The relation of lactate level and carbon dioxide pressure discrepancies between transcutaneous and arterial measurements

Introduction: Partial carbondioxide pressure of the arterial blood (PaCO2) is used to evaluate alveolar ventilation. Transcutaneous carbon dioxide pressure (TcCO2) monitoring has been developed as a non-invasive (NIV) alternative to arterial blood gas analysis (ABG). Studies have shown that decreased tissue perfusion leads to increased carbondioxide (CO2). The use of transcutaneous capnometry may be unreliable in patients with perfusion abnormalities. In this study, we aimed to evaluate the relation between TcCO2-PaCO2 and lactate level which is recognized as a marker of hypoperfusion. Materials and Methods: In this prospective cohort study in critical care patients with hypercapnic respiratory failure (PaCO2 >= 45 mmHg) who received NIV between April 2019 and January 2020 in the intensive care unit were enrolled in the study. Patients' simultaneously measured TcCO2 and PaCO2 values of hypercapnic patients were recorded. Each paired measurement was categorized into two groups; normal lactate (<2 mmol/L) and increased lactate (>= 2 mmol/L). Results: A total of 116 paired TcCO2 and PaCO2 measurements of 29 patients were recorded. Bland -Altman analysis showed the mean bias between the TcCO2 and PaCO2 and 95% limits of agreement (LOA) in all measurements (1.75 mmHg 95% LOA -3.67 to 7.17); in the normal lactate group (0.66 mmHg 95% LOA -1.71 to 3.03); and in the increased lactate group (5.17 mmHg 95% LOA -1.63 to 11.97). The analysis showed a correlation between lactate level and the difference between TcCO2 and PaCO2 (r= 0.79, p< 0.001) and a negative correlation between mean blood pressure and the difference between TcCO2 and PaCO2 (r= -0.54, p= 0.001). Multiple regression analysis results showed that lactate level was independently associated with increased differences between TcCO2 and PaCO2 (Beta= 0.875, p< 0.001). Conclusion: TcCO2 monitoring may not be reliable in patients with increased lactate levels. TcCO2 levels should be checked by ABG analysis in these patients.