Arterial blood gas can be obtained from a needle puncture from the radial, brachial, femoral, axillary, dorsalis pedis artery; or from an indwelling arterial catheter. It can measure the partial pressure of carbon dioxide (PaCO2), partial pressure of O2 (PaO2), and acidity. Then from those values we can calculate: bicarbonate and Arterial oxygen saturation (SaO2).Contraindications for obtaining ABG include: a failed Allen’s test, and infection/scarring/AV fistulas at the sample location. Its collection can be very painful, and risk scarring/ clotting/ aneurysms at the collection site. Also, it may be difficult to collect when patients have low pulses.

Some error may be a falsely lowered partial pressure of O2 (PaO2) when the sample is in room temperature. One reason for this is that O2 in the sample can be used up by the leukocytes or platelets, especially if the patients have an abnormally excessive level of them. Placing the sample on ice and having it analyzed within 15min helps avoid this error. Also, gas may diffuse out and give erroneous readings if a plastic syringe, and not a glass syringe is used.
Heparin can be added to the sample syringe to avoid coagulation, however, it can also cause issues. Its acidity may contribute to further reducing the sample’s pH. To avoid this, minimal heparin should be used, and at least 2mL of arterial blood should be collected.
Also, we must try to remove air bubbles after collecting the sample, since their presence can falsely elevate the partial pressure of O2 (PaO2), and falsely decrease the partial pressure of carbon dioxide (PaCO2).

Venous blood gas can be obtained from different sites: via venipuncture, central venous catheter, or from a pulmonary artery catheter. The correlation with ABG versus VBG is best studied with central venous blood gases, but venipuncture is typically used for those critically ill patients with no central access.
VBG can measure venous carbon dioxide tension (PvCO2), acidity (pH), oxyhemoglobin saturation (SvO2), and serum bicarbonate (HCO3) concentration.
Checking for the venous CO2, bicarbonate, and pH helps with checking acid-base status and ventilation. Oxyhemoglobin saturation (SvO2) is used to check the effectiveness of resuscitation in patients with sepsis or shock.

The values between ABG and VBG correlate well, and are helpful in determining acid base imbalances. Only exception is that VBG cannot measure one’s oxygenation like an ABG can, so its use is usually combined with pulse oximetry. With that said, ABG is especially preferred in patients who are hypoxemic, in shock, has ARDs, or has poor circulation that result in inaccurate pulse ox readings.

• https://www.uptodate.com/contents/venous-blood-gases-and-other-alternatives-to-arterial-blood-gases?search=blood%20gas%20interpretation&source=search_result&selectedTitle=2~150&usage_type=default&display_rank=2
• https://www.uptodate.com/contents/arterial-blood-gases?search=blood%20gas%20interpretation&topicRef=16991&source=see_link#H10
• https://www.youtube.com/watch?v=oVoJgd3DR5U
• https://epmonthly.com/article/blood-gases-abg-vs-vbg/