Acid-Base Analysis examples.presentation

Acid-Base Analysis examples

1. Evaluation and Analysis of Acid-Base Disorders Taylor Sawyer DO Resident Pediatrics TAMC

2. Why Acid-Base ? Complicated Confusing Time consuming

3. Reference: Western Journal of Medicine. Aug 1991; 155: 146-151

4. Objectives: Introduction to equipment used and variables involved in acid-based problems Simplified discussion on acid-base disorders Systematic approach to acid-base interpretation 3 Quick and easy rules on acid-base disorders

5. Acid-Base Analysis, What do You Need? Blood gas (pH, CO 2 ) Serum chemistry (Na, Cl, HCO 3 ) Calculator 30 seconds

6. The Tools: Blood Gas Few drops of blood (< 100 μl) into cartridge Cartridge placed into analyzer Foil pouch in the cartridge containing a calibrated buffered solution with analytes in know concentration is punctured and flows over sensors (calibration) Blood sample then pushed onto sensors Measurements performed

7. i-STAT 7 I stat 7: Sodium Potassium Ionized calcium pH PCO 2 PO 2 Hematocrit Bicarbonate* Total carbon dioxide* Base excess* O2 saturation* Hemoglobin* (* denotes calculated result)

8. The Tools: Serum Chemistry COBAS INTERGA 800 Uses four separate methods of analysis on each sample Can perform up to 72 different tests Can handle 185 samples tubes Can do up to 850 serum chemistries per hour Direct measurements of electrolytes and HCO 3

9. ABG : 7.40 / 40 / 80 / 24 / pH PaCO 2 PaO 2 HCO 3 BE

10. Acid-Base Normals: pH= 7.40 (7.35 - 7.45) PCO 2 = 40 (35 - 45) HCO 3 = 24 (22 - 26)

11. Acidemic vs. Alkalemic pH < 7.35 = Acidemic pH > 7.45 = Alkalemic Separate term for pH to allow description of the net effect of multiple respiratory and metabolic abnormalities

12. Rule 1 Look at the pH. Whichever side of 7.40 the pH is on, the process (CO 2 , HCO 3 ) that caused it to shift that way is the primary abnormality. Principle: The body does not fully compensate for a primary acid-base disorder

13. Keep It Simple: CO 2 = Acid CO 2 =  pH (acidemia)  CO 2 =  pH (alkalemia) HCO 3 = Base  HCO 3 =  pH (alkalemia)  HCO 3 =  pH (acidemia)

14. Four Primary Disorders: PCO 2 < 35 = respiratory alkalosis PCO 2 > 45 = respiratory acidosis HCO 3 < 22 = metabolic acidosis HCO 3 > 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!

15. Simple Acid-Base Disorders

16. Example # 1: Blood gas: 7.50 / 29 / 22 Alkalemic Low PCO 2 is the primary (respiratory alkalosis) No metabolic compensation = acute process Acute Respiratory Alkalosis

17. Acute Respiratory Alkalosis

18. Example # 2: Blood gas: 7.25 / 60 / 26 Acidemic Elevated CO 2 is primary (respiratory acidosis) No metabolic compensation= acute process Acute Respiratory Acidosis

19. Acute Respiratory Acidosis

20. Acidemic Elevated CO 2 is primary (respiratory acidosis) Metabolic compensation has occurred = chronic process Chronic Respiratory Acidosis with Metabolic Compensation * * true metabolic compensation takes 3 days (72hrs) Example # 3: Blood gas: 7.34 / 60 / 31

21. Chronic Respiratory Acidosis with Metabolic Compensation

22. Example # 4: Blood gas: 7.50 / 48 / 36 Alkalemic Elevated HCO 3 is primary (metabolic alkalosis) Respiratory compensation has occurred = acute /chronic ? Metabolic Alkalosis with Respiratory Compensation* *Respiratory compensation takes only minutes

23. Metabolic Alkalosis with Respiratory Compensation

24. Example # 5: Blood gas: 7.20 / 21 / 8 Acidemic Low HCO 3 Is primary (metabolic acidosis) Respiratory compensation is present Metabolic Acidosis with Respiratory Compensation

25. Anion Gap (AG): The calculated difference between the positively charged ( cations ) and negatively charged ( anions ) electrolytes in the body: AG= Na + - (Cl - + HCO 3 - ) Normal AG = 12 ± 2 (10 – 14)

26. Anion Gap Also can be though of as the concentration of the excess unmeasured anion in the serum Total body cations = total body anions (net 0) Normal Measured : Na - (Cl + HCO 3 ) = + 12 Normal Unmeasured : anions - Cations = - 12 -------- net = 0

