Impaired arterial oxygenation is the result of abnormal gas

Transcription

1 AUSTRALIA S PREMIER VETERINARY SCIENCE TEXT Effet of reuing inspire oxygen onentrtion on oxygention prmeters uring generl nesthesi in horses in lterl or orsl reumeny E Uquills, CM Drt, NR Perkins n AJ Drt * Ojetive To ompre the effets of two onentrtions of oxygen elivere to the nestheti rething iruit on oxygention in mehnilly ventilte horses nesthetise with isoflurne n positione in orsl or lterl reumeny. Methos Selete respirtory prmeters n loo ltte were mesure n oxygention inies lulte, efore n uring generl nesthesi, in 24 lterlly or orslly reument horses. Horses were rnomly ssigne to reeive 100% or 60% oxygen uring nesthesi. All horses were nesthetise using the sme protool n intermittent positive pressure ventiltion (IPPV) ws ommene immeitely following nestheti inution n enotrhel intution. Arteril loo gs nlysis ws performe n oxygention inies lulte efore premeition, immeitely fter inution, t 10 n 45 min fter the ommenement of mehnil ventiltion, n in reovery. Results During nesthesi, the rteril prtil pressure of oxygen ws equte in ll horses, regrless of position of reumeny or the onentrtion of oxygen provie. At 10 n 45 min fter ommening IPPV, the rteril prtil pressure of oxygen ws lower in horses in orsl reumeny ompre with those in lterl reumeny, irrespetive of the onentrtion of oxygen supplie. Bse on oxygention inies, pulmonry funtion uring generl nesthesi in horses ple in orsl reumeny ws more ompromise thn in horses in lterl reumeny, irrespetive of the onentrtion of oxygen provie. Conlusion During generl nesthesi, using oxygen t onentrtion of 60% inste of 100% mintins equte rteril oxygention in horses in orsl or lterl reumeny. However, it will not reue pulmonry funtion normlities inue y nesthesi n reumeny. Keywors horses; inhltion nesthesi; inspire oxygen onentrtion; oxygention inies; pulmonry shunts; reumeny; ventiltion n perfusion rtio Arevitions P A =(F i (P P H2O ) (P C /R)), lveolr gs eqution; C, ron ioxie; DAP, istoli rteril pressure F, onentrtion of oxygen elivere to the nestheti rething iruit; F i, inspire oxygen onentrtion; HR, hert rte; IPPV, intermittent positive pressure ventiltion; IPPV10, 10 min fter IPPV ommene; IPPV45, 45 min fter IPPV *Corresponing uthor. Reserh n Clinil Trils Unit, University Veterinry Tehing Hospitl Cmen, Shool of Veterinry Siene, University of Syney, 410 Weromi Ro, Cmen, New South Wles 2570, Austrli; nrew.rt@syney.eu.u Shool of Veterinry Siene, The University of Queensln, Gtton, Queensln, Austrli Impire rteril oxygention is the result of norml gs exhnge. It is well-reognise omplition uring inhltion nesthesi in horses tht my le to reue tissue oxygention n ontriute to nestheti moriity. 1,2 Both reumeny n inhltion nesthesi ffet the mthing of ventiltion n perfusion, whih is the most importnt eterminnt of pulmonry gs exhnge. 3 6 Horses ple in orsl reumeny uring inhltion nesthesi evelop more severe ventiltion perfusion rtio (V/Q) normlities thn horses ple in lterl reumeny, in prt euse of teletsis, whih retes right-to-left shunts n ultimtely poor rteril oxygention. 1,7,8 In humns, n some niml speies, there is orreltion etween high inspire frtion of oxygen (F i ) n n inrese likelihoo of V/Q normlities uring inhltion nesthesi. 9 High inspire F i my le to wshout of nitrogen from the lveoli n use loss of lveolr pteny s oxygen iffuses into the pulmonry vsulture. 9 Computer moelling suggests lveoli ventilte with high inspire oxygen onentrtion will evelop teletsis muh erlier thn those ventilte with low inspire oxygen onentrtion. 10 Reent stuies hve shown enefits of using lower F i in humns n nimls A numer of stuies hve evlute the effet of reuing F i on V/Q n rteril oxygen prtil pressure (P ) uring inhltion nesthesi in horses positione in orsl reumeny or in lterl reumeny However, none of those stuies ompre the effets of reuing F i on the V/Q reltionship in horses positione in lterl reumeny to those positione in orsl reumeny. It is possile tht in horses sorption teletsis my exerte teletsis use y other ftors suh s ltere hest wll mehnis n ompression of the lungs, n further ontriute to reution in funtionl resiul pity. 2,8 ommene; MAP, men rteril pressure; OTIs, oxygen tension se oxygention inies; P C, rteril prtil pressure of ron ioxie; P A P, lveolr rteril oxygen tension ifferene; P A P /P, respirtory inex; P A, lveolr prtil pressure; P, rteril prtil pressure of oxygen; P / F i, rteril to inspire oxygen tension rtio; P /P A, rteril to lveolr oxygen tension rtio; PostIND, immeitely fter nestheti inution; PrePRE, immeitely efore premeition; Qs/Qt, shunt loo flow/totl ri output; REC, 10 min fter spontneous respirtion ommene n isonnetion from the nestheti mhine in reovery; RR, respirtory rte; S, rteril hemogloin oxygen sturtion; SAP, systoli rteril pressure; V/ Q, ventiltion perfusion rtio Aust Vet J 2018;96:46 53 oi: /vj Austrlin Veterinry Journl Volume 96 No 1-2, Jnury/Ferury Austrlin Veterinry Assoition

