Acid Base Physiology Overview

1. Acid Base PhysiologyOverview Jeff Kaufhold, MD FACP 2010

2. Acid Base Physiology • Where does Acid come from? • Physiologic control • Regulation • Rules of Thumb • Cases

3. Source of Acid • Cellular Metabolism 32,000 mEq/day • Converted to CO2 and Water in the Cell • Organic 16,000 milliMole • Cleared by Lung • Inorganic 70 mEq • Cleared by Kidneys

4. Physiologic Control • Normal pH is 7.3-7.5 • Equivalent to only 40 Nanomole/liter of free H+ (0.0017 mEq) • Compared to 2000 mEq of sodium • Must be buffered and eliminated

5. Buffer System • Bicarbonate • Anion Gap is a measure of buffer capacity • Proteins (predominantly albumin) • RBC’s carry hemoglobin which can conveniently bind acid and carry away from tissue to lungs

6. Excretion of Acid • Lung blows off CO2 and traps H+ as H2O • Kidney • Lumenal carbonic anhydrase in proximal tubule to reclaim filtered bicarb • Basolateral Carbonic Anhydrase in Distal tubule to extract H+ from blood and excrete it • Ammoniagenesis in interstitium to buffer urine pH. • Disorders here lead to RTA

7. Acid base Regulation • Henderson hasselbach equation: • pH = 6.1 + log (HCO3/0.3pCO2) • Rearranged to Linear relationship: • H+ = 24 (pCO2/HCO3) • Based on the reaction mediated by Carbonic Anhydrase: • Co2 + H2O < H2CO3 > H+ + HCO3

8. Rules of Thumb • metabolic acidosis • pCO2 = 1.5 (HCO3) + 8 (+/- 2) • metabolic alkalosis • pCO2 = 0.9 (HCO3) + 9 (+/- 2) • pCO2 should be same as last two digits of pH (pCO2 = 40 when pH is 7.40) for simple metabolic disorder

9. Rules of THumb • respiratory acidosis • acute HCO3 increases 1 mEq for each 10 mmHg pCO2 • chronic HCO3 up 3.5 mEq for each 10 mmHg pCO2

10. Rules of Thumb • respiratory alkalosis • acute HCO3 decreases 2 mEq/ each 10 mmHg pCO2 • chronic HCO3 down 5 mEq/ each 10 mmHg pCO2

11. HOW TO EVALUATE CLINICAL PROBLEMS IN ACID-BASE • 1. calculate the anion gap and potential bicarb • 2. is pH acidic or basic? • 3. is pCO2 alkalotic (<40) or acidotic (>40) • 4. is HCO3 calculated by ABG machine consistent with measured HCO3 on lytes? • If not, samples are not simultaneous and conclusions may be invalid • 5. Apply rules of thumb - if values are consistent, a simple disorder is present with appropriate compensation, if not, suspect a second disorder.

12. Simple Acid Base Case 1 • 38 y.o. male with CKD due to chronic GN. • Increased weakness and lethargy • BP 135/75, T 98 P 75 R 22 • No edema, no rales, no rub. • Na 134 Cl 100 BUN 159 • K 5.6 CO2 14 creat 15 • pH 7.26/pCO2 27 pO2 85

13. Simple Case 1 • Na 134 Cl 100 BUN 159 • K 5.6 CO2 14 creat 15 • pH 7.26/pCO2 27 pO2 85 • Step 1 Anion Gap • Potential bicarb? • Step 2 Acidemia or Alkalemia? • Step 3 Which way is pCO2 going?

14. Simple Case 1 • Na 134 Cl 100 BUN 159 • K 5.6 CO2 14 creat 15 • pH 7.26/pCO2 27 pO2 85 • Step 3 Note: can only hyper or HypO-ventilate, can’t do both at same time! • Step 4 make sure ABG and lytes are drawn at same time/bicarb is consistent • Step 5 apply rules of thumb.

15. Rules of Thumb • metabolic acidosis • pCO2 = 1.5 (HCO3) + 8 (+/- 2) • metabolic alkalosis • pCO2 = 0.9 (HCO3) + 9 (+/- 2) • pCO2 should be same as last two digits of pH (pCO2 = 40 when pH is 7.40) for simple metabolic disorder

16. Rules of Thumb • metabolic acidosis • pCO2 = 1.5 (HCO3) + 8 (+/- 2) • This case 27 = 1.5(14) +8 (+-2) • Therefore simple metabolic acidosis with respiratory compensation.

17. Causes of Anion Gap Metabolic Acidosis • Methanol • Uremia • Diabetic Ketoacidosis • Paraldehyde • Iron or Isoniazid • Lactic acidosis • Ethanol/ethylene Glycol • Salicylates

18. Causes of Non GappedMetabolic Acidosis • GI losses • Diarrhea • Pancreatic fistula • Renal Losses • RTA • Acetazolamide/Diamox • Addison’s disease • Iatrogenic infusions: Normal saline, TPN

19. Metabolic Alkalosis • GI losses of HCl (N/V) or Ileostomy • Renal disorders eg, Bartter syndrome, • Drug use • Loop or thiazide diuretics • Licorice • Tobacco chewing • Glucocorticoids • Antacids (eg, magnesium hydroxide) • Calcium carbonate • Chronic Respiratory ACIDosis • COPD, Sleep apnea

20. Simple Acid Base Case 2 • 40 y.o. female with status asthmaticus. • Temp 101, P 108, R 28, BP 138/85 • 1st gas drawn. Started treatment. • 4 hours later is less alert, disoriented • 2nd gas drawn. • Place on vent and resptx/steroids • 3rd gas drawn 24 hours later.

