The use of capnometry to predict arterial partial pressure of CO(2) in non-intubated breathless patients in the emergency department

BACKGROUND: Capnometry measures carbon dioxide in expired air and provides the clinician with a noninvasive measure of the systemic metabolism, circulation and ventilation. This study was carried out on patients with acute breathlessness to define the utility and role of capnometry in the emergency department. AIM: 1. To determine the correlation between end tidal CO(2) and PaCO(2) in non-intubated acutely breathless patients. 2. To determine factors that influence the end tidal carbon dioxide (ETCO(2)). 3. To determine the correlation between ETCO(2) with PaCO(2) in patients presenting with pulmonary disorders. METHODS: One hundred fifty acutely breathless patients arriving at the emergency department and fulfilling the inclusion and exclusion criteria were chosen during a 6-month study period. The patients gave written or verbal consent, and were triaged and treated according to their presenting complaints. Demographic data were collected, and the ETCO(2) data were recorded. Arterial blood gas was taken in all patients. The data were compiled and analyzed using various descriptive studies from the Statistics Program for Social Studies (SPSS) version 12. Correlation between ETCO(2) and PaCO(2) was analyzed using the Pearson correlation coefficient. Other variables also were analyzed to determine the correlation using simple linear regression. The agreement and difference between ETCO(2) and PaCO(2) were analyzed using paired sample t-tests. RESULTS: There is a strong correlation between ETCO(2) and PaCO(2) using the Pearson correlation coefficient: 0.716 and p value of 0.00 (p < 0.05). However, the paired t-test showed a mean difference between the two parameters of 4.303 with a p value < 0.05 (95% CI 2.818, 5.878). There was also a good correlation between ETCO(2) and acidosis state with a Pearson correlation coefficient of 0.374 and p value 0.02 (p < 0.05). A strong correlation was also observed between ETCO(2) and a hypocapnic state, with a Pearson correlation coefficient of 0.738 (p < 0.05). Weak correlation was observed between alkalosis and ETCO(2), with a Pearson correlation coefficient of 0.171 (p < 0.05). A strong negative correlation was present between ETCO(2) and hypercapnic patients presenting with pulmonary disorders, with a Pearson correlation coefficient of -0.738 (p < 0.05) and of -0.336 (p < 0.05), respectively. CONCLUSION: This study shows that ETCO(2) can be used to predict the PaCO(2) level when the difference between the PaCO(2) and ETCO(2) is between 2 to 6 mmHg, especially in cases of pure acidosis and hypocapnia. Using ETCO(2) to predict PaCO(2) should be done with caution, especially in cases that involve pulmonary disorders and acid-base imbalance.