Download - Acid Base Disorders Prof. Tahir Shafi
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Acid Base DisordersProf. Tahir Shafi
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Why should we know about acid base disorders
What are acid base disordersHow to interpret acid base
disorderHow to establish the cause
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Why to worry about acid base disordersAcid base homeostasis
fundamental for maintaining life Extreme acidemia or alkalemia
can◦alter tertiary protein structure
affecting enzyme activities ion transport
◦alter almost every metabolic pathway
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Extreme acidemia can◦depress cardiac function, impair the
vascular response to catecholamines, and cause arteriolar vasodilation and venoconstriction, with resultant systemic hypotension and pulmonary edema.
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Alkalemia can cause ◦cardiac arrhythmias, neuromuscular
irritability, and contribute to tissue hypoxemia.
◦fall in cerebral and myocardial blood flow
◦respiratory depression◦potassium abnormalities a common
accompaniment of acid-base disorders, also contribute to the morbidity.
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Identification of mild to moderate disorders can give you important clue to the presence of unsuspected but may be otherwise important medical conditions
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A patient has the following lab tests:ABG pH 7.4 PO2 100 mmHgPCO2 40 Bicarb 24 meq/lSodium 140 Potassium 4,2Chloride 80 Bicarb 24Creatinine 0.9 Glucose 110Which one of the following statements is correctA. He has normal acid base parametersB. He has metabolic acidosis and respiratory
alkalosisC. He has metabolic alkalosis and respiratory
acidosisD. He has metabolic acidosis and metabolic alkalosis
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Regulation of acid base statusAcidity in the ECF measured by
[H]+ H+ = 38-42 nEq/L Average = 0.00004 mEq/L = 40
nEq/LpH - negative log of H+ in moles Maintenance of arterial pH within
normal range 7.38-7.42 with average pH 7.4
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Henderson Hasselbach equation
pH = pk + logBaseAcid
6.1 + log HCO3
H2CO3
24apCO2
241.2
1.3=7.4
a =0.03 Log 20
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Kassirer - Bleich Equation
H+ = 24 x pCO2
HCO3
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pH and H+ are inversely relatedh H+ results in i pH - Acidemia i H+ and hpH – AlkalemiaNormal pH or H+ - Euphemia
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Types of acid base disorders
H+ = 24 x pCO2
HCO3
RespiratoryAcidosis
Alkalosis
MetabolicAlkalosis
AcidosisStabilization of H+ for every primary disorder there must be physiological compensation by the opposite system
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Compensations are predictable and can calculatedRespiratory compensation
Primary
pH Initial event
CompResp
Expected Cpmpensation
Met. Acidosis
i iHCO3 ipCO2 For 1 meq iHCO3, pCO2 i by 1-1.3 mmHgPCO2 = HCO3+ 15PCO2 = last 2 digits of pHpCO2 = (HCO3) x 1.5+8+2 Winter’s equation
Met. Alkalosis
h hHCO3 hpCO2 For 1 meq h HCO3,PCO2 h by 0.6 mmHgPCO2 =(HCO3 x 0.9) + 9PCO2 =(HCO3 x 0.7) + 20
H+ = 24 x pCO2
HCO3
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Metabolic compensation
Primary pH Initial event
CompResp
Expected Cpmpensation
Ac. Resp. Acidosis i hpCO2 hHCO3 For every 10 mmHg h PCO2 ,HCO3 h
by 1
Ch. Resp. Acidosis
i hPCO2 hHCO3 For every 10 mmHg h PCO2, HCO3 h by 3.5
Acute Resp Alkalosis
h ipCO2 iHCO3 For every 10 mmHg i PCO2, HCO3 i by 2
Ch. Resp Alkalosis h ipCO2
iHCO3 For every 10 mmHg i PCO2, HCO3 i by 5
H+ = 24 x pCO2
HCO3
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15
Rates of correctionBuffers function almost
instantaneouslyRespiratory mechanisms take
several minutes to hoursRenal mechanisms may take
several hours to days
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Classification acid base disorders Simple
◦ Metabolic acidosis◦ Metabolic alkalosis◦ Respiratory acidosis◦ Respiratory alkalosis
Primary change in one component Secondary change in the other component within physiological
range Change in ph in the direction of primary change
Mixed◦ Double
Primary change with incomplete or over compensation with other component
◦ Triple◦ Quadruple
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Normal acid base homeostasis
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Metabolic acidosisExogenous
load HClOther acids
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Types of metabolic acidosisAddition of acid other than HCl
◦Serum chloride normal, bicarbonate low
◦High anion metabolic acidosisAddition of HCl
◦Serum chloride high, bicarbonate low◦Normal anion or hyperchloremic
metabolic acidosis
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Anion gap
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Albumin is a major source of unmeasured anions!
