Arterial Blood Gas and Venous Blood Gas Analysis

Introduction to ABG and VBG

The American College of Physicians recommends that ABG samples arterial blood and provides the gold standard for assessing oxygenation, ventilation, and acid-base status, while VBG samples venous blood and can reliably assess pH and CO₂ but cannot accurately measure oxygenation 1

The American Thoracic Society states that ABG remains essential when precise PaO₂ measurement is required, as VBG cannot accurately measure arterial oxygenation 1
The Society of Critical Care Medicine recommends that ABG is mandatory in all critically ill patients requiring assessment of oxygenation, patients in shock, on vasopressor therapy, or with severe peripheral edema 2, 1

The American Heart Association suggests that ABG is mandatory in patients with cardiogenic shock requiring precise acid-base assessment, and in patients on ECMO, samples should come from the right radial arterial line to represent cerebral perfusion 2, 1
The American College of Emergency Physicians recommends that a normal SpO₂ does not rule out significant acid-base disturbances or hypercapnia, and pulse oximetry will appear normal in patients with normal PO₂ but abnormal pH or PCO₂ 2, 1

The American Society of Anesthesiologists states that for ABG, local anesthesia should be used except in emergencies, and Allen's test must be performed before radial artery puncture 1
The Society of Critical Care Medicine recommends that samples from arterial sites are most similar to laboratory plasma or blood gas analyzer values, and when using VBG, avoid contamination from IV fluid infusing through multilumen catheters 3, 4

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, as both types can show acidosis and venous samples cannot accurately assess oxygenation 5

The American College of Critical Care Medicine recommends that all blood samples should be drawn from an indwelling arterial catheter as the primary sampling site for critically ill patients requiring blood gas analysis 5, 6
If an arterial catheter is unavailable, sampling from a central venous catheter is the second-line alternative, taking care to avoid contamination from IV fluid infusing through multilumen catheters 5, 6

In patients with shock, hypotension, or on vasopressor therapy, arterial blood gas is mandatory because the arterio-venous difference becomes unpredictable and clinically significant 7
For patients on ECMO support, samples must come from the right radial arterial line to represent cerebral perfusion and identify differential oxygenation syndromes, with serial arterial blood gas sampling in the first 24 hours of ECMO support being mandatory 7

Patients with severe peripheral edema, mottled skin, or hypoperfusion have unreliable venous samples that can differ dramatically from arterial values 8, 6

ABG provides the definitive measurement of arterial pH for assessing acid‑base status in patients undergoing respiratory evaluation. 9, 10
Long‑term oxygen therapy assessment requires two ABG samples taken at least three weeks apart while the patient is clinically stable. 9
During non‑invasive ventilation (NIV), an ABG should be obtained 1–2 hours after initiation and again at 4–6 hours if the initial result shows minimal improvement. 11, 10, 12

Perform Allen’s test before radial artery puncture to confirm adequate collateral hand circulation and prevent ischemic complications. 9
Obtain informed consent that outlines potential risks such as arterial injury, thrombosis, hematoma, and reflex sympathetic dystrophy. 9
In the first 24 hours of critical illness, consider placing an indwelling arterial line to avoid repeated painful arterial punctures. 10, 12

Capillary blood gases may be used instead of ABG for re‑measuring arterial pCO₂ and pH during oxygen titration, but they cannot reliably assess oxygenation. 9
Cutaneous capnography can replace ABG for re‑measuring arterial pCO₂ alone; it is not suitable for pH determination. 9

All patients on NIV should undergo clinical reassessment and blood‑gas analysis approximately 1 hour after NIV is established. 10, 12
If there is no improvement in pCO₂ and pH after 4–6 hours of NIV, discontinue NIV and consider invasive mechanical ventilation. 11, 10