Relationship between arterial carbon dioxide and end-tidal carbon dioxide when a nasal sampling port is used

End-tidal carbon dioxide (ETCO₂) values obtained from awake nonintubated patients may prove to be useful in estimating a patient’s ventilatory status. This study examined the relationship between arterial carbon dioxide tension (PaCO₂) and ETCO₂ during the preoperative period in 20 premedicated patients undergoing various surgical procedures. ETCO₂ was sampled from a 16-gauge intravenous catheter pierced through one of the two nasal oxygen prongs and measured at various oxygen flow rates (2, 4, and 6 L/min) by an on-line ETCO₂ monitor with analog display. Both peak and time-averaged values for ETCO₂ were recorded. The results showed that the peak ETCO₂ values (mean = 38.8 mm Hg) correlated more closely with the PaCO₂ values (mean = 38.8 mm Hg; correlation coefficient r = 0.76) than did the average ETCO₂ values irrespective of the oxygen flow rates. The time-averaged PaCO₂-ETCO₂ difference was significantly greater than the PaCO₂-peak ETCO₂ difference (P < 0.001). Values for subgroups within the patient population were also analyzed, and it was shown that patients with minute respiratory rates greater than 20 but less than 30 and patients age 65 years or older did not differ from the overall studied patient population with regard to PaCO₂-ETCO₂ difference. A small subset of patients with respiratory rates of 30/ min or greater (n = 30) did show a significant increase in the PaCO₂-ETCO₂ difference (P < 0.001). It was concluded that under the conditions of this study, peak ETCO₂ values did correlate with PaCO₂ values and were not significantly affected by oxygen flow rate. However, obtaining peak ETCO₂ values is clinically more difficult, especially when partial air-way obstruction is present.