Step Four: Oxygenation Is a Separate Question

A gas can tell you two unrelated things at once, and step four is where the second one gets read. The pH, the carbon dioxide and the bicarbonate describe acid base status. The oxygen values describe something else, and a patient can be in trouble on one axis while looking fine on the other.

The two numbers

StatPearls gives a normal PaO2 of 75 to 100 mmHg and a normal oxygen saturation of 95 to 100%. Those are the values this step runs on.

They are not the same measurement. The PaO2 is the pressure of oxygen dissolved in arterial blood. The saturation is the percentage of haemoglobin binding sites carrying oxygen, and the two can move differently in the same patient.

Reference ranges vary slightly between laboratories and across age groups, which the source says in its own words. Read the range printed on your patient's report before you trust the one in your notes.

The rounding difference you will notice

Some sources round the lower end of the PaO2 range upward and publish 80 to 100 mmHg instead. That is a rounding convention rather than a disagreement about physiology, and the difference is trivial in practice.

It is worth naming anyway. You will meet both figures, and without an explanation you might assume you learned it wrong. Two sources, one fact, slightly different floors.

That is ordinary in reference material, and it is the same reason the laboratory variation caveat exists at all.

Two axes, read side by side

Here is why step four is separate rather than an afterthought. Acid base status and oxygenation come out of overlapping machinery, and they are not the same finding.

A patient can have a perfectly normal pH and a PaO2 well below its range. Another can have an ugly pH and oxygenate adequately. Neither value predicts the other, so reading one and inferring the other is guessing.

This is also why a normal pH never ends the reading. Step one gave you a side, steps two and three gave you a mechanism, and step four is still outstanding. The full order sits in arterial blood gases in a four step reading order.

When oxygenation outranks the acid base finding

Sometimes the oxygen value is the urgent one and the label can wait. Airway and breathing sit ahead of almost everything else in the standard prioritisation frames, and a failing oxygen value is a breathing problem before it is anything else.

Ask yourself two things. Is this patient moving air adequately right now? Is the oxygen value trending down across the results in front of you?

If either answer is bad, interpreting the pH is not what needs to happen next. The framework behind that ordering is in prioritization framework, ABCs and safety.

Trends beat snapshots here more than anywhere else on the gas. One PaO2 is a number. Two of them with a time between them is a direction.

What these two numbers do not tell you

Oxygenation and ventilation are not the same thing, and this is where the confusion usually begins. The oxygen values tell you how much oxygen is reaching the blood. The carbon dioxide tells you how well air is being moved.

A patient can oxygenate acceptably on supplemental oxygen while retaining carbon dioxide badly. That is the trap. The saturation on the monitor reads reassuringly while the ventilation fails underneath it.

So a saturation is never a substitute for the carbon dioxide on the gas, and never a substitute for looking at the patient. Look at the patient. Watch the effort, the rate, the chest rise and the level of consciousness alongside the number.

A worked step four

Take a gas with a pH of 7.50, a PaCO2 of 28 mmHg, a bicarbonate of 23 mEq/L, a PaO2 of 55 mmHg and a saturation of 88%.

The acid base reading is straightforward. High pH, low carbon dioxide, bicarbonate untouched, so this is an uncompensated respiratory alkalosis. But the oxygen values are the story, because both sit well below their published ranges.

That combination has a recognisable shape. A hypoxic patient breathing hard blows off carbon dioxide as a consequence, so the alkalosis is downstream of the oxygen problem rather than the thing to treat.

Read it in the order the method gives you and you still arrive there. Read the pH alone and you would have called it anxiety.

Where this hands off

Recognising a patient heading toward respiratory failure is a bigger job than reading two values, and it leans on the bedside picture as much as on the gas. Recognizing impending respiratory failure covers that ground.

Once oxygenation is assessed, the acid base findings still need a name and a clinical story. Respiratory acidosis and what the patient looks like and respiratory alkalosis and the hyperventilating patient are the two disorders that most often sit alongside an abnormal oxygen value.

Practising step four

Add one line to the end of every gas you interpret, even when the oxygen values are unremarkable. Say the PaO2 with its unit, say the saturation with its unit, and say whether they are adequate for this patient.

Saying it out loud when it is normal is what makes you notice when it is not. That is the whole discipline. Two numbers, one sentence, every single time.

There is a second habit worth building alongside it. Write down what the patient was receiving when the gas was drawn, because a PaO2 inside its published range on a high oxygen delivery is a very different finding from the same value on room air.

Stems do this deliberately. The oxygen delivery is mentioned once, early, and the values arrive several sentences later, by which point most readers have stopped holding it. Go back and check what the patient was on before you decide the oxygenation is adequate.