c r e a s e of PAO2-PaO 2, and not p r i m a r i l y by the ra te of ca rbon

d i o x i d e accumula t ion i n the a lveol i . In both ponies and ho r se s , t h e

r a t e of i n c r e a s e was r ap id d u r i n g the f i r s t 3 m i n u t e s , t h e n b e c a m e

s lower a f t e r tha t t ime. The PAO2-PaO 2 a c t u a l l y d e c r e a s e d s l o w l y f r o m

15 to 30 m i n u t e s , a l t h o u g h th i s decrease was not s ign i f i can t . The i n i t i a l

PAO2-PaO2 was s l i gh t ly , bu t i n s i g n i f i c a n t l y , l a r g e r in ho r ses t h a n in pon ies ,

bu t the r a t e of i n c r e a s e was s imilar in both g r o u p s , s u g g e s t i n g t h a t t h i s

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7.7

+-1

.26

9

3.8

+1

.72

" 8

9.6

+-2

.36

*

85

.5+

-3.1

9"

85

.2_

+2

.35

*

85

.6+

1.8

9.

PA

O2

-PaO

2 m

mH

g

29

5.6

-+1

6.9

4

53

.3+

-26

.8*

489.3

-+19.7

*

51

6.0

+-1

2.8

" 5

10

.2+

-12

.7"

501.0

+10.2

*

482.0

_+

II.0

*

pH

7

.49

+-0

.03

7

.26

+_

0.0

1"

7

.15

+_

0.0

1"

7

.10

+_

0.0

1"

7

.05

_+

0.0

2

*

7.0

2+

_0

.01

" 6

.99

+0

.02

*

PaC

O 2

mm

Hg

36

.4+

_2

.6

69.8

+3.5

*

89.5

+3.1

*

103.7

+3.8

*

119.2

+4.1

*

130.0

+4.3

*

14

6.6

+7

.4"

HC

O 3

- m

Eq/L

2

7.7

+-0

.6

29

.2_

+1

.0"

29.1

+-0

.8"

29

.7+

_0

.7

*

30.5

+-0

.8*

30

.8+

_1

.0"

32

.3_

+1

.2

*

B.E

. m

Eq/L

4

.0+

1.0

1

.5_

+0

.7

0.0

2+

0.7

*

-1.7

+1

.2"

-1.9

-+0

.6"

-2.1

+-1

.3"

-2.1

_+

1.5

*

PaO

2

PaC

O 2

PA

O 2

- P

aO 2

% S

AT

.

HC

O 3

-

B.E

.

= a

rte

ria

l o

xy

gen

te

nsi

on

.

-- a

rte

ria

l c

arb

on

dio

xid

e t

en

sio

n.

= a

lve

ola

r-a

rte

ria

l o

xy

ge

n d

iffe

ren

ce

.

= p

er

ce

nt

sa

tura

tio

n o

f h

em

og

lob

in.

= s

eru

m b

ica

rbo

na

te.

= b

ase

e

xc

ess

.

= i

nd

ica

tes

sig

nif

ica

nt

dif

fere

nc

e f

rom

b

ase

lin

e v

alu

es.

CO

288

g r a d i e n t is i n d e p e n d e n t of body size. The f inal PaO 2 was not co r re l a t ed with

the in i t i a l PAO2-PaO 2, or with the in i t ia l PaO 2, making it impossible to p red i c t

in which ho r se s apne ic o x y g e n a t i o n can be u sed safe ly .

TABLE 2

Acid-base status, and oxyhemoglobin saturation, during minutes of apneic oxygenation in six horses (mean ± S.E.)

Time in minu tes a f te r i n d u c t i o n of apnea

0 5 i0

pH 7.51±0.03 7.25±0.03* 7.19±0.03"

HCO 3 25.0±0.63 29.9±0.79* 30.9±0.83*

BE 2.45±0.60 1.56±0.88 0.5±0.01"

% S a t u r a t i o n 99.8±0.06 93.1±2.41" 89.4±3.02*

* Ind ica te s s i g n i f i c a n t l y d i f f e r en t from base l ine va lue .

It was e x p e c t e d tha t the ra te of i n c r e a s e of PAO2-PaO 2 would be c ons i s -

t e n t l y g r e a t e d in ho r se s t han in pon ies , l e ad ing to a more r a p i d deve lopment of

hypoxemia . It is known tha t a l a rge PAO2-PaO 2 g r a d i e n t develops soon a f te r

i n d u c t i o n in ho r ses and remains fa i r ly c o n s t a n t d u r i n g a n e s t h e s i a , whether

ven t i l a t i on is s p o n t a n e o u s or con t ro l led (Hall et a l . , 1968; Gillespie et a l . , 1969;

Hall, 1971). Recumbency alone can cause a PAO2-PaO 2 g r a d i e n t to develop in

pon ie s , e v e n when u n a n e s t h e t i z e d (Hall, 1984). It was expec t ed tha t func t iona l

r e s i d u a l capac i ty would dec rease more r ap id ly and tha t ven t i l a t ion pe r f u s i on

600

5OO I

E E 400

50(~

PONIES

I I 0 5 I0

M inutes

80 600-

"tO

0

50

40

500- :I: "5

4 0 0 -

I 0 5

Minutes

HORSES

IO

30

"tO

6O "O o

50

40

Fig. 3. A l v e o l a r - a r t e r i a l o x y g e n t en s ion d i f f e r e n c e dur ing 10 minu tes of apne ic o x y g e n a t i o n in pon ies .