2 The im of the present stuy ws to ompre the effets of two onentrtions of oxygen, elivere to the rething iruit (F ), on the oxygention in mehnilly ventilte horses nesthetise with isoflurne n positione in orsl or lterl reumeny. We hypothesise tht hnging the onentrtion elivere to the rething iruit from mximum F = 1.0 to F = 0.6 woul reue V/Q normlities n improve oxygention, ut not use hypoxemi, irrespetive of the position of reumeny. Mterils n methos The t for nlysis in the urrent stuy were ollete from horses while they were nesthetise for two seprte experimentl stuies performe for other resons. A totl of 24 horses were use in the two stuies: 16 horses were nesthetise one n 8 horses were nesthetise twie, 2 weeks prt, n this provie 32 sets of t. The t for the horses in orsl reumeny were ollete from 16 horses use in n experimentl rthrosopy stuy n inlue 12 gelings n 4 mres ge yers (men SD), weighing kg (men SD). The t for the horses in lterl reumeny were ollete from 8 horses use in n experimentl woun heling stuy n inlue 6 gelings n 2 mres, ge yers (men SD), weighing kg (men SD). The woun heling stuy require horses to e nesthetise on two osions, 2 weeks prt, whih me up the t set for 16 nesthetis performe in lterl reumeny. All horses were Stnrres n the helth sttus in oth groups ws ssesse y mens of physil exmintion n omplete loo ount n serum iohemistry. All vlues were within referene rnges. All the stuies were pprove y the institution s niml ethis ommittee. Stuy esign Horses were rnomly ssigne to one of two onentrtions of oxygen (F = 1.0 or 0.6) in one of two reumeny positions (lterl or orsl). Rnomistion (F = 1.0 or 0.6) ws hieve y pulling numers out of ht, suh tht there were 8 horses ssesse using eh omintion of oxygen onentrtion n position of reumeny. Prior to surgery loo smple from the roti rtery ws ollete through 21G neele into heprinise 1 ml loo gs syringe. Following septi plement of 14G, 14 m theter (BD Angioth, Beton Dikenson, NSW, Aust) into the left jugulr vein, gentmiin sulfte (6.6 mg/kg IV) n phenylutzone (4.4 mg/kg IV) were ministere. Proine peniillin (22 mg/kg, IM), tetnus ntitoxin (SC) n tetnus toxoi (IM) were lso given. At 5 min prior to inution of nesthesi, ll nimls were premeite with xylzine hyrohlorie (0.5 mg/kg IV) n methone (0.1 mg/kg IV). Anesthesi ws inue with guifenesin ministere to effet followe y olus of thiopentone (5 mg/kg, IV). Anestheti inutions were performe using tilt tle n horses were ple in right lterl reumeny. Immeitely following inution nother rteril loo smple ws ollete from the greter mettrsl rtery n the horses were intute with uffe orotrhel tue of 26, 28 or 30 mm internl imeter, hosen for est fit. The orotrhel tue ws onnete to lrge niml nesthesi irle-type rething system n ventiltor (SurgiVet DVH 1000 Lrge Animl Ventiltor, Smiths Meil, Dulin, OH, USA) n mehnil ventiltion ommene immeitely. The inspirtory flow on the ventiltor ws set to eliver til volume of pproximtely 15 ml/kg, whih ws set y using the sle on the ventiltor ellows, to mintin n inspirtory pressure etween 20 n 25 mmhg. Respirtory rte ws set t 6 reths/min. The rething system, inluing ventiltor ellows, ws prime with either 100% oxygen or 60% oxygen in ir, prior to inution of nesthesi. For oth F, totl fresh gs flow of 5 L/min ws use to fill the system n ir n oxygen flow meters were juste to eliver the respetive F. A gs nlyser (Dtsope Gs Moule II, Future Helth Conepts, Snfor, FL, USA) onnete to the ommon gs outlet n the Y-piee of the rething iruit ws use to just the F n to onfirm ompletion of priming of the iruit, respetively. For mintenne of nesthesi, isoflurne ws use, elivere from preision vporiser (Isoflurne Te 3, DLC Austrli, VIC, Aust) in oxygen t the ssigne F (either 1.0 or 0.6) n t the sme flow s use to prime the system. F i, entil ron ioxie (C ) n isoflurne onentrtion were mesure ontinuously using the gs nlyser (Dtsope Gs Moule II) onnete y smpling line to the Y-piee. Following onnetion of the nestheti rething iruit, horses ssigne to orsl reumeny were repositione n horses ssigne to lterl reumeny were left in situ. Hert rte (HR), eletroriogrm, hemogloin oxygen sturtion (S ), expire onentrtion of C n isoflurne, inspire onentrtion of oxygen, n rteril loo pressure n oy temperture were ontinuously mesure n isplye using multiprmeter vitl signs monitoring evie (Dtsope Pssport 2, Sn Jun Cpistrno, CA, USA) with gs nlyser. Boy temperture ws otine using temperture proe inserte 2 m into the retum. A 20G, 4.8 m theter ple septilly into the left trnsverse fil rtery, t the level of the lterl nthus of the eye, n onnete to pressure trnsuer n multi-prmeter vitl signs monitoring evie ws use for mesuring rteril loo pressure n olleting rteril loo smples. An nesthesi reor ws kept n the following prmeters were reore t 5-min intervls: HR, respirtory rte (RR), systoli (SAP), istoli (DAP) n men (MAP) rteril loo pressure, en-til C, inspire onentrtion of oxygen (F i ), en-til isoflurne, rteril hemogloin oxygen sturtion (S ) n retl temperture. The en-til isoflurne onentrtion ws juste oring to ptient nee, whih ws etermine using physil signs n riopulmonry prmeters n responses to surgil stimultion. All horses reeive IV Hrtmnn s solution t rte of 10 ml/kg per h throughout the proeure n outmine hyrohlorie t rte of 2 5 μg/kg per min if neee to mintin MAP of t lest 70 mmhg. All horses were mintine nesthetise for minimum of 50 min. At the en of nesthesi the horses were wene off the mehnil ventiltor y reuing the RR to 2 reths/min. One spontneous ventiltion h returne the horses were isonnete from the nestheti system, move to eite reovery stll n ple in right lterl reumeny. Supplementl oxygen (15 L/min) ws provie through nsotrhel tue ple one the orotrhel tue h een remove n fter the lst rteril loo smple h een ollete Austrlin Veterinry Assoition Austrlin Veterinry Journl Volume 96 No 1-2, Jnury/Ferury

3 Arteril loo smples were ollete from eh horse t five time points n inlue roti rteril smple ollete immeitely efore premeition (PrePRE), mettrsl rtery smple immeitely post inution n prior to onnetion to the nestheti irle system (PostIND) n fil rtery smples t 10 (IPPV10) n 45 min (IPPV45) fter ommening intermittent positive pressure ventiltion (IPPV), n uring reovery one spontneous ventiltion h returne n the horses h een isonnete from the nesthesi system for 10 min (REC). A loo gs nlyser lirte for equine loo ws use to mesure the rteril prtil pressure of oxygen (P ), the rteril prtil pressure of C (P C ), ph, onentrtion n oxygen sturtion, n ltte onentrtion. All loo gs results were orrete to the oy temperture of the respetive niml t the time of smple olletion. Oxygention ws lso ssesse y lulting the lveolr rteril oxygen tension ifferene (P A P ), the respirtory inex P A P /P, rteril to lveolr oxygen tension rtio P / P A n the rteril to inspire oxygen tension rtio P /F i. The P A P ws lulte using the lveolr gs eqution P A =(F i (P P H2O ) (P C /R)),whereRistherespirtory exhnge rtio, stnrise to 0.8, P is the tmospheri pressure (754 mmhg t Cmen, NSW, Aust) n P H2O is the sturte vpour pressure of wter t oy temperture (47 mmhg t 37 C). 21 Sttistil nlysis Generl liner moels were use to nlyse the t, with seprte moels for eh outome. Eh moel inlue the F i otine with F = 1.0 n 0.6, position (lterl, orsl), the oxygen/position intertion, time n rnom effet oing for horse ientity to ount for the lk of inepenene ssoite with repete mesures for eh horse. Resiuls were inspete for moel fit. Follow-up tests on mrginl mens were then performe to investigte pirwise omprisons etween group mens. All omprisons use stnr error terms erive from the generl liner moels n involve t-tests with Benjmini-Hoherg justment to orret for the risk of elevte type I error use y multiple omprisons. Anlyses were performe in STATA using n lph threshol of Results The P,P C, loo ltte, HR n MAP t tken from horses t the five smpling time points (PrePRE, PostIND, IPPV10, IPPV45 n REC), while rething F (either 0.6 or 1.0) n in orsl or lterl reumeny, re presente in Tle 1. Similrly, the oxygen tension-se oxygention inies (OTIs), P A P (Figure 1), P /P A (Figure 2), P /F i (Figure 3) n P A P /P (Figure 4) lulte from the horses rething F = 0.6 or 1.0 Tle 