Final ilExam-Study ilGuide
Week il6 iland il7
1. Interpret ilarterial ilblood ilgases il(ABG). ilDifferentiate ilalkalosis/
ilacidosis iland ilrespiratory il/ ilmetabolic
2. Identify ila ilventilation il– ilperfusion ilmismatch iland ilhow ilto iltreat ilit
If ilthere ilis ila ilmismatch ilbetween ilthe ilalveolar ilventilation iland ilthe
ilalveolar ilblood ilflow, ilthis ilwill ilbe ilseen ilin ilthe ilV/Q ilratio. ilIf ilthe
ilV/Q ilratio ilreduces ildue ilto ilinadequate ilventilation, ilgas ilexchange
ilwithin ilthe ilaffected ilalveoli ilwill ilbe ilimpaired. ilAs ila ilresult, ilthe
ilcapillary ilpartial ilpressure ilof iloxygen il(pO2) ilfalls iland ilthe ilpartial
ilpressure ilof ilcarbon ildioxide il(pCO2) ilrises.
To ilmanage ilthis, ilhypoxic ilvasoconstriction ilcauses ilblood ilto ilbe
ildiverted ilto ilbetter ilventilated ilparts ilof ilthe illung. ilHowever, ilin
ilmost ilphysiological ilstates ilthe ilhemoglobin ilin ilthese ilwell-
ventilated ilalveolar ilcapillaries ilwill ilalready ilbe ilsaturated. ilThis
ilmeans ilthat ilred ilcells ilwill ilbe ilunable ilto ilbind iladditional iloxygen
ilto ilincrease ilthe ilpO2. ilAs ila ilresult, ilthe ilpO2 illevel ilof ilthe ilblood
, remains illow, ilwhich ilacts ilas ila ilstimulus ilto ilcause
ilhyperventilation, ilresulting ilin ileither ilnormal ilor illow ilCO2 illevels.
A ilmismatch ilin ilventilation iland ilperfusion ilcan ilarise ildue ilto ileither
ilreduced ilventilation ilof ilpart ilof ilthe illung ilor ilreduced ilperfusion.
Ventilation/perfusion ilmismatch il— ilMechanical ilventilation ilcan
ilalter iltwo ilopposing ilforms ilof ilventilation/perfusion ilmismatch il(V/Q
ilmismatch), ildead ilspace il(areas ilthat ilare iloverventilated ilrelative
ilto ilperfusion; ilV>Q) iland ilshunt il(areas ilthat ilare ilunderventilated
ilrelative ilto ilperfusion; ilV<Q). ilBy ilincreasing ilventilation il(V), ilthe
ilinstitution ilof ilpositive ilpressure ilventilation ilwill ilworsen ildead
ilspace ilbut ilimprove ilshunt.
Increased ildead ilspace il— ilDead ilspace ilreflects ilthe ilsurface
ilarea ilwithin ilthe illung ilthat ilis ilnot ilinvolved ilin ilgas ilexchange. ilIt ilis
ilthe ilsum ilof ilthe ilanatomic ilplus ilalveolar ildead ilspace. ilAlveolar
ildead ilspace il(also ilknown ilas ilphysiologic ildead ilspace) ilconsists
ilof ilalveoli ilthat ilare ilnot ilinvolved ilin ilgas ilexchange ildue ilto
ilinsufficient ilperfusion il(ie, iloverventilated ilrelative ilto ilperfusion).
ilPositive ilpressure ilventilation iltends ilto ilincrease ilalveolar ildead
ilspace ilby ilincreasing ilventilation ilin ilalveoli ilthat ildo ilnot ilhave ila
ilcorresponding ilincrease ilin ilperfusion, ilthereby ilworsening ilV/Q
ilmismatch iland ilhypercapnia.
