Correlation of end-tidal CO2 measurements to arterial Paco2 in nonintubated patients1
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INTRODUCTION
Patients presenting to the emergency department often require assessment of their oxygenation, ventilation, and acid-base status. Noninvasive rapid assessment of these measures would greatly facilitate evaluation and reassessment of ED patients. The assessment of oxygenation by oximetry has become commonplace and, in fact, is viewed by many practitioners as a standard of care in emergency medicine for monitoring patients with disorders (real or potential) of oxygenation.
In recent years,
MATERIALS AND METHODS
All adult patients (older than 18 years) presenting to the ED at the University of California, San Francisco, who had an arterial blood gas determination as part of their evaluation in the ED were eligible for enrollment in the study. The study was conducted between January 1990 and April 1991.
Patients who agreed to participate in the study had a bedside determination of end-tidal carbon dioxide tension made using nasal cannulae adapted to deliver oxygen through one cannula and measure CO2
RESULTS
Seventy-six patients (50 men, 28 women) with a mean age of 56.8 years were enrolled during the study period. The difference in end-tidal carbon dioxide and PaCO2 in all patients averaged 3.5 mm Hg and was significant (P = .007). End-tidal carbon dioxide measurements were made in 65 patients with the endotracheal tube adapter and with the nasal cannulae in 11 patients. The correlation between PaCO2 and end-tidal carbon dioxide was good with r2 = .772 in all patients. The correlation was best in
DISCUSSION
This pilot study suggests that end-tidal carbon dioxide correlates well with PaCO2 in patients presenting to the ED with a wide variety of diagnoses. The correlation was best in patients with an acidosis or hypocapnia, whereas the correlation was weakest in patients with an alkalosis and hypercapnia.
The correlation between end-tidal carbon dioxide and PaCO2 was good in both patients with a primary pulmonary disorder as well as nonpulmonary disorders. This was surprising to us, because one might
CONCLUSION
End-tidal carbon dioxide measurements correlate well with PaCO2 in nonintubated patients presenting with a variety of underlying problems. Determinations are rapid, inexpensive, and noninvasive, and may obviate the need for arterial blood gases in selected groups of patients. Further study is necessary to validate its usefulness before applying this method in clinical practice.
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