1. Arteril prtil pressure of oxygen (P ) n ron ioxie (P C ), loo ltte, hert rte (HR) n men rteril pressure (MAP) otine in the horses efore premeition (PrePRE), immeitely following nestheti inution (PostIND), 10 min (IPPV10) n 45 min (IPPV45) fter nestheti inution n ommenement of mehnil ventiltion, n in the reovery room (REC) Prmeter Reumeny F F i PrePRE PostIND IPPV10 IPPV45 REC P (mmhg) Lterl F CG AF BG Dorsl D CE AD BE P C (mmhg) Lterl C D A B Dorsl C D A B Ltte (mmol/l) Lterl AB B Dorsl A HR (ets/min) Lterl A AC D Dorsl B C B D MAP (mmhg) Lterl Dorsl Vlues re for horses ple in lterl or orsl reumeny n reeiving two ifferent onentrtions of oxygen elivere to the rething iruit (F ). The inspire onentrtions of oxygen (F i ) were mesure. Dt re presente s men n stnr error of the men (SE). Vlues with sme supersripts re signifintly ifferent (P < 0.05). IPPV, intermittent positive pressure ventiltion. 48 Austrlin Veterinry Journl Volume 96 No 1-2, Jnury/Ferury Austrlin Veterinry Assoition

4 f 250 e P A - P (mmhg) e f g g 0 PrePre PostIND IPPV 10 IPPV 45 REC Lterl (F 0.6) Lterl (F 1.0) Dorsl (F 0.6) Dorsl (F 1.0) Figure 1. Men stnr error of the men of the lveolr rteril prtil pressure of oxygen ifferene (P A P ) lulte for the horses efore premeition (PrePRE), immeitely following nestheti inution (PostIND) n 10 min (IPPV10) n 45 min (IPPV45) fter inution n ommenement of mehnil ventiltion, n ple in lterl or orsl reumeny n onnete to the nestheti rething iruit supplie with 60% (F = 0.6) or 100% (F = 1.0) oxygen. Vlues with sme supersripts re signifintly ifferent (P < 0.05). IPPV, intermittent positive pressure ventiltion P / P A (rtio) ef ef PrePre PostIND IPPV 10 IPPV 45 REC Lterl (F 0.6) Lterl (F 1.0) Dorsl (F 0.6) Dorsl (F 1.0) Figure 2. Men stnr error of the men of the rtio of rteril to lveolr rteril prtil pressure of oxygen ifferene (P /P A ) lulte for the horses efore premeition (PrePRE), immeitely following nestheti inution (PostIND) n 10 min (IPPV10) n 45 min (IPPV45) fter inution n ommenement of mehnil ventiltion, n ple in lterl or orsl reumeny n onnete to the nestheti rething iruit supplie with 60% (F = 0.6) or 100% (F = 1.0) oxygen. Vlues with sme supersripts re signifintly ifferent (P < 0.05). IPPV, intermittent positive pressure ventiltion Austrlin Veterinry Assoition Austrlin Veterinry Journl Volume 96 No 1-2, Jnury/Ferury

5 700 P / F i (mmhg) PrePre PostIND IPPV 10 IPPV 45 REC Lterl (F 0.6) Lterl (F 1.0) Dorsl (F 0.6) Dorsl (F 1.0) Figure 3. Men stnr error of the men of the rtio rteril prtil pressure of oxygen to inspire onentrtion of oxygen (P /Fi ) lulte for the horses efore premeition (PrePRE), immeitely following nestheti inution (PostIND) n 10 min (IPPV10) n 45 min (IPPV45) fter inution n ommenement of mehnil ventiltion, n ple in lterl or orsl reumeny n onnete to the nestheti rething iruit supplie with 60% (F = 0.6) or 100% (F = 1.0) oxygen. Vlues with sme supersripts re signifintly ifferent (P < 0.05). IPPV, intermittent positive pressure ventiltion. while in orsl or lterl reumeny t eh of the five smpling time points, re presente grphilly. The men ( SE) F i ws n when horses were rething F = 0.6 n F = 1.0, respetively. Aprt from horses in lterl reumeny rething F = 0.6, ll horses were hypoxemi (S < 90%, P < 53 mmhg) 22 immeitely following inution of nesthesi, prior to onnetion to the rething iruit. In reovery, 10 min fter isonnetion from the nestheti rething iruit n rething spontneously, horses mintine on F = 0.6 were hypoxemi irrespetive of the position of reumeny uring nesthesi (Tle 1). At 10 n 45 min 2 P A - P / P (rtio) PrePre PostIND IPPV 10 IPPV 45 REC Lterl (F 0.6) Lterl (F 1.0) Dorsl (F 0.6) Dorsl (F 1.0) Figure 4. Men stnr error of the men of the rtio lveolr rteril prtil pressure of oxygen ifferene to rteril prtil pressure of oxygen (P A P /P ) lulte for the horses efore premeition (PrePRE), immeitely following nestheti inution (PostIND) n 10 min (IPPV10) n 45 min (IPPV45) fter inution n ommenement of mehnil ventiltion, n ple in lterl or orsl reumeny n onnete to the nestheti rething iruit supplie with 60% (F = 0.6) or 100% (F = 1.0) oxygen. Vlues with sme supersripts re signifintly ifferent (P < 0.05). IPPV, intermittent positive pressure ventiltion. 50 Austrlin Veterinry Journl Volume 96 No 1-2, Jnury/Ferury Austrlin Veterinry Assoition

