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Improving the Validity of Peripheral Venous Blood Gas Analysis as an Estimate of Arterial Blood Gas by Correcting the Venous Values with SvO2

https://doi.org/10.1016/j.jemermed.2012.07.041Get rights and content

Abstract

Background

Peripheral venous blood gas (pVBG) analysis in replacement of arterial blood gas (ABG) is limited by the unpredictable differences between arterial and venous values, especially for PCO2 and pH (ΔPCO2 and ΔpH).

Objectives

We hypothesized that, using the theoretical relationship linking SvO2 and blood flow, we could diminish the effect of local circulatory conditions on ΔPCO2 and ΔpH and thereby increase pVBG validity.

Methods

This was a prospective cross-sectional study performed in emergency patients requiring a blood gas analysis in which ABG and pVBG were performed simultaneously. The data of 50 randomly selected patients (model group) were used for developing two equations to correct PvCO2 and pHv according to the peripheral SvO2 (SpvO2) level. The formulas derived were PvCO2cor = PvCO2 − 0.30 × (75 − SpvO2), and pHvcor = pHv + 0.001 × (75 − SpvO2). The validity of the corrected values was then tested on the remaining population (validation group).

Results

There were 281 patients included in the study, mainly for dyspnea. ΔPCO2 and ΔpH were strongly correlated with SpvO2 (r2 = 0.62 and r2 = 0.53, respectively, p < 0.001). Using the data of the model group, we developed equations that we applied on the validation group. We found that the corrected values were more valid than the raw values for detecting a PaCO2 > 45 mm Hg (AUC ROC = 0.96 ± 0.01 vs. 0.89 ± 0.02, p < 0.001), a PaCO2 < 35 mm Hg (AUC = 0.95 ± 0.02 vs. 0.84 ± 0.03, p < 0.001), a pHa < 7.35 (AUC = 0.97 ± 0.01 vs. 0.95 ± 0.02, p < 0.05), or a pHa > 7.45 (AUC = 0.91 ± 0.02 vs. 0.81 ± 0.04, p < 0.001).

Conclusions

The variability of ΔPCO2 and ΔpH is significantly lowered when the venous values are corrected according to the SpvO2 value, and pVBG is therefore more accurate and valid for detecting an arterial abnormality.

Introduction

Arterial blood gas analysis plays an important role in interpreting metabolic and respiratory consequences of severe acute illness in the Emergency Department (ED). It is also an important tool for assessing the respiratory status of severe patients with a history of chronic obstructive pulmonary disease (COPD) admitted to the ED, regardless of the cause of their admission. Arterial puncture, however, is more time-consuming, more painful, and may lead to more complications when compared to venous puncture. Several studies have proposed the use of peripheral venous blood gas in replacement of arterial blood gas in the Emergency Medicine setting 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14. However, most of these studies have shown that this method seems to be somewhat unreliable, especially for evaluating PCO2, due to unpredictable discrepancies between arterial and venous values 2, 7, 9, 13, 14, 15, 16, 17, 18. Therefore, the substitution of venous blood gas for arterial blood gas could be limited for patients in whom assessment of PaCO2 is of great importance, such as those presenting to the ED with acute respiratory failure. Indeed, a method for correction of this discrepancy between arterial and venous PCO2 would be of great interest, particularly in this category of patients.

Several factors may be at the origin of the variability of arteriovenous differences in PCO2. First, tissue CO2 production may be different from one patient to another. However, this factor is unlikely to be important when the venous sample is taken from the forearm, because the CO2 production inducing change in PvCO2 is then limited to the hand and the forearm tissues. The second factor that could explain this variability is linked to the local blood flow. According to the Fick equation, CO2 may stagnate in the venous blood stream in the case of low blood flow, thereby increasing the gap between the arterial and the venous values (19). The decrease in forearm blood flow may be due to poor circulatory conditions, but may also be the consequence of tourniquet placement on the arm during venous blood sampling (20). In the case of low forearm blood flow, the local venous saturation of oxygen should decrease because local oxygen extraction increases to maintain local oxygen consumption.

We hypothesized that the gradient between arterial and venous PCO2 (and pH) is mainly dependent on the local blood flow, which may be evaluated by the peripheral SvO2 value (SpvO2). Therefore, the goals of our study were:

  • •

    to check if the gradient between arterial and venous PCO2 (and pH) is related to the SpvO2 values

  • •

    to propose a correction of the venous PCO2 and pH values according to the SpvO2 level to control for the effect of local circulatory conditions on the variability of venous values

  • •

    to test the validity of this calculation in a sample of emergency patients

Section snippets

Study Design

This was a cross-sectional study performed prospectively in patients presenting to the ED who required blood gas analysis. The patients included in the study were randomly divided into two groups: data from the first group of patients was used to construct a model for correction of PCO2 and pH values (model group), and data from the second group was used to validate that model (validation group).

This study was approved by our local ethics committee and informed consent was obtained from each

Overall Patients

Two hundred eighty-one patients were included in the study from September 2009 to January 2010. The mean age was 76 ± 16 years, and 48% were male. The diagnoses for which blood gas analysis was indicated are summarized in Table 1. The mean SpvO2 value was 66.1 ± 23%, with extremes ranging from 14% to 99%.

There was a strong relationship between the SpvO2 and the ΔPCO2 and ΔpH (Figure 1). The determination coefficient showed that the level of SpvO2 could explain 62% and 53% of the variability of

Discussion

Arterial blood gas analysis plays an important role in the evaluation of patients presenting to the ED for shortness of breath. However, it is a time-consuming, painful, and potentially unsafe procedure. Venous blood gases have been shown to be well related to arterial values, but the arteriovenous difference is highly variable and unpredictable, which makes the substitution not feasible 13, 15, 16, 17, 18. We have shown that the variability in the arteriovenous difference is, in part,

Conclusion

In conclusion, the arterial forearm blood flow is probably highly variable from one patient to the other, which is at the origin of a wide discrepancy of the arteriovenous differences in PCO2 and pH. The venous oxygen saturation value is a simple and convenient tool to assess this discrepancy. The correction of raw venous blood gas values according to the SpvO2 value allows a great improvement of venous screening test to detect arterial blood gas abnormalities. We suggest that this correction

Acknowledgments

The authors would like to thank Alison Killelea for syntax correction. Alison Killelea was a visiting resident in Emergency Medicine (Warren Alpert Medical School, Brown University, Providence, Rhode Island).

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