Which way did pH move?
Below 7.35 is acidemia. Above 7.45 is alkalemia. A normal pH does not clear the patient—check both drivers.
Free interactive simulatorMove CO₂ or bicarbonate.
Move CO₂ or bicarbonate.
Watch the other system answer.
Load a patient or build any ABG, then read it in three calls: pH, cause, compensation. The explanation fills in as you get each one right.
One relationship, not four listsCO₂ is respiratory acid. HCO₃⁻ is metabolic base. Whichever changes first, the other system moves in the same direction to protect pH.
Arterial blood gas
Drag either driver
Arterial pH7.40
PaCO₂Carbon dioxideLungs · minutesNormal
normal 35–45
HCO₃⁻BicarbonateKidneys · daysNormal
normal 22–26
Your ABG
A custom gas. Read it in three calls.
Make the three calls first. What to look for, what to do and what to expect ordered fill in for the disorder you find.
Normal values
Drag both grips in to where normal starts and ends, then check. Switch to View to see the bands.
pHArterial pH7.00–7.70
PaCO₂Carbon dioxide15–80 mmHg
HCO₃⁻Bicarbonate5–45 mEq/L
PaO₂Oxygen40–120 mmHg
SaO₂Saturation70–100 %
Reference ranges vary slightly between laboratories and textbooks. These match the ranges used across NurseSavvy.
Teaching model, not a clinical calculator. pH comes from Henderson–Hasselbalch; compensation bands use the common bedside rules. Real interpretation depends on the sample, albumin, oxygenation, timing, clinical context and your lab’s own reference ranges.
Stop memorizing four disconnected boxes
Acid–base is one ratio with two owners. The lungs control CO₂, the kidneys control bicarbonate, and pH tells you which side is winning.
Which value explains it?
ROME: Respiratory Opposite, Metabolic Equal. CO₂ moves opposite pH; bicarbonate moves with it. The matching value is the cause.
Did the other system answer?
Name it uncompensated, partially or fully compensated. Then check the size of the response: outside the expected band exposes a mixed disorder.
Clinical reference points: normal ranges, directional rules, and expected compensation were checked against the Merck Manual compensation table, American Thoracic Society ABG guide, and NCBI’s acid–base physiology review.
The compensation rules this simulator uses
These are screening rules for simple disorders. The simulator shows the expected band and places the measured response on it, so the arithmetic becomes a picture.
| Primary disorder | Primary change | Expected response |
|---|---|---|
| Metabolic acidosis | ↓ HCO₃⁻ | PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 |
| Metabolic alkalosis | ↑ HCO₃⁻ | PaCO₂ rises 0.6–0.75 per 1 mEq/L HCO₃⁻ rise |
| Respiratory acidosis | ↑ PaCO₂ | HCO₃⁻ rises 1–2 acute; 3–4 chronic per 10 mmHg CO₂ |
| Respiratory alkalosis | ↓ PaCO₂ | HCO₃⁻ falls 1–2 acute; 4–5 chronic per 10 mmHg CO₂ |
Rules of thumb vary slightly by reference. Compensation is physiologically limited and does not normalize every severe disturbance.
The two “normal pH” traps
One problem, enough time
A chronic CO₂ retainer can have high PaCO₂ and high HCO₃⁻ with pH close to normal. The abnormal pair tells the story even when pH looks comfortable.
Two problems, opposite pulls
Low HCO₃⁻ and a PaCO₂ lower than Winter’s range can pull pH in opposite directions. A near-normal result can hide two active disorders.
Questions students ask
How do I read an ABG in three steps?
First decide whether the pH is acidemic, normal, or alkalemic. Second identify whether PaCO₂, the respiratory acid, or bicarbonate, the metabolic base, explains that direction. Third calculate the expected response of the other system. A value outside that expected range indicates a second acid–base process.
What does compensation mean in acid–base balance?
Compensation is the other organ system moving in a direction that limits the pH change. The lungs alter PaCO₂ within minutes when a metabolic disorder starts. The kidneys change bicarbonate more slowly when a respiratory disorder persists. Simple compensation moves pH toward normal but does not overshoot to the opposite side.
What is the difference between uncompensated, partially compensated and fully compensated?
Uncompensated means the pH is abnormal and the other system has not moved out of its normal range yet. Partially compensated means the pH is still abnormal but the other value has moved in the helping direction. Fully compensated means both PaCO₂ and bicarbonate are abnormal while the pH has returned to 7.35–7.45. Compensation never overshoots pH to the opposite side.
How does Winter’s formula detect a mixed disorder?
For metabolic acidosis, expected PaCO₂ is 1.5 times bicarbonate plus 8, with a range of plus or minus 2 mmHg. Measured PaCO₂ above that range means an added respiratory acidosis; below the range means an added respiratory alkalosis.
How can the pH be normal when the patient has an acid–base disorder?
The primary problem and compensation may pull pH close to normal, as in chronic respiratory acidosis. Two opposing primary disorders can also create a deceptively normal pH. Always inspect PaCO₂ and bicarbonate even when pH is between 7.35 and 7.45.
What is the anion gap?
The common potassium-free calculation is sodium minus the sum of chloride and bicarbonate. A high gap in metabolic acidosis suggests unmeasured anions from added acids, such as lactate or ketoacids. A normal gap often reflects bicarbonate loss replaced by chloride. Albumin and the laboratory’s own reference range affect interpretation.
Is this ABG simulator a clinical calculator?
No. It is a nursing education model that makes directional relationships and common compensation rules visible. Real interpretation depends on the specimen, timing, albumin, oxygenation, comorbidities, and the full clinical picture.
Turn the pattern into exam-day instinct
Use the simulator for the relationship, then test it against 4,600+ NCLEX questions with rationales. Free to start, no card required.