Metabolic acid–base status of critically ill septic patients: a quantitative longitudinal study

Background

Septic patients frequently present with severe acid–base alterations, and the nature is not completely elucidated. The use of Stewart's approach may bring new light to this field.

Objectives

The aim of this study is to understand the nature of acid–base disturbance of early phase septic patients. Specifically, we have proposed to identify the components of strong ion difference (SID) that account for its variation in this clinical scenario.

Methods

This study took place in a mixed 14-bed ICU of a university hospital. Patients who had severe sepsis with recently diagnosed (<24 hours) organ dysfunction were considered eligible for the study. We prospectively collected plasma Na⁺, K⁺, Cl^-, ionic Ca²⁺, Mg²⁺, phosphate, albumin, lactate, arterial pH and pCO₂ for five consecutive days. Standard base excess (SBE) was calculated according to the Van-Slyke equation. SID effective (SIDe), SID apparent (SIDa) and strong ion gap (SIG) values were calculated using a computer program. The Pearson correlation index was used and P < 0.05 was considered statistically significant.

Results

From September 2004 to December 2004, 20 patients were enrolled. The observed SIDe value for a neutral SBE was 33.4, lower than the 'normal' SID (approximately 40) due to the low albumin serum level seen in our patients (2.34 ± 0.48 g/dl). The median SBE value increased from -6.56 (-16.05 to -0.27) on the first day to -3.15 (-14.71 to 6.93) on the fifth day of observation. This increase was strongly correlated with the median SIDa daily level (R² = 0.84) that increased from 33.41 (coincident with the observed 'neutral' SIDe value) to 35.16. However, the median daily SBE did not show good correlation with SIG levels, which were kept almost constant during the study period (4.9 ± 2.7). On the fifth study day, metabolic acidosis had disappeared in 11 (55%) patients but persisted in nine (45%) of them. This was strongly dependent on their final SIDa value (R² = 0.90) and specifically on the serum Cl level (R² = 0.66).

Conclusion

During the first days of severe sepsis a moderate-degree metabolic acidosis is caused by unmeasured anions. The SID apparent value can be considered neutral on the first day and becomes more positive during the next few days. This could be the first compensatory mechanism for restoration of a normal metabolic acid–base status.