Acid–Base Status in Critically Ill Patients: Physicochemical vs. Traditional Approach

<b>Background/Objectives</b>: Critically ill patients can often present acid-base alterations. The aim of this study was to evaluate the prevalence and the time-course of acid-base alterations on intensive care unit (ICU) admission and on day one by the traditional standard base excess (SBE)-based and the Stewart methods in mechanically ventilated patients. <b>Methods</b>: A prospective observational study enrolling mechanically ventilated patients in the ICU was conducted. Arterial blood gas analysis, blood and urine samples were obtained on ICU admission and on day one. Plasmatic and urinary acid-base variables were compared among acidemic, alkalemic and patients with normal pH. The agreement between the SBE-based and Stewart methods was assessed at ICU admission and on day one. <b>Results</b>: One hundred and seventy-two patients were enrolled. On ICU admission, 55 (32%), 29 (17%) and 88 (51%) patients had acidemia, alkalemia and a normal pH, respectively. On day one, 12 (7%), 48 (28%) and 112 (65%) patients had acidemia, alkalemia and a normal pH with lower values of paCO2 and albumin. According to the SBE and Stewart approaches, the occurrence of metabolic acidosis was similar (24% vs. 35%), as well as the rate of metabolic alkalosis (16% vs. 23%) on ICU admission; on day one, the occurrence of metabolic acidosis was different (12% vs. 35%), as well as the rate of metabolic alkalosis (35% vs. 14%). The agreement between methods was estimated to be low both on ICU admission and on day one. <b>Conclusions</b>: Up to 50 % of mechanically ventilated patients presented acid-base derangements, mainly due to acidemia on ICU admission and to alkalemia after 24 h, secondary to alterations in carbon dioxide and plasma albumin. The agreement between the traditional and Stewart approaches was poor. The Stewart approach could be more accurate in detecting the acid-base disturbances in critically ill patients characterized by changes of mechanical ventilation and fluid administration.