Original contributions
Validation of venous pCO2 to screen for arterial hypercarbia in patients with chronic obstructive airways disease

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Abstract

To validate a previously derived venous pCO2 (pvCO2) cut-off for ruling out arterial hypercarbia in patients with chronic obstructive pulmonary disease (COPD), matched arterial and venous blood gas samples were taken from a convenience sample of patients who presented to the Emergency Department (ED) with COPD deemed by their treating doctor to require arterial blood gas (ABG) analysis as part of their care. The screening cut-off was defined as pvCO2 of > 45 mm Hg and arterial hypercarbia was defined as arterial pCO2 (paCO2) of > 50 mm Hg. Descriptive statistics were employed. Sensitivity, specificity and negative predictive value were calculated. There were 112 patients enrolled in the study, of whom 107 had complete data for analysis. Forty-three patients had arterial hypercarbia (range of 51 to 90mm Hg, median 60 mm Hg). All cases of arterial hypercarbia were detected by the screening cut-off (sensitivity 100%; 43/43; 95% CI 91–100%; specificity 47%, 95% CI 35–59%). The negative predictive value of pvCO2 < 45 mm Hg was 100% (30/30, 95% CI 89–100%). Assuming the ABG was performed to assess hypercarbia, 29% of ABGs potentially could have been avoided if a venous screening test was employed. In conclusion, pvCO2 can be used as a screening test for arterial hypercarbia, and if employed, can potentially reduce the requirement for ABG sampling.

Introduction

Management of acute exacerbations of chronic obstructive pulmonary disease (COPD) is challenging. Blood gas analysis, in particular measurement of pCO2 for the detection of CO2 retention, is useful in guiding oxygen therapy and the need for ventilatory support. But arterial blood gas samples are not without problems. They are painful and have a small incidence of complications including local hematoma, infection, occlusion and embolization with consequent ischemic injury to the digits. There is also a risk of needlestick injury to health care workers associated with an additional vascular puncture.

Previous research has shown excellent agreement between pH measured on arterial and venous samples and good agreement between oxygen saturation measured by pulse oximeter and by arterial blood gas (ABG) (1, 2, 3, 4, 5, 6, 7, 8, 9). We have previously reported a derivation study exploring a potential screening cut-off for pvCO2 in the detection of arterial hypercarbia (10). That study showed that a pvCO2 screening cutoff of > 45 mm Hg was 100% sensitive in detecting cases of arterial hypercarbia with a specificity of 51.7%. The objective of this study was the validation of that screening cut-off.

Section snippets

Methods

This prospective study was conducted in the Emergency Department (ED) of Western Hospital, a 250-bed community teaching hospital in Melbourne, Australia. The ED has an annual census of approximately 32,000 adult patients.

Patients were eligible for entry into the study if they presented with an acute exacerbation of COPD (judged and documented by the treating clinician) and were deemed by the treating doctor to require an ABG analysis to determine their ventilatory status. Confirmation of COPD

Results

There were 112 patients enrolled in the study, of whom 107 had complete data for analysis. The median age of patients was 73 years. The median paCO2 was 45 mm Hg (range 24–90 mm Hg). Forty-three patients (40%) had arterial hypercarbia (range of 51–90 mm Hg, median 60 mm Hg). Agreement between arterial and venous pCO2 was poor with a bias of 6 mm Hg and 95% limits of agreement of −14 to +26 mm Hg.

All cases of arterial hypercarbia were detected by the screening cut-off (sensitivity 100%; 43/43;

Discussion

ABG analysis is the standard method for obtaining an estimation of pCO2 in the clinical evaluation of patients suffering exacerbations of COPD. ABGs, however, are unpleasant and carry a small risk of complications for both patients and staff. If there was an alternative method of obtaining this information without an additional puncture, there would be potential benefits for staff and patients.

Although several studies have shown good correlation between arterial and venous pCO2, this study and

Conclusion

Venous CO2 can be used as a screening test for arterial hypercarbia and, if employed, can potentially reduce the requirement for ABG sampling.

Acknowledgments

The investigators would like to thank Denise Green, Darren Gaut and Lyndal Morgan (nurse educators) for their role in promoting the study and the nursing and medical staff of the Emergency Department for their participation.

References (12)

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Original Contributions is coordinated by John Marx, MD, of Carolinas Medical Center, Charlotte, North Carolina

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