Corrected AG = Observed AG + 2.5 x (4.5 – measured albumin)
If serum albumin is high, corrected anion gap will be lower
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Metabolic AcidosisCauses of high Anion Gap metabolic acidosisK Diabetic KetoacidosisU UremiaS Salicylate intoxicationS Starvation ketosisM Methanol ingestionA Alcohol ketoacidosisU Unmeasured Osmoles, Ethylene Glycol, ParaldehydeL Lactic acidosis
MUDPILES Methanol, Uremia, DKA, Paraldehyde, INH/Iron toxicity, Lactic acidodis, Ethanol/Ethlene glycol, Salicylate toxicity
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Metabolic AcidosisCauses of normal Anion Gap metabolic
acidosis
H HyperalimentationA AcetazolamideR Renal tubular acidosis D DiarrhoeaU Uretero sigmoidostomyP Pancreatic fistula
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D – DiarrheaU – Ureteral diversionR – Renal tubular acidosisH – HyperalimentationA – AcetazolamideM - Miscellaneous (toluene,
amphotericin B
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Hyperchloremic metabolic acidosis Normal or high ammonia
excretionDecreased ammonia excretion
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Urine anion gap
Negative urine anion gap positive urine anion gap
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Urine osmolal gapMeasured urine osmolality – calculated
urine osmolalityCalculated urine osmolality
= 2 (UNa + UK )+
< 150 mosm /l= impaired NH4 excretion>150 mosm/l = increased NH4 excretionUrine ammonia level = half urine osmolal gap
Urea6
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Metabolic acidosis & osmolal gapEthyl Alcohol Acetaldehyde Acetic AcidMethyl Alcohol Formaldhyde Formic
AcidEthylene Alcohol Glycoaldehyde
Glycolic acid Glycooxalic acid Oxalic acid
OSMOLAL GAP = Measured osmolality- calculated osmolality
Normal = < 10-15 mOsmol/Kg of H2OCalculated serum osmolaliy = 2(Na) + + Urea
6BS18
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Metabolic alkalosisAddition of base or loss of acid
from bodyHigh pH, high bicarbonate, high
pCO2
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Metabolic alkalosisSaline responsive alkalosisLow urinary Cl- (<10 mmol/l)
Normal or low BP
vomitingPost diureticsAfter hypercapniaK+ depletionRefeeding alkalosis
Saline resistant alkalosis High Urinary Cl- (>10 mmol/l)
Normal BP High BP
Bartter’s syndrome Reno vascularDiuretics diseaseMg++ depletion Conn’s syndromeSevere K+ depletion Cushing’s syndrome
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More then one problem?The “gap-gap” or “delta-delta” ratioIn the presence of a high AG
metabolic acidosis, ? another metabolic acid base disorder
Comparing the AG excess to the HCO3 deficit - D AG =D HCO3
D AG = (Observed AG – 12) D HCO3 = (Observed – Normal HCO3)
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Mixed metabolic acid base disorders
D AGD HCO3 hh
CoexistentHypercholemic acidosis
delta/ delta <1
Coexistent Metabolic alkalosisdelta/delta > 1 hh
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Mixed metabolic acid base disordersCorrected bicarbonate =
observed biacrbonate + > 24 meq/l g Metabolic Alkalosis< 24 meq/l g Metabolic Acidosis
D AG
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Respiratory acid base disorders
H+ = 24 x pCO2
HCO3
Respiratory acidosis Respiratory alkalosisHypoventilation Hyperventilation
Primary eventChange in pCO2
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Respiratory Alkalosis or AcidosisNeuro respiratory control
◦Central hypo or hyperventilation◦Intrinsic lung disease
A-a gradient (Alveolar arterial gradient)Alveolar O2 =(760 – 47) * 0.21* PCO2/0.8 =100Atmospheric pressure – water vapor pressure * FiO2* PaCO2
Arterial (a) =100 A-a gradient normal is < 10 in adults and < 20 in elderly
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Respiratory AlkalosisLow pCO2, low bicarbonate, high pH
◦A–a difference >10 mm Hg (>20 mm Hg in elderly) - Hyperventilation with intrinsic lung disease, ventilation–perfusion mismatch, or both (e.g., pneumonia, pulmonary embolism)
◦A–a difference <10 mm Hg (<20 mm Hg in elderly) Hyperventilation without intrinsic lung disease (e.g., fever, pregnancy)
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Respiratory AcidosisHigh pCO2, high bicarbonate, low
pH◦A–a difference >10 mm Hg (>20 mm
Hg in elderly) Hypoventilation with intrinsic lung disease, ventilation–perfusion mismatch, or both (e.g., pneumonia, pulmonary embolism)
◦A–a difference <10 mm Hg (<20 mm Hg in elderly) - Hypoventilation without intrinsic lung disease (CNS suppression)
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Approach to Acid Base Disorders
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HistoryPhysical examinationSerum electrolytes
◦Anion gap◦Delta/delta ratio
Arterial blood gasesUrinary electrolytes – Urinary
anion gap urine osmolal gapSerum osmolal gap
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Acid Base AnalysisStep 1 : Look at pH. acidemic or
alkalemic?Step 2 : Look at pCO2
High – respiratory acidosisLow – respiratory alkalosis
Step 3: Look at HCO3 High – metabolic alkalosisLow – metabolic acidosis
Step 4: Determine primary event - direction of pH
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Step 5: Is it simple or mixed disorder. compensatory response within physiological range ?
