A Practical Approach to Acid-Base DisordersMadeleine V. Pahl, M.D., FASN

Professor of MedicineDivision of Nephrology

Course Objectives

What will I learn from this lecture? I will come away knowing that acid-base

disorders are:a) boring and irrelevantb) relevant but obtusec) fascinating and clinically important

Introduction

• Daily acid production: 15,000 mmol of CO2 and 50-100 meq of non-volatile acid (mostly sulfuric acid from metabolism of amino acids)

• Balance maintained by renal and pulmonary excretion

• Renal excretion: combination of H+ with titratable acids, mainly phosphate and ammonia

• Balance assessed in terms of bicarbonate-carbon dioxide buffer system, Henderson-Hasselbalch equation – pH = 6.10 x log ([HCO3] / [0.03 x pCO2])

Introduction

• Acid-base homeostasis critically affects tissue and organ performance

• Both acidosis and alkalosis can have severe and life threatening consequences

• It is the nature of the responsible condition that determines the prognosis

Definitions

• Acidosis: process that lowers the extracellular fluid pH (reduction in HCO3 or elevation in pCO2)– Metabolic acidosis: low pH and low HCO3

– Respiratory acidosis: low pH and high pCO2

• Alkalosis: process that raises extracellular pH (elevation in HCO3 or fall in pCO2)– Metabolic alkalosis: high pH and high HCO3

– Respiratory alkalosis: high pH and low pCO2

Compensatory Responses

• Metabolic acidosis: respiratory compensation begins in the first hour, 1.2 mmHg fall for 1 meq/L HCO3 reduction (Winter’s equation)– pCO2 = 1.5 x (HCO3) + 8 + 2

• Metabolic alkalosis: rise of 0.6 mmHg pCO2 for 1 meq/L HCO3 elevation

Compensatory Responses

• Respiratory acidosis: – acute: HCO3 increases 1meq/L for every 10 mmHg rise of

pCO2

– Chronic (renal compensation complete in 3-5 days): HCO3 increases 3.5meq/L for every10 mmHg rise of pCO2

• Respiratory alkalosis: – acute: HCO3 increases 2 meq/L for every 10 mmHg fall of

pCO2

– Chronic: HCO3 increases 4 meq/L for every 10 mmHg fall of pCO2

Compensatory Responses

PrimaryPrimary CompensatioCompensationn

RespiratoryRespiratory::

AcidosisAcidosis pCOpCO22 HCOHCO33

AlkalosisAlkalosis pCOpCO22 HCOHCO33

Metabolic:Metabolic:

AcidosisAcidosis HCOHCO33 pCOpCO22

AlkalosisAlkalosis HCOHCO33 pCOpCO22

Metabolic acidosis: anion gap

• AG (12) = Na+ - (HCO3- + Cl-)

• Normal AG: – HCO3 loss

– RTA

• Elevated AG:– ketoacidosis – lactic acidosis – drugs and toxins – uremia

Na+

Cl-

HCO3-

Unmeasured anions

Unmeasured cations

Delta HCO3

• Calculate anion gap• Obtain the difference from expected anion

gap (12)• Add the difference to the measured HCO3• If > 24 then there is a ‘hidden’ metabolic

alkalosis

Problem Solving

• Is an acid-base disorder present?• What is the primary or dominant

abnormality?• Is the disorder simple or mixed?• What is the cause?

Case: 1

67 yo male s/p gastrectomy returns 12 days later with nausea decreased mental status

PE: T 39o BP 86/50, P 130Drowsy, minimally responsiveLungs: clearCor: tachy, RR , no rubAbd: diffusely tenderExt: no edema

pHpH 7.247.24

pCOpCO22 2424

HCOHCO33 1010

pOpO22

131322

9595

8.28.2 1212

BUN = 86

Cr = 5.4

Problem Solving: Case 1• pH: 7.24 low, acidosis• pCO2: 24 low, respiratory alkalosis• HCO3: 12 low, metabolic acidosis• Anion Gap: 132- (95+12) = 25• Winter’s equation (expected pCO2): (12 x 1.5 = 18)

+ 8 = 26 (observed = 24) • Delta change HCO3: (25-12= 13)+12(observed) =

25 (a normal HCO3)• Answer: – anion gap anion gap metabolic acidosismetabolic acidosis – compensatory compensatory respiratory alkalosisrespiratory alkalosis

Case 2:

18 yo male c/o lethargy, SOB for 3-4 days, had ‘stomach flu’ 1 week ago

PE: T 37o BP 100/60, P 129 RR: 30Lungs: clearCor: tachy, RR Abd: normalExt: no edemaSkin: poor turgor, dry mucous membranes

pHpH 7.17.100

pCOpCO22 1515

HCOHCO33 55

pOpO22 111100

141400

101000

6.56.5 55

Problem Solving: Case 2• pH: 7.1 low, acidosis• pCO2: 15 low, respiratory alkalosis• HCO3: 5 low, metabolic acidosis• Anion Gap: 140 - (100+5) = 35 • Winter’s equation (expected pCO2): (5 x 1.5 = 7.5) + 8

=15.5 (observed = 15)• Delta change HCO3: (35-12= 23)+5 (observed) = 28 (an

elevated HCO3)• Answer:

• anion gap metabolic acidosis • compensatory respiratory alkalosis • metabolic alkalosis

