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FINAL EXAM-STUDY GUIDE QUESTIONS AND CORRECT ANSWERS

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Final Exam-Study Guide Week 6 and 7 1. Interpret arterial blood gases (ABG). Differentiate alkalosis/ acidosis and respiratory / metabolic 2. Identify a ventilation – perfusion mismatch and how to treat it If there is a mismatch between the alveolar ventilation and the alveolar blood flow, this will be seen in the V/Q ratio. If the V/Q ratio reduces due to inadequate ventilation, gas exchange within the affected alveoli will be impaired. As a result, the capillary partial pressure of oxygen (pO2) falls and the partial pressure of carbon dioxide (pCO2) rises. To manage this, hypoxic vasoconstriction causes blood to be diverted to better ventilated parts of the lung. However, in most physiological states the hemoglobin in these well-ventilated alveolar capillaries will already be saturated. This means that red cells will be unable to bind additional oxygen to increase the pO2. As a result, the pO2 level of the blood remains low, which acts as a stimulus to cause hyperventilation, resulting in either normal or low CO2 levels. A mismatch in ventilation and perfusion can arise due to either reduced ventilation of part of the lung or reduced perfusion. Ventilation/perfusion mismatch — Mechanical ventilation can alter two opposing forms of ventilation/perfusion mismatch (V/Q mismatch), dead space (areas that are overventilated relative to perfusion; VQ) and shunt (areas that are underventilated relative to perfusion; VQ). By increasing ventilation (V), the institution of positive pressure ventilation will worsen dead space but improve shunt. Increased dead space — Dead space reflects the surface area within the lung that is not involved in gas exchange. It is the sum of the anatomic plus alveolar dead space. Alveolar dead space (also known as physiologic dead space) consists of alveoli that are not involved in gas exchange due to insufficient perfusion (ie, overventilated relative to perfusion). Positive pressure ventilation tends to increase alveolar dead space by increasing ventilation in alveoli that do not have a corresponding increase in perfusion, thereby worsening V/Q mismatch and hypercapnia. Reduced shunt — An intraparenchymal shunt exists where there is blood flow through pulmonary parenchyma that is not involved in gas exchange because of insufficient alveolar ventilation. Patients with respiratory failure frequently have increased intraparenchymal shunting due to areas of focal atelectasis that continue to be perfused (ie, regions that are underventilated relative to perfusion). Treating atelectasis with positive pressure ventilation can reduce intraparenchymal shunting by improving alveolar ventilation, thereby improving V/Q matching and oxygenation. This is particularly true if PEEP is added. (See "Positive end-expiratory pressure (PEEP)" and "Measures of oxygenation and mechanisms of hypoxemia", section on 'V/Q mismatch'.) 3. Be able to calculate an Aa gradient. Be able to interpret an Aa gradient. The alveolar to arterial (A-a) oxygen gradient is a common measure of oxygenation ("A" denotes alveolar and "a" denotes arterial oxygenation). It is the difference between the amount of the oxygen in the alveoli (ie, the alveolar oxygen tension [PAO2]) and the amount of oxygen dissolved in the plasma (PaO2): A-a oxygen gradient = PAO2 - PaO2 PaO2 is measured by arterial blood gas, while PAO2 is calculated using the alveolar gas equation: PAO2 = (FiO2 x [Patm - PH2O]) - (PaCO2 ÷ R)

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FINAL EXAM-STUDY GUIDE QUESTIONS AND
CORRECT ANSWERS
1. Interpretqarterialqbloodqgasesq(ABG).qDifferentiateqalkalosis/qacidosi
sqandqrespiratoryq/qmetabolic

2. Identifyqaqventilationq–qperfusionqmismatchqandqhowqtoqtreatqit

Ifqthereqisqaqmismatchqbetweenqtheqalveolarqventilationqandqtheqalveol
arqbloodqflow,qthisqwillqbeqseenqinqtheqV/Qqratio.qIfqtheqV/Qqratioqreduce
sqdueqtoqinadequateqventilation,qgasqexchangeqwithinqtheqaffectedqalv
eoliqwillqbeqimpaired.qAsqaqresult,qtheqcapillaryqpartialqpressureqofqoxy
genq(pO2)qfallsqandqtheqpartialqpressureqofqcarbonqdioxideq(pCO2)qris
es.

