The correlation between end-tidal carbon dioxide and arterial blood gas parameters in patients evaluated for metabolic acid-base disorders

BACKGROUND The analysis of arterial blood gas (ABG) is an invasive procedure that is used frequently in the emergency department (ED) to evaluate the acid-base status of critically-ill patients. However, capnometry is an alternative procedure that has been used in recent years to determine the metabolic status of patients' blood. Considering the correlation between end-tidal carbon dioxide (ETCO2) and arterial partial pressure of carbon dioxide (PaCO2) identified in the previous studies and the strong correlation between PaCO2 and bicarbonate (HCO3(-) ), we assumed that ETCO2 might be a useful parameter in predicting the presence of metabolic acidosis. The aim of this study was to determine the correlation between ETCO2 and the parameters of ABG in adult patients who were likely present metabolic acid-base disturbances in the Emergency Department of Imam Reza Hospital, the largest academic hospital in Mashhad in northeast Iran. METHODS This was a cross-sectional study conducted during six months on 62 adult patients who presented with suspected metabolic acid-base disorders to the ED. The exclusion criteria were patients with chronic obstructive pulmonary diseases, loss of consciousness, intubated patients, and those who were unable to tolerate capnography. The patients' demographic information and vital signs were recorded. Also, ABG and ETCO2 results were recorded. The Pearson product moment correlation analysis and linear regression were used to determine the correlation between ETCO2 and ABG parameters. RESULTS Sixty-four patients were enrolled, consisting of 37 men and 27 women with a mean age of 55.4 ± 22.7 years. The most common complaints presented were nausea and vomiting (n = 24). The average value for ETCO2 was 26.2 ± 6.1. There were significant linear correlations between ETCO2 level, pH (r = 0.368), HCO3(-) (r = 0.869), PaCO2 (r = 0.795), and Base Excess (B.E.) (r = 0.346). HCO3 and PaCO2 were the significant predictor values for ETCO2 (linear regression analysis). CONCLUSION ETCO2 can be an appropriate indicator to estimate HCO3(-) and PaCO2 in critical emergency situations, but it cannot be used as an indicator to estimate all ABG variables.