6 fter ommenement of IPPV, P ws greter in horses rething F = 1.0 thn in horses rething F = 0.6 (P < 0.001) in either lterl or orsl reumeny. However, P ws lwys greter in horses in lterl ompre with orsl reumeny when rething either of the F (P < 0.001). Despite these ifferenes, ll horses were le to mintin P > 100 mmhg regrless of F n irrespetive of the position of reumeny one mehnil ventiltion ommene. At IPPV10, horses reeiving F = 0.6 in lterl reumeny h lower P C (P = 0.035) thn horses in orsl reumeny (Tle 1). At IPPV45, horses reeiving F = 1.0 or 0.6 in lterl reumeny h lower P C ompre with horses in orsl reumeny (P = 0.001). However, the ifferenes in P C ssoite with position of reumeny were not linilly signifint. When position of reumeny ws exlue, there ws no ifferene in P C etween horses reeiving F = 1.0 or F = 0.6. One move into reovery, horses tht were mintine in lterl reumeny n reeiving F = 1.0 uring nesthesi h P C greter thn tht in horses mintine in orsl reumeny (P < 0.001). Whether the horses were ple in lterl or orsl reumeny, the F i not hve sttistilly signifint effets on OTIs. However, in the horses rething F = 1.0, P A P ws notiely greter thn in the horses rething F = 0.6, regrless of position of reumeny. Ltte inrese over time from se vlues to IPPV45, regrless of position of reumeny n F (P < 0.001). At IPPV45 there ws higher loo ltte in horses in orsl reumeny ompre with horses in lterl reumeny when reeiving F = 1.0 (P = 0.007). At IPPV45, horses in lterl reumeny reeiving F = 0.6 h higher ltte thn horses reeiving F = 1.0 (P = 0.04). There were signifint effets of position of reumeny on ll OTIs whether horses were reeiving F = 1.0 or 0.6. At IPPV10 n IPPV45, P A P,P /P, n P A P /P were greter in horses in orsl reumeny ompre with those in lterl reumeny (P = 0.001), n P /F i ws lower for horses in orsl reumeny ompre with horses in lterl reumeny, regrless of F (P 0.001). In generl, one horses were nesthetise n mehnilly ventilte, the oxygen inies were onsistent with ompromise pulmonry funtion ompre with vlues otine from horses prior to ministrtion of premeition. Disussion The results of this stuy showe tht elivering oxygen t onentrtion of 60% into the nestheti rething system will mintin equte rteril oxygention for t lest 50 min in helthy ult horses, nesthetise with isoflurne, mehnilly ventilte n positione in lterl or orsl reumeny. However, it woul pper tht utilising F = 0.6 oes not reue pulmonry funtion normlities n onsequently improve oxygention, in either lterl or orslly reument horses uring nesthesi, ompre with F = 1.0. Positioning of horses in orsl reumeny, however, oes hve greter negtive effet on pulmonry funtion n oxygention ompre with horses positione in lterl reumeny. Oxygen is routinely use in veterinry nesthesi s the sole rrier gs n therefore ptients re ommonly expose to ner 100% oxygen. This optimises P,S n tissue oxygen elivery, 23 ut eviene suggests tht high F i, ompre with lower F i, preisposes to pulmonry teletsis, V/Q normlities n venous mixture. 9,14 The reue pity of horses to optimise lveolr to rteril oxygen trnsfer uring generl nesthesi hs prompte severl stuies into possile uses n potentil mngement strtegies over the pst few ees ,20 Those stuies hve investigte the effets of mixing lternte gses with oxygen, with the im of reuing F i n minimising sorption teletsis s use of poor oxygention. However, the reporte enefits of reuing the F i on oxygen trnsfer pity in horses uring generl nesthesi re vrile, possily relte to ifferenes in stuy esign, inluing the F i, the nestheti protool, moe of ventiltion n ptient positioning uring nesthesi. Nonetheless, numer of reports investigting the effet of reuing the F i gree tht in nesthetise, spontneously rething or mehnilly ventilte horses, oxygention remins equte to ompletely sturte hemogloin even t F i of ,20 The results of those stuies re onsistent with the results of the present stuy. In this stuy, the horses were reeiving two onentrtions of oxygen, elivere into the rething iruit t totl flow of 5 L/min. The resulting F i ws lower thn the respetive F. The ifferene etween F i n F is result of the wshout of nitrogen from the ptient into the rething iruit n hs long een reognise s hving notle signifine in lrge niml speies onnete to onventionl rerething nestheti iruits similr to the system use in the urrent stuy. 24 This ifferene etween F n F i oul hve een minimise y inresing the fresh gs flow or, lterntively in horses reeiving F of 0.6, y ontinuously justing the oxygen ir mixture. Stuies investigting the effets of reue F i on the V/Q reltionship, if onute in horses in lterl reumeny either rething spontneously or mehnilly ventilte, hve shown tht using low F i uring nesthesi my reue V/Q normlities. 