Toqmanageqthis,qhypoxicqvasoconstrictionqcausesqbloodqtoqbeqdive
rtedqtoqbetterqventilatedqpartsqofqtheqlung.qHowever,qinqmostqphysiolo
gicalqstatesqtheqhemoglobinqinqtheseqwell-
ventilatedqalveolarqcapillariesqwillqalreadyqbeqsaturated.qThisqmeans
qthatqredqcellsqwillqbequnableqtoqbindqadditionalqoxygenqtoqincreaseqth

eqpO2.qAsqaqresult,qtheqpO2qlevelqofqtheqblood

, remainsqlow,qwhichqactsqasqaqstimulusqtoqcauseqhyperventilation,qres
ultingqinqeitherqnormalqorqlowqCO2qlevels.

Aqmismatchqinqventilationqandqperfusionqcanqariseqdueqtoqeitherqreduc
edqventilationqofqpartqofqtheqlungqorqreducedqperfusion.

Ventilation/perfusionqmismatchq—
qMechanicalqventilationqcanqalterqtwoqopposingqformsqofqventilation/pe

rfusionqmismatchq(V/Qqmismatch),qdeadqspaceq(areasqthatqareqoverv
entilatedqrelativeqtoqperfusion;qV>Q)qandqshuntq(areasqthatqarequnderv
entilatedqrelativeqtoqperfusion;qV<Q).qByqincreasingqventilationq(V),qth
eqinstitutionqofqpositiveqpressureqventilationqwillqworsenqdeadqspaceqb
utqimproveqshunt.

Increasedqdeadqspaceq—
qDeadqspaceqreflectsqtheqsurfaceqareaqwithinqtheqlungqthatqisqnotqinvol

vedqinqgasqexchange.qItqisqtheqsumqofqtheqanatomicqplusqalveolarqdea
dqspace.qAlveolarqdeadqspaceq(alsoqknownqasqphysiologicqdeadqspac
e)qconsistsqofqalveoliqthatqareqnotqinvolvedqinqgasqexchangeqdueqtoqins
ufficientqperfusionq(ie,qoverventilatedqrelativeqtoqperfusion).qPositiveqp
ressureqventilationqtendsqtoqincreaseqalveolarqdeadqspaceqbyqincreasi
ngqventilationqinqalveoliqthatqdoqnotqhaveqaqcorrespondingqincreaseqinq
perfusion,qtherebyqworseningqV/Qqmismatchqandqhypercapnia.

Reducedqshuntq—
qAnqintraparenchymalqshuntqexistsqwhereqthereqisqbloodqflowqthroughq

pulmonaryqparenchymaqthatqisqnotqinvolvedqinqgasqexchangeqbecaus
eqofqinsufficientqalveolarqventilation.qPatientsqwithqrespiratoryqfailureqfr
equentlyqhaveqincreasedqintraparenchymalqshuntingqdueqtoqareasqofqf
ocalqatelectasisqthatqcontinueqtoqbeqperfusedq(ie,qregionsqthatqareqund
erventilatedqrelativeqtoqperfusion).qTreatingqatelectasisqwithqpositiveqp
ressureqventilationqcanqreduceqintraparenchymalqshuntingqbyqimprovi
ngqalveolarqventilation,qtherebyqimprovingqV/Qqmatchingqandqoxygen
ation.
ThisqisqparticularlyqtrueqifqPEEPqisqadded.q(Seeq"Positiveqend-
expiratoryqpressureq(PEEP)"qandq"Measuresqofqoxygenationqandqme
chanismsqofqhypoxemia",qsectionqonq'V/Qqmismatch'.)