21. Simple Acid base case 2 • Admit: • Na K Cl CO2 pH pCO2 pO2 HCO • 135 3.5 100 21 7.50 30 80 20 • Step 1 Anion Gap • Potential bicarb? • Step 2 Acidemia or Alkalemia? • Step 3 Which way is pCO2 going?

22. Simple case 2 • Na K Cl CO2 pH pCO2 pO2 HCO • 135 3.5 100 21 7.50 30 80 20 • respiratory alkalosis • acute HCO3 decreases 2 mEq/ each 10 mmHg pCO2 • chronic HCO3 down 5 mEq/ each 10 mmHg pCO2

23. Simple Acid base case 2 • Deteriorated: • Na K Cl CO2 pH pCO2 pO2 HCO • 137 4.2 97 26 7.25 60 48 25 • Step 1 Anion Gap • Potential bicarb? • Step 2 Acidemia or Alkalemia? • Step 3 Which way is pCO2 going?

24. Simple case 2 • Na K Cl CO2 pH pCO2 pO2 HCO • 137 4.2 97 26 7.25 60 48 25 • respiratory acidosis • acute HCO3 increases 1 mEq for each 10 mmHg pCO2 • chronic HCO3 up 3.5 mEq for each 10 mmHg pCO2

25. Simple Acid base case 2 • 24 hours later: • Na K Cl CO2 pH pCO2 pO2 HCO • 138 4.0 95 31 7.45 43 140 30 • Step 1 Anion Gap • Potential bicarb? • Step 2 Acidemia or Alkalemia? • Step 3 Which way is pCO2 going?

26. Simple case 2 • Na K Cl CO2 pH pCO2 pO2 HCO • 138 4.0 95 31 7.45 43 140 30 • respiratory acidosis • acute HCO3 increases 1 mEq for each 10 mmHg pCO2 • chronic HCO3 up 3.5 mEq for each 10 mmHg pCO2 • Kidneys reabsorb bicarb in response to increased pCO2, but it takes time.

27. Case 3Mixed acid base disturbance • 32 y.o. female with chronic pyelonephritis and CKD. Admitted with N/V and DOE. • Right pleural effusion, S3. • 1st set of labs obtained. • Thoracentesis performed and within minutes dyspnea markedly increased. Pt intubated and 2nd set of labs drawn.

28. Mixed Acid base case 3 • Admitted: • Na K Cl CO2 pH pCO2 pO2 HCO • 130 5.0 94 15 7.32 32 48 16 • Step 1 Anion Gap • Potential bicarb? • Step 2 Acidemia or Alkalemia? • Step 3 Which way is pCO2 going? • Step 4 compare • Step 5 which rule of thumb to use?

29. Rules of Thumb • metabolic acidosis • pCO2 = 1.5 (HCO3) + 8 (+/- 2) • This case 32 = 1.5(15) +8 (+-2) • Therefore simple metabolic acidosis with respiratory compensation.

30. Mixed Acid base case 3 • After thoracentesis: • Na K Cl CO2 pH pCO2 pO2 HCO • 131 7.8 92 9 6.90 50 48 10 • Step 1 Anion Gap • Potential bicarb? • Step 2 Acidemia or Alkalemia? • Step 3 Which way is pCO2 going? • Step 4 compare • Step 5 None of the rules of thumb apply!

31. Mixed acid base case 3 • Bonus points: • Why did the Anion Gap rise so quickly? • Why did the K concentration rise so quickly?

32. Mixed Disturbance Case 4 • 44 y.o male with alcoholism and indiscriminate taste in the nature of the alcohol, admitted with intoxication and severe abdominal pain. • N/V, Dyspnea, Cough with rales in right base, abd diffuse tenderness, no rebound. • Diagnosed with pneumonia and pancreatitis.

33. Mixed case 4 • Na K Cl CO2 pH pCO2 pO2 HCO • 142 3.4 98 20 7.28 41 58 20 • Step 1 Anion Gap • Potential bicarb? • Step 2 Acidemia or Alkalemia? • Step 3 Which way is pCO2 going? • Step 4 compare • Step 5 which rule of thumb to use?

34. Mixed case 4 • Na K Cl CO2 pH pCO2 pO2 HCO • 142 3.4 98 20 7.28 41 58 20 • Step 1 Anion Gap 24 • Potential bicarb? 12 so actual bicarb = 32 • Step 2 Acidemia or Alkalemia? • Step 3 Which way is pCO2 going? • Not dropping as it should (pt can’t hyperventilate effectively due to pneumonia)

35. Mixed Disturbances • Usually due to: • Nausea and vomiting causing GI induced alkalosis (NG suction does same thing) • Respiratory compromise leads to insufficient respiratory compensation, and may lead to hypoxia and Lactic acidosis superimposed on underlying condition. • Key to recognizing these: Potential bicarb!