- base excess is used as an indicator of the degree of metabolic disturbance
The problem with bicarbonate
- Bicarbonate levels are not an ideal indicator of either metabolic or respiratory components of acid-base disturbance because it is affected by both
- Furthermore the relationship between metabolic acidosis and bicarbonate is neither consistent nor linear
- the concentration of the bicarbonate ion (HCO3–) (in mEq/L) is not measured, it is calculated from the PCO2 and pH
- Standard base excess (SBE) is the best measure of the metabolic disturbance
BASE EXCESS AND STANDARD BASE EXCESS
- introduced in 1958
- base excess is dose of acid or alkali to return in vitro blood to normal pH (7.40) under standard conditions ( at 37C at a PCO2 of 40 mm Hg)
- Normal Base excess is between -3 and +3mEq/L
Standard Base Excess
- standard base excess is dose of acid or alkali to return the ECF to normal pH (7.40) under standard conditions ( at 37C at a PCO2 of 40 mm Hg)
- this is the base excess calculated for anaemic blood (Hb = 50g/L)
- based on the principle that this closely represents the behaviour of the whole body, as Hb effectively buffers the plasma as well as the ECF
Abnormal base excess with normal anion gap
- normal anion gap metabolic acidosis (e.g. acetazolide, hypercholoremia, GI losses of HCO3, renal tubular acidosis)
Normal base excess with abnormal anion gap
- lactic acidosis (or other high anion gap metabolic acidosis) with pre-existing metabolic alkalosis
- HAGMA masked by hypoalbuminemia (if anion gap is uncorrected)
- salicylate toxicity – respiratory alkalosis plus in increased anion gap metabolic acidosis
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.
After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.
He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.
His one great achievement is being the father of three amazing children.
On Twitter, he is @precordialthump.