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The PCCN respiratory domain tests more than memorizing normal ABG values. It tests whether you can interpret a gas quickly, decide when oxygen delivery needs to escalate, and spot early deterioration in a progressive-care patient. These questions map to the current AACN PCCN test plan and are written for step-down and telemetry nurses. For the broader exam blueprint, start with the AACN PCCN handbook; to drill under timed conditions, use the free PCCN practice set.
This article is a study aid, not a substitute for unit policy, provider orders, or qualified clinical review. Arterial-blood-gas interpretation and oxygen titration should always be performed within your scope of practice and local protocols.
ABG interpretation step by step
The sequence below is one structured approach. Interpret the gas with clinical history and expected compensation; a pH inside the reference range does not exclude a mixed disorder.
- pH: Is the blood acidemic (<7.35), alkalemic (>7.45), or within 7.35–7.45? This tells you the direction of the primary problem.
- PaCO2: Is the respiratory component acidotic (>45 mmHg) or alkalotic (<35 mmHg)? PaCO2 is the respiratory acid.
- HCO3−: Is the metabolic component acidotic (<22 mEq/L) or alkalotic (>26 mEq/L)? Bicarbonate is the metabolic base.
- Compensation: Compare measured values with the expected response. Use a baseline PaCO2 of 40 mmHg and HCO3− of 24 mEq/L for the calculations below. A bicarbonate value within its reference interval can still represent appropriate acute compensation.
The normal ranges below are the values most commonly cited in critical-care and respiratory references and are the ranges used for every worked item in this article.
| Parameter | Normal range | Notes |
|---|---|---|
| pH | 7.35–7.45 | Unitless; reflects hydrogen-ion concentration |
| PaCO2 | 35–45 mmHg | Respiratory acid; inversely related to alveolar ventilation |
| HCO3− | 22–26 mEq/L | Metabolic base; renal compensation takes days |
| PaO2 | 80–100 mmHg | On room air at sea level; decreases with age and altitude |
| SaO2 | 95–100% | Hemoglobin saturation; the SpO2 pulse-oximeter estimate should usually be within a few percent |
| Base excess | −2 to +2 mEq/L | Quantifies metabolic component independent of PaCO2 |
The four primary disturbances + compensation patterns
For each primary disturbance, the body attempts to compensate. The table shows the expected direction and magnitude of compensation. Values outside these ranges suggest a mixed disorder.
| Primary disorder | Expected compensation |
|---|---|
| Acute respiratory acidosis | HCO3− increases ~1 mEq/L for every 10 mmHg rise in PaCO2 |
| Chronic respiratory acidosis | HCO3− increases 3.5–4 mEq/L for every 10 mmHg rise in PaCO2 |
| Acute respiratory alkalosis | HCO3− decreases ~2 mEq/L for every 10 mmHg fall in PaCO2 |
| Chronic respiratory alkalosis | HCO3− decreases 4–5 mEq/L for every 10 mmHg fall in PaCO2 |
| Metabolic acidosis | PaCO2 decreases ~1.2 mmHg per 1 mEq/L fall in HCO3− (Winter’s formula: expected PaCO2 ≈ 1.5 × [HCO3−] + 8, ±2) |
| Metabolic alkalosis | PaCO2 increases ~0.7 mmHg per 1 mEq/L rise in HCO3− |
A common PCCN trap is calling a value “normal” without asking whether it is appropriate for the primary disorder. A PaCO2 of 40 mmHg is normal in isolation, but in a patient with HCO3− of 15 mEq/L it represents a failure of respiratory compensation — a clinically different finding.
Respiratory scenarios beyond the ABG
Oxygen-delivery escalation
Choose oxygen delivery and ventilatory support according to hypoxemia, work of breathing, airway protection and response to treatment. Devices are not a mandatory ladder: severe deterioration may require immediate emergency airway support rather than sequential trials of every device.
The BTS oxygen guideline recommends an initial SpO2 target of 88–92% for patients with COPD or another risk of hypercapnic respiratory failure pending blood gases, and 94–98% for many other acutely ill adults. Use the patient-specific order and relevant condition guideline, reassess promptly and do not delay treatment of severe hypoxemia.
Asthma and COPD exacerbation priorities
An acute asthma exacerbation requires rapid assessment, inhaled short-acting bronchodilator treatment and oxygen when indicated, with early systemic corticosteroids under the treatment plan. Use the prescribed saturation target; a silent chest, fatigue or rising PaCO2 requires urgent escalation.
For COPD exacerbation, prioritize controlled oxygen titration (target 88–92%), bronchodilators, corticosteroids, and antibiotics when bacterial infection is suspected. Escalate to NIV early if respiratory acidosis or increased work of breathing persists despite initial therapy.
Post-extubation monitoring on stepdown
After extubation, monitor respiratory rate, SpO2, work of breathing, ability to manage secretions, voice quality, and swallowing before oral intake. Stridor, neck swelling, or a new hoarse voice should raise concern for airway edema or vocal-cord injury and should be reported promptly.
