GE CARESCAPE - Page 344

Gasexchange
productionandO
2
consumption,aswellastheenergyexpenditure,EE,whichindicates
thenumberofcaloriesofenergythepatientisusing.Themeasurementofpulmonary
gasexchangecorrespondstothereleaseofenergyfromthebodyinasteadystate.
Asteadystateconditioncanbedenedasaperiodoftimeafterthepatienthas
stabilizedfromanychangesandwillnotincurfurtherchangesintreatmentthatmay
affecttheirgasexchangeorincreasemetabolism.Wheneverthehomeostasisofa
patientischanged,thesteadystateconditionisdisrupted,andacertainperiodof
timehastopassbeforeanewsteadystateisre-established.Thisshouldbenoted
inshort-timemeasurement.Incontinuousmeasurement,obtainingaverageresults
overlongerperiodshelpseliminatingtheeffectsofvaryingsteadystate.
Oxygenconsumption(VO
2
)
Indirectcalorimetrymeasuresoxygenconsumptionastheuptakeofoxygenfromthe
respiratorygases.Acutechangesinventilation,hemodynamics,andphysicalactivity
mayinducewidevariationsintheVO
2
measuredbyanymethod.SinceVO
2
canbe
measuredcontinuously,thetransientchangesinthemeasuredVO
2
canbereadily
observedinprolongedmeasurements.
Underaerobicconditions,VO
2
dependsonthemetabolicactivityofthetissues.Ata
givenmetabolicrate,thesubstratesofenergymetabolismalsohaveanimpacton
theVO
2
,sincetheamountofoxygenrequiredtoproducethesameamountofenergy
fromdifferentsubstratesvaries.Theamountofoxygenneededtoproduce1kcalof
energyfromcarbohydrateis207ml,fromfat213ml,andfromprotein223ml.
Iftheamountofoxygendeliveredtothetissuesisinadequateformetabolicneeds,
tissueoxygenconsumptionbecomesdependentonoxygendeliveryandanaerobic
metabolismwithlacticacidproductionwillensue.Duringanaerobicmetabolism,the
VO
2
measuredfromtherespiratorygasesdoesnotreectthetissueoxygenneeds,
sinceanoxygendebtdevelopsinthetissues.Whenaerobicconditionsarerestored,
theoxygendebtwillbereectedasincreasedoxygenconsumption.
Carbondioxideproduction(VCO
2
)
Measurementofcarbondioxideproduction(VCO
2
)byindirectcalorimetryis
susceptibletomajorerrorsunlessthecloserelationshipbetweenVCO
2
,alveolar
ventilation(VA),andarterialCO
2
(PaCO
2
)istakenintoaccount.Accordingtothe
classicalBohr'sequation,VCO
2
=VAxPaCO
2
/k,wherekisaconstantthatdependson
theunitsandtheconditions(pressure,temperature,humidity)ofthemeasurement.
Theconstantisequalto0.1150when:
●VCO
2
isgiveninml/min,standardtemperature(0°C)anddrygas(STPD),
●VAisgiveninl/min,37°C,andfullysaturatedwithwatervapor(BTPS),
●andPaCO
2
isgiveninkPa,
TheBohr'sequationdemonstratesthatthemeasurementofVCO
2
issensitiveto
changesinventilation:anychangeinalveolarventilationwillbedirectlyreectedin
VCO
2
untilanewsteadystateofPaCO
2
hasbeenachieved.
Insteadystate,VCO
2
dependsonthemetabolicactivityofthetissuesand,similarly
toVO
2
,onthesubstratesoftheenergymetabolism.Productionof1kcalofenergy
fromcarbohydrateproduces207mlofCO
2
,fromfatitproduces151ml,andfrom
proteinitproduces181ml.IfanyofthevariablesintheBohr'sequationchanges,
thebodyCO
2
poolwillchange.Underthesecircumstances,enoughtimeshouldbe
allowedforthebodyCO
2
pooltostabilizeifthemeasuredVCO
2
shouldreectthe
metabolicproductionofCO
2
.Continuousmeasurementofgasexchangefacilitates
thevericationofasteadystate.
344CARESCAPEModularMonitors2062971-001