Question 9

Created on Fri, 05/29/2015 - 01:35
Last updated on Fri, 04/28/2017 - 16:43
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Other SAQs in this paper

Other SAQs on this topic

a)  Define base excess.
b)  List 2 conditions in which there is a negative base excess without any changes in the anion gap.
c)  List 1 condition in which there is an increase in the anion gap without a negative base excess.

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College Answer

a) Base excess is defined as the amount of strong acid or base required to titrate pH of an in vitro sample of blood back to 7.40 at 37C at a PCO2 of 40 mm Hg.

b) Negative base excess without anion gap elevation

1)  Dilutional acidosis from saline resuscitation
2)  Renal tubular acidosis
3)  Ureterosigmoidostomy
4)  Acetazolamide therapy

c)  Lactic acidosis in a patient with pre-existing metabolic alkalosis

Discussion

Base excess definition

  • Dose of acid or base required to return the pH of a blood sample
  • Measured at standard conditions: 37°C and 40mmHg PaCO2
  • Thus, isolates the metabolic disturbance from the respiratory

Standard base excess

  • Dose of acid or base required to return the pH of an anaemic blood sample
  • Calculated for a Hb of 50g/L
  • Haemoglobin buffers both the intravascular and the extravascular fluid
  • Thus, SBE assesses the buffering of the whole extracellular fluid, not just the haemoglobin-rich intravascular fluid

"Base Excess" is the amount of acid or base required to titrate a blood sample (of whole blood) to a pH of 7.40, at standard temperature and pressure, with a standard PaCO2 of 40mmHg.

The "Standard Base Excess" is different because it uses extracellular fluid rather than whole blood. (of course, you dont sample the extracellular fluid - the ABG machine calculates the SBE for anaemic blood, with a Hb of 50g/L). The argument for this is the buffering ability of haemoglobin. It would be inappropriate to extrapolate whole blood findings to the total extracellular fluid, because though circulating haemoglobin buffers all extracellular fluid, it does so from the intravascular compartment to which it is confined.

An example of a base deficit in the presence of a normal anion gap is essentially any cause of normal anion gap metabolic acidosis- take your pick.

Examples of a normal base excess in the presence of a raised anion gap would include any situation where the high anion gap metabolic acidosis occurs in the setting of a chronic metabolic alkalosis. A favourite example is a raised lactate in the diuretic-using CCF patient, but one could just as easily use the case of the torrentially vomiting methanol drinker. LITFL also mention salicylate toxicity and HAGMA masked by uncorrected hypoalbuminaemia.

References

SIGGAARD‐ANDERSEN, O., and N. FOGH‐ANDERSEN. "Base excess or buffer base (strong ion difference) as measure of a non‐respiratory acid‐base disturbance." Acta Anaesthesiologica Scandinavica 39.s107 (1995): 123-128.

Ole Siggard-Andersen has his own website, which is an excellent anthology of acid-base information. This man has pioneered the concept of base excess in 1958, two years after his graduation from University of Copenhagen as candidatus medicinae (laudabilis præ ceteris et quidem egregie).

Kraut, Jeffrey A., and Nicolaos E. Madias. "Serum anion gap: its uses and limitations in clinical medicine." Clinical Journal of the American Society of Nephrology 2.1 (2007): 162-174.

Shock, Nathan W., and A. Baird Hastings. "Studies of the acid-base balance of the blood IV. Characterization and interpretation of displacement of the acid-base balance." Journal of Biological Chemistry 112.1 (1935): 239-262.