Correlation of end-tidal CO2 measurements to arterial Paco2 in nonintubated patients1

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Study objective: To determine the accuracy of end-tidal carbon dioxide levels as a measure of arterial carbon dioxide levels in non-intubated patients presenting to an emergency department for care.

Design: A prospective, cross-sectional analysis.

Setting: University hospital ED.

Type of participant: Nonintubated adult patients presenting to the ED for care of a variety of problems.

Interventions: Patients who had arterial blood gas samples taken as part of their ED evaluation were asked to breathe normally through an endotracheal tube adapter or a modified nasal cannula connected to a side port sampling capnometer while a sample for arterial blood gas was drawn from the radial artery.

Measurements: End-tidal carbon dioxide levels (mm Hg) were recorded at the time of arterial blood gas sampling. The difference between end-tidal carbon dioxide and Paco2 was tested with the paired t-test at a significance level of .05. The corrlation of end-tidal carbon dioxide to Paco2 was tested in all patients and in subgroups using simple linear regression.

Results: Seventy-six patients were enrolled. In all patients, end-tidal carbon dioxide was 3.5 mm Hg lower than Paco2 and correlated well with Paco2 (r2=.772). In patients with hypocapnia, there was no significant difference between end-tidal carbon dioxide and Paco2 (P=.17), and the correlation of end-tidal carbon dioxide to Paco2 was stronger (r2=.838). In patients with a respiratory or metabolic acidosis, the difference between end-tidal carbon dioxide and Paco2 was 6 mm Hg (P=.005), but end-tidal carbon dioxide correlated well to Paco2 (r2=.899).

Conclusion: Measurements of end-tidal carbon dioxide concentrations correlate well with Paco2 values in nonintubated patients presenting with a variety of conditions to EDs. End-tidal carbon dioxide measurements may be sufficient measures of Paco2 in selected patients and obviate the need for repeat arterial blood gas determination. Further study is warranted.

Section snippets

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.

References (10)

  • AB Sanders

    Capnometry in emergency medicine

    Ann Emerg Med

    (1989)
  • D Raemer et al.

    Variation in PCO2 between arterial blood and peak expired gas during anesthesia

    Anesth Analg

    (1983)
  • J Niehoff et al.

    Efficacy of pulse oximetry and capnometry in postoperative ventilatory weaning

    Crit Care Med

    (1988)
  • AB Sanders et al.

    End-tidal carbon dioxide monitoring during cardiopulmonary resuscitation: A prognostic indicator for survival

    JAMA

    (1989)
  • M Callaham et al.

    Prediction of outcome of cardiopulmonary resuscitation from end-tidal carbon dioxide concentration

    Crit Care Med

    (1990)
There are more references available in the full text version of this article.

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