Renal and respiratory

Acid-base map

Drag the numbers and watch where the patient lands. The compensation formulas are here, but they appear at the end of the reasoning rather than the start, which is the order you will need them in.

Where the patient sits

The map

Normal

The curved lines are carbon dioxide isobars: every point on one has the same PaCO2. The dashed one is the patient’s. Change carbon dioxide and the point slides to a different curve; change bicarbonate and it moves along the one it is on.

Drag these

The four numbers

PaCO240 mm HgWhat the lungs are doingBicarbonate24 mEq/LWhat the kidneys and the acid load are doingSodium140 mEq/LOnly used for the anion gapChloride104 mEq/LThe value that decides whether the gap opens

How long has this been going on?

This changes nothing about the gas and everything about its meaning. The kidney needs days, so the same PaCO2 is an emergency on day one and a baseline on day five.

Step by step

What these numbers mean

7.40

within range

Anion gap

Na − Cl − HCO3, normal up to 12

Compensation

Not applicable

no primary disorder

One. The pHAt 7.40 the pH is inside the normal range, which does not by itself mean nothing is wrong. Two abnormal values either side of normal are either full compensation or two opposing disorders.
Two. The primary disorderNo primary disorder
Four. The anion gapA gap of 12 is normal. If there is an acidosis here it is the kind where bicarbonate is swapped for chloride: gut losses or a renal tubular acidosis.

The case

Normal

A healthy adult at rest.

Every other panel is read against this one. Anchor to 7.40, 40 and 24.

The formulas here are the conventional teaching versions and the ranges are the ones a question stem will use. They are for learning the method, not for treating a patient, and nothing here is medical advice. How this content is written.