LungDistension:TheMajorCauseofDecreasedPulmonaryVascularResistanceinNear-temFetalSheep

Introduction

Atbirth,asthelungsreplacetheplacentaasthemainorganofgasexchange,pulmonaryvascularresistancemustdecreasedramatically,allowingpulmonarybloodflowtoincreaseandoxygenexchangetooccurinthelungs.Ifpulmonaryvascularresistancedoesnotdecrease,thesyndromeofpersistentpulmonaryhypertensionofthenewbornoccurs,oftenleadingtodeath.

Threemajoreventsthatcouldcausethisdecreasearerhythmicgaseousdistensionofthelungs,oxygenationofthelungs,andocclusionoftheumbilicalcord.Twooftheseevents-distensionandoxygenation--havebeenstudiedinacutelyexteriorizedfetalsheep.Thestudiessuggestedthatoxygenationratherthandistensionofthefetallungsisthemajorcauseofthedecreaseinpulmonaryvascularresistance.However,themetaboliceffectsofacuteanesthesiaandsurgeryusedtoexteriorizethefetalsheepmayhavealteredthepulmonaryvascularresponseinthesestudies,becausethisresponseisconsideredtobeatleastpartlymediatedbyvasoactivemetabolites.

Therefore,inthisstudy,weaskedwhetherdistensionofthelungsoroxygenationofthelungsorocclusionoftheumbilicalcordisthemajorcauseofthedecreaseinpulmonaryvascularresistancethatnormallyoccursatbirth.Toanswerthisquestion,weassessedthecumulativeeffectsofthethreeeventsonpulmonaryvascularresistancein16near-termfetalsheepinutero.Toavoidthesuperimposedeffectsofacuteanestheticandsurgicalstressesandofothercomponentsofthebirthprocess,suchasprenatalhormonalsurges,labor,delivery,andco1dexposure,wedidtheseexperimentsinnear-termfetalsheepinutero2-3daysaftersurgeryforcatheterplacement.

MaterialsandMethods

Animals

Sixteenfetalsheepwerestudiedat134.19±1.2(SD)daysofgestation(termisabout145days).Thefetuseswereofnormalweig