C74-12 Interpreting Arterial Blood Gases in Cardiothoracic Surgery: Implications of Altitude Adjusted Thresholds for ICU Outcomes

Rationale At high altitudes such as Bogotá (2,640 meters above sea level), hypobaric conditions alter baseline arterial blood gas (ABG) values, leading to potential misclassification of acid-base disorders when sea-level reference ranges are applied. The Siggaard-Andersen method provides altitude-adjusted thresholds that better reflect physiological conditions under chronic hypoxia. Accurate interpretation of these values is essential in cardiothoracic surgery, where perioperative acid-base balance strongly influences hemodynamic stability and recovery. This study aimed to quantify the diagnostic disagreement between traditional and altitude-adjusted (Siggaard-Andersen) ABG classifications and to explore whether either system correlates more accurately with prolonged intensive care unit (ICU) stay. Methods We conducted a retrospective cohort study of 196 adult patients who underwent cardiothoracic surgery between 2022 and 2024 at a tertiary care center in Bogotá. ABG parameters (pH, PaCO2, HCO31) were classified using two systems: Traditional thresholds: pH (<7.35 acidemia; >7.45 alkalemia), PaCO2 (<35 mmHg alkalosis; >45 mmHg acidosis), HCO31 (<22 mmol/L acidosis; >28 mmol/L alkalosis). Siggaard-Andersen thresholds: pH (<7.39 acidemia; >7.47 alkalemia), PaCO2 (<28.3 mmHg alkalosis; >38.7 mmHg acidosis), HCO31 (<19.1 mmol/L acidosis; >24.7 mmol/L alkalosis). The secondary outcome was prolonged ICU stay (>5 days). Concordance between both methods and their association with ICU length of stay were analyzed using Chi-square tests. Results Application of altitude-adjusted thresholds produced a significant reclassification of acid-base disorders. The prevalence of acidemia increased from 19.4% to 39.3%, and respiratory alkalosis from 19.9% to 49.0% under the Siggaard-Andersen criteria. Overall discordance between traditional and altitude-adjusted classifications was statistically significant across all parameters (p < 0.001). Despite this diagnostic shift, neither classification system showed a significant association with prolonged ICU stay (p = 0.412). These findings emphasize that standard sea-level reference values underestimate acid-base disturbances in high-altitude surgical populations. Conclusion The use of sea-level ABG reference ranges at high altitude leads to systematic underdiagnosis of acidemia and respiratory alkalosis. Implementing altitude-adjusted Siggaard-Andersen thresholds significantly modifies clinical interpretation but does not independently predict ICU length of stay. Accurate, context-specific ABG analysis is essential for perioperative decision-making and should be integrated into institutional protocols for high-altitude cardiothoracic anesthesia and critical care.References: 1. Estrella D, León-Sarmiento FE. Interpretation of arterial blood gases in Bogotá (2640 meters above sea level) based on the Siggaard-Andersen nomogram: A proposal for simplifying and unifying reading. Biomédica. 2018;38(1):54-63. doi:10.7705/biomedica.v38i1.3767 This abstract is funded by: None