15,17 In spontneously rething horses in lterl reumeny, n F i = 0.3 reue the evelopment of hypoventiltion n V/Q normlities when ompre with F i 0.85, 15 while in mehnilly ventilte horses in lterl reumeny, rething n F i rnging etween 0.25 n 0.9, there ws less eviene of V/Q normlities n venous mixture in omprison with horses rething n F i = In ontrst, stuies onute in horses in orsl reumeny, either rething spontneously or mehnilly ventilte, reuing F i h no meliorting effet on V/Q normlities. 18,20 Mehnilly ventilte horses in orsl reumeny, rething F i = 0.5, i not emonstrte reue pulmonry shunt frtion n lveolr e spe in omprison with horses rething F i Similrly, spontneously rething horses in orsl reumeny, rething F i = 0.5, were more likely to evelop hypoxemi ompre with horses rething F i lose to OTIs otine in the present stuy from the horses in lterl reumeny, irrespetive of the F, showe eviene of less severe V/Q normlities 2017 Austrlin Veterinry Assoition Austrlin Veterinry Journl Volume 96 No 1-2, Jnury/Ferury

7 ompre with inies otine from horses in orsl reumeny. These results suggeste tht ifferenes in the position of reumeny my explin the vriility in the results from erlier stuies. Our results lso onfirm tht reumeny, n prtiulrly orsl reumeny, uring generl nesthesi signifintly ontriutes to V/Q normlities in horses. 3 6 OTIs, suh s those pplie in this stuy, re wiely epte s mens of ssessing venous mixture in humns with ute respirtory istress synrome, 25 euse they provie n esy, reltively non-invsive metho of ssessing pulmonry funtion normlities, inluing V/Q normlities. However, the Berggren shunt eqution, Qs/Qt (shunt loo flow/totl ri output), is the most urte ssessment of V/Q mismth n venous mixture. 26 The etermintion of Qs/Qt requires olletion of mixe venous loo smple from the pulmonry rtery in ition to n rteril loo smple, to ssess the rteril to mixe venous oxygen ontent ifferene, whih forms n essentil prt of the eqution. However, in omprison with OTIs, Qs/Qt etermintion is more invsive n, in prtiulr, requires plement of pulmonry rtery theter. Within the sope of the present stuy it ws not possile to ple pulmonry rtery theters in the horses. As n lterntive to the Berggren shunt eqution the F-shunt n e use, in whih the rteril to venous oxygen ontent ifferene is sustitute y fixe vlue, mking olletion of pulmonry rtery smple osolete. In stuy using sheep, the F-shunt ws shown to e more urte estimte of Qs/Qt ompre with OTIs. 13 In horses uring generl nesthesi, the orreltion etween the F-shunt n Qs/Qt ws foun to e greter thn the F-shunt n OTIs. 27 When use for repet lultions of venous mixture in ptient, flututions in ri output n hemogloin onentrtion my erese the reliility of Qs/Qt n F-shunt results. In the horses in the present stuy, nestheti rugs, surgil stimultion n ministrtion of the exogenous teholmine, outmine, woul hve resulte in flututions in ri output n hemogloin onentrtion, thus ffeting the reliility of results for omprison etween respetive tretments. Therefore, in this stuy the pplition of severl OTIs, s estimtes of venous mixture, ws onsiere resonle n equte metho of ompring venous mixture etween the respetive tretments. In the horses in this stuy rething F = 1.0, the P A P ws greter thn in the horses rething F = 0.6, regrless of position of reumeny, n espite higher P t n F = 1.0 ompre with F = 0.6. This is onsistent with other stuies of nesthetise horses where P A P ws greter with F i 0.95 ompre with F i = 0.2 or 0.5, n suggeste tht venous mixture my e greter in horses rething high F i. However, solute vlues for P A P hve to e interprete with some ution euse P A P inreses in non-liner fshion with inresing F i (P A ), espite greter P with high F i n even when venous mixture remins unhnge. 