Acid Base Physiology Overview Jeff Kaufhold, MD FACP 2010.

Acid Base PhysiologyAcid Base PhysiologyOverviewOverview

Jeff Kaufhold, MD FACPJeff Kaufhold, MD FACP

20102010

Acid Base PhysiologyAcid Base Physiology

Where does Acid come from?Where does Acid come from?Physiologic controlPhysiologic controlRegulationRegulationRules of ThumbRules of ThumbCasesCases

Source of AcidSource of Acid

Cellular Metabolism 32,000 mEq/dayCellular Metabolism 32,000 mEq/dayConverted to CO2 and Water in the CellConverted to CO2 and Water in the Cell

OrganicOrganic 16,000 milliMole16,000 milliMoleCleared by LungCleared by Lung

InorganicInorganic 70 mEq70 mEqCleared by KidneysCleared by Kidneys

Physiologic ControlPhysiologic Control

Normal pH is 7.3-7.5Normal pH is 7.3-7.5Equivalent to only 40 Nanomole/liter of Equivalent to only 40 Nanomole/liter of

free H+ (0.0017 mEq)free H+ (0.0017 mEq)Compared to 2000 mEq of sodiumCompared to 2000 mEq of sodiumMust be buffered and eliminatedMust be buffered and eliminated

Buffer SystemBuffer System

BicarbonateBicarbonateAnion Gap is a measure of buffer Anion Gap is a measure of buffer

capacitycapacityProteins (predominantly albumin)Proteins (predominantly albumin)RBC’s carry hemoglobin which can RBC’s carry hemoglobin which can

conveniently bind acid and carry away conveniently bind acid and carry away from tissue to lungs from tissue to lungs

Excretion of AcidExcretion of Acid

LungLungblows off CO2 and traps H+ as blows off CO2 and traps H+ as H2OH2O

KidneyKidneyLumenal carbonic anhydrase in proximal tubule Lumenal carbonic anhydrase in proximal tubule

to reclaim filtered bicarbto reclaim filtered bicarbBasolateral Carbonic Anhydrase in Distal tubule Basolateral Carbonic Anhydrase in Distal tubule

to extract H+ from blood and excrete itto extract H+ from blood and excrete itAmmoniagenesis in interstitium to buffer urine Ammoniagenesis in interstitium to buffer urine

pH.pH.Disorders here lead to RTADisorders here lead to RTA

Acid base RegulationAcid base Regulation

Henderson hasselbach equation:Henderson hasselbach equation:pH = 6.1 + log (HCO3/0.3pCO2)pH = 6.1 + log (HCO3/0.3pCO2)

Rearranged to Linear relationship:Rearranged to Linear relationship:H+ = 24 (pCO2/HCO3)H+ = 24 (pCO2/HCO3)

Based on the reaction mediated by Carbonic Based on the reaction mediated by Carbonic Anhydrase:Anhydrase:

Co2 + H2O <Co2 + H2O < H2CO3 H2CO3 > H+ + HCO3> H+ + HCO3

Rules of ThumbRules of Thumb

metabolic acidosis metabolic acidosis pCO2 = 1.5 (HCO3) + 8 (+/- 2)pCO2 = 1.5 (HCO3) + 8 (+/- 2)metabolic alkalosismetabolic alkalosis pCO2 = 0.9 (HCO3) + 9 (+/- 2)pCO2 = 0.9 (HCO3) + 9 (+/- 2)

pCO2 should be same as last two pCO2 should be same as last two digits of pH (pCO2 = 40 when pH is digits of pH (pCO2 = 40 when pH is 7.40) for simple metabolic disorder7.40) for simple metabolic disorder

