Acid-Base Disturbances Clinical Approach 2006 Pravit Cadnapaphornchai

Simple vs Mixed Simple When compensation is appropriate Mixed When compensation is inappropriate

Simple Acid-Base Disturbances When compensation is appropriate Metabolic acidosis (↓ HCO3, ↓ pCO2) Metabolic alkalosis (↑ HCO3, ↑ pCO2) Respiratory acidosis (↑ pCO2, ↑ HCO3) Respiratory alkalosis (↓ pCO2, ↓ HCO3)

Stepwise Approaches History & physical examination Arterial blood gas for pH, pCO2, (HCO3) Use the HCO3 from ABG to determine compensation Serum Na, K, Cl, CO2 content Use CO2 content to calculate anion gap Calculate anion gap Anion gap = {Na - (Cl + CO2 content)} Determine appropriate compensation Determine the primary cause

Organ dysfunction CNS – respiratory acidosis (suppression) and alkalosis (stimulation) Pulmonary – respiratory acidosis (COPD) and alkalosis (hypoxia, pulmonary embolism) Cardiac – respiratory alkalosis, respiratory acidosis, metabolic acidosis (pulmonary edema) GI – metabolic alkalosis (vomiting) and acidosis (diarrhea) Liver – respiratory alkalosis, metabolic acidosis (liver failure) Kidney – metabolic acidosis (RTA) and alkalosis (1st aldosteone)

Organ Dysfunction Endocrine Drugs/toxins Diabetes mellitus – metabolic acidosis Adrenal insufficiency – metabolic acidosis Cushing’s – metabolic alkalosis Primary aldosteronism – metabolic alkalosis Drugs/toxins Toxic alcohols – metabolic acidosis ASA – metabolic acidosis and respiratory alkalosis Theophylline overdose – respiratory alkalosis

Stepwise Approaches History & physical examination Arterial blood gas for pH, pCO2, (HCO3) Use the HCO3 from ABG to determine compensation Serum Na, K, Cl, CO2 content Use CO2 content to calculate anion gap Calculate anion gap Anion gap = {Na - (Cl + CO2 content)} Determine appropriate compensation Determine the primary cause

pH < 7.35 7.4 >7.45 Acidosis Mixed Alkalosis Metabolic < 7.35 7.4 >7.45 Acidosis Metabolic Respiratory Mixed Alkalosis Metabolic Respiratory

Stepwise Approaches History & physical examination Arterial blood gas for pH, pCO2, (HCO3) Use the HCO3 from ABG to determine compensation Serum Na, K, Cl, CO2 content Use CO2 content to calculate anion gap Calculate anion gap Anion gap = {Na - (Cl + CO2 content)} Determine appropriate compensation Determine the primary cause

CO2 content Low Normal High Metabolic acidosis Normal Metabolic alkalosis Resp alkalosis Mixed Resp acidosis A normal CO2 content + high anion gap = metabolic acidosis + Metabolic alkalosis or metabolic ac + compensatory respiratory ac.

Stepwise Approaches History & physical examination Arterial blood gas for pH, pCO2, (HCO3) Use the HCO3 from ABG to determine compensation Serum Na, K, Cl, CO2 content Use CO2 content to calculate anion gap Calculate anion gap Anion gap = {Na - (Cl + CO2 content)} Determine appropriate compensation Determine the primary cause

Calculation of Anion Gap in Metabolic Acidosis Anion gap = Na – (Cl + HCO3) Normal 8 ± 2 Correction for low serum albumin Add (4-serum albumin g/dL) X 2.5 to the anion gap

Stepwise Approaches History & physical examination Arterial blood gas for pH, pCO2, (HCO3) Use the HCO3 from ABG to determine compensation Serum Na, K, Cl, CO2 content Use CO2 content to calculate anion gap Calculate anion gap Anion gap = {Na - (Cl + CO2 content)} Determine appropriate compensation Determine the primary cause

Compensations for Metabolic Disturbances Metabolic acidosis pCO2 = 1.5 x HCO3 + 8 ( ± 2) Metabolic alkalosis pCO2 increases by 7 for every 10 mEq increases in HCO3

How does the kidney compensate for metabolic acidosis?