18 This is euse of the sigmoi shpe of the oxyhemogloin issoition urve, whih limits the rte of rise of P t high F i n results in n inrese in P A P. In humns rething n F i = 0.21 (room ir), norml P A P is pproximtely 10 mmhg ut inreses up to 40 mmhg if F i is The uthors re not wre of similr informtion ville in wke, stning horses. Beuse of the non-liner inrese in P A P with inresing F i,p A P nnot e use lone for ompring venous mixture t ifferent F i. In the present stuy, it ws not possile to ifferentite etween the ontriutions of venous mixture n high F i to P A P t F = 1.0, in the horses in orsl reumeny. However, greter venous mixture in the horses in orsl reumeny n rething F = 1.0, ompre with lterl reumeny, is very likely euse P A P ws lso inrese in orslly reument horses rething F = 0.6. Similrly, other inies inite greter venous mixture in orslly reument horses regrless of F i. In humns with ute respirtory istress synrome, P /F i hs een use to ssess the severity of respirtory ompromise, n P /F i < 300 is initive of inequte respirtory funtion. 25 In horses, P /F i hs not een use for similr pplitions. Most horses in the present stuy h P /F i < 300 immeitely post inution n in reovery, ut while they were onnete to the rething iruit n mehnilly ventilte the P /F i ws >300 in ll horses, with the exeption of those in orsl reumeny reeiving F = 0.6. The P /F i my reflet the negtive effets of nesthesi n reumeny on respirtory funtion in horses n emonstrte the enefiil effets of ministering n oxygen-enrihe gs mixture. Immeitely following inution of nesthesi when horses were positione in lterl reumeny, the P in ll horses erese y pproximtely 50%. Anesthesi n ssoite rupt hnges in oy position from stning to lterl reumeny my hve ontriute to hypoventiltion n hemoynmi n pulmonry hnges leing to poor oxygention. 29 The susequent ministrtion of oxygen t the two onentrtions improve P oringly; however, the OTIs i not iffer etween the two F ministere, regrless of position of reumeny. This my inite tht, in horses, venous mixture is more likely ssoite with ftors other thn sorption teletsis. Erly stuies showe tht pulmonry ysfuntion in horses uring nesthesi is use y irwy losure, prtiulrly in the epenent lung res, euse of ltere hest wll mehnis, inluing the iphrgm, n mehnil ompression of the lungs ssoite with intrpulmonry grvittionl fores. 2,8,30 Reuing the F i my therefore hve limite n unrelile enefits for improving V/Q normlities in horses uring generl nesthesi. In omprison, the use of mehnil ventiltion for improving the V/Q reltionship, inluing positive en-expirtory pressure 2,23,31 n ontinuous positive irwy pressure, 32,33 my hve more relile, leit limite, enefiil effets. A likely use of the inrese in ltte onentrtion is neroi gluoneogenesis resulting from poor musle perfusion n ssoite poor tissue oxygen elivery uring generl nesthesi. 34 Aoring to our finings, F = 0.6 ppers to not mrkely ffet oxygen elivery to tissue, in prtiulr to musle. This is similr to previous stuies tht ssesse tissue oxygen elivery in horses rething less thn mximum F i uring generl nesthesi. 15,16,18 Arteril loo smples were ollete from the roti rtery prior to inution of nesthesi n from the greter mettrsl rtery n irumflex fil rtery immeitely post inution n uring nesthesi n reovery, respetively. This my hve presente 52 Austrlin Veterinry Journl Volume 96 No 1-2, Jnury/Ferury Austrlin Veterinry Assoition

8 soure of vriility of loo gs results, whih woul hve een of gretest signifine for the results otine uring nesthesi, use for omprison etween tretments. However, unpulishe results of stuy onute t our hospitl in nesthetise horses revele no signifint ifferene in loo gs vlues in smples ollete from the fil n mettrsl rteries. Venous mixture, oring to the OTIs use in this stuy, is greter in orslly reument horses thn in lterlly reument horses n oes not improve y reuing F to 0.6, regrless of position of reumeny. The itionl use of oxygen ontent-se inies suh s the F-shunt, espite the limittions note, my hve provie more etile informtion on the potentil effets of reuing F i on venous mixture in nesthetise orslly n lterlly reument horses. Conflits of interest n soures of funing The uthors elre no onflits of interest or soures of funing for the work presente here. Referenes 1. Nymn G, Heenstiern G. Ventiltion perfusion reltionships in the nesthetise horse. Equine Vet J 1989;21: Nymn G, Funkquist B, Kvrt C et l. Ateletsis uses gs exhnge impirment in the nesthetise horse. Equine Vet J 1990;22: Hll LW. Criovsulr n pulmonry effets of reumeny in two onsious ponies. Equine Vet J 1984;16: Sorenson PR, Roinson NE. Posturl effets on lung volumes n synhronous ventiltion in nesthetize horses. J Appl Physiol 1980;48: Hornof WJ, Dunlop CL, Prestge R et l. Effet