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ACID –BASE DISTURBANCES Dr. A. Zolfaghari Assistant Professor of EM SSU

Learn about acid-base analysis and the principles behind pH, PCO2, and HCO3 levels. Identify the primary abnormalities and recognize mixed acid-base disorders. Includes useful calculations and key concepts.

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ACID –BASE DISTURBANCES Dr. A. Zolfaghari Assistant Professor of EM SSU

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  1. ACID –BASE DISTURBANCES Dr. A. ZolfaghariAssistant Professor of EM SSU

  2. Acid-Base Analysis, What do You Need? • Blood gas (pH, CO2) • Serum chemistry (Na, Cl, HCO3) • Calculator

  3. Acid-Base Normals: • pH= 7.40 (7.35 - 7.45) • PCO2 = 40 (35 - 45) • HCO3= 24 (22 - 26)

  4. Henderson-Hasselbach Equation pH 7.20 7.30 7.40 7.50 7.60 [H+] 60 50 40 30 20

  5. [H+] pH • 20 7.7 • 25 7.6 • 32 7.5 • 40 7.4 • 50 7.3 • 64 7.2 • 80 7.1 • 101 7.0 • 128 6.9 • 160 6.8

  6. 7.20 PCO2 60 HCO3 24 • PH=log(H+)=24 × PCO2/HCO3 • 7.40 >>> 40=24 × 40/24 • PH= 24× 60/24

  7. Important concepts • -emia refers to a pH • -osis refers to an abnormal condition or process

  8. Acidemic vs. Alkalemic • pH < 7.35 = Acidemic • pH > 7.45 = Alkalemic

  9. Respiratory Acidosis: Acute: 10 mm Hg PCO2 >>> 1 mEq/L HCO3 Chronic (>5 days of hypercapnia): 10 mm Hg PCO2 >>>4 mEq/L HCO3 • Respiratory Alkalosis: Acute: 10mmHg  in PCO2>>>2 mEq/L HCO3 Chronic: 10mmHg  in PCO2 >>>  4 mEq/L HCO3 • Metabolic Acidosis: PCO2 = 1.5(HCO3) +8  2 • Metabolic Alkalosis: PCO2 = 0.9 (HCO3) + 9 1 mEq/L  HCO3 >>> 0.6  PCO2

  10. Rule 1 • Look at the pH. Whichever side of 7.40 the pH is on, the process (CO2, HCO3) that caused it to shift that way is the primary abnormality. Principle: The body does not fully compensate for a primary acid-base disorder

  11. Keep It Simple: • CO2 = Acid • CO2 =  pH (acidemia) •  CO2 =  pH (alkalemia) • HCO3 = Base •  HCO3 =  pH (alkalemia) •  HCO3 =  pH (acidemia)

  12. Four Primary Disorders: • PCO2 < 35 = respiratory alkalosis • PCO2 > 45 = respiratory acidosis • HCO3 < 22 = metabolic acidosis • HCO3 > 26 = metabolic alkalosis • Can have mixed pictures with compensation • Can have up to 3 abnormality simultaneously (1 respiratory + 2 metabolic) • The direction of the pH will tell you which is primary!

  13. Simple Acid-Base Disorders

  14. Example # 1:Blood gas: 7.50 / 29 / 22

  15. Example # 1:Blood gas: 7.50 / 29 / 22 • Alkalemic • Low PCO2 is the primary (respiratory alkalosis) • Acute Respiratory Alkalosis

  16. Acute Respiratory Alkalosis

  17. Example # 2:Blood gas: 7.25 / 60 / 26

  18. Example # 2:Blood gas: 7.25 / 60 / 26 • Acidemic • Elevated CO2 is primary (respiratory acidosis) • Acute Respiratory Acidosis

  19. Acute Respiratory Acidosis

  20. Example # 3:Blood gas: 7.34 / 60 / 31

  21. Example # 3:Blood gas: 7.34 / 60 / 31 • Acidemic • Elevated CO2 is primary (respiratory acidosis) • Metabolic compensation has occurred = chronic process • Chronic Respiratory Acidosis *true metabolic compensation takes 3 days (72hrs)

  22. Chronic Respiratory Acidosis with Metabolic Compensation

  23. Example # 4:Blood gas: 7.50 / 48 / 36

  24. Example # 4:Blood gas: 7.50 / 48 / 36 • Alkalemic • Elevated HCO3 is primary (metabolic alkalosis) • 0.9*36+ 9=41 • 1HCO3 >>> 0.6  CO2 (12* 0.6=7.2) 48 • Metabolic Alkalosis with Respiratory Compensation* *Respiratory compensation takes only minutes

  25. Metabolic Alkalosis with Respiratory Compensation

  26. Example # 5:Blood gas: 7.20 / 21 / 8

  27. Example # 5:Blood gas: 7.20 / 21 / 8 • Acidemic • Low HCO3 Is primary (metabolic acidosis) • Respiratory compensation is present • Metabolic Acidosis with Respiratory Compensation • PH=7.20 >>> PCO2=20

  28. Anion Gap (AG): • The calculated difference between the positively charged (cations) and negatively charged (anions) electrolytes in the body: AG= Na+ - (Cl- + HCO3 -) • Normal AG = 12 ± 2 (10 – 14)

  29. Rule 2 • Calculate the anion gap. If the anion gap is  20, there is a primary metabolic acidosis regardless of pH or serum bicarbonate concentration Principle: The body does not generate a large anion gap to compensate for a primary disorder (anion gap must be primary)

  30. Rule 3 • Calculate the excess anion gap (total anion gap – normal anion gap) and add this value to the measured bicarbonate concentration: • if the sum is > than normal bicarbonate (> 30) there is an underlying metabolic alkalosis(MAL) • if the sum is less than normal bicarbonate (< 23) there is an underlying nonanion gap metabolic acidosis (NAGMA) • Excess AG = Total AG – Normal AG (12) • Excess AG + measured HCO3= > 30 or < 23?

