Dynamic changes in the direction of blood flow through the ductus arteriosus at birth

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1 J Physiol (2009) pp Dynmic chnges in the irection of bloo flow through the uctus rteriosus t birth Kelly J. Crossley 1,BethJ.Allison 1,GremeR.Polglse 2,ColinJ.Morley 3,4, Peter G. Dvis 3 n Sturt B. Hooper 1 1 Deprtment of Physiology, Monsh University, Clyton, Victori 3800, Austrli 2 School of Women s n Infnt s Helth, The University of Western Austrli, Crwley, Western Austrli 6009, Austrli 3 Neontl Services, Royl Women s Hospitl, Melbourne, Victori 3053, Austrli 4 Muroch Chilren s Reserch Institute, Melbourne, Victori 3052, Austrli Mjor criovsculr chnges occur t birth, incluing increse pulmonry bloo flow (PBF) n closure of the uctus rteriosus (DA), which cts s low resistnce shunt between the fetl pulmonry n systemic circultions. Although the pressure grient between these circultions reverses fter birth, little is known bout DA bloo flow chnges n whether reverse DA flow contributes to PBF fter birth. Our im ws to escribe the chnges in PBF n DA flow before, uring n fter the onset of pulmonry ventiltion t birth. Flow probes were implnte on the left pulmonry rtery (LPA) n DA in preterm fetl sheep (n = 8) 3ysbeforetheywere elivere n ventilte. Bloo flow ws mesure in the LPA n DA, before n fter umbilicl cor occlusion (UCO) n for 2 h fter ventiltion onset. Following UCO, DA flow ecrese from 534 ± 57 ml min 1 to 237 ± 29 ml min 1 which reflecte similr reuction in right ventriculr output. Within 5 min of ventiltion onset, PBF increse from 11 ± 6mlmin 1 to 230 ± 13 ml min 1 wheres DA flow ecrese to 172 ± 54 ml min 1 ; negtive vlues inicte reverse DA flow (left-to-right shunting). Reverse flow through the DA contribute up to 50% of totlpbft30 minnecreseinthiscontributionccountefor71 ± 13%ofthetime-relte ecrese in PBF fter birth. DA bloo flow is very ynmic fter birth n epens upon the pressure grient between the pulmonry n systemic circultions. Following ventiltion, reverse DA flow provie significnt contribution to totl PBF fter birth. (Receive 4 My 2009; ccepte fter revision 4 August 2009; first publishe online 12 August 2009) Corresponing uthor S. Hooper: Deprtment of Physiology, PO Box 13F, Monsh University, Vic. 3800, Austrli. Emil: sturt.hooper@me.monsh.eu.u Abbrevitions DA, uctus rteriosus; F IO2, frction of inspire oxygen; HR, hert rte; LPA, left pulmonry rtery; PBF, pulmonry bloo flow; PEEP, positive en expirtory pressure; PGE 2, prostglnin E 2 ; PVR, pulmonry vsculr resistnce; RVO, right ventriculr output; UCO, umbilicl cor occlusion. The trnsition to ir-brething t birth requires substntil chnges in criovsculr physiology, prticulrly lrge increse in pulmonry bloo flow (PBF) n closure of vsculr shunts tht llow bloo to by-pss the lungs (Ruolph, 1979, 1985; Rei & Thornburg, 1990; Teitel et l. 1990; Friemn & Fhey, 1993; Heymnn, 1999). In the fetus, lmost 90% of right ventriculr output (RVO) by-psses the lungs n enters the systemic circultion vi the uctus rteriosus (DA), which connects the min pulmonry rtery with the escening thorcic ort (Fig. 1). Flow through the DA is etermine by the pressure grient cross it n, s pressure in the fetl pulmonry circultion is 5 mmhg bove systemic rteril pressure (Ruolph, 1979; Rei & Thornburg, 1990; Hooper, 1998), bloo flows from the pulmonry circultion through the DA n into the systemic circultion, referre to s right-to-left shunting. In the fetus, pulmonry rteril pressure is high n PBF is low becuse pulmonry vsculr resistnce (PVR) is high (Ruolph, 1979; Rei & Thornburg, 1990). The high PVR, in combintion with the presence of the DA, confers unique chrcteristics to the bloo flow wveform in the left n right pulmonry rteries ownstrem from the junction with the DA (Ruolph, 1979; Rei & Thornburg, 1990; Polglse et l. 2004, 2005). In prticulr, forwr flow (towrs the lungs) only occurs briefly uring systole, wheres throughout most of istole, flow is retrogre with bloo flowing wy from the lungs n exiting the pulmonry circultion vi the DA (Ruolph, 1979; Rei & Thornburg, 1990; Polglse et l. 2004, 2005). DOI: /jphysiol