3. BeqableqtoqcalculateqanqAaqgradient.qBeqableqtoqinterpretqanqAaqgradient.

Theqalveolarqtoqarterialq(A-
a)qoxygenqgradientqisqaqcommonqmeasureqofqoxygenationq("A"qdenot
esqalveolarqandq"a"qdenotesqarterialqoxygenation).qItqisqtheqdifferenceq
betweenqtheqamountqofqtheqoxygenqinqtheqalveoliq(ie,qtheqalveolarqoxy
genqtensionq[PAO2])qandqtheqamountqofqoxygenqdissolvedqinqtheqplas
maq(PaO2):

A-aqoxygenqgradientq=qPAO2q-qPaO2

PaO2qisqmeasuredqbyqarterialqbloodqgas,qwhileqPAO2qisqcalculatedqusi
ngqtheqalveolarqgasqequation:

PAO2q=q(FiO2qxq[Patmq-qPH2O])q-q(PaCO2q÷qR)

, whereqFiO2qisqtheqfractionqofqinspiredqoxygenq(0.21qatqroomqair),qPatm
qisqtheqatmosphericqpressureq(760qmmHgqatqseaqlevel),qPH2Oqisqtheqp

artialqpressureqofqwaterq(47qmmHgqatq37ºC),qPaCO2qisqtheqarterialqcar
bonqdioxideqtension,qandqRqisqtheqrespiratoryqquotient.qTheqrespirator
yqquotientqisqapproximatelyq0.8qatqsteadyqstate,qbutqvariesqaccordingqt
oqtheqrelativequtilizationqofqcarbohydrate,qprotein,qandqfat.

TheqA-
aqgradientqcalculatedqusingqthisqalveolarqgasqequationqmayqdeviateqfr
omqtheqtrueqgradientqbyqupqtoq10qmmHg.qThisqreflectsqtheqequation'sq
simplificationqfromqtheqmoreqrigorousqfullqcalculationqandqtheqimprecis
ionqofqseveralqindependentqvariablesq(eg,qFiO2qandqR).

TheqnormalqA-
aqgradientqvariesqwithqageqandqcanqbeqestimatedqfromqtheqfollowingqe
quation,qassumingqtheqpatientqisqbreathingqroomqair:

A-aqgradientq=q2.5q+q0.21qxqageqinqyears

TheqA-
aqgradientqincreasesqwithqhigherqFiO2.qWhenqaqpatientqreceivesqaqhig
hqFiO2,qbothqPAO2qandqPaO2qincrease.qHowever,qtheqPAO2qincreases
qdisproportionately,qcausingqtheqA-

aqgradientqtoqincrease.qInqoneqseries,qtheqA-
aqgradientqinqmenqbreathingqairqandq100qpercentqoxygenqvariedqfromq
8qtoq82qmmHgqinqpatientsqyoungerqthanq40qyearsqofqageqandqfromq3qt
oq120qmmHgqinqpatientsqolderqthanq40qyearsqofqageq[5].

ProperqdeterminationsqofqtheqA-
aqgradientqrequireqexactqmeasurementqofqFiO2qsuchqasqwhenqpatient
sqareqbreathingqroomqairqorqareqreceivingqmechanicalqventilation.qThe
qFiO2qofqpatientsqreceivingqsupplementalqoxygenqbyqnasalqcannulaqor

qmaskqcanqbeqestimatedqandqtheqA-

aqgradientqapproximatedqbutqlargeqvariationsqmayqexistqandqtheqA-
aqgradientqmayqsubstantiallyqvaryqfromqtheqpredicted,qlimitingqitsqusef
ulness.qThequseqofqaq100qpercentqnon-
rebreathingqmaskqreasonablyqapproximatesqactualqdeliveryqofq100qp
ercentqoxygenqandqcanqbequsedqtoqmeasureqshunt.

WhyquseqtheqAaqgradient:
TheqA-
aqGradientqcanqhelpqdetermineqtheqcauseqofqhypoxia;qitq
pinpointsqtheqlocationqofqtheqhypoxiaqasqintra-qorqextra-
qpulmonary.

WhenqtoquseqtheqAaqgradient:
Patientsqwithqunexplainedqhypoxia.
Patientsqwithqhypoxiaqexceedingqtheqdegreeqofqtheirqclin
icalqillness.

, 4. Identifyqclinicalqsymptomsqorqconditionsqindicatingqaqneedqtoqintubateq
andqventilateqaqpatient

Neuromuscularqdepressionqorqfailure
A. Drugs
OpiodsqSed
ativesqNMq
Blockers
B. Trauma
SpinalqCordqinjuryq
Phrenicqnerveqinjur
y
C. Disease
GuillainqBarreqsyndrome

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