Arterial vs. Venous Blood Gas: Key Differences and Clinical Applications
Arterial blood gas (ABG) samples arterial blood and provides the gold standard for assessing oxygenation, ventilation, and acid-base status, while venous blood gas (VBG) samples venous blood and can reliably assess pH and CO₂ but cannot accurately measure oxygenation. 1
Fundamental Physiological Differences
ABG measures blood directly from the arterial system (typically radial artery), reflecting gas exchange at the alveolar-capillary interface and providing accurate assessment of:
- PaO₂ (arterial oxygen tension) - the definitive measure of oxygenation 1
- PaCO₂ (arterial carbon dioxide tension) - reflects ventilatory adequacy 2, 3
- pH - arterial acid-base status 4
VBG measures blood from the venous system (peripheral or central veins), reflecting tissue metabolism and oxygen extraction:
- Cannot accurately measure oxygenation - venous PO₂ reflects tissue oxygen extraction, not arterial oxygenation 1, 5
- pH and PCO₂ correlate reasonably with arterial values in stable patients 6
- Provides insights into tissue perfusion through parameters like ScvO₂ and venous-arterial CO₂ difference 5
When ABG is Mandatory
ABG remains essential when precise PaO₂ measurement is required 1. Specific clinical scenarios where ABG is non-negotiable include:
- All critically ill patients requiring assessment of oxygenation 1
- Patients in shock, on vasopressor therapy, or with severe peripheral edema - the arterio-venous difference becomes unpredictable and clinically significant 1
- Cardiogenic shock requiring precise acid-base assessment 1
- Patients on ECMO - samples must come from the right radial arterial line to represent cerebral perfusion 1
- Long-term oxygen therapy (LTOT) assessment - requires two ABG measurements at least 3 weeks apart during clinical stability 4
- Non-invasive ventilation (NIV) monitoring - ABG should be measured after 1-2 hours of NIV and after 4-6 hours if earlier sample showed little improvement 4
When VBG May Be Acceptable
VBG can serve as a less invasive alternative for acid-base status and metabolic function assessment in hemodynamically stable patients 5. The evidence supports VBG use when:
- Screening for hypercapnia - central VBG can identify arterial hypercapnia 6
- Assessing acid-base disturbances in stable patients - mean difference between arterial and central venous pH is approximately 0.03 units 6
- Evaluating metabolic parameters - lactate, electrolytes, and bicarbonate correlate well 7
Conversion Formulas for Stable Patients
In hemodynamically stable patients without severe acid-base disturbances, you can estimate arterial values from central VBG 6:
- Arterial pH = venous pH + 0.05 units
- Arterial PCO₂ = venous PCO₂ - 5 mm Hg
Critical caveat: In patients with circulatory failure, the difference between central venous and arterial pH/PCO₂ is 4-fold greater, making these conversions unreliable 6.
Clinical Pitfalls to Avoid
A normal SpO₂ does not rule out significant acid-base disturbances or hypercapnia - pulse oximetry appears normal in patients with normal PO₂ but abnormal pH or PCO₂ 1. This is a common error that delays appropriate intervention.
In critically ill patients with severe metabolic acidosis and hypoxemia, the sampling site is the only definitive way to identify whether the blood sample is arterial or venous - both types can show acidosis, but venous samples cannot accurately assess oxygenation 1.
Patients with severe peripheral edema, mottled skin, or hypoperfusion have unreliable venous samples that can differ dramatically from arterial values 1.
Technical Considerations for ABG Sampling
Allen's test must be performed before radial artery puncture to ensure dual blood supply to the hand from both radial and ulnar arteries 4, 8. This prevents ischemic complications if the radial artery becomes occluded.
Obtain informed consent with discussion of possible risks including arterial injury, thrombosis, hematoma, and reflex sympathetic dystrophy 4, 7.
During the first 24 hours of critical illness, use of an indwelling arterial line should be considered to avoid repeated painful punctures 4.
Alternative Sampling Methods
When ABG is not practical, capillary blood gases (CBG) can replace ABG for re-measuring PaCO₂ and pH during oxygen titration 4, 8. However, CBG cannot accurately assess oxygenation.
Cutaneous capnography can replace ABG for re-measuring PaCO₂ alone but not pH 4, 8.
Monitoring Frequency in Critical Care
All patients must have clinical re-assessment approximately 1 hour after being established on NIV, together with blood gas analysis 4. If there has been no improvement in PaCO₂ and pH after 4-6 hours, NIV should be discontinued and invasive ventilation considered 4.
Patients with baseline hypercapnia must have ABG monitoring after each flow rate titration - a rise in PaCO₂ > 1 kPa (7.5 mm Hg) indicates clinically unstable disease 8.