Reduced ilshunt il— ilAn ilintraparenchymal ilshunt ilexists ilwhere
ilthere ilis ilblood ilflow ilthrough ilpulmonary ilparenchyma ilthat ilis ilnot
ilinvolved ilin ilgas ilexchange ilbecause ilof ilinsufficient ilalveolar
ilventilation. ilPatients ilwith ilrespiratory ilfailure ilfrequently ilhave
ilincreased ilintraparenchymal ilshunting ildue ilto ilareas ilof ilfocal
ilatelectasis ilthat ilcontinue ilto ilbe ilperfused il(ie, ilregions ilthat ilare
ilunderventilated ilrelative ilto ilperfusion). ilTreating ilatelectasis ilwith
ilpositive ilpressure ilventilation ilcan ilreduce ilintraparenchymal
ilshunting ilby ilimproving ilalveolar ilventilation, ilthereby ilimproving
ilV/Q ilmatching iland iloxygenation.
This ilis ilparticularly iltrue ilif ilPEEP ilis iladded. il(See il"Positive ilend-
expiratory ilpressure il(PEEP)" iland il"Measures ilof iloxygenation iland
ilmechanisms ilof ilhypoxemia", ilsection ilon il'V/Q ilmismatch'.)
3. Be ilable ilto ilcalculate ilan ilAa ilgradient. ilBe ilable ilto ilinterpret ilan ilAa ilgradient.
The ilalveolar ilto ilarterial il(A-a) iloxygen ilgradient ilis ila ilcommon
ilmeasure ilof iloxygenation il("A" ildenotes ilalveolar iland il"a" ildenotes
ilarterial iloxygenation). ilIt ilis ilthe ildifference ilbetween ilthe ilamount
ilof ilthe iloxygen ilin ilthe ilalveoli il(ie, ilthe ilalveolar iloxygen iltension
il[PAO2]) iland ilthe ilamount ilof iloxygen ildissolved ilin ilthe ilplasma
il(PaO2):
A-a iloxygen ilgradient il= ilPAO2 il- ilPaO2
PaO2 ilis ilmeasured ilby ilarterial ilblood ilgas, ilwhile ilPAO2 ilis
ilcalculated ilusing ilthe ilalveolar ilgas ilequation:
,PAO2 il= il(FiO2 ilx il[Patm il- ilPH2O]) il- il(PaCO2 il÷ ilR)
, where ilFiO2 ilis ilthe ilfraction ilof ilinspired iloxygen il(0.21 ilat ilroom
ilair), ilPatm ilis ilthe ilatmospheric ilpressure il(760 ilmmHg ilat ilsea
illevel), ilPH2O ilis ilthe ilpartial ilpressure ilof ilwater il(47 ilmmHg ilat
il37ºC), ilPaCO2 ilis ilthe ilarterial ilcarbon ildioxide iltension, iland ilR ilis
ilthe ilrespiratory ilquotient. ilThe ilrespiratory ilquotient ilis
ilapproximately il0.8 ilat ilsteady ilstate, ilbut ilvaries ilaccording ilto ilthe
ilrelative ilutilization ilof ilcarbohydrate, ilprotein, iland ilfat.
The ilA-a ilgradient ilcalculated ilusing ilthis ilalveolar ilgas ilequation
ilmay ildeviate ilfrom ilthe iltrue ilgradient ilby ilup ilto il10 ilmmHg. ilThis
ilreflects ilthe ilequation's ilsimplification ilfrom ilthe ilmore ilrigorous ilfull
ilcalculation iland ilthe ilimprecision ilof ilseveral ilindependent
ilvariables il(eg, ilFiO2 iland ilR).
The ilnormal ilA-a ilgradient ilvaries ilwith ilage iland ilcan ilbe ilestimated
ilfrom ilthe ilfollowing ilequation, ilassuming ilthe ilpatient ilis ilbreathing
ilroom ilair:
A-a ilgradient il= il2.5 il+ il0.21 ilx ilage ilin ilyears
The ilA-a ilgradient ilincreases ilwith ilhigher ilFiO2. ilWhen ila ilpatient
ilreceives ila ilhigh ilFiO2, ilboth ilPAO2 iland ilPaO2 ilincrease. ilHowever,
ilthe ilPAO2 ilincreases ildisproportionately, ilcausing ilthe ilA-a ilgradient
ilto ilincrease. ilIn ilone ilseries, ilthe ilA-a ilgradient ilin ilmen ilbreathing
ilair iland il100 ilpercent iloxygen ilvaried ilfrom il8 ilto il82 ilmmHg ilin
ilpatients ilyounger ilthan il40 ilyears ilof ilage iland ilfrom il3 ilto il120
ilmmHg ilin ilpatients ilolder ilthan il40 ilyears ilof ilage il[5].