of lterl reumeny on regionl lung funtion in nesthetize horses. Am J Vet Res 1986;47: Shtzmnn U, Koehli M, Dun F et l. Effet of posturl hnges on ertin irultory n respirtory vlues in the horse. Am J Vet Res 1982;43: Steffey EP, Whet JD, Megher DM et l. Boy position n moe of ventiltion influenes rteril ph, oxygen, n ron ioxie tensions in hlothnenesthetize horses. Am J Vet Res 1977;38: MDonell WN, Hll LW, Jeffott LB. Riogrphi eviene of impire pulmonry funtion in lterlly reument nesthetise horses. Equine Vet J 1979;11: Rothen HU, Sporre B, Engerg G et l. Prevention of teletsis uring generl nesthesi. Lnet 1985;345: Stffieri F, De Monte V, De Mrzo C et l. Effets of two frtions of inspire oxygen on lung ertion n gs exhnge in ts uner inhlnt nesthesi. Vet Anesth Anlg 2010;37: Stffieri F, Frnhini D, Giuseppin L et l. Compute tomogrphi nlysis of the effets of two inspire oxygen onentrtions on pulmonry ertion in nesthetize n mehnilly ventilte ogs. Am J Vet Res 2007;68: Stffieri F, Driessen B, Monte VD et l. Effets of positive en-expirtory pressure on nesthesi-inue teletsis n gs exhnge in nesthetize n mehnilly ventilte sheep. Am J Vet Res 2010;71: Aros JD, Lrenz MP, Boston RC et l. Use of the oxygen ontent-se inex, Fshunt, s n initor of pulmonry venous mixture t vrious inspire oxygen frtions in nesthetize sheep. Am J Vet Res 2012; 73: Joye CJ, Bker AB, Kenney FR. Gs uptke from unventilte res of lung: omputer moel of sorption teletsis. J Appl Physiol 1993;74: Cuvelliez SG, Eiker SW, MLuhln C et l. Criovsulr n respirtory effet of inspire oxygen frtion on hlothne-nesthetize horses. Am J Vet Res 1990;51: Mrntell S, Nymn G, Heenstiern G. High inspire oxygen onentrtions inrese intrpulmonry shunt in nesthetize horses. Vet Anesth Anlg 2005;32: Stffieri F, Buquier SH, Mote PJ et l. Pulmonry gs exhnge in nesthetise horses mehnilly ventilte with oxygen or helium/oxygen mixture. Equine Vet J 2009;41: Huell JA, Arnes TK, Benrski RM et l. Effet of 50% n mximl inspire oxygen onentrtions on respirtory vriles in isoflurnenesthetize horses. BMC Vet Res 2011;7: Crumley MN, MMurphy RM, Hogson DS et l. Effets of inspire oxygen onentrtion on ventiltion, ventiltory rhythm, n gs exhnge in isoflurne-nesthetize horses. Am J Vet Res 2013;74: Shuvliege S, Svvs I, Gsthuys F. The effet of the inspire oxygen frtion on rteril loo oxygention in spontneously rething, isoflurne nesthetize horses: retrospetive stuy. Vet Anesth Anlg 2015;42: Rossier PH, Men H. L insuffisne pulmonire. Shweiz Me Wohenshr 1943;73: Clerux TH, Gustin P, Detry B et l. Comprtive stuy of the oxyhemogloin issoition urve of four mmmls: mn, og, horse n ttle. Comp Biohem Physiol 1993;106A: Dy TK, Gynor J, Muir W et l. Bloo gs vlues uring intermittent positive pressure ventiltion n spontneous ventiltion in 160 nesthetize horses positione in lterl or orsl reumeny. Vet Surg 1995;24: Grny JL, Steffey EP, Miller M. Arteril loo Po 2 n Po 2 in horses uring erly hlothne oxygen nesthesi. Equine Vet J 1987;19: Goul MK, Ruoss SJ, Rizk NW et l. Inies of hypoxemi in ptients with ute respirtory istress synrome: reliility, vliity, n linil usefulness. Crit Cre Me 1997;25: Berggren SM. The oxygen efiit of rteril loo use y non-ventilting prts of the lung. At Physiol Sn 1942;4: Brignti A, Portel DA, Grsso S et l. Aury of ifferent oxygention iniesinestimtingintrpulmonryshunting t inresing infusion rtes of outmine in horses uner generl nesthesi. Vet J 2015;204: Si SI, Bnerjee CM. Venous mixture to the pulmonry irultion in humn sujets rething 100 per ent oxygen. J Clin Invest 1963;42: Steffey EP, Kelly AB, Hogson DS et l. Effet of oy posture on riopulmonry funtion in horses uring five hours of onstnt-ose hlothne nesthesi. Am J Vet Res 1990;51: Wtney GCG. Riogrphi eviene of pulmonry ysfuntion in nesthetize ttle. Res Vet Si 1986;41: Moens Y, Lgerweij E, Gootjes P et l. Differentil rtifiil ventiltion in nesthetize horses positione in lterl reumeny. Am J Vet Res 1994;55: Mosing M, MFrlne P, Brell D et l. Continuous positive irwy pressure (CPAP) erese pulmonry shunt in nesthetize horses. Vet Anesth Anlg 2016;43: MFrlne PD, Mosing M. Erly experiene with ontinuous positive irwy pressure (CPAP) in 5 horses: se series. Cn Vet J 2012;53: Tylor PM. Equine stress responses to nesthesi. Br J Anesth 1989;63: (Aepte for pulition 22 June 2017) 2017 Austrlin Veterinry Assoition Austrlin Veterinry Journl Volume 96 No 1-2, Jnury/Ferury