2 4696 K. J. Crossley n others J Physiol The successful trnsition to pulmonry gs exchnge t birth is lrgely epenent on irwy liqui clernce n lrge ecrese in PVR so tht the entire output of the right ventricle cn pss through the lungs (Friemn & Fhey, 1993; Hooper & Hring, 2005). As result, the DA must close to prevent bloo by-pssing the lungs n to seprte the pulmonry n systemic circultions, llowing men pulmonry rteril pressure to ecrese towrs ult levels ( 15 mmhg) (West, 2003). However, ny reuction in pulmonry rteril pressure while the DA remins ptent must result in bloo flow from the systemic into the pulmonry circultion through the DA; this is referre to s left-to-right shunting. Ptency of the DA is reltively common in infnts born premturely or with pulmonry hypoplsi n mnifests s either right-to-left (prticulrly in infnts with pulmonry hypertension) or left-to-right shunting (Kluckow & Evns, 2000; Clymn, 2006; Hermes-DeSntis & Clymn, 2006). However, reltively little is known of the fctors controlling the shunting of bloo through the DA fter birth or the effect tht ptent DA n ynmic chnges in the irection of shunting hs on pulmonry n criovsculr function in the newborn. Although it is well estblishe tht PBF increses fter birth ue to lrge ecrese in PVR (Ruolph, 1979; Heymnn, 1999), the reltive contributions of the right-ventricle n left-to-right shunting through the DA (bloo erive from the left ventricle) to the increse in PBF fter birth re unknown. Furthermore, Figure 1. An imge from fetl sheep emonstrting the ntomy of the uctus rteriosus (DA) in reltion to the min pulmonry rtery (PA), the left pulmonry rtery (LPA), the right ventricle (RV) n escening thorcic ort (DTA) Flow probes were plce roun the LPA n the DA, t the positions inicte by the otte lines, for the simultneous mesurement of bloo flow in the LPA n DA, respectively. the ynmics of the chnges in the irection of bloo flow through the DA fter birth re lso unknown. We hve previously emonstrte tht, following the lrge increse in PBF immeitely fter birth, PBF grully ecreses by 50% within 2 h of ventiltion onset in preterm lmbs (Polglse et l. 2005, 2008, 2009; Crossley et l. 2007). In this stuy, we hypothesize tht rpi reversl of the pressure grient cross the DA reverses flow through the DA (cusing left-to-right shunting) following ventiltion onset n tht left-to-right flow through the DA significntly contributes to the increse in PBF fter birth. We lso hypothesise tht the grul ecrese in PBF following ventiltion onset is lrgely ue to reuce left-to-right flow through the DA cuse by its prtil closure. To test these hypotheses, we hve simultneously mesure bloo flow in the left pulmonry rtery (LPA) n the DA before, uring n fter elivery n with the onset of mechnicl ventiltion in preterm lmbs. Methos Experimentl protocol All experimentl proceures on nimls were pprove by the Monsh University Animl Ethics Committee, ccoring to the guielines of the Ntionl Helth n Meicl Reserch Council of Austrli Coe of PrcticefortheCrenUseofAnimlsforScientific Purposes. Aseptic surgery ws conucte on eight pregnnt ewes (Borer Leicester Merino) t 125± 2 ys of gesttion (term is 147 ys) s escribe previously (Polglse et l. 2004). Anesthesi ws inuce vi n intrvenous bolus of 5% soium thiopentone (Pentothl; 1 g in 20 ml) n, following intubtion, nesthesi ws mintine with inhltion of 1.5 3% hlothne in O 2. Antibiotics (1 g of mpicillin, I.V.) were lso ministere to the ewe prior to ny incisions n the ewes were mechniclly ventilte throughout surgery. Post-opertive nlgesi ws mintine for 3 ys using trnserml fentnyl ptches (75 ug h 1 ; Jnssen-Cilg, North Rye, NSW, Austrli). Ctheters were inserte into fetl croti rtery n jugulr vein, the mniotic sc n left pulmonry rtery; the tip of the croti rtery ctheter ws locte just istl of the junction with the ortic rch. Ultrsonic flow probes (4 mm; Trnsonic Systems; Ithc, NY, USA) were plce roun the left pulmonry rtery n the DA. Surgery ws performe prentlly, rther thn following cesren elivery, to voi extensive thorcic surgery fter birth, which coul hve influence the respirtory n criovsculr mesurements me before n following ventiltion onset. Before experimenttion, fetl well-being ws monitore ily by mesuring fetl rteril P O2, P CO2,pHnper cent oxygen sturtion of hemoglobin (S O2 ) (ABL30,

