Original Contribution
Venous vs arterial blood gases in the assessment of patients presenting with an exacerbation of chronic obstructive pulmonary disease

https://doi.org/10.1016/j.ajem.2011.06.011Get rights and content

Abstract

Objective

The purpose of this study was to investigate the clinical correlation between arterial and venous blood gas (VBG) values in patients presenting to the emergency department (ED) with acute exacerbation of chronic obstructive pulmonary disease.

Methods

A prospective study of patients with chronic obstructive pulmonary disease presenting to the ED with acute ventilatory compromise was done. Patients were included if their attending physician considered arterial blood gas sampling important in their initial assessment. Data from arterial and venous samples were compared using Spearman correlation and bias plot (Bland-Altman) methods.

Results

Ninety-four patients were enrolled in the study. Eighty-nine patients had complete data sets for analysis. Arterial hypercarbia was present in 30 patients (33.7%; range, 51-140.19 mm Hg). All cases of arterial hypercarbia were detected using VBG sampling when a screening cutoff of 45 mm Hg was applied (sensitivity, 100%; 95% confidence interval, 88.7%-100% and specificity, 34%; 95% confidence interval, 23.1%-46.6%). Bias plot revealed moderate agreement between arterial and venous Pco2 with an average difference of 8.6 mm Hg and 95% limits of agreement of −7.84 to 25.05 mm Hg. For pH, mean difference between each group was 0.039 (range, −0.12 to 0.03). Linear regression analysis for pH demonstrated very close equivalence with a regression coefficient of 0.955, and Spearman correlation showed significant correlation of 0.826 (P = .001).

Conclusion

Venous pH and HCO3 values show excellent correlation with arterial values. Using a previously validated screening cutoff of 45 mm Hg, venous CO2 has 100% sensitivity in detecting arterial hypercarbia. There is insufficient agreement between venous and arterial CO2 for VBG to replace arterial blood gas in determining the degree of hypercarbia.

Introduction

Arterial blood gas (ABG) sampling is the gold standard test for assessing ventilatory status in patients presenting with acute exacerbations of chronic obstructive pulmonary disease (COPD). In addition to assisting the clinician to determine both ventilatory and acid-base status, it aids in the decision making regarding the need for continuing pharmacologic management and noninvasive ventilation (NIV) [1]. Several studies to date have looked at the possibility of replacing ABG sampling with venous blood gas (VBG) sampling for patients in the emergency setting [2], [3], [4], [5]. They have shown good correlation between venous and arterial values for pH but less so for Paco2. Kelly et al [6], [7] reported 100% sensitivity of Pvco2 in detecting cases of arterial hypercarbia with a specificity of 51% when a screening cutoff of greater than 45 mm Hg was applied. There is a growing body of evidence supporting the diagnostic use of VBG sampling in the assessment of patients presenting to the emergency department (ED) with acute respiratory illness.

Arterial samples are painful to acquire and carry greater risk than venous punctures. Risks include bleeding, hematoma, infection, nerve injury, and digital ischemia. The additional blood-sampling requirement carries a further opportunity of inoculation injury to staff. Venous blood gases can be drawn at the time the patient is cannulated, which is generally at an early time point in patient assessment. Furthermore, in the assessment of patients for NIV, serial blood gases are required to assess the response to optimum medical management.

Acute exacerbation of COPD can range from self-limited episodes to florid respiratory failure requiring ventilation. Exacerbations can be triggered by bacterial infection, heart disease, or other lung diseases (eg, pulmonary emboli, pneumothorax). The initial clinical focus is therefore directed by an adequate assessment of the patient and includes initiation of controlled oxygen therapy and pharmacologic management including bronchodilators, corticosteroids, rehydration, and antibiotics when indicated. Noninvasive ventilation is indicated for patients with acute exacerbations of COPD, when respiratory acidosis (pH <7.35, Paco2 >45 mm Hg) persists despite optimum medical management [1]. In this situation, noninvasive positive pressure ventilation may allow time for other therapies to work and thus avoid endotracheal intubation. The identification of patients suitable for NIV is therefore one of the treatment priorities for these patients. We were interested in evaluating the level of agreement between ABG vs VBG, using the screening cutoff of Pvco2 greater than 45 mm Hg and a threshold of 0.05 pH units or less in patients presenting with acute exacerbations of COPD [6], [7]. In this study, we undertook a comparison of paired ABG and VBG results to determine if a VBG could accurately substitute for an ABG sample in the early assessment of patients presenting to an ED with acute exacerbation of COPD.

Section snippets

Methodology

Patients aged 16 and older were eligible for the study if they had a previously documented diagnosis of COPD and were presenting with an acute exacerbation of COPD.

The study was undertaken in an urban ED from October 1, 2009, to March 1, 2010. We prospectively correlated paired ABG and VBG results from a consecutive convenience cohort of patients presenting with an exacerbation of COPD. Patients were included in the study if an arterial blood sample was clinically indicated to evaluate their

Results

A total of 94 patients were enrolled in the study. Five patients were excluded because of incomplete data sets, leaving 89 sample pairs for analysis. Of the 89 patients, 48% were male and 51% were female. The mean age of patients was 69 years. All patients included had a previously documented diagnosis of COPD. Of 89 patients, 16 (18%) were treated with NIV in addition to standard medical management for acute exacerbation of COPD. No patients required endotracheal intubation either in ED or

Discussion

Arterial blood gas sampling provides important information about the ventilatory status in patients presenting with acute respiratory illness to an emergency setting. There is a growing body of evidence to suggest that this can be safely replaced by a more convenient VBG sample. In our study, comparative analysis of paired VBG and ABG results in patients presenting with acute exacerbations of COPD revealed that VBG results accurately identified arterial hypercarbia when a previously derived Pvco

Conclusion

Correlation between venous and arterial values is excellent for both pH and HCO3. When a screening cutoff of 45 mm Hg was applied to venous CO2, VBG had 100% sensitivity in detecting arterial hypercarbia. There is insufficient agreement between venous CO2 and arterial CO2 for VBG to replace ABG in determining the degree of hypercarbia. Respiratory acidosis can be accurately identified using VBG sampling, reducing the need for arterial sampling in the early assessment of patients presenting with

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Competing Interests: None.

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