Worked practice questions
Each item below is original and written for progressive-care scope. Work the ABGs in the same four-step order every time; the answer callout shows the full math.
Question 1 — ABG interpretation
Q1. A 58-year-old postoperative patient on a morphine PCA has the following ABG on room air: pH 7.29, PaCO2 55 mmHg, HCO3− 25.5 mEq/L. What is the primary acid-base disturbance, and is it compensated?
- A. Metabolic acidosis, uncompensated
- B. Acute respiratory acidosis with an appropriate acute compensatory response
- C. Respiratory acidosis, fully compensated
- D. Metabolic alkalosis, partially compensated
Answer: B. pH 7.29 indicates acidemia and PaCO2 55 indicates a respiratory acidifying process. Using 40 mmHg and 24 mEq/L as baselines, expected acute HCO3− = 24 + (55 − 40)/10 × 1 = 25.5 mEq/L. The bicarbonate response fits an acute respiratory acidosis; it need not leave the reference interval to represent compensation. The pH is still low, so “fully compensated” is misleading. Clinical assessment must also address possible opioid-related hypoventilation.
Question 2 — ABG interpretation
Q2. A patient with severe COPD presents with the following ABG: pH 7.32, PaCO2 70 mmHg, HCO3− 35 mEq/L. How would you classify this disturbance?
- A. Acute respiratory acidosis
- B. Chronic respiratory acidosis with renal compensation
- C. Metabolic alkalosis with respiratory compensation
- D. Mixed respiratory and metabolic acidosis
Answer: B. pH 7.32 indicates acidemia with PaCO2 70 mmHg. Relative to 40 mmHg, the rise is 30 mmHg. Expected chronic HCO3− = 24 + 3 × (3.5–4) = 34.5–36 mEq/L. The measured 35 is consistent with chronic respiratory acidosis with the expected renal response. History and prior gases help confirm the time course.
Question 3 — ABG interpretation
Q3. An anxious patient is hyperventilating after receiving bad news. ABG shows pH 7.49, PaCO2 30 mmHg, HCO3− 22 mEq/L. What is the disturbance?
- A. Acute respiratory alkalosis with an appropriate acute compensatory response
- B. Metabolic alkalosis, uncompensated
- C. Respiratory alkalosis, fully compensated
- D. Metabolic acidosis, partially compensated
Answer: A. pH 7.49 is alkalemic and PaCO2 30 mmHg is low. Expected acute HCO3− = 24 − 2 × (40 − 30)/10 = 22 mEq/L, matching the measured value. This fits acute respiratory alkalosis with its expected acute response; a value within the bicarbonate reference interval does not mean compensation is absent. In practice, assess other causes of hyperventilation rather than assuming anxiety alone.
Question 4 — ABG interpretation
Q4. A patient with diabetic ketoacidosis has pH 7.32, PaCO2 30 mmHg, HCO3− 15 mEq/L. What is the disturbance and is compensation appropriate?
- A. Metabolic acidosis with appropriate respiratory compensation
- B. Metabolic acidosis with inadequate respiratory compensation
- C. Respiratory alkalosis with metabolic compensation
- D. Mixed respiratory acidosis and metabolic acidosis
Answer: A. Step 1: pH 7.32 is acidemia. Step 2: PaCO2 30 mmHg is low, not high, so the primary problem is not respiratory. Step 3: HCO3− 15 mEq/L is low → metabolic acidosis. Step 4: Winter’s formula gives expected PaCO2 = 1.5 × 15 + 8 ± 2 = 30.5 ± 2 = 28.5–32.5 mmHg. The measured PaCO2 of 30 falls within that range, so the hyperventilation is an appropriate respiratory compensation for metabolic acidosis.
Question 5 — ABG interpretation
Q5. A patient with persistent vomiting has pH 7.49, PaCO2 48 mmHg, HCO3− 35 mEq/L. What is the disturbance?
- A. Respiratory acidosis, partially compensated
- B. Metabolic alkalosis with respiratory compensation
- C. Respiratory alkalosis, fully compensated
- D. Metabolic acidosis with respiratory compensation
Answer: B. Alkalemia with HCO3− 35 mEq/L supports metabolic alkalosis. Using baselines of 24 mEq/L and 40 mmHg, the estimated compensatory PaCO2 is 40 + 0.7 × (35 − 24) = 47.7 mmHg. Measured PaCO2 48 is consistent with that approximate response. Compensation rules are estimates and must be interpreted with the clinical picture.
Question 6 — Oxygen-therapy escalation
Q6. A 68-year-old with COPD is on 2 L/min nasal cannula. You find him diaphoretic, using accessory muscles, with SpO2 84% and respiratory rate 28. What is the nurse’s first action?
- A. Draw a stat ABG before changing oxygen
- B. Increase supplemental oxygen and titrate toward the COPD target range
- C. Begin sedation to reduce work of breathing
- D. Wait for the provider to round in 30 minutes
Answer: B. The patient has increased work of breathing and hypoxemia. In COPD, titrate oxygen toward the usual SpO2 target of 88–92% while reassessing work of breathing and mental status. Drawing an ABG first (A) delays treatment of hypoxemia; sedation (C) can precipitate respiratory failure; and waiting (D) is unsafe. Escalate oxygen now, reassess, notify the provider, and prepare for possible noninvasive ventilation if he does not improve.