3 J Physiol Bloo flow through the uctus rteriosus t birth 4697 Riometer, Denmrk). Instntneous bloo flows in the LPA n the DA were recore igitlly using t cquisition system (Powerlb; ADInstruments, Cstle Hill, Austrli). Arteril pressures were mesure using pressure trnsucers (PD10; DTX Plus Trnsucer; Becton Dickinson, Singpore) n lso recore igitlly. At 128 ± 2 ys of gesttion, ewes n fetuses were nesthetise by n intrvenous bolus of 5% soium thiopentone (20 ml Pentothl) n ws mintine following intubtion with 1.5 3% hlothne in O 2 throughout elivery. The fetl he n neck were expose vi hysterotomy n the trche ws intubte with 3.5 mm cuffe tube n lung liqui rine pssively before the umbilicl cor ws clmpe n cut. The lmbs were then elivere, rie, weighe, plce uner rint heter n ventilte with Bbylog ventiltor (Dräger, Lübeck, Germny)using volume gurntee moe with set expire til volume (V T )of5mlkg 1,t 60 infltions min 1, vrible frction of inspire oxygen (F IO2 ) n positive en expirtory pressure (PEEP) of 4cmH 2 O s previously escribe (Probyn et l. 2004; Polglse et l. 2005). The inspirtory n expirtory times n F IO2 were ltere to mintin rteril P CO2 between 40 n 55 mmhg n S O2 between 90 n 95%. Lmbs receive 5% extrose infusion (I.V.) n were sete (pentobrbitone, I.V.) to prevent spontneous brething. All lmbs were ventilte for 120 min fter birth; men elivery weight ws 3.3 ± 0.2 kg n the gener istribution ws three mles n five femles. Following elivery of the lmb, the ewe ws kille using n overose of soium pentobrbitone (130 mg kg 1, I.V.), while it ws still uner generl nesthesi. Anlyticl methos Men PBF n DA flow, hert rte (HR), bloo pressures in both the left pulmonry n croti rteries n pulse height of the bloo flow wveform in both PA n DA were verge over 10 s perio before n fter the umbilicl cor occlusion (UCO) n t 30 s, 1 min, 2 min, 5 min, 10 min, 30 min, 1 h n 2 h fter the onset of ventiltion. Perios of recoring with obvious rtefcts (e.g. cuse by bloo smpling or boy movement) were voie uring these times. Totl PBF ws clculte by ssuming tht flow in the left pulmonry rtery equlle 40% of totl PBF, which is bse on the weight ifference between the right n left lungs (Moessinger et l. 1990; Kermiris et l. 1996). RVO ws then clculte by summing DA flow n totl PBF, wheres the percentge contribution of DA flow to totl PBF fter birth ws clculte by expressing DA flow s percentge of totl PBF when men DA flow ws left to right (i.e. t 3 min fter ventiltion onset, see below). Sttisticl nlysis One-wy repete mesures ANOVAs were use to nlyse the chnges in men PBF n DA flow, percentge of PBF erive from the DA, hert rte n DA n PBF wveform pulse heights with time fter birth. Two-wy repete mesures ANOVAs were use to nlyse the time-relte chnges in pressure n ifferences between the croti n pulmonry rteril pressures. Where significnt effects were foun, lest significnt ifference (LSD) post hoc test ws use to etect ifferences between groups n time points. Dt in the text re presente s the men ± S.E.M. The level of sttisticl significnce ws P < 0.05 for ll sttisticl nlyses. Results Before ventiltion onset During fetl life (before UCO) bloo flow in the DA ws significntly greter (534 ± 57 ml min 1 )thn PBF (11 ± 6mlmin 1 ), with forwr flow occurring throughout the cric cycle (Figs 2 n 3). In contrst, PBF ws irecte towrs the lungs only briefly uring systole, wheres throughout istole, bloo flowe wy from the lungs (retrogre, ssigne negtive vlue), thereby contributing to flow in the DA (Fig. 3). Assuming