Arterial Blood Gas (ABG) Test, Animation

ABG vs VBG. How to read ABG results. How to assess acid-base status from ABG results.

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Arterial blood gas, ABG, is a blood test that measures the levels of blood oxygen and carbon dioxide, as well as blood pH, in a sample drawn from an artery, most often from the radial artery or femoral artery. ABG is typically ordered in emergency medicine, intensive care, or by pulmonologists, to evaluate respiratory, circulatory, and metabolic functions.
For routine blood tests, veins are usually favored over arteries because they sit closer to the skin’s surface, have thinner walls, larger lumens, and thus are more accessible. More importantly, the lower blood pressure in veins makes it easier to stop bleeding, reducing the chance of significant blood loss. Arterial blood sampling requires greater technical competency to avoid complications.
However, when it comes to carbon dioxide and oxygen measurements, arterial blood is generally more reliable because it comes directly from the heart, after being oxygenated in the lungs, whereas venous blood is pooled from various body tissues after delivering oxygen and collecting metabolic waste. Venous pH is slightly lower than arterial pH, but the 2 values often correlate well.
Arterial blood samples must be placed on ice and analyzed immediately to avoid errors. Typically, automated blood gas analyzers are used and results are delivered within 15 min. The following parameters are directly measured: pH, oxygen partial pressure (PaO2), and carbon dioxide partial pressure (PaCO2). Bicarbonate and base excess/deficit are then calculated from measured values of pH and PaCO2. Oxygen saturation can be calculated, or measured directly with an oximeter.
PaO2 is an indicator of the patient’s oxygenation status, while PaCO2 provides information on ventilation and acid-base status.
Normal value ranges differ depending on the altitude of the location, and may vary slightly between labs.
A pH below, or above, the normal range indicates acidemia or alkalemia, respectively. However, a pH in the normal range does not automatically rule out acid-base imbalance unless PaCO2 and bicarbonate levels are both normal. PaCO2 and bicarbonate represent respiratory and metabolic functions, respectively:
A high PaCO2 value indicates a respiratory acidosis, while a low PaCO2 signifies a respiratory alkalosis.
Conversely, a high bicarbonate value indicates a metabolic alkalosis, and a low bicarbonate level corresponds to a metabolic acidosis.
When PaCO2 and bicarbonate values indicate OPPOSING processes, one of them must be the result of compensation. The value that is consistent with pH is the primary process, and the one that is inconsistent with pH is the compensation. For example, an ABG with a low pH, high PaCO2 and high bicarbonate indicates a respiratory acidosis with metabolic compensation.
While oxygen partial pressure represents the patient’s oxygenation status, a normal PaO2 value does not completely rule out respiratory disorder, especially when the patient is receiving supplemental oxygen. PaO2 must be considered in conjunction with PaCO2 value, which reflects pulmonary ventilation, when making conclusion about respiratory functions.