Golden rules: 1- Mixed problem present if only one component abnormal2- Normal pH – both components
abnormal3-Both components in same direction
Step 6 : Look at base excess if availableStep 7: If the respiratory disturbance is
present, is it acute or chronic?
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Step 8 : Is there increased anion gap?
Step 9 : If anion gap increased, calculate corrected HCO3 or delta/delta
Step 10 : Calculate osmolal gap if ingestion of alcohols suspected
Step 11: Calculate urine anion gap or osmolol gap
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Acid Base Disorders
pH 7.54PCO2 30 mmHgHCO3 25 meq/lNa+ 140 meq/lCl- 92 meq/lK+ 3.6 meq/l
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Acid Base Disorders
pH 7.25pO2 70 mmHgPCO2 40 mmHgHCO3 17 meq/lNa+ 140 meq/lCl- 89 meq/lK+ 3.6 meq/l
A 55 years old man with renal failure presented with vomiting altered consciousness and fever
A. Respiratory acidosisB.Metabolic AcidosisC.Metabolic alkalosis
+ respiratory acidosis
D. Metabolic acidosis + Respiratory acidosisE. D + Metabolic alkalosis
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Acid Base Disorders
pH 7.25pO2 70 mmHgPCO2 40 mmHgHCO3 17 meq/lNa+ 140 meq/lCl- 89 meq/lK+ 3.6 meq/lAG 140-(89+17) =34D AG = 34-10=24D HCO3 = 24-17=7 High A-a gradient
A ss years old man with renal failure presented with vomiting and pleuritic chest pain
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Acid Base Disorders
pH 7.4PCO2 40 mmHgHCO3 24 meq/lNa+ 143 meq/lCl- 95 meq/lK+ 3.6 meq/lS. Albumin 1 gram/dl
16 years old boy , a case of FSGS on diuretics and Steroids presented with BP 70/40 mmHg
A. no acid base disorder
B. Respiratory acidosis
C. Metabolic alkalosis + resp acidosis
D. Metabolic acidosis +
resp alkalosisE, None of the above
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Acid Base Disorders
pH 7.4PCO2 40 mmHgHCO3 24 meq/lNa+ 143 meq/lCl- 95 meq/lK+ 3.6 meq/lS. Albumin 1 gram/dl
16 years old boy , a case of FSGS on diuretics and Steroids presented with BP 70/40 mmHg
A. no acid base disorder
B. Respiratory acidosis
C. Metabolic alkalosis + resp acidosis
D. Metabolic acidosis +
resp alkalosisE, None of the above
AG =143-(95+24) = 24Corrected AG= 24+(4- Alb) * 2.5 = 31.5Delta AG = 21.5Delta bicarb = 0Mixed high anion gap metabolic acidosis + Alkalosis
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Glucose 90 mg/dl
Urea 30 mg/dl
Serum Sodium 136 meq/l
Serum potassium 4 meq/l
Calc osm 291 mosm/l
Serum osm 311 mosm/l
Osmolar gap =20 delta OG=10
460 mg/l of ethanol or320 mg/l of methanol
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Acid Base Disorders
HCO3 17 meq/lNa+ 140 meq/lCl- 89 meq/l
K+ 3.6 meq/l
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Berend K et al. N Engl J Med 2014;371:1434-1445
If delta delta ratio=1 only high AG Met Acidosis
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Berend K et al. N Engl J Med 2014;371:1434-1445
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Base Excess = 0.9287 (HCO3 - 24.4 + 14.83
(pH - 7.4))
Predicted pH = 7.4 + pCO2 * 0.08
pH = Actual pH – predicted pHBase Excess = pH *10/0.15
For each 0.15 unit difference in pH Base excess or deficit will be =10
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Approach to Acid Base DisordersTraditional or Boston approach Copen Hagen approach
◦Base excessStewart / Physiochemical approach
◦PCO2
◦Non-volatile weak ion acid buffer A Total Atotal = A- + AH (Albumin, Pi, SO4, Hb)
◦Strong ion difference (SID) (Na+K+Ca+Mg) – (Cl+Lac) = A- + HCO3 = 40-42
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The physiologic or “Boston” method ◦ uses measurements of arterial pH, pCO2, and [HCO3−]
together with an analysis of the anion gap (AG) and a set of compensation rules.
2. The Base Excess (BE) or “Copenhagen” method uses◦ measurements of arterial pH and pCO2, and calculation
of the BE and the AG.3. The physicochemical or “Stewart” method uses
◦ Measurements of arterial pH and pCO2 together with the calculated apparent (SIDa) and effective (SIDe) “Strong Ion Difference,” the “Strong Ion Gap” (SIG = SIDa-SIDe), and the total concentration of plasma weak acids (Atot).
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