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0535. Going beyond the base deficit to predict mortality
The base deficit, a measure that is available from routine blood gas
analysis, has been incorporated into mortality risk scores, used to assist
in clinical decision-making and incorporated into APLS guidelines. While
no one disputes the value of a significant base deficit, intensivists
continue to explore the bigger picture, looking into factors such as lactate
levels per se, non-lactate 'unmeasured' anions, strong ions and weak acids.
In a study from Cape Town, researchers measured blood gases, electrolytes
and lactate and calculated 'unmeasured' anions in order to assess whether
mortality was correlated better with the nature of the acidosis (hyperlactataemia,
hyperchchloraemia or occult anionaemia) than with the magnitude thereof.
Forty-six shocked children admitted to the PICU were studied. Diagnoses
included gastroenteritis, septicaemia, myocarditis, near-drowning and
poisoning. Mortality rate was 35%, with non-survivors differing from survivors
in terms of lactate levels (11,6 vs 3,3 mmol/l) but not by pH, base excess,
unmeasured anions or corrected chloride (although the latter showed a
trend towards higher chloride levels in survivors). Area under the receiver
operating curve (ROC) was best for lactate (0,83) followed by the PIM
score (Paediatric Index of Mortality) at 0,71. The authors comment that
the base excess may miss true metabolic acidosis in >15% of cases,
concealed by alkalinizing factors such as hypoalbuminaemia. On the other
hand they caution that endotoxin may inactivate pyruvate dehydrogenase,
thereby elevating lactate and giving a false impression of hyperlactataemia
from tissue hypoxia-ischaemia. The Cape Town conclusion was that lactate
may be a helpful measure but should not be used in isolation and in fact
on its own is not much more useful than the admission PIM score. Other
researchers have found different results, with corrected anion gap or
strong ion gap correlating better with tissue acidosis or mortality respectively.
Bottom line is that there is indeed more to interpretation of blood gas
analysis than simple use of the base excess/deficit figure. |