The use of capnometry to predict arterial partial pressure of CO₂ in non-intubated breathless patients in the emergency department

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.

The objectives of the study were:

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 ETCO₂ 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 ETCO₂ and PaCO₂ 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 ETCO₂ and PaCO₂ were analyzed using paired sample t-tests.

There is a strong correlation between ETCO₂ and PaCO₂ 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 ETCO₂ 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 ETCO₂ and a hypocapnic state, with a Pearson correlation coefficient of 0.738 (p < 0.05). Weak correlation was observed between alkalosis and ETCO₂, with a Pearson correlation coefficient of 0.171 (p < 0.05). A strong negative correlation was present between ETCO₂ 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.

This study shows that ETCO₂ can be used to predict the PaCO₂ level when the difference between the PaCO₂ and ETCO₂ is between 2 to 6 mmHg, especially in cases of pure acidosis and hypocapnia. Using ETCO₂ to predict PaCO₂ should be done with caution, especially in cases that involve pulmonary disorders and acid-base imbalance.