tht ll retrogre flow in the PA exits the pulmonry circultion vi the DA uring istole, up to 100% of flow in the DA uring istole ws erive from the pulmonry circultion, lthough this contribution to DA flow ws very vrible (Fig. 3). The fining of continuous forwr flow (right to left) through the DA is consistent with the finings of higher rteril pressures in the LPA compre to the croti rtery (Fig. 4; Tble 1). Immeitely following cor occlusion, men DA bloo flow ws significntly reuce from 534 ± 57 ml min 1 to 237 ± 29 ml min 1 (Fig. 2), wheres PBF tene to increse (from 11 ± 6mlmin 1 to 21 ± 11 ml min 1 ), but this ws not significnt. In contrst, the mplitue of the bloo flow wveform in the left pulmonry rtery ws significntly reuce wheres the pulse height of the DA bloo flow wve form ws unltere (Tble 1). The reuction in DA bloo flow cuse by UCO ws lso ssocite with lrge reuction in RVO (Tble 1). However, the pressure grient between the pulmonry n croti rteries ws not ltere by UCO (Fig. 4; Tble 1). Immeitely following ventiltion onset Immeitely fter ventiltion begn, PBF rpily increse from 21 ± 11 ml min 1 to 90 ± 19 ml min 1 within 30 s n increse further to 255 ± 10 ml min 1 within 10 min (Fig. 2). This increse in PBF coincie with mjor

4 4698 K. J. Crossley n others J Physiol verticl shift n chnge in shpe of the PBF wveform, resulting in forwr flow towrs the lungs throughout the cric cycle (Fig. 5). In contrst, following smll ely (30 s to 1 min), men DA flow ws mrkely reuce from 237 ± 29 ml min 1 (right-to-left irection) to 303 ± 47 ml min 1 fter 10 min of ventiltion (Fig. 2); negtive vlue reflects left-to-right shunting (from systemic towrs the pulmonry circultion) through the DA. These bloo flow chnges cuse mjor verticl shift in the DA bloo flow wveform, resulting in left-to-right shunting throughout the cric cycle, except for brief perio uring systole (Fig. 5). As result, left-to-right shunting through the DA provie significnt contribution to the increse in PBF fter birth. Left-to-right shunting begn to significntly contribute to PBF fter 3.0 ± 0.5 min of ventiltion n rpily increse to 47.5± 8.6% of totl PBF within 10 min (Fig. 6). Immeitely following ventiltion onset, the pressure grient between the pulmonry n croti rteries ws reuce, s pressures in both vessels were similr (Fig. 4; Tble 1). However, fter 10 min of ventiltion, men pulmonry rteril pressure tene to be lower thn men croti rteril pressure n by 30 min men pulmonry rteril pressure ws significntly lower thn men croti rteril pressure (Tble 1; Fig. 4). Time-relte chnges following ventiltion onset PBF (ml/min) Left pulmonry rtery c b b c bc The ventiltion-inuce increse in men PBF ws mximl t 10 min (255 ± 10 ml min 1 ), tene to be reuce t 30 min (221 ± 23 ml min 1 ) n ws significntly reuce (P < compre to the 10 min vlue) t 60 min (164 ± 23 ml min 1 ) n 120 min (144 ± 22 ml min 1 ; Fig. 2). Similrly, following 50 0 BCOACO DA bloo flow (ml/min) b b bc c e e -400 BCOACO Time (mins) Figure 2. Men bloo flow, verge over 10 s perio, mesure in the left pulmonry rtery (top pnel) n in the uctus rteriosus (DA; bottom pnel) before umbilicl cor occlusion (BCO), fter cor occlusion (ACO) n t selecte intervls fter ventiltion onset (esignte s time 0) For ech pnel, vlues tht o not shre common letter re significntly ifferent (P < 0.05) from ech other. For DA bloo flow, positive vlues reflect bloo flow from the pulmonry rtery into the ort (right-to-left shunting), wheres negtive vlues reflect bloo flow from the ort into the pulmonry circultion (left-to-right shunting). Figure 3 Instntneous mesures of bloo flow in the left pulmonry rtery (top pnel) n uctus rteriosus (mile pnel) cquire over 3 consecutive cric cycles from fetl sheep. The bottom pnel shows the percentge of totl pulmonry bloo flow (PBF) contributing to right-to-left (from pulmonry into systemic circultion) flow of bloo through the uctus rteriosus. Negtive PBF vlues (top pnel) inicte retrogre flow of bloo wy from the lungs.

