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Metabolic Acidosis From Henderson to Stewart ACCS training day 13/01/2015

Metabolic Acidosis From Henderson to Stewart ACCS training day 13/01/2015. Dr Josip Stosic ICU. Quiz 1. Lovely lady 85 y/o In ED Bowel obstruction Waiting for theatre to become available. 2, 45 yo drug abuser. pH 6.95 pCO2 1.19 pO2 17.02 Bic 2 Na 130 Cl 98 Alb 32 Lac 2.4

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Metabolic Acidosis From Henderson to Stewart ACCS training day 13/01/2015

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  1. Metabolic AcidosisFrom Henderson to StewartACCS training day13/01/2015 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 • pH 6.95 • pCO2 1.19 • pO2 17.02 • Bic 2 • Na 130 • Cl 98 • Alb 32 • Lac 2.4 • BE -34

  4. 3, 45 y/o Abdominal pain, obese • Fio2 90% • pH 7.177 • pCO2 7.81 • pO2 16.24 • Bic 21.2 • BE -7.6 • Na 138 • K 4.6 • Gluc 4.3 • Lac 2.8 • Chl 109 • Alb 22 • Creat 41

  5. 4, 60 y/o Septic shock • Fio2 28% • pH 7.31 • pCO2 4.48 • pO2 14.5 • Bic 16.7 • BE -8.4 • Na 134 • K 4.9 • Chl 106 • Lac 1.5 • Cr 323 • Alb 19

  6. 5, 11 y/o dehydrated, CBG • FiO2 21% • pH 7.18 • pCO2 2.46 • pO2 8.59 • Bic 6.7 • BE -19.3 • Na 138 • K 3.7 • Chl 107 • Gluc 15.7 • Lac 1.14 • Alb 36

  7. 6, Unwell, pancreatic pseudocyst • Fio2 40 • pH 7.38 • pCO2 5.25 • pO2 18.56 • Bic 23.1 • BE -1.7 • Na 123 • Chl 92 • K 3.72 • Lac 1.58 • Gluc 5.8 • Alb 24

  8. 7, Arrest call, ?episode, output remains • FiO2 85% • pH 7.205 • pCO2 5.15 • pO2 17.42 • Bic 14.9 • BE -12.3 • Na 138 • Cl 108 • Gluc 11.2 • Lact 11.5 • 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 BalanceReading 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 HasselbachWelcome • Equilibrium reaction for carbonic acid CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- • pK(H2CO3)=6.1 • SCO2= 0.03

  14. Problem • Acid base balance cannot be explained by HH: • CO2 effects Bicarb! • CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- • 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- + HCO3- ) 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 Gapcorr = AG + (albumin gap/4) Increase AG by 2.5 for every 10 g/l fall in Albumin from baseline

  23. Example of AGcorr Albumin = 18 AG = 15 (normal) AGcorr = 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 Water Strong ions in solution in water Buffer solutions in water CO2 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++, SO4-

  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+PO43- • 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 CO2 to anything, you get • Dissolved CO2 • Carbonic acid H2CO3 • Bicarbonate ions HCO3- • Carbonate ions CO32- • The final equilibrium is [H+] x [CO32-] = k x [HCO3-]

  34. Solving for all the equations... • The combination of water + strong ions + buffers + CO2 resembles plasma • The only independent variable which vary pH are • SID • ATOT Total weak acid concentration • pCO2 • 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 • FiO2 0.3 • pH 7.43 • pO2 8.9 • pCO2 8.6 • BIC 43 • BE 15 • St 92% • Lac 2.7 • Na 131 • Cl 78 • Cr 120 • Ur 14.5 • BM 11.6 • Alb 31

  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 • pH 6.95 • pCO2 1.19 • pO2 17.02 • Bic 2 • Na 130 • Cl 98 • Alb 32 • Lac 2.4 • 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% • pH 7.177 • pCO2 7.81 • pO2 16.24 • Bic 21.2 • BE -7.6 • Na 138 • K 4.6 • Gluc 4.3 • Lac 2.8 • Chl 109 • Alb 22 • Creat 41

  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 • pO2 14.5 • Bic 16.7 • BE -8.4 • Na 134 • K 4.9 • Chl 106 • Lac 1.5 • Cr 323 • Alb 19

  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% • pH 7.18 • pCO2 2.46 • pO2 8.59 • Bic 6.7 • BE -19.3 • Na 138 • K 3.7 • Chl 107 • Gluc 15.7 • Lac 1.14 • 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 • Fio2 40 • pH 7.38 • pCO2 5.25 • pO2 18.56 • Bic 23.1 • BE -1.7 • Na 123 • Chl 92 • K 3.72 • Lac 1.58 • Gluc 5.8 • Alb 24

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

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