0536. Partitioning the base deficit assists in understanding aetiology of acid-base disturbances

As indicated in summary 0534, base deficit alone is useful but other relatively simple measures are available and can add even more value in unraveling what is going on biochemically in high risk situations of shock and/or post-operative care in paediatrics. Many authors refer to the Stewart-Fencl approach that allows quantification of each component of acid-base status, but the physicochemical calculations are cumbersome and require the simultaneous measurement of many biochemical variables. Abbreviated versions have been derived for albumin and chloride, and these were used in a UK study of 60 shocked infants with meningococcal septicaemia to partition base deficit (BD) into BDtotal = BDalbumin + BDchloride + BDUnmeasured Anions. Total BD is influenced by weak acids (albumin is dominant here), strong ions (chloride concentration relative to sodium most important)), and net unmeasured anions from tissue acids. Lactate can be considered as a strong ion if measured or an unmeasured anion if not measured. In this study lactate was considered an unmeasured anion. Hyperalbuminaemia, hyperchloraemia and excess unmeasured anions can exert an acidifying influence whereas low albumin or chloride or excess cations will alkalinize. Applying this partitioning methodology as per formulae for each it was found that in this group of infants there was significant acidosis and that the unmeasured anion-related base deficit was greater than the total base deficit. This was predominantly due to the alkalinizing effect of hypoalbuminaemia. Partitioning also showed that the chloride effect may be in either direction (i.e. acidification or alkalinisation). Clinical significance of this approach is that it assists in situations such as hypoalbuminaemia where the true degree of acidosis may be masked. It will also assist where a biochemical acidosis results from resuscitation with albumin (which is high in chloride) or saline and could be misinterpreted as ongoing hypovolaemia and requirement for additional (high-chloride containing) fluid. The partitioning approach may offer information that is similar to the anion gap provided that the anion gap is corrected for albumin, but the anion gap cannot diagnose mixed acidosis (unmeasured anion plus hyperchloraemic). Inclusion of lactate measurement may refine the above process.

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Critical Care online: http://ccforum.com/content/9/4/R464
Br J Anaesth 2004;92:54-60
Paediatr Crit Care Med 2004; 5: s311