5 J Physiol Bloo flow through the uctus rteriosus t birth 4699 Tble 1. Men hert rte (HR, bets min 1 ), right ventriculr output (RVO, ml min 1 ), pulmonry rteril pressure (PAP, mmhg), systemic rteril pressure (SAP, mmhg), mplitue of the pulmonry bloo flow (PBF) wveform (PBF pulse, ml min 1 ) n mplitue of the uctus rteriosus (DA) bloo flow wveform (DA pulse) mesure before umbilicl cor occlusion (BCO), fter umbilicl cor occlusion (ACO) n t 0.5, 1, 2, 5, 10, 30, 60 n 120 min fter ventiltion onset Time HR (bets min 1 ) RVO (ml min 1 ) PAP (mmhg) SAP (mmhg) PBF pulse (ml min 1 ) DA pulse (ml min 1 ) BCO 167 ± ± ± ± ± ± 68 ACO 166 ± ± 14 b 56.3 ± ± ± 58 b 791 ± ± ± 65 b 55.4 ± ± ± 73 b 1043 ± 106 b ± ± 66 b 56.2 ± ± ± 72 b 1069 ± 124 b ± ± 54 b 53.9 ± ± ± 74 b 1078 ± 103 b ± ± 50 b 53.8 ± ± ± 47 b 1061 ± 69 b ± ± 45 b 49.6 ± 5.7 b 53.2 ± ± 36 b 911 ± 109 b ± ± 59 b 37.4 ± 3.5 b 44.9 ± ± 34 b 379 ± 103 c ± ± 64 b 42.7 ± 4.1 b 47.3 ± ± 43 b 368 ± 90 c ± ± 40 b 42.7 ± 3.2 b 45.0 ± ± 55 b 457 ± 92 c Vlues tht o not shre common letter within the sme column re significntly ifferent (P < 0.05) from ech other. the ventiltion-inuce reversl in irection of DA bloo flow (from right-to-left to left-to-right flow), the mount of left-to-right shunting through the DA ws significntly reuce from mximum of 303 ± 47 ml min 1 t 10 min to 96 ± 37 ml min 1 t 60 min n 99 ± 55 ml min 1 t 120 min (Fig. 2). As result, the contribution of DA flow to PBF flow ws reuce from 47.5 ± 8.6% of totl PBF t 10 min to 24.4 ± 8.5% t 60 min n 24.1 ± 14.0% t 120 min (Fig. 6). Thus, left-to-right shunting of bloo through the DA remine significnt contributor to PBF throughout the ventiltion perio (Fig. 2). Although the pulse mplitue of the DA flow wveform ws initilly increse fter ventiltion begn, it ws significntly reuce t 30 min (Tble 1) wheres the PBF pulse height ws not significntly ltere following UCO. Similrly, lthough the fetl hert rte tene to increse when ventiltion begn n then ecrese with ventiltion time, there were no significnt chnges in hert rte throughout the experimentl perio (Tble 1). Following the onset of ventiltion, men pulmonry rteril pressures grully ecrese to minimum t 30 min n remine low t 60 n 120 min (Tble 1), wheres croti men rteril pressures were not significntly ltere throughout the experimentl perio. As result, the pressure grient between the pulmonry n systemic circultions ws reverse, compre with before ventiltion (Fig. 4), ecresing from men of 6.7 ± 3.8 mmhg (pulmonry > systemic men rteril pressure) before ventiltion to 7.5 ± 2.0 mmhg (systemic > pulmonry men rteril pressure) fter 30 min of ventiltion (Fig. 4). The pressure grient between the pulmonry n systemic circultions grully reuce with time (Fig. 4); lthough systemic rteril pressures remine significntly bove pulmonry rteril pressures fter 60 min ( 4.6± 2.2 mmhg), they were not fter 120 min ( 2.3± 1.3 mmhg; P = 0.09) of ventiltion. Bloo gs chnges The onset of ventiltion significntly ltere rteril ph (ecrese), P O2 (increse) n S O2 (increse) levels, compre with fetl vlues, but none of these bloo gs prmeters were significntly ltere throughout the 2 h ventiltion perio (Tble 2). Discussion The finings of this stuy emonstrte tht the mount n irection of bloo flow through the DA (between the pulmonry n systemic circultions) is very ynmic immeitely fter birth n follows the pressure grient Pulmonry - systemic rteril pressure grient (mmhg) bc b c BCO ACO Time (mins) Figure 4. The pressure grient between the pulmonry n systemic circultions verge over 10 s perio n mesure before umbilicl cor occlusion (BCO), fter cor occlusion (ACO) n selecte intervls fter the onset of ventiltion (inicte s time 0) Vlues tht o not shre common letter re significntly ifferent (P < 0.05) from ech other. c bc

