Venous Blood Gas Interpretation


Blood Gases, Venous


Blood flux transports gases such as oxygen gas (O2) and carbon dioxide gas (CO2). This test determines the concentration of these gases on the venous blood, and if so, an imbalance in the amount of O2 or CO2 or acid-base imbalance, indicative of a respiratory, metabolic or kidney problem.

Arterial blood gas analysis has been a useful tool for doctors managing acutely ill patients with presumed acid-base imbalances since automated blood gas analyzers appeared in the early 1960s. Despite this, in specific scenarios, a Venous blood gas analysis can provide enough information to make correct diagnosis, with some advantages over ABG analysis.

The most important advantage of a VBG instead of an ABG is decreased pain for the patient. Also a VBG sample can be drawn using the same intravenous line used to extract blood for other lab tests. No need of extra puncture. Consequently less costs, work, risk of arterial laceration and pain.

Normal venous pH, PCO2, and HCO3 are useful to check severe acid-base disturbances without the necessity of an ABG analysis.

On the contrary, in some cases such as cardiac arrest, the disparity between arterial and venous values is important. In this situation, tissue hypoxia is reflected by the lower pH and higher PCO2 on the venous analysis in comparation with ABG.

Also, in general, Venous PO2 values do not reflect arterial PO2 levels very accurately.

Purpose of the test

This test is ordered when you have symptoms such us difficulty breathing, shortness of breath, or rapid breathing in order to determine your oxygenation and acid/base status.

It is also ordered to monitor the effectiveness of oxygen therapy and during certain surgeries.

If you have some respiratory problem your body can not restore gas and pH imbalance by itself. Then you may need for example therapy with pure O2. Then your doctor may order blood gas tests in order to monitor the effectiveness of the therapy.

Reference range values

pH 7.32 – 7.45

PCO2: 5.05kPa – 6.65kPa


Abnormal findings

Abnormal gas concentrations may mean that your body is not able to get enough oxygen, or is not able to get rid of enough carbon dioxide. Also can mean there is a problem with kidney function.
A pH imbalance may be cause by respiratory disease.
Lower pH and an increased PCO2 can mean respiratory depression (not enough oxygen in and CO2 out), caused by:
• Pneumonia
• chronic obstructive pulmonary disease (COPD)
• over-sedation from narcotics
Raised pH and a decreased PCO can mean over ventilation caused by:
• hyperventilating
• pain
• emotional distress
• certain lung diseases
Lower pH and decreased HCO3- can mean the blood is too acidic on a metabolic/kidney level, due to:
• diabetes
• shock
• renal failure
Elevated pH and increased HCO3- can mean:
• hypokalemia
• chronic vomiting
• sodium bicarbonate overdose


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