CCRN ABG and Acid-Base Practice: Primary Disorder, Compensation, and Winter's Formula

Acid-base on the CCRN (Adult) test plan

The Adult CCRN test plan for exams on and after November 12, 2025, lists acid-base imbalance and fluid and electrolyte imbalances under Multisystem, which is 16% of the exam. Blood gases also appear inside other categories: a ventilated COPD patient is a respiratory problem, and a patient in septic shock is a multisystem one. AACN says most items are written at the application and analysis levels, so expect a gas embedded in a patient story rather than a bare set of numbers.

The thresholds and formulas below come from the MSD Manual's acid-base chapter (full review March 2025). Use the reference ranges an item gives you when it gives them.

A four-step ABG method for ICU stems

1. Read the pH. MSD defines acidemia as a pH below 7.35 and alkalemia as a pH above 7.45. The pH names the primary direction, even after compensation pulls it toward normal.
2. Find the value that explains the pH. A high PaCO2 explains acidemia (respiratory acidosis), and a low HCO3− explains it too (metabolic acidosis). For alkalemia, look for a low PaCO2 or a high HCO3−.
3. Check the compensation. The other value should move in the same direction by a predictable amount. If it moves too much or too little, a second primary disorder is present.
4. For any metabolic acidosis, calculate the anion gap, then the delta gap, to look for a hidden metabolic alkalosis.

Compensation moves the pH toward its baseline without overshooting into the opposite primary disturbance. A pH within 7.35–7.45 does not by itself establish either complete compensation or a mixed disorder. Check the expected PaCO2 or HCO3− response and the clinical timeline before naming the pattern.

Expected compensation for each primary disorder

Winter's formula worked three times

All three patients have a metabolic acidosis. The question in each is whether breathing is keeping up.

Case 1: appropriate compensation. A patient in septic shock has pH 7.32, PaCO2 32 mm Hg, HCO3− 16 mEq/L.

• Expected PaCO2 = 1.5 × 16 + 8 = 32, so the range is 30 to 34.
• Measured PaCO2 is 32, inside the range. This is a metabolic acidosis with appropriate respiratory compensation.

Case 2: a hidden respiratory acidosis. A patient with severe lactic acidosis, who has been given repeated opioid doses for pain, has pH 7.12, PaCO2 38 mm Hg, HCO3− 12 mEq/L.

• Expected PaCO2 = 1.5 × 12 + 8 = 26, so the range is 24 to 28.
• Measured PaCO2 is 38. A PaCO2 near 40 looks normal, but for this bicarbonate it is far too high. The patient is not ventilating enough to compensate, which is a second, respiratory acidosis. On an exam, that finding points to ventilatory failure and an urgent report, not to "normal breathing."

Case 3: a hidden respiratory alkalosis. An agitated, hyperventilating patient has pH 7.37, PaCO2 18 mm Hg, HCO3− 10 mEq/L.

• Expected PaCO2 = 1.5 × 10 + 8 = 23, so the range is 21 to 25.
• Measured PaCO2 is 18, below the range: a metabolic acidosis plus a primary respiratory alkalosis. The near-normal pH is the trap. MSD warns that in mixed disorders the values may look deceptively normal.

The anion gap and the delta gap, with one worked example

MSD defines the anion gap as Na⁺ − (Cl⁻ + HCO3−), with a typical value of about 12 mEq/L. An elevated gap almost always means a metabolic acidosis, most commonly from ketones, lactate, or toxic alcohols or salicylate. A low albumin lowers the expected gap: MSD adjusts the normal range down by 2.5 mEq/L for each 1 g/dL fall in albumin.

When the gap is high, the delta gap checks for a second process. The rise in the gap (the delta) is added back to the measured HCO3−. If the corrected HCO3− lands above the normal range, a metabolic alkalosis is also present.

Worked with Case 1's chemistry: Na⁺ 138, Cl⁻ 100, HCO3− 16.

• Anion gap = 138 − (100 + 16) = 22, which is elevated.
• Delta gap = 22 − 12 = 10. Corrected HCO3− = 16 + 10 = 26, which is within the normal range. There is no hidden metabolic alkalosis, and the picture fits an anion gap acidosis such as lactic acidosis.

Three ICU cases: shock, vomiting, and COPD on the ventilator

Lactic acidosis in shock. Case 1 is the common one. The 2026 Surviving Sepsis Campaign guidelines suggest measuring lactate, and they suggest against giving sodium bicarbonate to improve hemodynamics in patients with lactic acidemia. That is not a blanket prohibition: SSC 2026 separately suggests bicarbonate for septic shock with pH ≤ 7.2 and AKIN stage 2 or 3 acute kidney injury. For Case 1, the priority is treating the cause of hypoperfusion, which the sepsis and shock guide covers, rather than to correct the number.

Vomiting or nasogastric suction. A patient after three days of high-output nasogastric drainage has pH 7.48, PaCO2 47, HCO3− 34. Starting from an HCO3− of about 24 and a PaCO2 of 40 (Passy's assumed baseline), the HCO3− rose by 10, so MSD's rule predicts a PaCO2 rise of 6 to 7.5, to about 46 to 47.5. The measured 47 fits a metabolic alkalosis with appropriate respiratory compensation, well under the 55 ceiling.

COPD on the ventilator. A patient with severe COPD arrives intubated with PaCO2 70 and HCO3− 36. The PaCO2 is 30 above 40, so chronic compensation predicts an HCO3− rise of 9 to 12, to about 33 to 36. A value of 36 fits a chronic respiratory acidosis with metabolic compensation, which is compatible with chronic compensation but must be checked against earlier gases and the clinical history; one gas does not establish the patient's baseline. If the same PaCO2 came with an HCO3− of 26, the rise would be smaller than even acute compensation predicts (27 to 30), which suggests that there has been no time to compensate or that a metabolic acidosis is also present. The ventilator plan belongs to the provider and respiratory therapist. The nurse's exam task is to recognize which pattern the numbers show and report a change. The respiratory review covers the ventilator side.

Traps that turn a correct formula into a wrong answer

• Using the wrong bicarbonate. MSD notes that the HCO3− on a blood gas is calculated, while the chemistry panel measures it directly. When they disagree, the measured value is considered more accurate.
• Forgetting the albumin before calling a gap normal in a malnourished or critically ill patient.
• Letting the PaO2 name the disorder. Oxygenation matters for the patient, but it does not decide acidosis or alkalosis.
• Calling a normal-range pH reassuring without checking whether PaCO2 and HCO3− fit the expected compensation.

The free 20-question CCRN (Adult) set includes one shock-patient blood gas that asks whether the respiratory response is appropriate. Once the three worked cases above feel routine, run Winter's formula on the shock-patient gas in the free set → and compare your reading with the explanation for each of the four interpretations.