VBG + Vibes

Our ED has experienced several times when the lab goes down and none of our labs return. On days like these, our team coined the term “VBG + Vibes” to describe the clinical atmosphere. But all jokes aside: is our reliance on VBGs truly justified?

When ICU teams arrive, their residents and fellows sometimes request arterial blood gases (ABGs) instead. While there are certain indications for ABGs—particularly when assessing oxygenation or in unstable patients—a VBG typically provides enough clinical information for most ED decisions.

We also routinely turn to VBGs for electrolytes, especially when a hemolyzed BMP gives us a suspiciously elevated potassium level (e.g., K = 7). If the VBG potassium is normal, we often proceed based on that value. But how accurate and dependable are these VBG electrolytes?

In this POTD, we’ll explore the evidence behind VBG use—its accuracy, limitations, and when it truly matters to make the distinction. Let’s clarify when “VBG + Vibes” is solid clinical practice—and when it might not be enough.

Origins and Uses of VBGs

• Faster, less painful alternative to ABGs for acid–base assessment

• Widely adopted in EDs for evaluating pH, bicarbonate, CO₂, and lactate

• Particularly useful in DKA, sepsis, and undifferentiated shock

• Modern analyzers also provide electrolytes and hemoglobin, expanding utility

VBG vs. ABG 

• Advantages of VBG

  • Easier, safer, and faster to obtain than ABG

  • Less painful; no arterial puncture needed

  • Suitable for most acid–base and perfusion assessments

• Limitations of VBG

  • Cannot measure PaO₂ – use SpO₂ for oxygenation (unless severe hypoxemia suspected)

  • Less reliable at extremes (e.g., shock, hypercapnia >45 mmHg)

  • If VBG PCO₂ is elevated or oxygenation is unclear, follow with ABG

Accuracy of VBG Parameters

• pH & Bicarbonate

  • pH difference vs ABG: ~0.03–0.05 units → clinically negligible

  • Bicarbonate (HCO₃⁻) closely mirrors ABG and serum values (within ~2 mEq/L)

  • Reliable for assessing acidosis/alkalosis, including in DKA and renal failure

• CO₂ (PCO₂)

  • Correlates well when normal or mildly elevated

  • PCO₂ >45 mmHg → correlation worsens, may under- or overestimate PaCO₂ by ±10 mmHg

  • Use VBG to rule out hypercapnia; ABG needed if VBG CO₂ is high or patient is unstable

• Electrolytes

  • Potassium & sodium from VBG strongly correlate with serum values (r > 0.9)

  • Good enough to trust for clinical decisions (e.g., hyperkalemia)

  • Caution: Blood gas analyzers don’t detect hemolysis – falsely elevated K⁺ possible

• Hemoglobin & Hematocrit

  • Reliable from VBG analyzers (via co-oximetry)

  • Sufficient for ED decision-making, including transfusions and anemia eval

• Lactate

  • Venous lactate is accurate for diagnosis and trending in sepsis and shock

  • Normal VBG lactate rules out tissue hypoperfusion

  • Trending must be done using same sample type (venous vs arterial)

Common ED Use Cases

• Diabetic Ketoacidosis (DKA)

  • Venous pH and HCO₃⁻ are adequate to diagnose and monitor DKA

  • VBG electrolytes (Na⁺, K⁺, bicarb, AG) reliably match BMP

  • No need for ABG unless coexisting respiratory failure

• Sepsis

  • VBG lactate + base deficit reflect perfusion status

  • Serial VBGs are reliable for lactate clearance

  • ABG only if oxygenation unclear or patient in severe distress

• Undifferentiated Shock

  • VBG rapidly identifies metabolic or respiratory acidosis

  • Helps differentiate between shock types early

  • ABG may follow if VBG is highly abnormal or clinical picture unclear

When to Use ABG Instead

• Need exact PaO₂ (e.g., ARDS, unclear oxygenation despite high SpO₂)

• Suspected severe hypercapnia or respiratory failure (e.g., altered COPD)

• Mixed acid–base disorder with unclear etiology

• Consultant or protocol explicitly requires it (e.g., trauma base deficit)

Takeaways

• VBG + SpO₂ + clinical judgment is accurate, safe, and efficient in most ED patients

• ABG should be selective, not routine

• Embracing VBGs reduces pain, speeds care, and provides reliable data in:

  • DKA

  • Sepsis

  • Shock

  • COPD/asthma exacerbations

• Knowing when a VBG is enough is essential; use ABG only when it changes management

References:

Kelly, A. M., McAlpine, R., Kyle, E., & Klim, S. (2018). How accurate are blood gas electrolyte measurements? REBEL EM. https://rebelem.com/how-accurate-are-blood-gas-electrolyte-measurements/

LITFL. (2023). VBG versus ABG. Life in the Fast Lane. https://litfl.com/vbg-versus-abg/

Koul, P. A., Khan, U. H., Wani, A. A., Ahangar, A. G., & Ahmad, M. (2024). Venous versus arterial blood gases in patients with COPD exacerbation: A clinical equivalence study. Journal of Emergency Medicine, 66(2), 142–148. https://doi.org/10.1016/j.jemermed.2024.01.003

Zeserson, E., Goodgame, B., Hess, J. D., Satty, T., Walker, C., Diercks, D. B., & Clancy, T. V. (2018). Correlation of venous blood gas and pulse oximetry with arterial blood gas in the undifferentiated critically ill patient. Western Journal of Emergency Medicine, 19(2), 403–408. https://doi.org/10.5811/westjem.2017.11.35191

Gokel, Y., Paydas, S., Koseoglu, Z., Seydaoglu, G., & Tan, M. E. (2000). Comparison of blood gas and electrolyte values in arterial and venous blood samples in patients with uremic acidosis and other acid-base disturbances. American Journal of Nephrology, 20(4), 319–323. https://doi.org/10.1159/000013601