Fig. 4. A lveo l a r - a r t e r i a l o x y g e n tens ion d i f ference d u r i n g I0 minutes of apne ic oxyge na t i on in ho r se s .

289

inequalities would be greater in larger and heavier horses. There was consid-

erable variability in the rate of increase of the PAO2-PaO 2 gradient and in the

baseline PAO2-PaO 2 within each group of animals. The rate of rise of PAO 2-

PaO 2 could not be predicted from the baseline value.

An important determinant of the duration of apnea before gross hypoxemia

occurs is the composition of the alveolar gas. If alveoli are filled with air,

alveolar PO 2 decreases rapidly towards the mixed venous PO 2, which itself is

declining continuously as the PaO 2 decreases. Gross hyp0xia and hypoxemia

occur in about 90 seconds. However, when the lungs are ventilated with oxygen

for periods greater than 30 minutes before apnea is induced, most of the nitro-

gen is eliminated from the body, and PAO 2 does not reach inadequate levels for

a longer period. When oxygen diffusing across the alveolar capillary membrane

is being replaced constantly by a stream of oxygen flowing down the trachea,

no nitrogen is added to the alveoli from the ambient gas, and PAO 2 decreases

only as fast as the PACO 2 increases. This greatly prolongs the duration of

apnea before gross hypoxia develops. The deficiency of nitrogen in the alveoli

tends to increase the likelihood of alveolar collapse, which may contribute to the

rapid increase in PAO2-PaO 2 gradient which occurred in this study.

The mean increase in PaCO2.during I0 minutes was 5.31+_0.31 mmHg/min in

ponies and 5.34_+0.64 mmHg/min in horses, which compares well with values re-

ported by Hubbell and Muir (1985) during several minutes of apnea after con-

trolled ventilation. This slow and steady rise results from the rate of pro-

duction of CO 2 and the capacity of the body stores (Nahas and Verosky, 1966;

Nunn, 1969). Although the decrease in pH associated with hypercarbia would

displace the oxyhemoglobin dissociation curve to the right (Bohr effect), this

should not compromise oxygen delivery to the tissues. Even at a pH of 7.19,

hemoglobin in arterial blood was still 89.4% saturated in horses, and the lowest

degree of arterial hemoglobin saturation in ponies was 85.2% at a pH of 7.02.

The Bohr effect of acidosis facilitates the delivery of oxygen to the tissues.

The increase in cardiac output that is normally associated with hypercarbia

should also improve perfusion of the tissues and increase oxygen delivery.

Therefore, apneic oxygenation should not compromise tissue oxygen delivery.

Flow rates of 0 2 into the trachea during apnea were set at 2 1/min for

ponies and 15 I/min for horses, which were adequate for the metabolic 0 2 re-

quirement, which Thomas (1981) reported as 2.2-+0.3 ml/kg/min in horses and

Parks (1983) reported as 9.2+-1 ml/kg/min in ponies. The flow rate for ponies

was set to exceed metabolic 0 2 requirement. The flow rate in horses was that

which is generally recommended for oxygen insufflation during recovery from

general anesthesia.

290

A major p rob lem with apne ic o x y g e n is the r e t e n t i o n of CO 2 a n d p r o -

g r e s s i v e r e s p i r a t o r y a c i d o s i s . The mean PaCO 2 of 86.6 mmHg o b s e r v e d in

h o r s e s at 10 minu te s of a p n e a would no t be u n u s u a l in t hose a n e s t h e t i z e d and

b r e a t h i n g s p o n t a n e o u s l y . However , h y p e r c a r b i a may c o n t r i b u t e to a r r h y t h m i a ,

especially during halothane anesthesia (Eberly et al., 1968). Even though mean

oxygenation in ponies was adequate at 30 minutes (84.6 mmHg), the concomitant

hypercarbia and possibility of severe arrhythmia may be the limiting factor to

the safe duration of apnea. The narcotic effect of high CO 2 in horses is not

known, but it is possible that in ponies, where the mean PaCO 2 was above

90 mmHg at 15 minutes of apnea, the requirement for anesthetic agent may have

been considerably reduced.

Apnea was maintained in both groups of animals by infusion of succinyl-

choline. A previous report (Hildebrand and Hewitt, 1983) has demonstrated a

rise in body temperature and metabolic acidosis associated with succinylcholine

infusion in ponies. These changes were not observed in this study.

ACKNOWLEDGMENTS

The a u t h o r s wish to t h a n k Dr . A. O r t e n b u r g e r , Dr . D. Daun t , and

Mr. S. Ba i ley for t h e i r a s s i s t a n c e . Th is s t u d y was f u n d e d b y the A g r i c u l t u r a l

E x p e r i m e n t S t a t i on of Michigan S ta te U n i v e r s i t y .

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