Rules of THumbRules of THumb

respiratory acidosis respiratory acidosis acute HCO3 increases 1 mEq for acute HCO3 increases 1 mEq for

each 10 mmHg pCO2each 10 mmHg pCO2chronic HCO3 up 3.5 mEq for each 10 chronic HCO3 up 3.5 mEq for each 10

mmHg pCO2mmHg pCO2

Rules of ThumbRules of Thumb

respiratory alkalosisrespiratory alkalosisacute HCO3 decreases 2 mEq/ each 10 acute HCO3 decreases 2 mEq/ each 10

mmHg pCO2mmHg pCO2chronic HCO3 down 5 mEq/ each 10 chronic HCO3 down 5 mEq/ each 10

mmHg pCO2mmHg pCO2

HOW TO EVALUATE CLINICAL PROBLEMS HOW TO EVALUATE CLINICAL PROBLEMS IN ACID-BASEIN ACID-BASE

1. calculate the anion gap and potential bicarb1. calculate the anion gap and potential bicarb 2. is pH acidic or basic?2. is pH acidic or basic? 3. is pCO2 alkalotic (<40) or acidotic (>40)3. is pCO2 alkalotic (<40) or acidotic (>40) 4. is HCO3 calculated by ABG machine consistent 4. is HCO3 calculated by ABG machine consistent

with measured HCO3 on lytes? with measured HCO3 on lytes? If not, samples are not If not, samples are not simultaneous and simultaneous and

conclusions may be invalidconclusions may be invalid 5. Apply rules of thumb - if values are consistent, a 5. Apply rules of thumb - if values are consistent, a

simple disorder is present with appropriate simple disorder is present with appropriate compensation, if not, suspect a second disorder. compensation, if not, suspect a second disorder.

Simple Acid Base Case 1Simple Acid Base Case 1

38 y.o. male with CKD due to chronic GN.38 y.o. male with CKD due to chronic GN. Increased weakness and lethargyIncreased weakness and lethargyBP 135/75, T 98 P 75 R 22BP 135/75, T 98 P 75 R 22No edema, no rales, no rub.No edema, no rales, no rub.Na 134Na 134 Cl 100 BUN 159Cl 100 BUN 159K 5.6 CO2 14 creat 15K 5.6 CO2 14 creat 15pH 7.26/pCO2 27 pO2 85pH 7.26/pCO2 27 pO2 85

Simple Case 1Simple Case 1

Na 134Na 134 Cl 100 BUN 159Cl 100 BUN 159K 5.6 CO2 14 creat 15K 5.6 CO2 14 creat 15pH 7.26/pCO2 27 pO2 85pH 7.26/pCO2 27 pO2 85

Step 1 Anion GapStep 1 Anion GapPotential bicarb?Potential bicarb?

Step 2 Acidemia or Alkalemia?Step 2 Acidemia or Alkalemia?Step 3 Which way is pCO2 going?Step 3 Which way is pCO2 going?

Simple Case 1Simple Case 1

Na 134Na 134 Cl 100 BUN 159Cl 100 BUN 159K 5.6 CO2 14 creat 15K 5.6 CO2 14 creat 15pH 7.26/pCO2 27 pO2 85pH 7.26/pCO2 27 pO2 85Step 3 Note: can only hyper or HypO-Step 3 Note: can only hyper or HypO-

ventilate, can’t do both at same time!ventilate, can’t do both at same time!Step 4 make sure ABG and lytes are Step 4 make sure ABG and lytes are

drawn at same time/bicarb is consistentdrawn at same time/bicarb is consistentStep 5 apply rules of thumb.Step 5 apply rules of thumb.

