Base excess can be misleading in acute respiratory acidosis
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KeywordsHCO3 High Specificity Data Distribution Metabolic Acidosis Metabolic Parameter
Base excess (BE) is the measure of nonrespiratory change of acid-base status in the body. It is calculated after correcting the blood sample's pH to 7.4, temperature to 37°C and pCO2 to 40 mmHg. Actual HCO3 level is a metabolic parameter derived directly from the Henderson-Hasselbalch equation. There is some evidence that temporary changes in pCO2 affect BE [1, 2], but little is known about the response of HCO3. Therefore, the aim of this study was to investigate the relationship between BE and HCO3 in critically ill patients immediately after admission to the ICU.
The first arterial blood gas samples (within 1 hour of admission) of patients admitted to our ICU were retrospectively evaluated and pH, HCO3, pCO2 and BE were registered and analysed. After testing the data distribution, correlation was determined with Pearson's correlation.
Arterial blood gas samples from 88 patients were analysed. There was a strong, significant correlation between BE and HCO3 (r2 = 0.93, P < 0.001) in the whole sample. In blood samples with pCO2 > 45 mmHg, in 26 cases the pH was >7.3, and in 15 cases pH was <7.3 (that is, acute respiratory acidosis). In these cases with a cut-off BE <0 mmol/l, the BE had sensitivity = 73% and specificity = 85% for predicting acidosis. With a cut-off for HCO3 < 24 mmol/l, the HCO3 had sensitivity = 27% and specificity = 100% for acidosis. Choosing a cut-off for BE <-2 mmol/l, sensitivity = 47%, specificity = 100%; for HCO3 < 22 mmol/l, sensitivity = 13%, specificity = 100%.
Although BE and HCO3 had very good correlation in the whole sample, in acute respiratory acidosis BE indicated metabolic acidosis with high sensitivity, while the high specificity and low sensitivity of HCO3 showed that there was no metabolic component of the acid-base imbalance. Therefore, in accord with previous studies, our preliminary results give further evidence that HCO3 is a more reliable parameter to analyse acid-base balance in acute circumstances, especially in acute respiratory acidosis, than BE.
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