VBG vs ABG in Hypercarbia

The blood gas analysis offers important data to guide the treatment and prognostication of critically ill patients. Providers are able to quickly assess acid-base status, tissue perfusion, and respiratory mechanics. Traditionally, providers relied on arterial blood gas (ABG) for these data; however, a relatively new consensus of data supports venous blood gas (VBG) pH, bicarbonate, and lactate as surrogates for arterial blood values^1,2,3.

In a recent prospective study, Middleton et al  matched 168 arterial and venous blood samples and determined that VBG and ABG could be used interchangeably (95% limits of agreement- This means that the difference did not have meaningful clinical variation) for pH, bicarbonate, base excess, and lactate¹. This is good news for patients because arterial punctures are often painful, more difficult to obtain, and have associated complications (although rare) such as nerve injury and digital ischemia. However, we can’t relegate the ABG to the part of the bitter past we’d like to forget (much like I’ve already done to the Cruz-Fiorina campaign and life before the TSA PreCheck option) because there is little consensus on the VBG’s reliability as a surrogate to assess CO₂ levels, particularly when the value is moderate to severely elevated.

The Studies

There are 5 recent studies comparing ABG to VBG CO₂ inter-reliability that publish their 95% limits of agreement.

In summation, these studies suggest peripheral venous CO₂ (PvCO₂) is a useful screening tool. Low levels of PvCO₂ (<30 mmHg) may help rule out hypercarbia and high levels (>45 mm Hg) strongly correlate with presence of hypercarbia on the ABG. Determining the presence of hypercarbia based on the VBG may negate the need for ABG evaluation in some instances. However, PvCO₂ cannot fully replace arterial peripheral arterial CO₂ (PaCO₂) because of the wide and unpredictable variation of the credibility interval. Moreover a “correction factor” cannot be used to calculate the PaCO₂ from PvCO₂ because the actual value may be higher or lower⁹. The average mean difference in ABG vs VBG CO₂ is large enough that it may change patient management, such as the decision to start non-invasive ventilation (NIV) in hypercapneic acute respiratory failure.

High Stakes for COPD patients

COPD patients represent one particular population where a reliable CO₂ is critical to your decision-making process. You may be able to rule in or out hypercarbia based on the VBG, but knowing with accuracy that your patient’s CO₂ is worsened from baseline may trigger early NIV (however, I would like to emphasize most decisions to begin NIV should be made clinically, even before a VBG is resulted). Early appropriate NIV in patients with arterial pH < 7.35 and hypercapnea has demonstrated lower mortality, reduced rates of intubation, reduction in total number of hospital admission days, and fewer complications such as ventilator-induced lung injury and nosocomial lung infections^10,11,12.

Trending PCO₂ to Guide Duration of NIV  
Monitoring CO₂ during trials of self-ventilation may help clinicians appropriately titrate down NIV use. The 2016 British Thoracic Society guidelines recommend trending transcutaneous, arterial, or capillary pCO₂ along with pH. Once these lab values have normalized and the patient is clinically improved, NIV can be discontinued¹³. While their recommendation does not specifically address venous blood sampling, a low-normal value would effectively rule out hypercarbia.

Conclusions

-Draw the PvCO₂ while you are getting your other labs. You may be able to avoid an arterial stick.

–PvCO₂ > 45 is a useful screening test for the presence of hypercarbia and PvCO₂ < 30 is a good “rule out” test.

-PvCO₂ does not accurately predict PaCO₂, and the true value may be significantly higher or lower.

-You may need to obtain an accurate CO₂ (ABG) measurement in some patients because it might change your management. In patients with COPD, early NIV in moderate to severe COPD exacerbations has been shown to reduce morbidity and mortality

-Trending CO₂ and pH to normal values can help guide discontinuation of NIV. BEWARE! VBGs  have limited utility in trending CO₂ until they reach a low-normal level.

Written by Wendy Chan MD

Peer-Review by Eden Kim DO and Raul Hernandez MD

Reviewed/Edited by Ian deSouza MD

Works Cited

1. Middleton P, Kelly AM, Brown J, et al. “Agreement between Arterial and Central Venous Values for PH, Bicarbonate, Base Excess, and Lactate.” Emergency Medicine Journal 23.8 (2006): 622-24.

2. Ma OJ, Rush MD, Godfrey MM, et al. “Arterial Blood Gas Results Rarely Influence Emergency Physician Management of Patients with Suspected Diabetic Ketoacidosis.” Academic Emergency Medicine 10.8 (2003): 836-41.

3. Kelly AM, McAlpine R, Kyle, E. “Agreement between Bicarbonate Measured on Arterial and Venous Blood Gases.” Emerg Med Australas Emergency Medicine Australasia 16.5-6 (2004): 407-09.

4. Kelly A, Kerr D, Middleton, P. “Validation of Venous PCO2 to Screen for Arterial Hypercarbia in Patients with Chronic Obstructive Airways Disease.” The Journal of Emergency Medicine 28.4 (2005): 377-79.

5. McCanny P, Bennett K, Staunton P, et al. “Venous vs Arterial Blood Gases in the Assessment of Patients Presenting with an Exacerbation of Chronic Obstructive Pulmonary Disease.” The American Journal of Emergency Medicine 30.6 (2012): 896-900.

6. Toftegaard M, Rees SE, Andreassen S. “Correlation between Acid–base Parameters Measured in Arterial Blood and Venous Blood Sampled Peripherally, from Vena Cavae Superior, and from the Pulmonary Artery.” European Journal of Emergency Medicine 15.2 (2008): 86-91. 

7. Ibrahim I, Ooi SB, Chan YH, et al. “Point-of-Care Bedside Gas Analyzer: Limited Use of Venous PCO2 in Emergency Patients.” The Journal of Emergency Medicine 41.2 (2011): 117-23.

8. Malatesha G, Singh NK, Bharija A, et al. “Comparison of Arterial and Venous PH, Bicarbonate, PCO2 and PO2 in Initial Emergency Department Assessment.” Emergency Medicine Journal 24.8 (2007): 569-71.

9. Byrne AL, Bennett M, Chatterji R, et al “Peripheral Venous and Arterial Blood Gas Analysis in Adults: Are They Comparable? A Systematic Review and Meta-analysis.” Respirology 19.2 (2014): 168-75.

10. Lightowler, JV. “Non-invasive Positive Pressure Ventilation to Treat Respiratory Failure Resulting from Exacerbations of Chronic Obstructive Pulmonary Disease: Cochrane Systematic Review and Meta-analysis.”BMJ 326.7382 (2003): 185.

11. Keenan, SP. “Which Patients with Acute Exacerbation of Chronic Obstructive Pulmonary Disease Benefit from Noninvasive Positive-Pressure Ventilation?” Annals of Internal Medicine Ann Intern Med,138.11 (2003): 861.

12. Ram FS, Picot J, Lightowler J, et al. “Non-invasive Positive Pressure Ventilation for Treatment of Respiratory Failure Due to Exacerbations of Chronic Obstructive Pulmonary Disease.” Cochrane Database of Systematic Reviews Reviews (2004).

13. Davidson A, Banham S, Elliott M, et al. “BTS/ICS Guideline for the Ventilatory Management of Acute Hypercapnic Respiratory Failure in Adults.” Thorax 71. Suppl 2 (2016): Ii1-i35.

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wendyrollerblades

Senior EM Resident at SUNY Downstate / Kings County Hospital, EM/Critical Care Blogger, Medical Student Education Curriculum Co-Chair, has a blackbelt in "keepin' it real"

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