Rules of ThumbRules of Thumb

metabolic acidosis metabolic acidosis pCO2 = 1.5 (HCO3) + 8 (+/- 2)pCO2 = 1.5 (HCO3) + 8 (+/- 2)metabolic alkalosismetabolic alkalosis pCO2 = 0.9 (HCO3) + 9 (+/- 2)pCO2 = 0.9 (HCO3) + 9 (+/- 2)

pCO2 should be same as last two pCO2 should be same as last two digits of pH (pCO2 = 40 when pH is digits of pH (pCO2 = 40 when pH is 7.40) for simple metabolic disorder7.40) for simple metabolic disorder

Rules of ThumbRules of Thumb

metabolic acidosis metabolic acidosis pCO2 = 1.5 (HCO3) + 8 (+/- 2)pCO2 = 1.5 (HCO3) + 8 (+/- 2)

This case 27 = 1.5(14) +8 (+-2)This case 27 = 1.5(14) +8 (+-2)

Therefore simple metabolic acidosis with Therefore simple metabolic acidosis with respiratory compensation.respiratory compensation.

Causes of Anion Gap Causes of Anion Gap Metabolic AcidosisMetabolic Acidosis

MethanolMethanol UremiaUremia Diabetic KetoacidosisDiabetic Ketoacidosis

ParaldehydeParaldehyde Iron or IsoniazidIron or Isoniazid Lactic acidosisLactic acidosis Ethanol/ethylene GlycolEthanol/ethylene Glycol SalicylatesSalicylates

Causes of Non GappedCauses of Non GappedMetabolic AcidosisMetabolic Acidosis

GI lossesGI lossesDiarrheaDiarrheaPancreatic fistulaPancreatic fistula

Renal LossesRenal LossesRTARTAAcetazolamide/DiamoxAcetazolamide/DiamoxAddison’s diseaseAddison’s disease

Iatrogenic infusions: Normal saline, TPNIatrogenic infusions: Normal saline, TPN

Metabolic AlkalosisMetabolic Alkalosis

GI losses of HCl (N/V) or IleostomyGI losses of HCl (N/V) or Ileostomy Renal disorders eg, Renal disorders eg, Bartter syndrome, , Drug use Drug use

Loop or thiazide diureticsLoop or thiazide diuretics LicoriceLicorice Tobacco chewingTobacco chewing GlucocorticoidsGlucocorticoids Antacids (eg, magnesium hydroxide)Antacids (eg, magnesium hydroxide) Calcium carbonateCalcium carbonate

Chronic Respiratory ACIDosis Chronic Respiratory ACIDosis COPD, Sleep apneaCOPD, Sleep apnea

Simple Acid Base Case 2Simple Acid Base Case 2

40 y.o. female with status asthmaticus.40 y.o. female with status asthmaticus.Temp 101, P 108, R 28, BP 138/85Temp 101, P 108, R 28, BP 138/8511stst gas drawn. Started treatment. gas drawn. Started treatment.4 hours later is less alert, disoriented4 hours later is less alert, disoriented22ndnd gas drawn. gas drawn.Place on vent and resp tx/steroidsPlace on vent and resp tx/steroids33rdrd gas drawn 24 hours later. gas drawn 24 hours later.

Simple Acid base case 2Simple Acid base case 2

Admit:Admit:Na K Cl CO2 pH pCO2 pO2 HCONa K Cl CO2 pH pCO2 pO2 HCO135 3.5 100 21 7.50 30 80 20135 3.5 100 21 7.50 30 80 20Step 1 Anion GapStep 1 Anion Gap

Potential bicarb?Potential bicarb?

Step 2 Acidemia or Alkalemia?Step 2 Acidemia or Alkalemia?Step 3 Which way is pCO2 going?Step 3 Which way is pCO2 going?

Simple case 2Simple case 2

Na K Cl CO2 pH pCO2 pO2 HCONa K Cl CO2 pH pCO2 pO2 HCO135 3.5 100 21 7.50 30 80 20135 3.5 100 21 7.50 30 80 20

respiratory alkalosisrespiratory alkalosisacute HCO3 decreases 2 mEq/ each 10 acute HCO3 decreases 2 mEq/ each 10

mmHg pCO2mmHg pCO2chronic HCO3 down 5 mEq/ each 10 chronic HCO3 down 5 mEq/ each 10

mmHg pCO2mmHg pCO2

Simple Acid base case 2Simple Acid base case 2

Deteriorated:Deteriorated:Na K Cl CO2 pH pCO2 pO2 HCONa K Cl CO2 pH pCO2 pO2 HCO137 4.2 97 26 7.25 60 48 25137 4.2 97 26 7.25 60 48 25Step 1 Anion GapStep 1 Anion Gap

Potential bicarb?Potential bicarb?

