Acid – Base Acid – Base Disturbance Disturbance Sri Sunarmiasih Sri Sunarmiasih
Acid – Base Acid – Base DisturbanceDisturbance
Sri SunarmiasihSri Sunarmiasih
Introduction Introduction
1887 : Arrhenius : 1887 : Arrhenius : acid = hydrogen (Hacid = hydrogen (H++) ion donor) ion donor base = hydroxyl (OHbase = hydroxyl (OH--) ion donor) ion donor
1948 : Singer and Hastings1948 : Singer and Hastings buffer base (BB) = a quantitative index of the surplus amount of buffer base (BB) = a quantitative index of the surplus amount of
fixed acid fixed acid or base in the bloodor base in the blood
1960 : Astrup1960 : Astrupstandard bicarbonat or base excess should be used as an index standard bicarbonat or base excess should be used as an index of the nonrespiratory acid-base status of the blood.of the nonrespiratory acid-base status of the blood.Henderson-Hasselbach : pH depends on the interaction of Henderson-Hasselbach : pH depends on the interaction of carbondioxide and bicarbonatcarbondioxide and bicarbonat
1983 : Stewart1983 : StewartSID (strong ion difference), based on electroneutrality and SID (strong ion difference), based on electroneutrality and mass conservationmass conservation
(Ferrer L. 2006. Critical Care Medicine)(Ferrer L. 2006. Critical Care Medicine)
Arterial Blood GasesArterial Blood Gases
Obtaining ABGs Obtaining ABGs
ABGs are ABGs are valuablevaluable only only ifif
obtained properlyobtained properly and and measured measured carefullycarefully
stable FiOstable FiO22 for at least 10 minutes for at least 10 minutes
body position body position supine = SpO supine = SpO22 worse worse
ventilatory pattern ventilatory pattern RR/ depth RR/ depth ~~ PaCOPaCO22 /PaO /PaO22
prolonged attempt prolonged attempt pain/hyperventilation pain/hyperventilation
Pitfalls in Pitfalls in Collection, Analysis, and Collection, Analysis, and
InterpretationInterpretation Timing of analysisTiming of analysis : do not delay : do not delay
PaCOPaCO22 rises 3 – 10 mmHg/hour in rises 3 – 10 mmHg/hour in un-icedun-iced specimens specimensPaOPaO22 remains stable in an iced sample for 1 – 2 hr remains stable in an iced sample for 1 – 2 hr
Pseudohypoxemiadiffusion of O2 through the wall of plastic syringes may lead to false reduction in measured PaO2
Pseudoacidosisexcessive amounts of acidic heparin in the sampling syringe may cause pseudoacidosis by diluting and/or neutralizing serum bicarbonate
Air bubbles room air : PO2 = 150/PCO2 = 0 when large air bubbles are mixed with arterial blood PaO2 rises/PaCO2 falls
Contamination of arterial samples with venous blood
Basic ABG ConceptsBasic ABG Concepts Normally, Normally,
arterial pH 7,35 – arterial pH 7,35 – 7,457,45
Breathing room air : Breathing room air : PaCOPaCO22 35 – 45 mmHg 35 – 45 mmHg
PaOPaO22 > 80 – 90 > 80 – 90 mmHgmmHg
Venous blood gases : Venous blood gases : lower pH (~ 7,35)lower pH (~ 7,35)
PvOPvO22 ~ 40 mmHg ~ 40 mmHg
PvCOPvCO22 ~ 45 mmHg ~ 45 mmHg
Buffer SystemBuffer System
Carbonic acid : Carbonic acid : COCO22 , HCO , HCO33
-- Noncarbonic : Noncarbonic :
albumin, albumin, hemoglobin, hemoglobin, phosphat, bone phosphat, bone carbonatescarbonates
Compensatory Compensatory mechanism : renal, mechanism : renal, respiratoryrespiratory
Acid-Base DerangementsAcid-Base Derangements
Terminology of acid-base disordersTerminology of acid-base disorders acidemia acidemia and and alkalemiaalkalemia refer to refer to blood blood
pHpH, ,
acidemiaacidemia = blood = blood pH < 7.35pH < 7.35
alkalemiaalkalemia = blood = blood pH > 7.45pH > 7.45 acidosisacidosis and and alkalosis alkalosis refer to basic refer to basic
pathophysiologicpathophysiologic processes or processes or tendencies favoring the development tendencies favoring the development of acidemia or alkalemiaof acidemia or alkalemia
Stepwise ABG AnalysisStepwise ABG Analysis
Henderson-Hasselbach approachHenderson-Hasselbach approach1. pH is analyze first : 1. pH is analyze first :
pH < 7.35 = acidemiapH < 7.35 = acidemiapH > 7.45 = alkalemiapH > 7.45 = alkalemiapH normal pH normal ?? no acid-base disorderno acid-base disorder acid-base disorders with perfectly acid-base disorders with perfectly offsettingoffsetting
pH effects exist pH effects exist near complete physiologic near complete physiologic
compensationcompensation
Henderson-hasselbach…Henderson-hasselbach…
2. In the acidemic patient, 2. In the acidemic patient,
an an elevatedelevated PaCOPaCO22 indicates that some indicates that some component of component of respiratoryrespiratory acidosisacidosis is is present.present.bicarbonatbicarbonat concentration can be used to concentration can be used to decide whether appropriate decide whether appropriate compensationcompensation is occurring or if concurrent is occurring or if concurrent metabolic metabolic disorderdisorder is present : is present :
HCOHCO33 = = (0.10 – 0.35) unit (0.10 – 0.35) unit PaCO PaCO2 2
appropriate metabolic compensation.appropriate metabolic compensation.
