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Reading an Arterial Blood Gas

The arterial blood gas is the single test that ties this all together. Learn what each number means — pH, PaCO2, PaO2, bicarbonate — and a simple order for reading it.

What the test measures

An arterial blood gas (ABG) is a small sample of blood taken from an artery — usually at the wrist — and analysed within minutes. Unlike a fingertip pulse oximeter, which estimates only oxygen saturation, the ABG measures the actual pressures of the gases and the blood's acidity directly. Four numbers carry most of the meaning.

  1. pH — the overall acidity. Normal is about 7.35–7.45. Below is acidic, above is alkaline.
  2. PaCO2 — the respiratory number. Normal about 35–45 mmHg. Set by breathing.
  3. bicarbonate (HCO3⁻) — the metabolic number. Normal about 22–26 mmol/L. Set mainly by the kidneys.
  4. PaO2 — the oxygen pressure. Normal about 80–100 mmHg breathing room air at sea level.

A simple reading order

Read an ABG the same way every time and it stops being intimidating. Look at pH first to know the direction of trouble, then ask whether the lungs or the kidneys explain it.

Worked example — reading an ABG

Given:  pH 7.30   PaCO2 60 mmHg   HCO3- 28 mmol/L   PaO2 70 mmHg

Step 1  pH 7.30 (< 7.35) -> ACIDIC. The problem is an acidosis.

Step 2  Which number explains a LOW pH?
        PaCO2 60 is HIGH (normal 35-45). High CO2 = acid.
        -> The lungs are the culprit: a RESPIRATORY acidosis.

Step 3  Is anyone compensating?
        HCO3- 28 is mildly HIGH (normal 22-26).
        A high bicarbonate pushes pH UP, opposing the acidosis.
        -> The kidneys are partially compensating.

Step 4  Oxygen?
        PaO2 70 is LOW (normal 80-100) -> mild hypoxemia,
        as expected when CO2 builds up and crowds out O2.

Reading:  Respiratory acidosis with partial metabolic
          compensation, plus mild hypoxemia.
          Classic picture of hypoventilation
          (e.g. a COPD flare or sedative effect).
A step-by-step ABG read. The same four-step order works for every gas.

Oxygen vs CO2 — two different problems

Beginners often blur oxygen and CO2 together, but they answer different questions. A low PaO2 (hypoxemia) tells you the lungs are not loading enough oxygen onto the blood. A high PaCO2 (hypercapnia) tells you the body is not clearing enough CO2 — that is, ventilation is failing. A patient can have one problem, the other, or both, and the treatment differs.