Acid – Base Acid – Base DisturbanceDisturbance

Teguh SantosoTeguh Santoso

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