Proper ildeterminations ilof ilthe ilA-a ilgradient ilrequire ilexact
ilmeasurement ilof ilFiO2 ilsuch ilas ilwhen ilpatients ilare ilbreathing
ilroom ilair ilor ilare ilreceiving ilmechanical ilventilation. ilThe ilFiO2 ilof
ilpatients ilreceiving ilsupplemental iloxygen ilby ilnasal ilcannula ilor
ilmask ilcan ilbe ilestimated iland ilthe ilA-a ilgradient ilapproximated ilbut
illarge ilvariations ilmay ilexist iland ilthe ilA-a ilgradient ilmay
ilsubstantially ilvary ilfrom ilthe ilpredicted, illimiting ilits ilusefulness.
ilThe iluse ilof ila il100 ilpercent ilnon-rebreathing ilmask ilreasonably
ilapproximates ilactual ildelivery ilof il100 ilpercent iloxygen iland ilcan
ilbe ilused ilto ilmeasure ilshunt.
Why iluse ilthe ilAa ilgradient:
The ilA-a ilGradient ilcan ilhelp ildetermine ilthe ilcause ilof
ilhypoxia; ilit ilpinpoints ilthe illocation ilof ilthe ilhypoxia
ilas ilintra- ilor ilextra- ilpulmonary.
When ilto iluse ilthe ilAa ilgradient:
Patients ilwith ilunexplained ilhypoxia.
Patients ilwith ilhypoxia ilexceeding ilthe ildegree ilof iltheir
ilclinical ilillness.
Week il6 iland il7
1. Interpret ilarterial ilblood ilgases il(ABG). ilDifferentiate ilalkalosis/
ilacidosis iland ilrespiratory il/ ilmetabolic
2. Identify ila ilventilation il– ilperfusion ilmismatch iland ilhow ilto iltreat ilit
If ilthere ilis ila ilmismatch ilbetween ilthe ilalveolar ilventilation iland ilthe
ilalveolar ilblood ilflow, ilthis ilwill ilbe ilseen ilin ilthe ilV/Q ilratio. ilIf ilthe
ilV/Q ilratio ilreduces ildue ilto ilinadequate ilventilation, ilgas ilexchange
ilwithin ilthe ilaffected ilalveoli ilwill ilbe ilimpaired. ilAs ila ilresult, ilthe
ilcapillary ilpartial ilpressure ilof iloxygen il(pO2) ilfalls iland ilthe ilpartial
ilpressure ilof ilcarbon ildioxide il(pCO2) ilrises.
To ilmanage ilthis, ilhypoxic ilvasoconstriction ilcauses ilblood ilto ilbe
ildiverted ilto ilbetter ilventilated ilparts ilof ilthe illung. ilHowever, ilin
ilmost ilphysiological ilstates ilthe ilhemoglobin ilin ilthese ilwell-
ventilated ilalveolar ilcapillaries ilwill ilalready ilbe ilsaturated. ilThis
ilmeans ilthat ilred ilcells ilwill ilbe ilunable ilto ilbind iladditional iloxygen
ilto ilincrease ilthe ilpO2. ilAs ila ilresult, ilthe ilpO2 illevel ilof ilthe ilblood
, remains illow, ilwhich ilacts ilas ila ilstimulus ilto ilcause
ilhyperventilation, ilresulting ilin ileither ilnormal ilor illow ilCO2 illevels.
A ilmismatch ilin ilventilation iland ilperfusion ilcan ilarise ildue ilto ileither
ilreduced ilventilation ilof ilpart ilof ilthe illung ilor ilreduced ilperfusion.
