How do you interpret a Venous Blood Gas (VBG)?

Interpreting Venous Blood Gas (VBG)

A venous blood gas (VBG) can reliably assess acid-base status and is an acceptable alternative to arterial blood gas (ABG) in many clinical scenarios, though with specific limitations for assessing oxygenation.

Normal VBG Reference Values

• pH: 7.30-7.43 (acidemia <7.30, alkalemia >7.43) 1
• pCO2: 38-58 mmHg (respiratory acidosis >58 mmHg, respiratory alkalosis <38 mmHg) 1
• HCO3-: 22-30 mmol/L (metabolic alkalosis >30 mmol/L, metabolic acidosis <22 mmol/L) 1
• Base excess: -1.9 to 4.5 mmol/L (metabolic acidosis <-1.9) 1
• pO2: 19-65 mmHg (not reliable for assessing oxygenation) 1
• Lactate: 0.4-2.2 mmol/L 1

Systematic Approach to VBG Interpretation

Step 1: Assess pH

• pH <7.30 indicates acidemia 2
• pH >7.43 indicates alkalemia 2

Step 2: Determine Primary Disorder

• Respiratory disorders: Assess pCO2

  • Elevated pCO2 (>58 mmHg) suggests respiratory acidosis 2
  • Low pCO2 (<38 mmHg) suggests respiratory alkalosis 2

• Metabolic disorders: Assess HCO3- and base excess

  • Elevated HCO3- (>30 mmol/L) suggests metabolic alkalosis or compensation for respiratory acidosis 2
  • Low HCO3- (<22 mmol/L) suggests metabolic acidosis or compensation for respiratory alkalosis 2
  • Negative base excess (<-1.9) indicates metabolic acidosis 2

Step 3: Evaluate Compensation

• In respiratory acidosis: Look for elevated HCO3- (metabolic compensation) 2
• In respiratory alkalosis: Look for decreased HCO3- (metabolic compensation) 2
• In metabolic acidosis: Look for decreased pCO2 (respiratory compensation) 2
• In metabolic alkalosis: Look for elevated pCO2 (respiratory compensation) 2

VBG vs ABG Correlation

• The mean difference between arterial and venous samples:

  • pH: arterial is 0.027-0.032 higher than venous 3, 4
  • pCO2: venous is 3.8-5.0 mmHg higher than arterial 3, 4
  • HCO3-: venous is 0.8-1.0 mmol/L higher than arterial 3, 4

• Conversion formulas from venous to arterial values:

  • Arterial pH = venous pH + 0.05 4
  • Arterial pCO2 = venous pCO2 - 5 mmHg 4

Clinical Applications and Limitations

• Acid-base assessment: VBG is reliable for evaluating acid-base status in most clinical scenarios 3, 5

• Ventilation assessment: VBG can be used for screening of hypercapnia with high sensitivity (91.89%) 5

• Metabolic disorders: VBG has high sensitivity (80.64%) and specificity (89.47%) for detecting metabolic acidosis 5

• Limitations:

  • VBG cannot reliably assess oxygenation; arterial samples are required when precise oxygenation assessment is needed 2
  • In shock or hypotension, arterio-venous differences may be greater than normal, making VBG less reliable 2
  • Only 72-80% of paired ABG-VBG samples are clinically equivalent in trauma patients 6

Special Considerations

• For patients with COPD at risk of hypercapnic respiratory failure, target oxygen saturation of 88-92% when using pulse oximetry alongside VBG 2, 7
• Proper sample handling is crucial - air bubbles, delayed analysis, or improper storage can significantly affect VBG results 2
• In critically ill patients, central venous blood gas parameters (pH and pCO2) can be used as surrogates for arterial values in hemodynamically stable patients 4
• For lactate, bicarbonate, and base excess, venous samples provide clinically acceptable results compared to arterial samples 5

Clinical Pitfalls to Avoid

• Do not rely on VBG for oxygenation assessment (pO2 or O2 saturation) 2
• Standard pulse oximetry cannot differentiate carboxyhemoglobin, potentially giving falsely normal readings in carbon monoxide poisoning 8
• Avoid using VBG for acid-base assessment in patients with shock or severe circulatory failure, as the arterio-venous difference may be 4-fold greater than normal 4
• Remember that the 95% limits of agreement between ABG and VBG are wider than the clinically acceptable thresholds in trauma patients 6