1. Metabolic Acidosis From Henderson to Stewart ACCS training day 13/01/2015 Dr Josip Stosic ICU

2. Quiz 1  Lovely lady  85 y/o  In ED  Bowel obstruction  Waiting for theatre to become available

3. 2, 45 yo drug abuser  pH6.95  pCO21.19  pO217.02  Bic2  Na 130  Cl98  Alb 32  Lac2.4  BE-34

4. 3, 45 y/o Abdominal pain, obese  Fio2 90%  pH7.177  pCO27.81  pO216.24  Bic21.2  BE-7.6  Na 138  K4.6  Gluc4.3  Lac2.8  Chl109  Alb 22  Creat41

5. 4, 60 y/o Septic shock  Fio2 28%  pH 7.31  pCO2 4.48  pO214.5  Bic16.7  BE-8.4  Na 134  K4.9  Chl106  Lac1.5  Cr323  Alb19

6. 5, 11 y/o dehydrated, CBG  FiO2 21%  pH7.18  pCO22.46  pO28.59  Bic6.7  BE-19.3  Na 138  K3.7  Chl107  Gluc15.7  Lac1.14  Alb 36

7. 6, Unwell, pancreatic pseudocyst  Fio240  pH7.38  pCO25.25  pO218.56  Bic23.1  BE-1.7  Na123  Chl92  K3.72  Lac1.58  Gluc5.8  Alb24

8. 7, Arrest call, ?episode, output remains  FiO285%  pH7.205  pCO25.15  pO217.42  Bic14.9  BE-12.3  Na 138  Cl108  Gluc11.2  Lact11.5  Alb 38

9. Some Concepts  We are Water  Water is OH - and H +

10. What Causes Acidaemia  CO2 obviously!  Anion Excess  The principle of electroneutrality applies  Anions are paired with Cations!  H + is dissociated from water  pH is H +  Weak acids

11. Approach to Acid Base Balance Reading ABG  Do ABG  What is pH?  What is pCO2?  What is Bicarbonate?  Or do I care?  What is the buffer status? (BE, SID)  What is the cause? Do Anion gap/Strong Ion Gap  Narrow it down if high AG/SIG- Osmolar Gap

12. Revelation  Our machines measure only pH and pCO2  Bic and BE are derived!

13. Henderson and tidy Mr Hasselbach Welcome  Equilibrium reaction for carbonic acid CO 2 + H 2 O ↔ H 2 CO 3 ↔ H + + HCO 3 -  pK(H2CO3)=6.1  SCO2= 0.03

14. Problem  Acid base balance cannot be explained by HH:  CO2 effects Bicarb!  CO 2 + H 2 O ↔ H 2 CO 3 ↔ H + + HCO 3 -  We need to quantify the acid/base metabolic contributors!

15. What is on the horizon?  1948 Singer and Hastings: “buffer base”  Sum of plasma anions (bicarb+weak acids)  1960 Antrup: “standard bicarbonate”  Bicarb when pCO2 is 5.3kPa  1960 Siggaard-Andersen: “base excess”  Concentration of H + required to titrate pH to 7.4 at pCO2 5.3

16. Base Excess  Another derived normogram value (Danish volunteers)

17. What now!  If BE negative, there is a metabolic component to acidaemia  Need to find the cause  Normal Anion Gap  High Anion Gap

18. Anion Gap  The principle of electroneutrality  (Na + + K + ) – (Cl - + HCO 3 - )  Usually 12-16 mEq/l  Low albumin will increase anion gap

19. Reduced anion gap  Increased ‘unmeasured’ cations  Hypermagnesaemia  Lithium toxicity  Xs protein  Myeloma  Waldenstrom’s macroglobulinaemia

20. Normal anion gap  Disorders of bicarbonate homeostasis  Hyperchloraemia causes the acidosis  GI losses  Vomitting  Diarrhoea  Renal losses  Renal tubular acidosis  Acetazolamide  Iatrogenic NaCl

21. Increased anion gap  Increased ‘unmeasured’ anions  Lactate  Ketones  Ethanol  Asprin  Cyanide  Methanol  Ethylene glycol

22. Corrected anion gap  Hypoproteinamia common in critical illness  Albumin has a lot of negative charge  Albumin Gap = 40 – apparent albumin  Anion Gap corr = AG + (albumin gap/4)  Increase AG by 2.5 for every 10 g/l fall in Albumin from baseline

23. Example of AG corr  Albumin = 18  AG = 15 (normal)  AG corr = 15 + (40-18)/4 = 20.5 (increased)  Ie. Look for an unmeasured anion!

24. Osmolar Gap  OG = measured serum osmolality − calculated osmolality  Calculated osmolality = 2 x [Na mmol/L] + [glucose mmol/L] + [urea mmol/L]  A normal osmol gap is < 10 mOsm/kg

25. The physico-chemical approach Stewart

26. Clever?  I will let you decide  Bottom line is that it measures contributors of the BE!  The concept of what contributors are is genius!

