From the Guidelines
The CO2 reading on a Venous Blood Gas (VBG) is generally reliable and correlates well with arterial CO2 levels in most clinical scenarios, with VBG CO2 values typically running 4-6 mmHg higher than arterial values 1. When interpreting VBG CO2 readings, it is essential to consider the physiological context, including the patient's acid-base status, perfusion, and respiratory function.
- In patients with normal or near-normal acid-base status and adequate perfusion, VBG CO2 values are a reasonable substitute for arterial blood gas (ABG) measurements.
- However, the reliability of VBG CO2 readings decreases in patients with severe respiratory distress, shock states, or significant acid-base disturbances, where an arterial sample may be necessary for precise assessment 1. The physiological principle that CO2 equilibrates well across tissue beds supports the reliability of VBG CO2 readings, as venous blood reflects tissue CO2 levels after gas exchange has occurred.
- For routine monitoring of patients with chronic respiratory conditions or for trending purposes, VBG CO2 measurements provide sufficient accuracy while offering the advantages of easier sampling, less patient discomfort, and fewer complications compared to arterial punctures. It is crucial to note that the mechanisms for carbon dioxide retention are complex, and high concentrations of oxygen can lead to carbon dioxide retention in vulnerable patients, such as those with COPD or obesity-hypoventilation syndrome (OHS) 1.
- Targeted lower concentration oxygen therapy, aiming for a target range of 88–92%, can help avoid oxygen-induced hypercapnia in these patients.
From the Research
Reliability of CO2 Reading on Venous Blood Gas (VBG)
- The reliability of the carbon dioxide (CO2) reading on a Venous Blood Gas (VBG) is a topic of ongoing research, with studies yielding mixed results 2, 3, 4, 5, 6.
- A study published in 2016 found that the agreement for pCO2 between arterial and venous blood gas samples was poor, with 95% limits of agreement of the order of 20 mmHg (2.67 kPa) 2.
- However, another study published in 2003 found that venous blood gas could accurately predict arterial blood gas values, including pCO2, in patients with acute respiratory failure receiving mechanical ventilation 3.
- A more recent study published in 2023 found that mathematical arterialisation of venous samples using a newer method (v-TAC) showed close agreement with arterial blood gas sampling for PaCO2 and pH, and could potentially replace routine ABG testing in patients with hypercapnic respiratory failure 4.
- A narrative review published in 2021 found that central venous blood gas parameters of pH and pCO2 were potentially good surrogates for determining arterial pH and pCO2 in stable patients without severe acid-base disturbances, but the difference between central venous and arterial pH/pCO2 was greater in patients with circulatory failure 5.
- Another study published in 2013 found that sequential venous blood gas measurement provided an unpredictable means for assessing pCO2 in patients undergoing flexible bronchoscopy, and that the limits of agreement between arterial and venous pCO2 widened further after physiological stress 6.
Key Findings
- The agreement between arterial and venous pCO2 values can be poor, with wide limits of agreement 2, 6.
- Venous blood gas can accurately predict arterial blood gas values, including pCO2, in certain patient populations, such as those with acute respiratory failure receiving mechanical ventilation 3.
- Newer methods, such as mathematical arterialisation of venous samples, may show closer agreement with arterial blood gas sampling for PaCO2 and pH 4.
- Central venous blood gas parameters of pH and pCO2 may be useful surrogates for determining arterial pH and pCO2 in stable patients without severe acid-base disturbances 5.