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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