What is a VBG used for?

A VBG is used to assess acid-base status and ventilation trends. It is commonly used in place of an ABG when oxygenation assessment is not required.

What are normal VBG values?

Normal VBG values include pH 7.31–7.41, PvCO2 41–51 mmHg, and HCO3 22–29 mEq/L. PvO2 is not used to assess oxygenation.

VBG can replace ABG for acid-base and ventilation assessment in many cases, but ABG is required for evaluating oxygenation and in critically ill patients.

Acid–Base Basics

VBG Interpretation: Step-by-Step Guide

2026-03-23 · ~6 min read

Practice with VBGenius VBG vs ABG Guide

Venous blood gases (VBG) are a fast, low-risk alternative to arterial blood gases (ABG) for many clinical questions. While they cannot assess oxygenation, they are extremely useful for evaluating acid-base status and trending ventilation.

Step 1: Check pH

Determine whether the patient is acidotic or alkalotic.

Normal VBG pH: 7.31–7.41
< 7.31 → Acidosis
> 7.41 → Alkalosis

Step 2: Evaluate CO₂

CO₂ reflects respiratory contribution.

Normal PvCO₂: 41–51 mmHg
High CO₂ → Respiratory acidosis
Low CO₂ → Respiratory alkalosis

Step 3: Check HCO₃⁻

Bicarbonate reflects metabolic contribution.

Normal HCO₃⁻: 22–29 mEq/L
Low → Metabolic acidosis
High → Metabolic alkalosis

Step 4: Determine Compensation

Assess whether the body is compensating appropriately.

Opposite system should move in response
Partial vs full compensation depends on pH normalization

Limitations of VBG

Cannot assess oxygenation (PvO₂ unreliable)
Less accurate in shock or poor perfusion
CO₂ differences widen in unstable states

When to Use ABG Instead

Assessing oxygenation (PaO₂, P/F ratio)
Severe illness or shock
Ventilator management decisions