What's in a Blood Gas? VBG vs ABG

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You’re deep into a busy shift. Pushing yourself to see more volume towards the end of the year, you find yourself actively managing 8 patients.  You have 2 patients with difficulty breathing you believe have COPD exacerbations and 1 patient with a history of T1DM who has a critical high finger stick blood sugar and ketones in their urine.  You send VBGs as part of the work up for all these patients finding hypercarbia for the patients who have COPD exacerbations and a significant metabolic acidosis in the patient with T1DM confirming your diagnosis of DKA. You are in the process of admitting these patients when you face questions from your colleagues in-house as to why you didn’t perform an ABG on these patients?

Venous or Arterial Blood?

How comparable are the components of arterial and venous blood gases?

pH:Prior to 2001 there were few studies, most done in the 1950-1970s, evaluating the comparison between venous and arterial blood. Kelly and colleagues changed the landscape with a paper comparing the pH of venous and arterial blood in 246 patients in the Emergency Department, finding a difference of 0.04 (1). Since this data was published there have been many papers published seeking to replicate this data. Most papers and subsequent meta-analyses show that the arterio-venous pH difference is between 0.03-0.04 (See Table 1 below). 

PvCO2:While the pH is accepted to have a close correlation between arterial and venous blood PvCO2 has been more hotly debated. This debate stems from the data showing that at higher levels of PvCO2 arterial and venous blood lose their correlation resulting in unacceptably wide confidence intervals (2,3,4,5). After finding the poor correlation, especially at high levels, there were studies examining if there was value in using PvCO2 as a screening for arterial hypercarbia. Kelly and colleagues showed that a venous PvCO2 >45mmHg has a 100% sensitivity and 100% NPV in predicting arterial hypercarbia (6,7).

Differences represent VBG-ABG. *Converted from kPa to mmHg

Differences represent VBG-ABG. *Converted from kPa to mmHg

Shock and the VBG

Whether or not venous and arterial blood gas values keep their correlation in shock states is an ongoing question. There is concern that the pathophysiology of shock will cause a deterioration of the relationship. The concern is that venous values will show an increased pCO2 and acidemia due to increased production by the tissues and impaired removal.

A recently published article by Zeserson et al of patient in both the ED and ICU examined if venous blood gas would still correlate in undifferentiated critically ill patients (9). The authors found in this article that the pH and PvCO2 correlate well, 0.03 (95% CI of 0.03-0.04) and 4.8mmHg (95% CI of 3.7-6.0) respectively. They did a subgroup analysis for patient in shock during the blood draw, and while it was a small number of patients, they still found a strong correlation.

What else is in a VBG?

There may be more to your blood gas than just pCO2 and pH (depending on your institution)

  • Methemoglobin: If your VBG has the ability to use co-oximetry then a methemoglobin level can be obtained. Otherwise this is an instance where you would want to use an ABG to obtain an accurate PaO2 to evaluate if it is normal in a cyanotic patient.
  • Carboxyhemoglobin: There have been studies showing that venous and arterial levels do correlate. Touger et al in 1994 showed a strong relationship between arterial and venous carboxyhemoglobin levels (10). This study was done at a hyperbaric center so they were able to analyze a large range of carboxyhemoglobin, showed strong correlation up to 25%. They could not comment on levels greater than 25% because during the study period they did not have enough patients to truly make a claim. 

Tourniquet Use and VBGs

One of the last things that need to be addressed is whether or not the way VBGs are drawn should be standardized across the hospital so that the values obtained in the ED match ones drawn on the floor or ICU.

A study in 2009 by Cengiz et al, tested whether the application of a tourniquet effected multiple values from venous blood sampling (11). In this study the authors applied a tourniquet to a healthy volunteer and drew off venous blood with the tourniquet applied and then at 30 second intervals after its removal. In their 10 healthy volunteers there was no difference in blood gas values with the tourniquet applied or at any interval after its removal. 

Summary

In conclusion, the VBG is a useful, less painful, and more easily gathered test than ABG to asses for acidosis and hypercarbia. Multiple studies have shown that pH correlates well between VBG and ABG. As for PvCO2, using a cutoff of 45mmHg is very sensitive in determining whether there is arterial hypercarbia. New studies are coming out showing that VBG can be useful even in shock states.

References

  1. Kelly, AM, McAlpine, R, Kyle, E. Venous pH can safely replace arterial pH in the initial evaluation of patients in the emergency department. Emerg Med J. 2001;18:340-342.
  2. Byrne, A, 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;19:168-175.
  3. Kelly, AM. Can VBG analysis replace ABG analysis in emergency care? Emerg Med J. 2016;33:152-154.
  4. 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. Am J Emerg Med. 2012;30:896-900.
  5. McKeever, TM, Hearson, G, Housley, G, et al. Using venous blood gas analysis in the assessment of COPD exacerbations: a prospective cohort study. Thorax. 2016;71:210-215.
  6. Kelly, AM, Kerr, D, Middleton, P. Validation of venous pCO2 to screen for arterial hypercarbia in patients with chronic obstructive airways disease. J Emerg Med. 2005;28(4):377-379
  7. Kelly, AM. Review article: Can venous blood gas analysis replace arterial in emergency medical care. Emerg Med Australas. 2010;22:493-498.
  8. Middleton, P, Kelly, AM, Brown, J, et al. Agreement between arterial and central venous values for pH, bicarbonate, base excess, and lactate. Emerg Med J. 2006;23:622-624.
  9. Zeserson, E, Goodgame, B, Hess, JD, et al. Correlation of venous blood gas and pulse oximetry with arterial blood gas in the undifferentiated critically ill patient. J Intensive Care Med. 2018;33(3):176-181.
  10. Touger, M, Gallagher, EJ, Tyrell, J. Relationship between venous and arterial carboxyhemoglobin levels in patients with suspected carbon monoxide poisoning. Ann Emerg Med. 1995;25(4):481-483.
  11. Cengiz et al. Influence of tourniquet application on venous blood sampling for serum chemistry, hematological parameters, leukocyte activation and erythrocyte mechanical properties. Clin Chem Lab Med. 2009;47(6):769. 

Written by Jessica Koehler, MD PGY-1 University of Cincinnati, Dept. of Emergency Medicine

Peer Review, Editing, and Posting by Jeffery Hill, MD MEd