How does the kidney compensate for metabolic acidosis? By reabsorbing all filtered HCO3 By excreting H+ as NH4+ (and H2PO4- ) Interpretations Urine pH < 5.5 Urine anion gap Negative

Compensations for Respiratory Acidosis Acute respiratory acidosis HCO3 increases by 1 for every 10 mm increases in pCO2 Chronic respiratory acidosis HCO3 increases by 3 for every 10 mm increases in pCO2 If you don’t have kidneys, can you have chronic respiratory acidosis?

Compensations for Respiratory Alkalosis Acute respiratory alkalosis HCO3 decreases by 2 for every 10 mm decrease in pCO2 Chronic respiratory alkalosis HCO3 decreases by 4 for every 10 mm decrease in pCO2 If you don’t have kidneys can you have chronic respiratory alkalosis?

Mixed Acid-Base Disorders Mixed respiratory alkalosis & metabolic acidosis ASA overdose Sepsis Liver failure Mixed respiratory acidosis & metabolic alkalosis COPD with excessive use of diuretics

Mixed Acid-Base Disorders Mixed respiratory acidosis & metabolic acidosis Cardiopulmonary arrest Severe pulmonary edema Mixed high gap metabolic acidosis & metabolic alkalosis Renal failure with vomiting DKA with severe vomiting

Stepwise Approaches History & physical examination Arterial blood gas for pH, pCO2, (HCO3) Use the HCO3 from ABG to determine compensation Serum Na, K, Cl, CO2 content Use CO2 content to calculate anion gap Calculate anion gap Anion gap = {Na - (Cl + CO2 content)} Determine appropriate compensation Determine the primary cause

Generation of Metabolic Acidosis Loss of HCO3 diarrhea Administration of HCl, NH4+Cl, CaCl2, lysine HCl Exogenous acids ASA Toxic alcohol Endogenous acids ketoacids DKA starvation alcoholic Lactic acid L-lactic D-lactate H+ HCO3- Compensations Buffers Lungs Kidneys High gap Normal gap If kidney function is normal, urine anion gap Neg

H HCO3 Loss of H+ from GI Vomiting, NG suction Congenital Cl diarrhea Loss of H+ from kidney 1st & 2nd aldosterone ACTH Diuretics Bartter’s, Gitelman’s, Liddle’s Inhibition of β – OH steroid deh H HCO3 Compensations Buffer Respiratory Forget the kidney Gain of HCO3 Administered HCO3, Acetate, citrate, lactate Plasma protein products

CASE 1 A 24 year old diabetic was admitted for weakness. Serum Na 140, K 1.8, Cl 125, CO2 6, anion gap 9. pH 6.84 (H+ 144) pCO2 30, HCO3 5

Interpretation of Case 1 Patient has normal gap metabolic acidosis

Interpretation of Case 1 Next determine the appropriateness of respiratory compensation pCO2 = 1.5 x HCO3 + 8 ( ± 2) pCO2 = 1.5 x 5 + 8 + 2 = 17.5 The patient’s pCO2 is 30 The respiratory compensation is inappropriate

Interpretation of Case 1 This patient has normal anion gap metabolic acidosis with inappropriate respiratory compensation The finding does not fit DKA but is consistent with HCO3 loss from the GI tract or kidney

How to differentiate normal gap acidosis resulting from GI HCO3 loss (diarrhea) vs dRTA?

