Handbook of Blood Gas/Acid–Base Interpretation

362 Pages·2009·20.78 MB·English

by  Ashfaq Hasan (auth.)

Ashfaq Hasan
Handbook of Blood
Gas/Acid–Base
Interpretation
1 3
AshfaqHasan
CareInstituteofMedicalSciences
Banjara,Hyderabad
India
ISBN:978-1-84800-333-0 e-ISBN:978-1-84800-334-7
DOI10.1007/978-1-84800-334-7
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LibraryofCongressControlNumber:2008938671
#Springer-VerlagLondonLimited2009
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Preface
Blood gas analysis has become the ‘‘...single most helpful
laboratory test in managing respiratory and metabolic disor-
ders.(Itis)...imperativetoconsideranABGforvirtuallyany
symptom..., sign..., or scenario... that occurs in a clinical
setting,whetheritbetheclinic,hospital,orICU.’’1
For the uninitiated, the analysis of blood gas can be a
daunting task. Hapless medical students, badly constrained
for time, have struggled ineffectively with Hasselbach’s mod-
ification of the Henderson equation; been torn between the
Copenhagen and the Boston schools of thought; and lately,
been confronted with the radically different strong-ion
approachofPeterStewart.
In the modern medical practice, the multi-tasking health
provider’s time has become precious—and his attention span
short.Itisthereforeimportanttoretainfocusonthoseaspects
ofclinicalmedicinethattrulymatter.Inthehandlingofthose
subjects rooted in clinical physiology (and therefore predicta-
bly difficult to understand), it makes perfect sense, in my
opinion, to adopt an ‘algorithmic’ approach. A picture can
say a thousand words; a well constructed algorithm can save
atleastahundred—nottosay,muchprecioustime—andmake
for clarity of thinking. I have personally found this method
1Canham,EM,Beuther,DA,InterpretingArterialBloodGases,PCCU
online,Chest
vii
viii Preface
relativelypainless—andeasytoassimilate.Thebookissetout
intheformofflowchartsinlogicalsequence,introducingand
graduallybuildingupontheunderlyingconcepts.
The goal of this book is to enable medical students, resi-
dents, nurses and respiratory care practitioners to quickly
grasptheprinciplesunderlyingrespiratoryandacid-basephy-
siology,andtoapplytheconceptseffectivelyinclinicaldecision
making. Each of these sections, barring a few exceptions, has
beendesignedtofitintoasinglepowerpointslide:thisshould
facilitateteaching.
Over the years, many excellent books and articles have
appearedonthesubject.IhavefoundthemanualsbyLawrence
Martin2 and Kerry Brandis3 thoroughly enjoyable as also the
online tutorials of Alan Grogono4 and Bhavani Shankar
Kodali5: I have tried to incorporate into my own book, some
oftheirenergyandcontent.
No matter how small, a project such as this can never be
accomplishedwithoutthesupportofwellwishersandfriends.
I would like to acknowledge the unwavering support of my
colleagues Dr TLN Swamy and Dr Syed Mahmood Ahmed;
myassistantsAShobaandPSudheer;andaboveall,myfamily
who had to endure the painstaking writing of yet another
manuscript.
AshfaqHasan
2Martin,L.Allyoureallyneedtoknowtointerpretbloodgases.1999,
LippincottWilliamsandWilkins.
3KerryBrandis.Acid-BasePhysiology;www.anaesthesiaMCQ.com
4Grogono,AW.www.acid-base.com
5BhavaniShankarKodali.2007.WelcometoCapnography.com
Contents
1 TheBloodGases ........................... 1
2 GasExchange ............................. 5
3 TheNoninvasiveMonitoringof
BloodGases............................... 63
4 AcidsandBases ............................ 117
5 BufferSystems............................. 143
6 ThepH................................... 165
7 AcidosisandAlkalosis ....................... 189
8 RespiratoryAcidosis ........................ 195
9 RespiratoryAlkalosis........................ 207
10 MetabolicAcidosis.......................... 213
11 MetabolicAlkalosis......................... 263
ix
x Contents
12 TheAnalysisofBloodGases&FactorsModifying
theAccuracyofABGResults.................. 279
13 CaseExamples............................. 297
Index........................................ 347
1
Chapter 1
The Blood Gases
Contents
1.1 TheUtilityofBloodGases ............................ 2
1.2 Electrodes.......................................... 3
A.Hasan,HandbookofBloodGas/Acid–BaseInterpretation, 1
DOI10.1007/978-1-84800-334-7_1,
(cid:2)Springer-VerlagLondonLimited2009
1
2 1 TheBloodGases
1.1 The Utility of Blood Gases
1
1 TheBloodGases 3
1.2 Electrodes
Thedesignoftheelectrodesinthebloodgasanalyserisbased
onthemodeloftheelectrochemicalcell.
Two half-cells are immersed in an electrolyte solution. An external connection which includes an
ammeter completes the circuit. Chemical reactions that consume electrons occur at each half-cell in
solution. The half-cell at which the stronger of the two reactions occurs becomes the cathode; the
other half cell (which is therefore negative relative to the first half-cell) becomes the anode.
The temperature of the chamber is held constant: usually at 37°C. The chemical reactions produce a
measurable flow of electrons through the external circuit.
The blood is analyzed by three separate electrodes:
CO electrode (The
2
pH electrode (The Sanz Severinghaus electrode)
O2 elecetlreocdtreo (dTeh)e Clark Compared teol ethcetr Coldaerk) eThleec tCrOod2 ee liemcmtroedrsee ids ian g alass
The working of the O electrode, the Sanz electrode bicarbonate buffer solution.
electrode is based on2 the is complex. The essence of The latter is contained in a
principle of polarography this electrode is a special nylon spacer and separated
hygroscopic glass membrane. by a membrane from the
blood sample.
The glass membrane separates
Tanhoe deele acntrdo da ep ilnatcilnuudmes c aa tshilovedre tehleec btrlooloydte s saomluptlieo nfr.o Tmhe an sCaOm2 pdlieff, uthseros uoguht othf eth seil ibcloonoed
immersed in potassium chloride membrane is kept completely membrane and into the
solution. A semipermeable hydrated. On contact with the bicarbonate buffer solution,
membrane separates this blood sample, hydrogen ions altering the pH of the latter. The
solution from the blood sample. dissociate from the membrane H+ are measured by a modified
and produce a measurable flow pH cell.
of electrons.
aOn2d m roealecctu wleitsh d tihffeu scea tinhtood teh. eT hceell A current is generated within the Tphoete dnitffiaelr ecrnecaet eins tah ceu errleecnttr. iTchale
number of electrons produced by electrode depending upon the number of hydrogen ions
this reaction is proportional to difference in the electrical generated within the bicarbonate
the pO of the blood sample. charges on either side. solution is proportional to the
2 PCO2.
Hansen,JE.Arterialbloodgases.In:MahlerDA,ed.PulmonaryFunc-
tionTesting.ClinChestMed1989;5:227–237
Madama, VC. In: Pulmonary Function Testing and Cardiopulmonary
StressTesting,2nded.,Delmar,1998