6 4700 K. J. Crossley n others J Physiol Figure 5. Exmples of the bloo flow wve forms recore in the left pulmonry rtery (top pnel) n in the uctus rteriosus (DA) in the fetus (before umbilicl cor occlusion; BCO) n in the neonte t 5 min n 1 h fter the onset of ventiltion Negtive vlues of pulmonry bloo flow inicte retrogre flow wy from the lungs, wheres negtive bloo flow vlues through the DA inicte flow tht is irecte from the systemic (ort) into the pulmonry circultion. between the two circultions. During fetl life, men pulmonry rteril pressures were 6.4 ± 2.3mmHg bove systemic rteril pressures n so flow through the DA ws exclusively right to left (from pulmonry into the % of PBF erive from DA Time (mins) Figure 6. The percentge of totl pulmonry bloo flow (PBF) tht is erive from left-to-right flow (from the systemic into the pulmonry circultion) through the uctus rteriosus (DA) before umbilicl cor occlusion (BCO), fter umbilicl cor occlusion n t selecte intervls fter the onset of pulmonry ventiltion (esignte s time 0) Vlues tht o not shre common letter re significntly ifferent (P < 0.05) from ech other. systemic circultion) throughout the cric cycle (Fig. 3). However, fter birth, the ecrese in PVR ssocite with the onset of ventiltion rpily (within 3 min) ltere the irection of bloo flow through the DA, cusing the mjority of flow to be left-to-right in irection. Right-to-left flow through the DA only occurre briefly uring systole n the reversl in men flow ws closely ssocite with reversl in the pressure grient between the pulmonry n systemic circultions. As result, the contribution of left-to-right shunting of bloo through the DA, which origintes from the left ventricle, provie lmost 50% of flow to totl PBF by 10 min fter birth. Although the reltive contribution of left-to-right shunting to PBF grully ecline with time, it remine significnt contributor to totl PBF up to 2 h fter birth. However, the time-relte reuction in the contribution of left-to-right shunting through the DA to PBF ccounte for 71.3 ± 13.2% (t 2 h) of the reuction in PBF tht is commonly observe uring ventiltion fter preterm birth (Polglse et l. 2005, 2008, 2009; Crossley et l. 2007). Before ventiltion onset During fetl life, the retrogre PBF (wy from the lungs) tht occurs throughout most of istole (Fig. 2) hs been well escribe (Rei & Thornburg, 1990; Heymnn, 1999; Polglse et l. 2004) n the bloo is thought to exit the

7 J Physiol Bloo flow through the uctus rteriosus t birth 4701 Tble 2. Systemic rteril (pre-uctl) bloo gs vlues mesure before umbilicl cor occlusion (fetl) n t 5, 20, 60 n 120 min fter ventiltion onset Time ph P CO2 (mmhg) P O2 (mmhg) Sturtion (%) Fetl 7.36 ± ± ± ± min 7.21 ± ± ± ± min 7.24 ± ± ± ± min 7.25 ± ± ± ± min 7.24 ± ± ± ± 4.2 Vlues inicte by re significntly ifferent to ll other time points within the sme column (P < 0.001). pulmonry circultion n enter the systemic circultion vi the DA (Ruolph, 1979). However, simultneous mesures of PBF n DA bloo flow hve not been reporte previously, in either the fetus or newborn. Thus, the reltive contributions of retrogre PBF to DA flow prentlly n the contribution of left-to-right DA flow to PBF postntlly, ws unknown. Our stuy shows tht, in the fetus, bloo flows through the DA from right to left throughout the cric cycle, which is consequence of the sustine 5 mmhg pressure grient between the pulmonry n systemic circultions (Fig. 4). Furthermore, our stuy shows tht retrogre flow exiting the pulmonry circultion uring istole significntly contributes (up to 100%; Fig. 3) to flow through the DA n ccounts for most of the continue flow uring istole. Retrogre PBF uring istole is thought to result from reflecte pressure wve bouncing off the vsoconstricte pulmonry vsculr be (Grnt et l. 1999) n is likely to be responsible for sustining the pressure grient cross the DA tht fcilittes flow from the pulmonry into the systemic circultion uring istole (Fig. 3). As the DA is short, lrge-imeter (Fig. 1), low-resistnce shunt between the pulmonry n systemic circultions, it is not surprising tht flow in the PA n DA re intimtely linke. Inee, the interction between flows in the PA n DA is clerly evient in the bloo flow wveforms epicte in Fig. 3. Pek PBF uring systole correspons to cler reuction (notch) in the rte