Step 2 Acidemia or Alkalemia?Step 2 Acidemia or Alkalemia?Step 3 Which way is pCO2 going?Step 3 Which way is pCO2 going?

Simple case 2Simple case 2

Na K Cl CO2 pH pCO2 pO2 HCONa K Cl CO2 pH pCO2 pO2 HCO137 4.2 97 26 7.25 60 48 25137 4.2 97 26 7.25 60 48 25

respiratory acidosis respiratory acidosis acute HCO3 increases 1 mEq for acute HCO3 increases 1 mEq for

each 10 mmHg pCO2each 10 mmHg pCO2chronic HCO3 up 3.5 mEq for each 10 chronic HCO3 up 3.5 mEq for each 10

mmHg pCO2mmHg pCO2

Simple Acid base case 2Simple Acid base case 2

24 hours later:24 hours later:Na K Cl CO2 pH pCO2 pO2 HCONa K Cl CO2 pH pCO2 pO2 HCO138 4.0 95 31 7.45 43 140 30138 4.0 95 31 7.45 43 140 30Step 1 Anion GapStep 1 Anion Gap

Potential bicarb?Potential bicarb?

Step 2 Acidemia or Alkalemia?Step 2 Acidemia or Alkalemia?Step 3 Which way is pCO2 going?Step 3 Which way is pCO2 going?

Simple case 2Simple case 2

Na K Cl CO2 pH pCO2 pO2 HCONa K Cl CO2 pH pCO2 pO2 HCO138 4.0 95 31 7.45 43 140 30138 4.0 95 31 7.45 43 140 30

respiratory acidosis respiratory acidosis acute HCO3 increases 1 mEq for each 10 acute HCO3 increases 1 mEq for each 10

mmHg pCO2mmHg pCO2chronic HCO3 up 3.5 mEq for each 10 mmHg chronic HCO3 up 3.5 mEq for each 10 mmHg

pCO2pCO2Kidneys reabsorb bicarb in response to Kidneys reabsorb bicarb in response to

increased pCO2, but it takes time.increased pCO2, but it takes time.

Case 3Case 3Mixed acid base disturbanceMixed acid base disturbance

32 y.o. female with chronic pyelonephritis 32 y.o. female with chronic pyelonephritis and CKD. Admitted with N/V and DOE.and CKD. Admitted with N/V and DOE.

Right pleural effusion, S3.Right pleural effusion, S3.11stst set of labs obtained. set of labs obtained.Thoracentesis performed and within Thoracentesis performed and within

minutes dyspnea markedly increased. Pt minutes dyspnea markedly increased. Pt intubated and 2intubated and 2ndnd set of labs drawn. set of labs drawn.

Mixed Acid base case 3Mixed Acid base case 3

Admitted:Admitted:Na K Cl CO2 pH pCO2 pO2 HCONa K Cl CO2 pH pCO2 pO2 HCO130 5.0 94 15 7.32 32 48 16130 5.0 94 15 7.32 32 48 16Step 1 Anion GapStep 1 Anion Gap

Potential bicarb?Potential bicarb?

Step 2 Acidemia or Alkalemia?Step 2 Acidemia or Alkalemia?Step 3 Which way is pCO2 going?Step 3 Which way is pCO2 going?Step 4 compareStep 4 compareStep 5 which rule of thumb to use?Step 5 which rule of thumb to use?