lesser lesser HCOHCO33 complicating metabolic complicating metabolic acidosis or insufficient time to compensate.acidosis or insufficient time to compensate.
greater greater HCOHCO33 super imposed super imposed metabolic alkalosismetabolic alkalosis
Henderson-Hasselbach…Henderson-Hasselbach…
3. In acidemic patient…3. In acidemic patient…
a a reduced PaCOreduced PaCO22 indicates indicates metabolic acidosismetabolic acidosis
Diagnosis is reached by comparing the observed Diagnosis is reached by comparing the observed PaCOPaCO2 2 to that predicted by directly measuring to that predicted by directly measuring the serum HCOthe serum HCO3 3 content. content.
the expected PaCOthe expected PaCO2 2 = (1.5 x HCO = (1.5 x HCO33) + (8 ) + (8 2) 2)
1.0 – 1.3 mmHg PaCO1.0 – 1.3 mmHg PaCO22 change in 1 mEq change in 1 mEq HCOHCO33
ifif : : observed PaCOobserved PaCO2 2 == expected value expected value
simple metabolic acidosis + simple metabolic acidosis +
appropriate respiratory compensation appropriate respiratory compensation
PaCOPaCO22 >> expected value expected value
respiratory + metabolic acidosisrespiratory + metabolic acidosis
Henderson-Hasselbach…Henderson-Hasselbach…
4. In the alakalemic patient,4. In the alakalemic patient,
a a low PaCOlow PaCO22 diagnoses diagnoses respiratory respiratory alakalosisalakalosis
reduction in reduction in HCOHCO33 of 0.2-0.5 x change inof 0.2-0.5 x change in
PaCOPaCO22 compensationcompensation
failure failure to lower HCOto lower HCO33 byby 0.2 x change in 0.2 x change in
PaCOPaCO22 superimposed metabolic superimposed metabolic alkalosisalkalosis
HCOHCO3 3 that that declines > 0.5 declines > 0.5 x change in x change in PaCOPaCO22
a component ofa component of metabolic acidosis metabolic acidosis
Expected compensation for acid-base Expected compensation for acid-base disordersdisorders
Primary Primary disorderdisorder
PrimarPrimary y
changechange
CompensatCompensatory changeory change
Expected Expected compensationcompensation
MetaboliMetabolic acidosisc acidosis
HCOHCO3 3 PaCOPaCO22 PaCOPaCO2 2 = 1.2 = 1.2 HCOHCO33
MetaboliMetabolic c alkalosisalkalosis
HCOHCO33 PaCOPaCO2 2 PaCOPaCO2 2 = 0.9 = 0.9 HCOHCO33
RespiratoRespiratory ry acidosisacidosis
PaCOPaCO2 2 HCOHCO33
AcuteAcute
ChronicChronicHCOHCO3 3 = 0.10 = 0.10 PaCOPaCO2 2
HCOHCO3 3 = 0.35 = 0.35 PaCOPaCO2 2
RespiratoRespiratory ry alkalosisalkalosis
PaCOPaCO22 HCOHCO3 3
AcuteAcute
chronicchronicHCOHCO3 3 = 0.2 = 0.2 PaCOPaCO2 2
HCOHCO3 3 = 0.5 = 0.5 PaCOPaCO2 2
N
Metabolic Alkalosis
Uncompensated Metab. Alk
Resp. Alkalosis
Metabolic Acidosis
Respiratory Acidosis
Stewart Approach (Strong Ion Stewart Approach (Strong Ion Difference)Difference)
Three components in Three components in biological fluids biological fluids arearesubjected to these principals :subjected to these principals :
1. 1. waterwater, weakly dissociated into H, weakly dissociated into H+ + and and OHOH--
2. 2. strong ionsstrong ions, completely dissociated , completely dissociated such as Nasuch as Na++, K, K++, Cl, Cl-- and certain and certain
molecules ormolecules or compounds such as lactatecompounds such as lactate
3. 3. weak acidsweak acids, incompletely dissociated , incompletely dissociated compoundscompounds
Stewart strictly distinguished between Stewart strictly distinguished between dependent dependent and and independentindependent variables in variables in accord with these principlesaccord with these principles
Dependent variables can only change Dependent variables can only change if independent variables allow this if independent variables allow this changechange(changes in pH, H(changes in pH, H+ + and HCOand HCO33 are only are only possible if either SID or ATOT itself possible if either SID or ATOT itself changes)changes)
Dependent Dependent variablesvariables
bicarbonate bicarbonate
pHpH
HH++
Independent Independent variablesvariables
pCOpCO22
weak acid (ATOT)weak acid (ATOT)
strong ion differencestrong ion difference
(SID)(SID)
Strong ion difference…Strong ion difference…
In healthy humans, In healthy humans, the the normal normal SID is SID is 40 40 – 42– 42 mEq/lt mEq/lt
SID SID < 40< 40 relates to relates to metabolic acidosismetabolic acidosis
SIDSID > 42> 42 indicates indicates metabolic alkalosismetabolic alkalosis
SID = (Na+K+Ca+Mg) –SID = (Na+K+Ca+Mg) –
(Cl + lactate)(Cl + lactate)
SID = SID = Na+KNa+K - - ClCl
Na+
KCaMg
Cl-
PAlb
HCO3
UA