Andersons pediatric cardiology 176

overonecardiaccycle.Theaorticfractionalareachange(%)isdefinedas
thedifferencebetweenminimumandmaximumareadividedbyminimum
area.(FromWohlmuthC,OseiFA,MoiseKJ,etal.Aorticdistensibilityasa
surrogateforintertwinpulsepressuredifferencesinmonochorionic
pregnancieswithandwithouttwin-twintransfusionsyndrome.Ultrasound
ObstetGynecol.2016;48[2]:193–199.)

Pressure-VolumeLoops
Pressure-volumeloopsremainthegoldstandardformeasuringventricular
functionindependentlyofload,buttheirmeasurementisinvasive.Itispossible
toconstructpressure-volumeloopsinthechickembryo(Fig.6.22).1Physiologic
differenceshavebeendemonstratedbetweenrightandleftventricularfunction
inthepostnataldog.198Theshapeofthepressure-volumerelationshipofthe
rightventriclediffersinseveralwaysfromthatoftheleftventricle.Right
ventricularejectionoccurslongafterpeakpressurehasbeenachieved,and,fora
givenintraventricularvolume,thepressureislessintherightventricle.199The
instantaneouspressure-relationshipatend-systoleislinearinbothventricles,but
thecorrectionvolume,asdefinedbytheinterceptofthislinewiththex-axis,is
constantintheleftventricle.Itdoesnotchangewithcontractilestate,incontrast
tothatoftherightventricle.198

FIG.6.22 Pressure-volumeloopsinastage21chickembryomeasured
duringpreloadinfusion.(FromKellerBB.Maturation/couplingofthe
embryoniccardiovascularsystem.In:ClarkeEB,MarkwaldRR,TakaoA,
eds.DevelopmentalMechanismsofHeartDisease.Armonk,NY:


Futura;1995:375.)

Similarassessmentofthepressure-volumerelationships,independentofload,
isnotyettechnicallypossibleinthehealthyhumanfetus.However,noninvasive

insightsintonormalcardiovascularphysiologicdevelopmenthavebeenobtained
usingecho-trackingequipmentandDopplerultrasoundtissueimaging.Although
therightventricledealswithagreatervolumeloadinfetallifethandoestheleft
ventricle,minoraxisventricularsystolicfunctionandsimultaneousdirect
pressuremeasurementsaresimilarinrightandleftventriclesbeforebirth.195
Studiesoffetallongaxisfunctionhavereportedthatmyocardialvelocitiesand
amplitudeofmotionareincreasedintherightventricularfreewallcompared
withtheleftortheventricularseptum.129,135–137
Thismaybeinresponsetotheincreasedvolumeloadingoftherightventricle
comparedwiththeleftventricleduringnormalmaturation,andtheincreased
numberofmyocytesalignedinlongitudinalfashion.Therelativevolume
loadingoftherightventricleduringfetallifemayalterthedeposition,orcause
thereexpressionof,essentialcytoskeletalandcertainheatshockproteinssuchas
desmin,thecytokeratins,vimentin,andHSP-72.Thesehavebeendescribedin
conditionsofvolumeandpressureloadingofventriclespostnatallyandmayact
adversely,alteringresponsesinthepostnatalsituation,thuspermittingthe
ventricletodilatemorereadilyinresponsetovolumeandpressureloads,andso
furtherprejudiceitsfunction.

HeartRateVariability
Thevariabilityinheartrateisdeterminedbythematurationoftheautonomic
system.Therearemajordifferencesbetweenspeciesinthetimeatwhichthe
balancebetweenneurotransmissionofthesympatheticandparasympathetic
systemisaccomplished.Thisisrelatedtotheindependenceoftheindividual
speciesbeforeandimmediatelyafterbirth.Maladaptation,orimmaturity,of
neuralcontrolmaymanifestinacutelife-threateningeventsininfancy.Its
antenatalassessmentismorechallengingbecauseexistingtechnologysuchas
thecardiotocographfailstomeasurebeat-to-beatvariability.Althoughthefull
electrocardiogramcanberecordedbyuseofscalpelectrodes,thisisfeasible
onlyonceruptureofmembraneshasoccurred.Nevertheless,thishasprovided

usefulinformationduringlaborbyanalyzingtheSTwaves.200Noninvasive
recordingsofthefullfetalelectrocardiogramhavebeenreportedfrom15weeks


ofgestationinthehumanfetususingblindsignalseparationofsignalsobtained
fromelectrodesplacedonthematernalabdomenandfrommagnetocardiography
(Figs.6.23and6.24).Referencerangesfortimeintervalsinthenormalfetus
havebeenpublishedusingthesetechniques,201–203andmagnetocardiography
haspermittedamoredetailedanalysisoffetalarrhythmias,particularlyinthe
diagnosisofemergingheartblockandlongQTsyndromeinthefetus.204–208
Thereareconsiderablecomputationalchallengestoseparatingthefetalfrom
maternalsignals,209–211withcontinuoustelemetricrecordingsofthefetalECG
intheambulatorypregnantwomanremainingthe“holygrail”offetal
monitoring.212