27. Components of human fluids 1.Water 2.Strong ions in solution in water 3.Buffer solutions in water 4.CO 2 containing solutions

28. Water  High dielectric constant  Things with electrostatic bonds dissociate in it  Water only dissociates slightly

29. Strong ions in solution with water  Effectively fully dissociated  Always present at conc that they were added at  Don’t participate in reactions  Most abundant are Na + and Cl -  Others include K +, Mg ++, Ca ++, SO 4 -

30.  To be electroneutral  [Na + ] + [K + ] + [H + ] – [Cl - ] – [OH - ] = 0  The Strong Ion Difference  SID = [Na + ] + [K + ] – [Cl - ]  [H + ] ie pH depends on SID  If you alter the value of SID, more or less water dissociates to maintain electroneutrality, hence altering [H + ]

31.  SID(app) = [Na + ] + [K + ] + [Mg ++ ] + [Ca ++ ] – [Cl - ]  SID(eff)= HCO3 - + Alb+PO4 3-  SIG= SID(app)-SID(eff)=0

32. SID  SID is an independent variable  Imposed externally on the system  Varied by other factors

33. Carbon dioxide  When you add CO 2 to anything, you get  Dissolved CO 2  Carbonic acid H 2 CO 3  Bicarbonate ions HCO 3 -  Carbonate ions CO 3 2-  The final equilibrium is [H + ] x [CO 3 2- ] = k x [HCO 3 - ]

34. Solving for all the equations...  The combination of water + strong ions + buffers + CO 2 resembles plasma  The only independent variable which vary pH are  SID  A TOT Total weak acid concentration  pCO 2  Altering these will cause an alteration in the degree of water dissociation into H + ions

35. Putting it all together Are we any wiser?

36. How I tickle/tackle metabolic acidosis  Do ABG  pH  PaCO2- ?appropriate  BE(negative)  Identify measured components  Lactate  Ketones  BE (NaCL) = (Na-Cl)-38  BE (alb) = (42-Alb)x0.25  Identify BEgap  Check Corrected anion gap- Can you account for all BE contributors?  Osmolar gap?- cyanide/antifreeze/salycillates.

37. Conclusion  It is all about Anions (chloride)  Bicarb and H+ are infinitely abundant  Subtract your lactate and Chloride excess from BE (base deficit)  What is left is unmeassured:  Usually ketones, renal acids  Rarely salycilates or sweet tasting antifreeze!

38. Quiz  Lovely lady  85 y/o  In ED  Bowel obstruction  Waiting for theatre to become available

39. bloods  FiO20.3  pH7.43  pO28.9  pCO28.6  BIC43  BE15  St92%  Lac2.7  Na 131  Cl78  Cr120  Ur14.5  BM11.6  Alb31

40. BE components  (Na-Cl)-38=+15  (42-Alb)x0.25=+2.7  Lac=-2.7  BE gap= 0  Hypochloraemic Metabolic Alkalosis  Respiratory compensation!

41. 45 yo drug abuser  pH6.95  pCO21.19  pO217.02  Bic2  Na 130  Cl98  Alb 32  Lac2.4  BE-34

42.  (130-98)-38= -6  (42-32)x0.25= +2.5  lac=-2.4  BE gap= -28.1  AG= 130-98-2= 30  Excess of unmeasured anions:  Ketones/renal/exogenous (toulene)

43. 45 y/o Abdominal pain, obese  Fio2 90%  pH7.177  pCO27.81  pO216.24  Bic21.2  BE-7.6  Na 138  K4.6  Gluc4.3  Lac2.8  Chl109  Alb 22  Creat41

44.  (138-109)-38= -9  lac=-2.8  (42-22)x0.25= +5  BEgap= -1  Mixed respiratory and metabolic acidaemia.  Metabolic contributors accounted for

45. 60 y/o Septic shock  Fio2 28%  pH 7.31  pCO2 4.48  pO214.5  Bic16.7  BE-8.4  Na 134  K4.9  Chl106  Lac1.5  Cr323  Alb19

46.  (134-106)-38= -10  (42-19)x0.25= +5.1  Lac=- 1.5  BEgap= -8.4- (-6.4)= -2  Metabolic acidaemia with poor respiratory compensation. Unmeassured ions possibly renal acids

47. 11 y/o dehydrated, CBG  FiO2 21%  pH7.18  pCO22.46  pO28.59  Bic6.7  BE-19.3  Na 138  K3.7  Chl107  Gluc15.7  Lac1.14  Alb 36

48.  (138-107)-38= -7  (42-36)x0.25= +1  Lac=1.14  BE gap = -19-(-5)= -14  AG 28 Metabolic acidaemia with partial respiratory compensation. Large component of unmeassured anions Blood Ketone= 8 (B-hydroxybuturate)

49. Unwell, pancreatic pseudocyst  Fio240  pH7.38  pCO25.25  pO218.56  Bic23.1  BE-1.7  Na123  Chl92  K3.72  Lac1.58  Gluc5.8  Alb24

50.  (123.5-92)-38= -6.5  (42- 24)x0.25= +4.5  Lac== -1.6  BE gap = -1.7-(-3.6)= -1.9

51. Arrest call, ?episode, output remains  FiO285%  pH7.205  pCO25.15  pO217.42  Bic14.9  BE-12.3  Na 138  Cl108  Gluc11.2  Lact11.5  Alb 38

52.  (138-108)-38= -8  (42-38)x0.25= +1  lac=-11.5  BEgap= -12.3-(-18.5)= +6.2  BE could not catch up with our calculation!