Ventilation/perfusion ilmismatch il— ilMechanical ilventilation ilcan
ilalter iltwo ilopposing ilforms ilof ilventilation/perfusion ilmismatch il(V/Q
ilmismatch), ildead ilspace il(areas ilthat ilare iloverventilated ilrelative
ilto ilperfusion; ilV>Q) iland ilshunt il(areas ilthat ilare ilunderventilated
ilrelative ilto ilperfusion; ilV<Q). ilBy ilincreasing ilventilation il(V), ilthe
ilinstitution ilof ilpositive ilpressure ilventilation ilwill ilworsen ildead
ilspace ilbut ilimprove ilshunt.
Increased ildead ilspace il— ilDead ilspace ilreflects ilthe ilsurface
ilarea ilwithin ilthe illung ilthat ilis ilnot ilinvolved ilin ilgas ilexchange. ilIt ilis
ilthe ilsum ilof ilthe ilanatomic ilplus ilalveolar ildead ilspace. ilAlveolar
ildead ilspace il(also ilknown ilas ilphysiologic ildead ilspace) ilconsists
ilof ilalveoli ilthat ilare ilnot ilinvolved ilin ilgas ilexchange ildue ilto
ilinsufficient ilperfusion il(ie, iloverventilated ilrelative ilto ilperfusion).
ilPositive ilpressure ilventilation iltends ilto ilincrease ilalveolar ildead
ilspace ilby ilincreasing ilventilation ilin ilalveoli ilthat ildo ilnot ilhave ila
ilcorresponding ilincrease ilin ilperfusion, ilthereby ilworsening ilV/Q
ilmismatch iland ilhypercapnia.
Reduced ilshunt il— ilAn ilintraparenchymal ilshunt ilexists ilwhere
ilthere ilis ilblood ilflow ilthrough ilpulmonary ilparenchyma ilthat ilis ilnot
ilinvolved ilin ilgas ilexchange ilbecause ilof ilinsufficient ilalveolar
ilventilation. ilPatients ilwith ilrespiratory ilfailure ilfrequently ilhave
ilincreased ilintraparenchymal ilshunting ildue ilto ilareas ilof ilfocal
ilatelectasis ilthat ilcontinue ilto ilbe ilperfused il(ie, ilregions ilthat ilare
ilunderventilated ilrelative ilto ilperfusion). ilTreating ilatelectasis ilwith
ilpositive ilpressure ilventilation ilcan ilreduce ilintraparenchymal
ilshunting ilby ilimproving ilalveolar ilventilation, ilthereby ilimproving
ilV/Q ilmatching iland iloxygenation.
This ilis ilparticularly iltrue ilif ilPEEP ilis iladded. il(See il"Positive ilend-
expiratory ilpressure il(PEEP)" iland il"Measures ilof iloxygenation iland
ilmechanisms ilof ilhypoxemia", ilsection ilon il'V/Q ilmismatch'.)
3. Be ilable ilto ilcalculate ilan ilAa ilgradient. ilBe ilable ilto ilinterpret ilan ilAa ilgradient.
The ilalveolar ilto ilarterial il(A-a) iloxygen ilgradient ilis ila ilcommon
ilmeasure ilof iloxygenation il("A" ildenotes ilalveolar iland il"a" ildenotes
ilarterial iloxygenation). ilIt ilis ilthe ildifference ilbetween ilthe ilamount
ilof ilthe iloxygen ilin ilthe ilalveoli il(ie, ilthe ilalveolar iloxygen iltension
il[PAO2]) iland ilthe ilamount ilof iloxygen ildissolved ilin ilthe ilplasma
il(PaO2):
A-a iloxygen ilgradient il= ilPAO2 il- ilPaO2
PaO2 ilis ilmeasured ilby ilarterial ilblood ilgas, ilwhile ilPAO2 ilis
ilcalculated ilusing ilthe ilalveolar ilgas ilequation:
,PAO2 il= il(FiO2 ilx il[Patm il- ilPH2O]) il- il(PaCO2 il÷ ilR)
, where ilFiO2 ilis ilthe ilfraction ilof ilinspired iloxygen il(0.21 ilat ilroom
ilair), ilPatm ilis ilthe ilatmospheric ilpressure il(760 ilmmHg ilat ilsea
illevel), ilPH2O ilis ilthe ilpartial ilpressure ilof ilwater il(47 ilmmHg ilat
il37ºC), ilPaCO2 ilis ilthe ilarterial ilcarbon ildioxide iltension, iland ilR ilis
ilthe ilrespiratory ilquotient. ilThe ilrespiratory ilquotient ilis
ilapproximately il0.8 ilat ilsteady ilstate, ilbut ilvaries ilaccording ilto ilthe
ilrelative ilutilization ilof ilcarbohydrate, ilprotein, iland ilfat.
