An elevated venous pC02 is a rough indicator of the presence of arterial hypercarbia but if you really want to know what the arterial pC02 is in your patient with hypercarbia, you should get an arterial blood gas (ABG) . A 2021 narrative review of the literature suggested the following ABG conversion from central VBG: A. Arterial pH = venous pH + 0.05 units; and B. Arterial Pco2 = venous Pco2 – 5 mm Hg (1).
A meta-analysis of studies involving patients with COPD presenting to the emergency department (ED) found a good agreement for pH and bicarbonate values between arterial and venous blood gases but not for pC02 or p02 (2). Although a venous pCO2 of ~45 mmHg or less correctly identified patients who were not hypercarbic based on ABGs, the 95% limit of agreement varied widely from -17 to +26 mmHg between venous and arterial pC02 (average difference ~6.0 mm). Similar results have been reported by other studies involving patients with COPD exacerbation (3,4).
Another meta-analysis involving all comers (COPD and non-COPD patients) concluded that venous pC02 should not be used as a substitute for arterial pC02 when accurate pC02 is required (5). In fact, they emphasized that venous pC02 was not always greater than arterial pC02!
Bonus pearl: Did you know that an unexpectedly low bicarbonate level in a patient with COPD and CO2 retention should alert us to the possibility of concurrent metabolic acidosis (eg, due to lactic acidosis, uremia)?
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References
1. Chong WH, Saha BK, Medarov BI. Comparing cenral venous blood gas to arterial blood gas and determining its utility in critically ill patients: narrative review. Anesth Anal 2021; 133:374-78. https://pubmed.ncbi.nlm.nih.gov/33780397/
2. Lim BL, Kelly AM. A meta-analysis on the utility of peripheral venous blood gas analyses in exacerbations of chronic obstructive pulmonary disease in the emergency department. Eur J Emerg Med 2010;17:246-48. https://journals.lww.com/euro-emergencymed/Abstract/2010/10000/A_meta_analysis_on_the_utility_of_peripheral.2.aspx
3. McCanny P, Bennett K, Staunton P, et a. Venous vs arterial blood gases in the assessment of patients presenting with an exacerbation of chronic obstructive pulmonary disease. Am J Emerg Med 2012;30:896-900. https://www.sciencedirect.com/science/article/abs/pii/S0735675711002865
4. McKeevere TM, Hearson G, Housely G, et al. Using venous blood gas analysis in the assessment of COPD exacerbations: a prospective cohort study. Thorax 2016;71:210-15. https://www.researchgate.net/publication/285545995_Using_venous_blood_gas_analysis_in_the_assessment_of_COPD_exacerbations_A_prospective_cohort_study
5. 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 2014;19:168-75. https://onlinelibrary.wiley.com/doi/full/10.1111/resp.12225
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