of rise in DA flow, wheres mximum retrogre PBF (most negtive flow) uring istole correspons to slowing in the rte of ecrese in DA flow. This provies further evience to emonstrte tht retrogre PBF contributes to flow through the DA n ssists in sustining high DA flows uring istole; t this time in the cric cycle bloo flow is zero in the min pulmonry rteril trunk (Ruolph, 1979). Following UCO, we observe rpi (within 30 s) n mrke reuction in DA flow (50% reuction; Fig. 2) which resulte from verticl ownwr shift in the bloo flow wveform s pulse mplitue ws not ffecte (Tble 1). This reuction in DA flow ws closely ssocite with 50% reuction in RVO which most probbly resulte from reuction in venous return to the right sie of the hert ue to the suen loss of bloo returning from the plcentl circultion. It is surprising tht this i not result in brycri, lthough this my hve occurre if the lmbs h been elivere vginlly n were not ventilte s soon s possible fter birth. In contrst, PBF ws not reuce following UCO, inicting tht the reuction in RVO only ffecte DA flow. An increse in ownstrem resistnce within the systemic circultion, ue to loss of the highly complint plcentl vsculr be, my hve lso contribute to the reuction in DA flow following UCO. Increse resistnce within the systemic circultion woul not be expecte to increse systemic rteril pressure uner these circumstnces, ue to the substntil reuction ( 50%) in RVO; RVO contributes to 60% of combine ventriculr output in the fetus (Ruolph, 1979). After ventiltion onset The initition of ventiltion cuse lrge rpi increse in PBF (from 21 ± 11 ml min 1 to 90 ± 19 ml min 1 in 30 s n to 255 ± 10 ml min 1 t 10 min), which ws ue to lrge reuction in PVR, s previously escribe (Ruolph, 1979, 1985; Rei & Thornburg, 1990; Teitel et l. 1990; Friemn & Fhey, 1993; Heymnn, 1999). The increse in PBF ws ssocite with mjor chnge in the shpe of the PBF wveform, resulting in forwr (positive) flow towrs the lungs throughout the cric cycle (Polglse et l. 2005). This increse in PBF is thought to result from ertion of the istl gs exchnge structures cuse by ventiltion onset n n ssocite increse in oxygention (Iwmotoet l. 1993;Teitel et l. 1990; Hooper & Hring, 2005). Although we coul not etect significnt pressure chnges in both pulmonry n systemic circultions fter 5 min of ventiltion, the pressure grient between the two circultions ws lost within 30 s (Fig. 4). Then, fter 5 min of ventiltion, rteril pressures tene to be lower in the pulmonry circultion thn in the systemic circultion, which ws significnt t 10 min. The reversl of the rteril pressure grient between the two circultions ws closely

8 4702 K. J. Crossley n others J Physiol ssocite with reversl in the irection of bloo flow through the DA. As result, within minutes of ventiltion onset, bloo flow through the DA ws left to right (from systemic into the pulmonry circultion) throughout most of the cric cycle, with right-to-left flow occurring only briefly uring systole. The reversl in irection of bloo flow through DA (from right-to-left to left-to-right irection) occurre rpily fter ventiltion begn, with men flow becoming left to right t 3.0 ± 0.5 min. As result, bloo flow from the systemic circultion (i.e. originting from the left ventricle), vi the DA, begn to contribute to PBF. We clculte tht this contribution rpily increse to lmost 50% of totl PBF within 10 min of ventiltion onset (Fig. 6), inicting tht both left n right ventricles contribute to the very lrge increse in PBF fter birth. Although it is reltively cler tht both ventricles contribute to PBF immeitely fter birth, contribution from the left ventricle is counterprouctive, forming short-circuit loop from left ventricle to left trium through the lungs. Although this my enhnce oxygention of rteril bloo, it reuces the efficiency of bloo flow istribution to istl structures (e.g. cerebrl circultion) n exposes the lung to high vsculr pressures t time when it is clering lrge volumes of irwy liqui (Hooper & Hring, 1995; Olver et l. 2004) n hs high interstitil