Question 7 — Asthma exacerbation priorities
Q7. A 32-year-old with asthma is admitted to stepdown with respiratory rate 30, SpO2 91% on room air, and inability to speak in full sentences. Wheezing is audible. An authorized asthma protocol permits immediate inhaled bronchodilator and oxygen treatment. Which listed intervention should the nurse start while obtaining urgent clinical help?
- A. Administer nebulized albuterol and supplemental oxygen
- B. Give an intravenous sedative as the only initial treatment
- C. Call the provider before giving any medications
- D. Obtain a chest X-ray before treatment
Answer: A. The findings require urgent assessment and treatment. Under the protocol stated in the stem, start inhaled albuterol and oxygen to the prescribed target while obtaining clinical help. Do not delay treatment for imaging or merely wait for a callback. Sedation alone does not treat bronchospasm and may worsen respiratory compromise. Additional treatment, including systemic corticosteroids and airway support, follows the urgent clinical assessment.
Question 8 — Post-extubation monitoring
Q8. Four hours after extubation, a stepdown patient develops a hoarse voice, neck swelling, and inspiratory stridor. SpO2 is 94% on 4 L/min nasal cannula. What is the priority nursing action?
- A. Offer the patient ice chips and continue routine monitoring
- B. Notify the provider/rapid response and prepare for possible airway intervention
- C. Increase the nasal cannula to 15 L/min
- D. Administer a sedative to reduce anxiety
Answer: B. Hoarseness, neck swelling, and inspiratory stridor after extubation suggest upper-airway edema or vocal-cord dysfunction. This is an airway emergency until proven otherwise. Notify the provider or rapid response, keep the patient upright, prepare for emergency airway treatment under the responding team’s orders and protocol, and monitor for deterioration. Ice chips (A) are unsafe until swallowing is evaluated; turning a standard nasal cannula to 15 L/min (C) neither creates a high-flow nasal cannula system nor addresses the mechanical airway problem; and sedatives (D) can worsen respiratory drive.
How to fit this into your PCCN study plan
For these teaching cases, connect the gas result with breathing, perfusion and mental status. Assessment, stabilization and emergency escalation may need to happen together; do not delay calling for help until every calculation is complete.
After working these items, move to the free PCCN practice set for a short sample of additional questions with explanations. Then review the full exam blueprint in the AACN PCCN handbook to make sure you are spending your study time proportionally to the test weights.
References
- [1] American Association of Critical-Care Nurses (AACN) (2026). PCCN (Adult) Direct Care Certification Handbook. aacn.org. aacn.org
- [2] American Association of Critical-Care Nurses (AACN) (2026). Frequently Asked Questions About PCCN Certification. aacn.org. aacn.org
- [3] Merck Manual Professional (2026). Acid–base disorders and expected compensation — Merck Manual Professional. Merck Manual Professional; accessed October 3, 2026 (undated web resources use access year). Merck Manual Professional; accessed October 3, 2026 (undated web resources use access year)
- [4] British Thoracic Society (2017). BTS guideline: oxygen in healthcare and emergency settings. British Thoracic Society; accessed October 3, 2026 (undated web resources use access year). British Thoracic Society; accessed October 3, 2026 (undated web resources use access year)
Frequently asked questions
Respiratory content accounts for 14% of scored items on the Adult PCCN exam under the test plan effective February 6, 2024. With 125 scored items, that is roughly 17–18 respiratory questions.
Read the ABG in this order: pH first to identify acidemia or alkalemia; PaCO2 second to identify the respiratory component; HCO3− third to identify the metabolic component; then check whether the opposite parameter has moved in the expected compensatory direction.
These examples use pH 7.35–7.45, PaCO2 35–45 mmHg and HCO3− 22–26 mEq/L. For compensation calculations, use baseline values of 40 mmHg and 24 mEq/L. Interpret laboratory ranges and oxygen values in context; this is not an AACN-mandated reference sheet.
Use the bicarbonate response. In acute respiratory acidosis, HCO3− rises about 1 mEq/L for every 10 mmHg rise in PaCO2. In chronic respiratory acidosis, renal compensation raises HCO3− by 3.5–4 mEq/L for every 10 mmHg rise in PaCO2.
For COPD or another risk of hypercapnic respiratory failure, BTS recommends 88–92% initially pending blood gases. Individualize after assessment and follow the relevant treatment plan. Do not withhold needed oxygen because of the oversimplified “hypoxic drive” explanation.
Hoarseness, neck swelling, inspiratory stridor, increased work of breathing, drooling, and anxiety can all signal upper-airway edema or vocal-cord injury after extubation. These findings should be reported promptly because airway obstruction can progress quickly.
Keep practicing PCCN questions
Try the free PCCN question sampler, then use the explanations to choose what to review next. This is a short practice set, not a full-length exam.
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