Rules of ThumbRules of Thumb

metabolic acidosis metabolic acidosis pCO2 = 1.5 (HCO3) + 8 (+/- 2)pCO2 = 1.5 (HCO3) + 8 (+/- 2)

This case 32 = 1.5(15) +8 (+-2)This case 32 = 1.5(15) +8 (+-2)

Therefore simple metabolic acidosis with Therefore simple metabolic acidosis with respiratory compensation.respiratory compensation.

Mixed Acid base case 3Mixed Acid base case 3

After thoracentesis:After thoracentesis:Na K Cl CO2 pH pCO2 pO2 HCONa K Cl CO2 pH pCO2 pO2 HCO131 7.8 92 9 6.90 50 48 10131 7.8 92 9 6.90 50 48 10Step 1 Anion GapStep 1 Anion Gap

Potential bicarb?Potential bicarb?

Step 2 Acidemia or Alkalemia?Step 2 Acidemia or Alkalemia?Step 3 Which way is pCO2 going?Step 3 Which way is pCO2 going?Step 4 compareStep 4 compareStep 5 None of the rules of thumb apply!Step 5 None of the rules of thumb apply!

Mixed acid base case 3Mixed acid base case 3

Bonus points:Bonus points:Why did the Anion Gap rise so quickly?Why did the Anion Gap rise so quickly?

Why did the K concentration rise so quickly?Why did the K concentration rise so quickly?

Mixed Disturbance Case 4Mixed Disturbance Case 4

44 y.o male with alcoholism and 44 y.o male with alcoholism and indiscriminate taste in the nature of the indiscriminate taste in the nature of the alcohol, admitted with intoxication and alcohol, admitted with intoxication and severe abdominal pain.severe abdominal pain.

N/V, Dyspnea, Cough with rales in right N/V, Dyspnea, Cough with rales in right base, abd diffuse tenderness, no base, abd diffuse tenderness, no rebound.rebound.

Diagnosed with pneumonia and Diagnosed with pneumonia and pancreatitis.pancreatitis.

Mixed case 4Mixed case 4

Na K Cl CO2 pH pCO2 pO2 HCONa K Cl CO2 pH pCO2 pO2 HCO142 3.4 98 20 7.28 41 58 20142 3.4 98 20 7.28 41 58 20Step 1 Anion GapStep 1 Anion Gap

Potential bicarb?Potential bicarb?

Step 2 Acidemia or Alkalemia?Step 2 Acidemia or Alkalemia?Step 3 Which way is pCO2 going?Step 3 Which way is pCO2 going?Step 4 compareStep 4 compareStep 5 which rule of thumb to use?Step 5 which rule of thumb to use?

Mixed case 4Mixed case 4

Na K Cl CO2 pH pCO2 pO2 HCONa K Cl CO2 pH pCO2 pO2 HCO142 3.4 98 20 7.28 41 58 20142 3.4 98 20 7.28 41 58 20Step 1 Anion GapStep 1 Anion Gap 2424

Potential bicarb?Potential bicarb? 12 so actual bicarb = 3212 so actual bicarb = 32

Step 2 Acidemia or Alkalemia?Step 2 Acidemia or Alkalemia?Step 3 Which way is pCO2 going?Step 3 Which way is pCO2 going?

Not dropping as it should (pt can’t Not dropping as it should (pt can’t hyperventilate effectively due to pneumonia)hyperventilate effectively due to pneumonia)

Mixed DisturbancesMixed Disturbances

Usually due to:Usually due to:Nausea and vomiting causing GI induced Nausea and vomiting causing GI induced

alkalosis (NG suction does same thing)alkalosis (NG suction does same thing)

Respiratory compromise leads to insufficient Respiratory compromise leads to insufficient respiratory compensation, and may lead to respiratory compensation, and may lead to hypoxia and Lactic acidosis superimposed on hypoxia and Lactic acidosis superimposed on underlying condition.underlying condition.

Key to recognizing these: Potential bicarb!Key to recognizing these: Potential bicarb!