Diarrhea vs RTA Diarrhea History Urine pH < 5.5 Negative urine anion gap dRTA History Urine pH > 5.5 Positive urine anion gap

Case 2 A 26 year old woman, complains of weakness. She denies vomiting or taking medications. P.E. A thin woman with contracted ECF. Serum Na 133, K 3.1, Cl 90, CO2 content 32, anion gap11. pH 7.48 (H+ 32), pCO2 43, HCO3 32. UNa 52, UK 50, UCl 0, UpH 8

Interpretation of Case 2 Determine the appropriateness of respiratory compensation For every increase of HCO3 by 1, pCO2 should increase by 0.7 pCO2 = 40 + (32-25) x 0.7 = 44.9 The patient’s pCO2 = 43

Interpretation of Case 2 This patient has metabolic alkalosis with appropriate respiratory compensation

Interpretation of Case 2 Urine Na+ 52, UK+ 50, Cl- 0, pH 8 Urine pH = 8 suggests presence of large amount of HCO3. The increased UNa and UK are to accompany HCO3 excretion. The kidney conserves Cl The findings are consistent with loss of HCl from the GI tract Final diagnosis = Self-induced vomiting

Vomiting vs Diuretic Active vomiting Remote vomiting Active diuretic ECF depletion Metabolic alkalosis High UNa, UK, low UCl Urine pH > 6.5 Remote vomiting Low UNa, high UK, low Cl Urine pH 6 Active diuretic ECF depletion Metabolic alkalosis High UNa, UK and Cl Urine pH 5-5.5 Remote diuretic Low UNa, high UK, low Cl Urine pH 5-6

Case 3 A 40 year old man developed pleuritic chest pain and hemoptysis. His BP 80/50. pH 7.4, pCO2 25, HCO3 15 and pO2 50

Interpretation of Case 3 A normal pH suggests mixed disturbances

Interpretation of Case 3 His pCO2 is 25, his HCO3 15 If this is acute respiratory alkalosis his HCO3 should have been 25-{(40-25) x 2/10}= 22 If this is chronic respiratory alkalosis, his HCO3 should have been 25 – {(40-25) x 4/10} = 19 If this is metabolic acidosis, his pCO2 should have been 1.5 x 15 + 8 = 30-31

Interpretation of Case 3 He has combined respiratory alkalosis and metabolic acidosis The likely diagnosis is pulmonary embolism with hypotension and lactic acidosis or pneumonia with sepsis and lactic acidosis Other conditions are ASA overdose, sepsis, liver failure

Case 4 A patient with COPD developed CHF. Prior to treatment his pH 7.35, pCO2 was 60 and HCO3 32. During treatment with diuretics he vomited a few times. His pH after treatment was 7.42, pCO2 80, HCO3 48.

Interpretation of Case 4 Pt’s data pH 7.35, pCO2 60 and HCO3 32 For acute respiratory acidosis For every 10 mm elevation of pCO2, HCO3 increases by 1, his HCO3 should have been 25 + (60-40) x 1/10 = 27 He did not have acute respiratory acidosis

Interpretation of Case 4 Pt’s data pH 7.35, pCO2 60 and HCO3 32. For chronic respiratory acidosis For every 10 mm elevation of pCO2, HCO3 increases by 3 His HCO3 should have been 25 + (60-40) x 3/10 = 31 His HCO3 is 32 He had well compensated chronic respiratory acidosis

Interpretation of Case 4 His pH is now 7.42, pCO2 80, HCO3 48 If pCO2 of 80 is due to chronic respiratory acidosis, HCO3 should only be 32 +(80-60) x 3/10=38 and not 48 He had combined metabolic alkalosis and respiratory acidosis after treatment of CHF

Case 5 A cirrhotic patient was found to be confused. Serum Na 133, K 3.3, Cl 115, CO2 content 14, anion gap 4 pH 7.44 (H+ 36), pCO2 20, HCO3 13

Interpretation of Case 5 Determine the respiratory compensation For chronic respiratory alkalosis, every 10 reduction in pCO2, HCO3 should decrease by 4 HCO3 should be 25 - (40-20) x 4/10=17 For acute respiratory alkalosis, HCO3 = 21 Patient’s HCO3 is 13, suggesting a metabolic acidotic component is present Anion gap is 4, even corrected for low albumin, is still low suggesting a normal gap metabolic acidosis Patient had combined metabolic acidosis and respiratory alkalosis