  31. Mixed Acid-Base Disorders

  32. Remember the Rules • Look at the pH: (< or > 7.40?) whichever caused the shift (CO2 or HCO3) is the primary disorder • Calculate the anion gap: if AG  20 there is a primary metabolic acidosis (regardless of pH or HCO3) • Calculate the excess anion gap, add it to HCO3: Excess AG = Total AG – Normal AG (12) Excess AG + HCO3= ? If sum > 30 there is an underlying metabolic alkalosis If sum < 23 there is an underlying nonanion gap metabolic acidosis

  33. GAP=(calculated AG-12)-(24-measured HCO3) • Values greater than +6 equate to either simultaneous metabolic alkalosis or respiratory acidosis.# Values less than −6 imply a greater loss of HCO3 − , suggesting concurrent respiratory alkalosis or rarely a low-AG state.

  34. Example # 1Blood gas: 7.50 / 20 / 15 Na= 140,Cl = 103

  35. Example # 1Blood gas: 7.50 / 20 / 15 Na= 140, Cl = 103 • Alkalemic • Low CO2 is primary (respiratory alkalosis) • Partial metabolic compensation for chronic condition? • AG = 22 (primary metabolic acidosis) • Excess AG (AG – 12) + HCO3= 25 (no other primary abnormalities) • Respiratory Alkalosis and Metabolic Acidosis The patient ingested a large quantity of ASA and had both centrally mediated resp. alkalosis and anion gap met. Acidosis associated with salicylate overdose

  36. Example # 2Blood gas: 7.40 / 40 / 24 Na= 145, Cl= 100

  37. Example # 2Blood gas: 7.40 / 40 / 24 Na= 145, Cl= 100 • pH normal • AG = 21 (primary metabolic acidosis) • Excess AG (AG – 12) + HCO3= 33 ( underlying metabolic alkalosis) • Metabolic Acidosis and Metabolic Alkalosis This patient had chronic renal failure (met. acidosis) and began vomiting (met. alkalosis) as his uremia worsened. The acute alkalosis of vomiting offset the chronic acidosis of renal failure = normal pH

  38. Example # 3Blood gas 7.50 / 20 / 15 Na= 145, Cl = 100

  39. Example # 3Blood gas 7.50 / 20 / 15 Na= 145, Cl = 100 • Alkalemic • Low CO2 is primary (respiratory alkalosis) • AG = 30 (primary metabolic acidosis) • Excess AG (AG – 12) + HCO3= 33 (underlying metabolic alkalosis) • Respiratory alkalosis & Metabolic Acidosis and Metabolic Alkalosis This patient had a history of vomiting (met. alkalosis), poor oral intake (met. acidosis) and tachypnea secondary to bacterial pneumonia (resp. alkalosis)

  40. How Many Primary Abnormalities Can Exist in One Patient? • Three primary abnormalities is the max because a person cannot simultaneously hyper and hypoventilate • One patient can have both a metabolic acidosis and a metabolic alkalosis – usually one chronic and one acute

  41. Example # 4Blood gas: 7.10 / 50 / 15 Na= 145, Cl= 100

  42. Example # 4Blood gas: 7.10 / 50 / 15 Na= 145, Cl= 100 • Acidemic • High CO2 and low HCO3- both primary (respiratory acidosis and metabolic acidosis) • AG = 30 (metabolic acidosis is anion gap type) • Excess AG + HCO3=33 (underlying metabolic alkalosis) • Respiratory Acidosis, AG Metabolic Acidosis and Metabolic Alkalosis This is an obtunded patient (resp. acidosis) with a history of emesis (metabolic alkalosis) and lab findings c/w diabetic ketoacidosis (metabolic acidosis w/ gap)

  43. Example # 5Blood gas: 7.15 / 15 / 5 Na= 140, Cl= 110

  44. Example # 5Blood gas: 7.15 / 15 / 5 Na= 140, Cl= 110 • Acidemic • Low HCO3- primary (metabolic acidosis) • AG= 25 (metabolic acidosis is anion gap type) • Excess AG + HCO3 = 18 (underlying nonanion gap metabolic acidosis) • Anion Gap and Nonanion gap Metabolic Acidosis Diabetic ketoacidosis was present (anion gap met. acidosis). Patient also had a hyperchloremicnonanion gap met. acidosis secondary to failure to regenerate bicarbonate from ketoacids lost in the urine.

  45. Conclusions: • To do accurate acid-base evaluations you need both blood gas and serum chemistry • Use a systematic approach • Remember the 3 rules • “normal” blood gases may not be normal • It is important to identify all the underlying acid-base in order to appropriately treat the patient

  46. Example # 6 • 44 yo M 2 weeks post-op from total proctocolectomy for ulcerative colitis. • Na+ 134, K+ 2.9, Cl- 108, HCO3- 16, BUN 31, Cr 1.5 PH=7.31 PCO2=33

  47. Answer 6 • Primary Metabolic acidosis 1.5*16+8=30 • AG= 132- (108+16)= 10 >>> NL AG • E. GAP= -2+16= 14 >>> NAGMA

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