How to correct pCO2 (partial pressure of carbon dioxide) in a mixed arterio-venous arterial blood gas (ABG) sample?

Correcting PCO2 in Mixed Arterio-Venous Blood Gas Samples

You cannot reliably "correct" a PCO2 value from a mixed arterio-venous sample to obtain an accurate arterial value—the sample should be discarded and a proper arterial sample obtained instead.

Why Mixed Samples Cannot Be Corrected

The fundamental problem is that the degree of venous contamination in a mixed sample is unknown and unpredictable, making mathematical correction impossible 1, 2. The arterio-venous PCO2 difference varies dramatically based on:

• Circulatory status: In hemodynamically stable patients, the arterial-venous PCO2 difference averages 4-6.5 mmHg, but in patients with circulatory failure, this difference increases 4-fold 1
• Local blood flow conditions: Peripheral venous PCO2 differences from arterial values are strongly correlated with peripheral venous oxygen saturation (SpvO2), with r² = 0.62 3
• Tissue metabolism: The mixed venous PCO2 reflects the metabolic CO2 production and cardiac output relationship 4

What the Literature Shows About Venous-to-Arterial Conversion

While some studies have derived conversion formulas for pure venous samples, these do not apply to mixed arterio-venous samples:

For Central Venous Blood (Not Mixed Samples)

• Stable patients: Arterial PCO2 = venous PCO2 - 5 mmHg 1
• Mechanically ventilated patients: Arterial PCO2 = 3.06 + 0.76 × venous PCO2 2

For Peripheral Venous Blood (Not Mixed Samples)

• Correction using SpvO2: Arterial PCO2 = venous PCO2 - 0.30 × (75 - SpvO2) 3
• Simple correction: Arterial PCO2 = venous PCO2 × 0.83 2

Critical limitation: All these formulas assume you know whether the sample is purely venous or purely arterial. With a mixed sample, you have neither, and the proportion of each is unknown 1, 2.

Practical Management Algorithm

Step 1: Recognize the Mixed Sample

• Blood obtained during difficult arterial puncture with aspiration of dark blood 5
• Uncertainty about catheter tip position 1
• Unexpectedly low PO2 for the inspired oxygen concentration 4

Step 2: Do Not Attempt Correction

• Discard the sample and obtain a proper arterial sample 5
• Use the radial or brachial artery with proper Allen's test first 5, 6
• Consider using local anesthesia to improve success rate 5

Step 3: Alternative Approaches If Arterial Access Is Difficult

• Arterialized capillary blood gas: Can accurately reflect arterial pH and PCO2 in non-critical patients 5, 6
• Central venous blood gas: In hemodynamically stable patients, can serve as a screening tool with the understanding that arterial PCO2 ≈ venous PCO2 - 5 mmHg 1
• Transcutaneous PCO2 monitoring: Highly accurate (bias 0.23 kPa, SD 0.28 kPa) for monitoring chronic hypercapnia 7

Common Pitfalls to Avoid

• Assuming you can "average" the values: The relationship between arterial and venous PCO2 is not linear and depends on hemodynamic status 1, 3
• Using peripheral venous formulas for central samples or vice versa: The arterio-venous differences are significantly different 1, 2
• Relying on the sample when clinical decisions depend on accurate PCO2: Particularly dangerous in patients being evaluated for NIV (pH <7.35, PCO2 >49 mmHg) or those with suspected CO2 retention 8, 6
• Not recognizing that normal oxygen saturation doesn't validate the sample: A mixed sample may have acceptable PO2 but grossly inaccurate PCO2 5, 6

When Accurate PCO2 Is Critical

The following clinical scenarios require a proper arterial sample and cannot rely on corrected or estimated values 5, 8, 6:

• Determining need for non-invasive ventilation (pH <7.35 and PCO2 >49 mmHg) 8, 6
• Titrating oxygen in patients at risk for CO2 retention (rise >7.5 mmHg indicates instability) 8, 6
• Calculating alveolar-arterial oxygen gradient for diagnosing gas exchange abnormalities 4
• Brain death determination (requires PCO2 ≥60 mmHg and ≥20 mmHg above baseline) 4