Gas Exchange

Venous Blood Gas Testing

In recent years there has been a increasing trend toward the use of venous blood gas (VBG) testing in place of arterial blood gas (ABG) testing in certain clinical scenarios. VBG testing involves obtaining a blood sample from a peripheral vein, typically in the arm, rather than from an artery.

While both Arterial and Venous Blood Gas testing are invasive, VBG testing is generally considered to be less painful and less risky than ABG testing. This is because arterial punctures can be more difficult to perform and carry a higher risk of complications such as bleeding, hematoma formation, and arterial injury.

The major limition of a VGB is that it is obtained from a regional venous system, and therefore does not accurately reflect the oxygenation status of the patient. However, VBGs can provide useful information about acid-base status and carbon dioxide levels, which can be helpful in certain clinical scenarios. While VBG pH levels tend to be slightly lower than ABG pH levels (by approximately 0.03-0.04 units), there is generally a good correlation between the two measurements. Similarly, VBG CO2 levels tend to be slightly higher than ABG CO2 levels (by approximately 4-6 mmHg), but again there is generally a good treatment related correlation between the two measurements.

VBG testing I feel is most useful in the serial monitoring of patients with known acid-base disturbances, such as those with diabetic ketoacidosis or chronic obstructive pulmonary disease (COPD). In these cases, VBG testing can provide valuable information about the patient's acid-base status and help guide treatment decisions.

A significant caviat of VBG testing is that the correlation between pH and CO2 levels between VBG and ABG can be significantly impacted by factors such as poor peripheral perfusion, shock, and sepsis. In these cases, ABG testing will likely be necessary for accurate assessment.

Given the limitations of VBG testing, our interpretation is limited to the assessment of overall acid-base status and the respiratory systems physiologic response to that derrangement.