Case 3:

40 yo male with history of duodenal ulcer c/o epigastric pain for 2 weeks, severe vomiting for 1 week, unable to keep anything down

PE: T 37o BP 100/70, P 120Neck veins flatLungs: clearCor: tachy, RR Abd: diffusely tender

pHpH 7.547.54

pCOpCO22 4848

HCOHCO33 4040

pOpO22 8080

141400

8080

2.02.0 4444

Urine: Na = 2 Cl = 3 K = 21

Problem Solving: Case 3• pH: 7.54 high, alkalosis• pCO2: 48 high, respiratory acidosis• HCO3: 44 high, metabolic alkalosis, increased by 20 • so expected pCO2

• 0.6 x 20 = 12 • 40 + 12 = 52 Slightly higher than observed

• Answer: • metabolic alkalosis (Cl- responsive with low

urinary Cl)• compensatory respiratory acidosis with a slight

respiratory alkalosis ?

Case 4:

24 yo white male s/p gunshot wound to the abdomen required splenectomy and ileostomy. The pt is intubated, sedated and paralyzed. He has an NG tube in place, is on multiple antibiotics and has required post-op pressors.

PE: T 39o BP 100/60, P 113Looks terrible!Lots of tubes and drains

pHpH 7.67.611

pCOpCO22 3030

HCOHCO33 2929

pOpO22

141400

9494

3.03.0 2929

Problem Solving: Case 4

• pH: 7.61 high, alkalosis• pCO2: 30 low, respiratory alkalosis• HCO3: 29 high, metabolic alkalosis• Anion Gap: 140 - ( 94 + 29) = 17 • Delta change HCO3: (17-12= 5)+29 (observed) =

34 (‘true’ value without acidosis)• Answer:

• metabolic alkalosis• anion gap metabolic acidosis • respiratory alkalosis

Case 5:

55 yo man collapsed in a bar and was brought to the ER. He was unresponsive, no BP was obtainable, a sinus tachycardia was present and he had peritoneal signs.

He was intubated, started on pressors and treated with HCO3

pHpH 6.86.866

pCOpCO22 8181

HCOHCO33 1414

131399

8484

3.93.9 1616

pHpH 7.047.04

pCOpCO22 3434

HCOHCO33 99

141488

9393

4.54.5 1010

Problem Solving: Case 5 Admission

• pH: 6.85 low, acidosis• pCO2: 81 high, respiratory acidosis• HCO3: 16 low, metabolic acidosis• Anion Gap: 139 – (84 + 16) = 39• Winter’s equation (expected pCO2): (16 x 1.5 =

24) + 8 = 32 (lower than observed, 81)

• Delta change HCO3: (39-12 = 27 )+16 (observed) = 43

• Answer: – anion gap metabolic acidosis – respiratory acidosis– metabolic alkalosis

Problem Solving: Case 5 After Intubation

• pH: 7.04 low, acidosis• pCO2: 34 low, respiratory alkalosis• HCO3:10 low, metabolic acidosis• Anion Gap: 148 – (93 + 10) = 45 (increasing)• Winter’s equation(expected pCO2): (10 x 1.5 = 15)

+ 8 = 23 (lower than observed, 34) • Delta change HCO3: (45-12 + 33)+10(observed) =

43 • Answer:

– anion gap metabolic acidosis (lactate was 24)

– respiratory alkalosis– metabolic alkalosis

Case 6:

32 yo hispanic female with a 1 week history of bloody diarrhea comes to the ER with SOB, weakness and a feeling of doom.

PE: T 38.7o BP 90/40, P 100

Abd: diffusely tender with hyperative bowel sounds and OB+ stools

pHpH 7.17.111

pCOpCO22 1616

HCOHCO33 55

141400

111155

3.73.7 55

Problem Solving: Case 6• pH: 7.11 low, acidosis• pCO2: 16 low, respiratory alkalosis• HCO3: 5 low, metabolic acidosis• Anion Gap: 140 – (115 + 5) = 20 • Winter’s equation (expected pCO2): (5 x 1.5 = 7.5) + 8

= 15.5 (same as observed, 16)• Delta change HCO3: (20-12 = 8)+5 (observed) = 13 (a

low HCO3)• Answer:

• anion gap metabolic acidosis • non-anion gap metabolic acidosis

(hyperchloremic)• compensatory respiratory alkalosis

Case 7:

18 yo female attempts suicide by taking pills found in mother’s medicine chest. Brought to ER alert but agitated.

PE: 148/60, P 126 T 37o

Cor: tachyNeuro: no focal findings

pHpH 7.567.56

pCOpCO22 1515

HCOHCO33 1313

pOpO22

141400

101077

4.54.5 1313

Problem Solving: Case 7

• pH: 7.56 high, alkalosis• pCO2: 15 low, respiratory alkalosis• HCO3: 13 low, metabolic acidosis• Anion Gap: 140 ( 107 – 13) = 20 • Winter’s equation (expected pCO2): (13 x 1.5 =19.5) +

8 = 27.5 (higher than observed, 15)• Delta change HCO3: (20-12 = 8)+13 (observed) = 21

(only minimally reduced HCO3)• Answer: salicylate poisoning

• anion gap metabolic acidosis • primary respiratory alkalosis