A Practical Approach to Acid-Base Disorders Madeleine V. Pahl, M.D., FASN Professor of Medicine Division of Nephrology

Course Objectives What will I learn from this lecture? I will come away knowing that acid-base disorders are: a) boring and irrelevant b) relevant but obtuse c) fascinating and clinically important

Introduction Daily acid production: 15,000 mmol of CO 2 and 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 ([HCO 3 ] / [0.03 x pCO 2 ])

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 HCO 3 or elevation in pCO 2 ) – Metabolic acidosis: low pH and low HCO 3 – Respiratory acidosis: low pH and high pCO 2 Alkalosis: process that raises extracellular pH (elevation in HCO 3 or fall in pCO 2 ) – Metabolic alkalosis: high pH and high HCO 3 – Respiratory alkalosis: high pH and low pCO 2

Compensatory Responses Metabolic acidosis: respiratory compensation begins in the first hour, 1.2 mmHg fall for 1 meq/L HCO 3 reduction (Winter’s equation) – pCO 2 = 1.5 x (HCO 3 ) Metabolic alkalosis: rise of 0.6 mmHg pCO 2 for 1 meq/L HCO 3 elevation

Compensatory Responses Respiratory acidosis: – acute: HCO 3 increases 1meq/L for every 10 mmHg rise of pCO 2 – Chronic (renal compensation complete in 3-5 days): HCO 3 increases 3.5meq/L for every10 mmHg rise of pCO 2 Respiratory alkalosis: – acute: HCO 3 increases 2 meq/L for every 10 mmHg fall of pCO 2 – Chronic: HCO 3 increases 4 meq/L for every 10 mmHg fall of pCO 2

Compensatory Responses PrimaryCompensation Respiratory: Acidosis pCO 2 pCO 2 HCO 3 HCO 3 Alkalosis pCO 2 pCO 2 HCO 3 HCO 3 Metabolic: Acidosis pCO 2 pCO 2 Alkalosis HCO 3 HCO 3 pCO 2 pCO 2

Metabolic acidosis: anion gap AG (12) = Na + - (HCO Cl - ) Normal AG: – HCO 3 loss – RTA Elevated AG: – ketoacidosis – lactic acidosis – drugs and toxins – uremia Na+ Cl- HCO 3 - Unmeasured anions Unmeasured cations

Delta HCO 3 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 39 o BP 86/50, P 130 Drowsy, minimally responsive Lungs: clear Cor: tachy, RR, no rub Abd: diffusely tender Ext: no edema pH7.24 pCO 2 24 HCO 3 10 pO BUN = 86 Cr = 5.4

Problem Solving: Case 1 pH: 7.24 low, acidosis pCO 2 : 24 low, respiratory alkalosis HCO 3 : 12 low, metabolic acidosis Anion Gap: 132- (95+12) = 25 Winter’s equation (expected pCO 2 ): (12 x 1.5 = 18) + 8 = 26 (observed = 24) Delta change HCO 3 : (25-12= 13)+12(observed) = 25 (a normal HCO 3 ) Answer: – anion gap metabolic acidosis – compensatory respiratory alkalosis

Case 2: 18 yo male c/o lethargy, SOB for 3-4 days, had ‘stomach flu’ 1 week ago PE: T 37 o BP 100/60, P 129 RR: 30 Lungs: clear Cor: tachy, RR Abd: normal Ext: no edema Skin: poor turgor, dry mucous membranes pH pCO 2 15 HCO 3 5 pO

Problem Solving: Case 2 pH: 7.1 low, acidosis pCO 2 : 15 low, respiratory alkalosis HCO 3 : 5 low, metabolic acidosis Anion Gap: (100+5) = 35 Winter’s equation (expected pCO 2 ): (5 x 1.5 = 7.5) + 8 =15.5 (observed = 15) Delta change HCO 3 : (35-12= 23)+5 (observed) = 28 (an elevated HCO 3 ) 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 37 o BP 100/70, P 120 Neck veins flat Lungs: clear Cor: tachy, RR Abd: diffusely tender pH7.54 pCO 2 48 HCO 3 40 pO Urine: Na = 2 Cl = 3 K = 21

Problem Solving: Case 3 pH: 7.54 high, alkalosis pCO 2 : 48 high, respiratory acidosis HCO 3 : 44 high, metabolic alkalosis, increased by 20 so expected pCO x 20 = = 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 39 o BP 100/60, P 113 Looks terrible! Lots of tubes and drains pH pCO 2 30 HCO 3 29 pO

Problem Solving: Case 4 pH: 7.61 high, alkalosis pCO 2 : 30 low, respiratory alkalosis HCO 3 : 29 high, metabolic alkalosis Anion Gap: ( ) = 17 Delta change HCO 3 : (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 HCO 3 pH6.86 pCO 2 81 HCO pH7.04 pCO 2 34 HCO

Problem Solving: Case 5 Admission pH: 6.85 low, acidosis pCO 2 : 81 high, respiratory acidosis HCO 3 : 16 low, metabolic acidosis Anion Gap: 139 – ( ) = 39 Winter’s equation (expected pCO2): (16 x 1.5 = 24) + 8 = 32 (lower than observed, 81) Delta change HCO 3 : (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 pCO 2 : 34 low, respiratory alkalosis HCO 3 :10 low, metabolic acidosis Anion Gap: 148 – ( ) = 45 (increasing) Winter’s equation(expected pCO 2 ): (10 x 1.5 = 15) + 8 = 23 (lower than observed, 34) Delta change HCO 3 : ( )+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 38.7 o BP 90/40, P 100 Abd: diffusely tender with hyperative bowel sounds and OB+ stools pH pCO 2 16 HCO

Problem Solving: Case 6 pH: 7.11 low, acidosis pCO 2 : 16 low, respiratory alkalosis HCO 3 : 5 low, metabolic acidosis Anion Gap: 140 – ( ) = 20 Winter’s equation (expected pCO 2 ): (5 x 1.5 = 7.5) + 8 = 15.5 (same as observed, 16) Delta change HCO 3 : (20-12 = 8)+5 (observed) = 13 (a low HCO 3 ) 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 37 o Cor: tachy Neuro: no focal findings pH7.56 pCO 2 15 HCO 3 13 pO

Problem Solving: Case 7 pH: 7.56 high, alkalosis pCO 2 : 15 low, respiratory alkalosis HCO 3 : 13 low, metabolic acidosis Anion Gap: 140 ( 107 – 13) = 20 Winter’s equation (expected pCO 2 ): (13 x 1.5 =19.5) + 8 = 27.5 (higher than observed, 15) Delta change HCO 3 : (20-12 = 8)+13 (observed) = 21 (only minimally reduced HCO 3 ) Answer: salicylate poisoning anion gap metabolic acidosis primary respiratory alkalosis