Acid Base Disturbance and Strong Ion Difference Dr Rob Stephens Joint Intensive Care Symposium 17/18 June 2010

contents  Introduction: Stewart  Basic definitions + crazy thoughts!  Water dissociation is the key  Talk through Stewarts ideas..  Physical principles  SID, Weak Acids, CO 2  Base Excess

Introduction: Peter Stewart  Why useful – an alternative explanation  No consistently demonstrable clinical benefit  Based on Physico-chemical principles  Combined with mathematical modeling, solving simultaneous equations

Basic Definitions  Acid  Base  Neutral  Electrolytes –dissociated into ions  Strong ions – completely dissociate H+/ Proton donor increases H+ concentration in solution H+/ Proton acceptor lowers H+ concentration in solution H+ = OH- Na + K + Mg ++ Ca ++ Cl-Lactate-

Some crazy thoughts  pH is a stupid measure  [H+] would be more sensible  [H+] is tiny nMol<<μMol<<mMol  [H+] is not controlled tightly! pH H

Stewart- Water  [Water] is high ~ 53Molar  Water dissociation H 2 0 H + + OH -

Stewart Things that have to be satisfied Electrical Neutrality In macroscopic aqueous solutions, the sum of all positively charged ions must equal the sum of all negatively charged ions An aqueous solution is always electrically neutral Conservation of Mass The amount of a substance remains constant unless it is added or removed or unless it is generated or destroyed The law of mass action: dictates the dissociation equilibrium of incompletely dissociated substances

Electrical Neutrality +- Na + K + Mg ++ Ca ++ H + = Cl-Lactate-SO 4 -- HCO 3 -OH- Weak Acids (Proteins, PO 4 --) 200mEq/L 150mEq/L 100mEq/L 50mEq/L 0

Variables  3 independent Strong Ion Difference pCO2 ‘volatile acid’ total weak non-volatile acids [ATOT]  6 Dependant H+ OH - HCO 3 - CO 3 --, HA, A - weak acids and ions

Strong Ion Difference Lactate Cl mEq/L 150mEq/L 100mEq/L 50mEq/L 0 Mg ++ Ca ++ K + Na + ? Weak Acids (Proteins, Albumin, PO 4 --) Unmeasured Anions SO 4 -- Ketoacids Salycylate etc - 40mEq/L

Strong Ion Difference  How does it affect H+  NaCl changes Na+Cl- Na+ Cl- OH- Na+ Cl- H+

SID = 40 SID = 32.3 Serum Na mEq/L Extracellular Na + : 140 x 15 = 2100 mEq/L Serum Cl mEq/L Extracellular Cl - : 100 x 15 = 1500 mEq/L Extracellular Na + : = /18 = mEq/L Extracellular Cl-: = /18 = 109 mEq/L Strong Ion Difference Add 3L of 0.9% saline

SID = 40 SID = 32.3 Serum Na mEq/L Extracellular Na + : 140 x 15 = 2100 mEq/L Serum Cl mEq/L Extracellular Cl - : 100 x 15 = 1500 mEq/L Extracellular Na + : = /18 = mEq/L Extracellular Cl-: = /18 = 109 mEq/L Strong Ion Difference Add 3L of 0.9% saline

pCO2 ‘volatile acid’ We are used to thinking about CO 2 Cause respiratory CO 2 H 2 OH 2 CO 3 HCO H + CO 2 dissolves in water to make H 2 CO 3 HCO 3 - and CO 3 --

[ATOT]  total weak non-volatile acids  inorganic phosphate, serum proteins, albumin  HProt H + + Prot -  Falls in Albumin, Haemoglobin Alkalosis  Rises in Phosphate Acidosis

[ATOT] Lactate Cl mEq/L 150mEq/L 100mEq/L 50mEq/L 0 Mg ++ Ca ++ K + Na + ? Weak Acids (Proteins, PO 4 --, Albumin) Unmeasured Anions SO 4 -- Ketoacids Salycylate etc - H+OH-

6 dependent Concentrations determined by concentrations of other ions / molecules. H+ OH - HCO 3 - CO 3 --, HA, A - : weak acids and ions

Does it really matter?  Similarity to Standard Base Excess  Explains some things better  Some unanswered questions  Not universally associated with ‘better outcome’

[ATOT] 200mEq/L 150mEq/L 100mEq/L 50mEq/L 0 Weak Acids (Proteins, PO 4 --, Albumin) Unmeasured Anions SO 4 -- Ketoacids Salycylate etc

Anion measures  Anion Gap = [Na + ] + [K + ] - [Cl - ] - [HCO 3 - ]  Corrected Anion Gap AG corr =AG (40-[albumin])-lactate  SID app = [Na + ] + [K + ] + [Ca 2+ ] + [Mg 2+ ] - [Cl - ] - [lactate - ]  Effective strong ion difference SID eff = 12.2 × pCO2 / (10 -pH ) + 10 × [albumin] × (0.123 × pH ) + [PO 4 - ] × (0.309 × pH ).  The strong ion gap: SIG = SID app - SID eff Weak Acids

Does it really matter R 2 = A/V SID A/V SBE Kellum et al. J Crit Care 1997; 12: 7-12

summary  Stewart uses physics + chemistry + mathematics  [H+]- SID, pCO 2 Total weak acids – independent  Useful in explaining some phenomena

Thanks + reading  Slides available at or google: ucl anaesthesia staphens  Prof Monty Mythen useful discussion  ‘Prof Grongono’  Stewarts book  Continuing Education in Anaesthesia, Critical Care & Pain Volume 8 Number Chawla, Drummond available at