Andersons pediatric cardiology 589

FIG.23.2 Cellularrespiration,whichresultsinthereleaseoftheenergyof
theterminalphosphatebondofadenosinetriphosphatetofuelthe
contractileandrelateddemandsoftheworkingskeletalmuscle.ATP,
Adenosinetriphosphate.(FromAstrandP,RodahlK.TheMuscleandIts
Contraction.TextbookofWorkPhysiology,PhysiologicalBasesof
Exercise.3rded.McGraw-Hill;1986:12–53.)

Anaerobicandaerobicmetabolicactivitiesuseglucose,whichismetabolized
topyruvate.Pyruvatethenhastwopossiblefates.Itmaybeconvertedintolactic
acidandexcretedintothebloodstream,whereitisbufferedbysodium
bicarbonateconvertingittolactate.Thisreactionresultsintheproductionof


carbondioxideandwater,alongwithsmallamountsofadenosinetriphosphate,
andtheformerareexcretedinthelungs.Thelactatemoleculeistakenupbythe
liverforresynthesistoglucoseandglycogen,whichcanthenbeusedagainfor
energyproduction.
Theotherfateofpyruvateisaerobicmetabolism.Pyruvateisconvertedinto
acetyl-coenzymeAandtransportedintothemitochondria,whereitentersthe
Krebscycle,againproducingcarbondioxideandwater.Adenosinetriphosphate
isproducedinlargequantitiesviatheelectrontransportchain,withoxygen
functioningastheterminalelectronacceptor.Unlikeanaerobicmetabolism,fats
andcarbohydratescanundergoaerobicmetabolism.Fatsenteraerobic
metabolismattheleveloftheKrebscycleanddonotundergoanaerobic
metabolism.4,5
Duringanyactivity,theavailabilityanduseofsubstrates,primarilyfatsor
carbohydrates,willvarydependinguponthetype,intensity,anddurationof
activity.Fatsaremorereducedthancarbohydrates,requiringmoreoxygenfor
completeoxidationcomparedwithcarbohydratesonamole-for-molebasis.4The
ratioofproductionofcarbondioxidetoconsumptionofoxygen,abbreviatedto
VCO/VO,iscalledtheratioofrespiratoryexchangeor,ifinasteadystate,the

respiratoryquotient.Inastateofhighuseoffat,theratioisapproximately0.7.
Conversely,duringpurecarbohydratemetabolism,theratiois1.0,reflectingthe
loweramountofoxygenneededtooxidizecarbohydrates.Thestoresof
glycogenintheadultbodyareseldommorethanapproximately1500Kcal.
Thereforesomeuseoffatisalmostalwaysrequired.Asaresult,therestingratio
ofrespiratoryexchange,eveninthewell-fedstate,willusuallyrangefrom0.85
to0.9.
Duringatypicalgradedmaximalexercisetest,theworkrateisgradually
increasedoverthecourseofapproximately10to15minutes,asexplainedinour
subsequentsectionsconcerningexerciseprotocols.Productionofadenosine
triphosphatewillneedtoincreaseasmechanicalworkincreases,andatlow
levelsofworkthisincreaseismetpredominatelybyincreasedaerobic
metabolism.Asworkrateincreases,consumptionofoxygenincreasesinalinear
fashion(Fig.23.3).Nearpeakworkrates,oxygenconsumptionwilltendto
plateau,asmaximalconsumptionisachieved.Thisphenomenonisoftenabsent
inchildren.4,6,7


FIG.23.3 Relationshipbetweenconsumptionofoxygen(VO2)andrateof
workduringprogressiveexerciseinahealthyandwell-conditioned
adolescent.Notethatwiththeonsetofexercisethereisanessentially
linearrelationshipbetweenthesetwofeaturesuntilnearthepeakof
exercisewhenVO2plateausdespitethecontinuedriseinrateofwork.

Asconsumptionofoxygenincreasesinresponsetoincreasedworkrate,there
isagradualriseintheratioofrespiratoryexchange.Thereasonforthisriseis
twofold.First,thereisagradualshiftinuseofcarbohydratescomparedwith
fats.Thisshiftallowsformoreefficientuseofoxygenbecausetheyieldof
adenosinetriphosphateperliterofoxygenisgreaterwithcarbohydrates
comparedwithfats.Secondly,theriseintheratiooccursasaresultofincreased

levelsoflacticacidintheblood.Atapproximately50%to60%ofthemaximal
consumptionofoxygen,levelsoflacticacidbegintoriseintheserum.This
pointisknownasthelactatethreshold.Theonsetofanaerobicmetabolismby
theexercisingmusclesisresponsibleforthisproductionoflacticacid.The
etiologyofthelactatethresholdappearstobethelimiteddeliveryofoxygento
theexercisingmuscles.8Theoxygentensionintheexercisingmusclecapillary
bedmustremainatalevelthatwillallowthediffusionofdissolvedoxygento
theexercisingmuscle'smitochondria.Atsealevel,barometricpressureanda
normalarterialhemoglobincontent,approximately6Lofbloodmustbe
deliveredtotheexercisingmusclesforeachliterofoxygenusedinaerobic
metabolism,tomaintainthiscriticalcapillaryoxygentensionthroughoutthe
courseofthecapillarybed.9
Abovethethreshold,levelsoflacticacidriseexponentiallyasworkrate
increases,necessitatingincreasedbufferingbysodiumbicarbonatetomaintain
bloodpHhomeostasis.Theby-productofthebufferingprocess,carbondioxide,