tissue pressure (Miserocchi et l. 1994). We foun tht PBF grully ecrese with time following ventiltion, which is consistent with our previous finings (Polglse et l. 2005, 2008, 2009; Crossleyet l. 2007). We hypothesise tht this reuction inpbfwsuetoreuctioninthecontributionof left-to-right shunting of bloo through the DA. Consistent with our hypothesis, we foun tht left-to-right shunting ws significntly reuce t 2 h fter ventiltion begn (Fig. 2), lthough it remine s significnt contributor to PBF ( 25% of totl) t this time (Fig. 6). Nevertheless, we clculte tht this reuction in left-to-right shunting ccounts for most (71.3 ± 13.2%) of the reuction in PBF t 2 h fter ventiltion onset. The remining 30% of this reuction my hve been ue to grully worsening lung isese, cuse by mechnicl ventiltion, or to grul reuction in cric output (both ventricles), s inicte by the tenency for rteril bloo pressures, hert rte n right ventriculr output to ecrese with time (Tble 1). Although the pulse height of the DA bloo flow wveform increse immeitely fter ventiltion begn, it ws mrkely reuce by 30, 60 n 120 min. We consier tht this reuction in bloo flow pulse height reflects grul constriction n prtil closure of the DA fter birth, which is consistent with the fining tht bloo flow (left to right) ws grully eclining t this time. The fctors regulting closure of the DA fter birth hve been investigte in etil n involve: (1) n increse in P O2, (2) ecrese in bloo pressure within the DA lumen, (3) ecrese in circulting prostglnins, prticulrly PGE 2 n(4)ecreseinpge 2 receptors in the DA wll (Kjino et l. 2001; Clymn, 2006; Hermes-DeSntis & Clymn, 2006). However, the potentil contribution me by the reversl of bloo flow through the DA hs not been wiely consiere s mjor unerlying fctor. Inee, s the junction of the DA n escening thorcic ort form reltively cute ngle (Fig. 1), left-to-right flow through the DA (from the ort into the pulmonry rtery) must cuse substntil turbulence t this junction s well s t the PA en. It is possible tht the resulting turbulence n increse sher-stress releses vsoconstrictive fctors from the enothelium tht my contribute to constriction of the DA fter birth. Thus, lrger ecrese in PVR my le to greter left-to-right shunting, which will hve more potent vsoconstrictor effect on the DA; this my prtilly explin higher rtes of ptent DA in preterm infnts. In conclusion, we hve shown, for the first time, tht the irection of bloo flow through the DA is very ynmic fter birth n probbly reflects the pressure grient between the pulmonry n systemic circultions. With ecrese n then reversl of the pressure grient between the pulmonry n systemic circultions, we foun tht the primry irection of bloo flow through the DA reverses. As result, immeitely (within 3 min) fter ventiltion begins, left-to-right flow through the DA provies lrge ( 50%) contribution to PBF n the grul reuction in this contribution, s the DA begins to close, results in grul n significnt reuction in PBF. The close inter-reltionship between PBF n DA flow inictes tht the fctors influencing both the systemic n pulmonry circultions nee to be consiere together when ttempting to unerstn problems ssocite with either circultions in the immeite newborn perio. References Clymn RI (2006). Mechnisms regulting the uctus rteriosus. Biol Neonte 89, Crossley KJ, Morley CJ, Allison BJ, Polglse GR, Drgville PA, Hring R & Hooper SB (2007). Bloo gses n pulmonry bloo flow uring resuscittion of very preterm lmbs trete with ntentl betmethsone n/or Curosurf: effect of positive en-expirtory pressure. Peitr Res 62, Friemn AH & Fhey JT (1993). The trnsition from fetl to neontl circultion: norml responses n implictions for infnts with hert isese. Semin Perintol 17, Grnt DA, Hollner E, Skuz EM & Fuchere JC (1999). Interctions between the right ventricle n pulmonry vsculture in the fetus. JApplPhysiol87, Hermes-DeSntis ER & Clymn RI (2006). Ptent uctus rteriosus: pthophysiology n mngement. JPerintol26, S14 S18.

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