27. Unmeasured Anions/Cations

28. 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)

29. Why is this true? AG > 20 is more than 4 standard deviations from the mean and therefore unlikely due to chance. Although a modest increase in anion gap does occur in patients with metabolic or respiratory alkalosis ( increase negatively charged serum proteins ), even in severe alkalosis this increase is almost never > 20 A specific cause for an anion gap can be found in less than 30% of cases with a anion gap less than 20, as compared to 77% of those with AG > 20, and 100% with AG > 30* * Gabow et al. Diagnostic Importance of an increased serum anion gap. N Engl J med. 1980; 303:854-858

30. So, The presence of an anion gap  20 is highly predictive of the presence of an underlying identifiable primary metabolic acidosis

31. 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 if the sum is less than normal bicarbonate (< 23) there is an underlying nonanion gap metabolic acidosis Excess AG = Total AG – Normal AG (12) Excess AG + measured HCO 3 = > 30 or < 23? Principle: 1 mmol of unmeasured acid titrates 1 mmol of bicarbonate (  anion gap =  [ HCO 3 ])

32. Why is this true? For each 1 mmol acid titrated by the carbonic acid buffer system, 1 mmol of HCO 3 is lost via conversion to CO 2 and H 2 O and 1 mmol of the sodium salt of the unmeasured acid is formed. 1 mmol  in HCO 3 = 1mmol in AG Therefore, the sum of the new (excess) anion gap and the remaining (measured) bicarbonate values should equal the normal bicarbonate concentration

33. Excess Anion gap

34. HCO 3 Added If : Excess AG + Measured HCO 3 = > normal HCO 3 (30) Then: Some additional disorder has added HCO 3 to the extracellular space ( metabolic alkalosis )

35. HCO 3 Removed If : Excess AG + Measured HCO 3 = < normal HCO 3 (23) Then: Some additional disorder has removed HCO 3 from the extracellular space ( nonanion gap metabolic acidosis ), e.g. renal or GI loses

36. Is This Really True? Published reports do indicate that a reciprocal relationship between increased anion gap and decreased HCO 3 does exist in uncomplicated organic acidosis* Due to multiple buffering systems in the body it may not always be a one-to-one relationship Bicarbonate is the major extracellular buffer * Naris et al . Anion gap and Serum Bicarbonate. N Engl J Med 1980; 303: 161

37. Mixed Acid-Base Disorders

38. Remember the Rules Look at the pH: (< or > 7.40?) whichever caused the shift (CO 2 or HCO 3 ) is the primary disorder Calculate the anion gap: if AG  20 there is a primary metabolic acidosis (regardless of pH or HCO 3 ) Calculate the excess anion gap, add it to HCO 3 : Excess AG = Total AG – Normal AG (12) Excess AG + HCO 3 = ? If sum > 30 there is an underlying metabolic alkalosis If sum < 23 there is an underlying nonanion gap metabolic acidosis

39. Example # 1 Blood gas: 7.50 / 20 / 15 Na= 140, Cl = 103 Alkalemic Low CO 2 is primary (respiratory alkalosis) Partial metabolic compensation for chronic condition? AG = 22 (primary metabolic acidosis) Excess AG (AG – 12) + HCO 3 = 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

40. Example # 2 Blood gas: 7.40 / 40 / 24 Na= 145, Cl= 100 pH normal AG = 21 (primary metabolic acidosis) Excess AG (AG – 12) + HCO 3 = 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

41. Example # 3 Blood gas 7.50 / 20 / 15 Na= 145, Cl = 100 Alkalemic Low CO 2 is primary (respiratory alkalosis) AG = 30 (primary metabolic acidosis) Excess AG (AG – 12) + HCO 3 = 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)

42. 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

43. Example # 4 Blood gas: 7.10 / 50 / 15 Na= 145, Cl= 100 Acidemic High CO 2 and low HCO 3 - both primary (respiratory acidosis and metabolic acidosis) AG = 30 (metabolic acidosis is anion gap type) Excess AG + HCO 3 = 33 (underlying metabolic alkalosis) Respiratory Acidosis, 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)

44. Example # 5 Blood gas: 7.15 / 15 / 5 Na= 140, Cl= 110 Acidemic Low HCO 3 - primary (metabolic acidosis) AG= 25 (metabolic acidosis is anion gap type) Excess AG + HCO 3 = 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 hyperchloremic nonanion 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