The ilA-a ilgradient ilcalculated ilusing ilthis ilalveolar ilgas ilequation
ilmay ildeviate ilfrom ilthe iltrue ilgradient ilby ilup ilto il10 ilmmHg. ilThis
ilreflects ilthe ilequation's ilsimplification ilfrom ilthe ilmore ilrigorous ilfull
ilcalculation iland ilthe ilimprecision ilof ilseveral ilindependent
ilvariables il(eg, ilFiO2 iland ilR).
The ilnormal ilA-a ilgradient ilvaries ilwith ilage iland ilcan ilbe ilestimated
ilfrom ilthe ilfollowing ilequation, ilassuming ilthe ilpatient ilis ilbreathing
ilroom ilair:
A-a ilgradient il= il2.5 il+ il0.21 ilx ilage ilin ilyears
The ilA-a ilgradient ilincreases ilwith ilhigher ilFiO2. ilWhen ila ilpatient
ilreceives ila ilhigh ilFiO2, ilboth ilPAO2 iland ilPaO2 ilincrease. ilHowever,
ilthe ilPAO2 ilincreases ildisproportionately, ilcausing ilthe ilA-a ilgradient
ilto ilincrease. ilIn ilone ilseries, ilthe ilA-a ilgradient ilin ilmen ilbreathing
ilair iland il100 ilpercent iloxygen ilvaried ilfrom il8 ilto il82 ilmmHg ilin
ilpatients ilyounger ilthan il40 ilyears ilof ilage iland ilfrom il3 ilto il120
ilmmHg ilin ilpatients ilolder ilthan il40 ilyears ilof ilage il[5].
Proper ildeterminations ilof ilthe ilA-a ilgradient ilrequire ilexact
ilmeasurement ilof ilFiO2 ilsuch ilas ilwhen ilpatients ilare ilbreathing
ilroom ilair ilor ilare ilreceiving ilmechanical ilventilation. ilThe ilFiO2 ilof
ilpatients ilreceiving ilsupplemental iloxygen ilby ilnasal ilcannula ilor
ilmask ilcan ilbe ilestimated iland ilthe ilA-a ilgradient ilapproximated ilbut
illarge ilvariations ilmay ilexist iland ilthe ilA-a ilgradient ilmay
ilsubstantially ilvary ilfrom ilthe ilpredicted, illimiting ilits ilusefulness.
ilThe iluse ilof ila il100 ilpercent ilnon-rebreathing ilmask ilreasonably
ilapproximates ilactual ildelivery ilof il100 ilpercent iloxygen iland ilcan
ilbe ilused ilto ilmeasure ilshunt.
Why iluse ilthe ilAa ilgradient:
The ilA-a ilGradient ilcan ilhelp ildetermine ilthe ilcause ilof
ilhypoxia; ilit ilpinpoints ilthe illocation ilof ilthe ilhypoxia
ilas ilintra- ilor ilextra- ilpulmonary.
When ilto iluse ilthe ilAa ilgradient:
Patients ilwith ilunexplained ilhypoxia.
Patients ilwith ilhypoxia ilexceeding ilthe ildegree ilof iltheir
ilclinical ilillness.
