Strong acid base

Peter Stewart – the inventor of the quantitative (“ignore H+”) approach to acid base

I grew up with the German approach to acid base (“no clue – ask someone else”), then learned the Boston way (with all their compensation formulas). In parallel I tried to understand the Stewart style of quantitative acid base treatment, always missing the easy rule-of-thumb calculations I so loved in the anion gap calculations. So I really enjoyed the developments of the last years, where simple formulas for the strong ion gap proved to be adequate.

I will not delve into the discussion of the pros and cons of Stewart’s “H+ is not the point of acid base”, but happily accept that the old anion gap (if corrected for albumin) and the physicochemical approach yield the same results, if by different means. Scott Weingart at¬†has devised a simple algorithm to deal with metabolic acid base disturbances, that I have adopted – you can enter all that stuff into’s calculator.

  1. Labs: BGA (Na, Cl, K, pH, pCO2, base deficit = -base excess, lactate) plus albumin
  2. Determine the sum effect of all acid base disorders:
    • pH < 7,35 = acidosis
    • pH > 7,45 = alkalosis
  3. Determine the respiratory effect (might be compensation)
    • pCO2 > 45 = respiratory acidosis
    • pCO2 < 35 = respiratory alkalosis
  4. Determine the strong ion effect: SID = Na – Cl
    • SID < 38 = low SID acidosis: get urinary anion gap (UAG) = urine Na + K – Cl
      • fluid administration with low SID fluids (saline, D5W)
      • renal tubular acidosis if UAG > 0
        • type I: urinary pH > 5,55
        • type II: urinary pH < 5,55
        • type IV: hyperkalemic, pH < 5,55
    • SID > 38 = high SID alkalosis (nasogastric suction, diuretics, hyperaldosteronism, fluid depletion)
  5. Determine the lactate effect (due to hypoxia or toxicity with Metformin, Propofol, Linezolid, amphetamines, Valproate, HIV drugs, …)
  6. Determine the albumine effect: 2,5 * (4,2 – [Alb in g/dl])
  7. Add all the metabolic components into the Strong Ion Gap:
    • SIG = base deficit + SID – 38 + Albumine effect – Lactate
    • SIG > 2 = SIG metabolic acidosis: calculate osmolar gap = measured – 2 * Na + Gluc/18 + Urea/2,8 (> 10 is pathological)
      • Uremia, diabetic ketoacidosis, alcoholic ketoacidosis
      • Toxins: ASS, alcohols (methanol, mannitol, propylene glycol as in benzo or dilantin preparations), iron
      • Short gut syndrome (D-Lactate)
    • SIG < 0 = overdose with cations (Lithium, Bromide, Hyper-K, Hyper-Mg, Hyper-Ca, Nitrates, Immunoglobulins)
  8. Check adequacy of compensation for chronic primary acid base disorders
    • In respiratory: expected delta SID = 0,4 * delta CO2
    • In metabolic acidosis: delta CO2 = base deficit
    • In metabolic alkalosis: delta CO2 = 0,6 * base excess

Recommended reading


Metabolic alkalosis

It was a cool case: called to admit to our stroke unit (which serves as an intermediary care unit as well) a patient from the general ward because of global respiratory insufficiency, we got an ABG that read pH 7,48, CO2 67. Her sats were in the 70ies without oxygen and she showed periodic breathing, yet without any dyspnea.

Obviously this is a case of metabolic alkalosis, so we used it to talk about this most common of all acid-base-disorders.

The take home messages are:

  • Metabolic alkalosis (like most of the physiological “excess” states such as hyponatremia – excess water, hyperkalemia and so forth) has 1. a cause and 2. a reason for being maintained – i.e. a kidney problem, usually hyperaldosteronism, hypovolemia, hypokalemia etc.
  • Differentiate the causes by GI, renal and intracellular.

As for procedural aspects,

  • always get an arterial ABG, serum elytes (Na, K, Cl, Mg, Ca) and urine chloride – and measure the anion gap
  • assess the volume status (hypo-, eu-, hypervolemic)

And therapy? If not sure, try pure chloride. You can get that as HCl, which is hard to get, or as KCl, which isn’t. Since potassium immediately goes into the cell (unless no insulin is present), you are left with chloride which removes an HCO3.

References: any textbook on intensive care.

In fact, we also mentioned the Stewart-Fencl-approach to acid-base-disorders, which I love and – as a mathematician – had to learn in my youth (4 years ago). If you find the time, go read Stewart’s original book (available on or this new textbook.