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Top 25 ABG and ventilator concepts for the RRT

The calculations and decision rules that show up again and again in RRT prep. They are written to textbook and guideline standards, not any one hospital's protocol. Each one cites a public source.

1. Normal adult ABG ranges. pH 7.35 to 7.45, PaCO₂ 35 to 45 mm Hg, HCO₃^- 22 to 26 mEq/L. Reference ranges vary slightly by lab and age.^[2]
2. Interpret in order. First the pH (acidemia or alkalemia), then PaCO₂ and HCO₃^- to find the primary problem, then compensation, then oxygenation.^[2]
3. Respiratory vs metabolic. Low pH with PaCO₂ above 40 points to respiratory acidosis. High pH with PaCO₂ below 40 points to respiratory alkalosis.^[2]
4. Acute vs chronic respiratory acidosis. HCO₃^- rises about 1 mEq/L for every 10 mm Hg rise in PaCO₂ when it is acute, and about 3.5 to 4 when it is chronic. That is how you spot acute-on-chronic CO₂ retention.^[3]
5. Winter's formula. In metabolic acidosis, expected PaCO₂ = (1.5 × HCO₃^-) + 8 ± 2. If the measured value is higher, there is also a respiratory acidosis. If it is lower, there is also a respiratory alkalosis.^[4]
6. Anion gap. Na⁺ − (Cl^- + HCO₃^-). A high gap points to added acids such as ketoacids or lactate. A normal-gap acidosis points to bicarbonate loss.^[5]
7. Alveolar air equation. PAO₂ = FiO₂(P_B − 47) − PaCO₂/0.8. On room air at sea level with a PaCO₂ of 40, that is about 100 mm Hg.^[6]
8. A–a gradient. PAO₂ − PaO₂. A widened gradient means a gas-exchange problem (V/Q mismatch, shunt, or diffusion). Pure hypoventilation keeps it normal.^[6]
9. Carbon monoxide fools the pulse oximeter. SpO₂ can read falsely normal with carboxyhemoglobin. Measure with CO-oximetry and give 100% oxygen.^[7]
10. Sampling errors. An air bubble pulls the PO₂ toward room-air values (about 150 mm Hg) and lowers the PCO₂. Expel bubbles and analyze promptly.^[8]
11. P/F ratio and ARDS severity (Berlin). On PEEP or CPAP of at least 5: mild 200 to 300, moderate 100 to 200, severe 100 or less.^[9]
12. Predicted body weight. Male 50 + 2.3 × (height in inches − 60). Female 45.5 + 2.3 × (height in inches − 60). The NBRC expects candidates to know these.^[10]
13. Tidal volume. Base it on PBW, not actual weight. The NBRC paper describes 6 to 8 mL/kg PBW for most ventilated patients, with plateau pressure kept under 30 cm H₂O. For ARDS, the ATS/ESICM/SCCM guideline recommends 4 to 8 mL/kg PBW.^[10][11]
14. Peak vs plateau. If peak pressure rises and plateau stays the same, airway resistance went up (secretions, bronchospasm, kinked or bitten tube). If both rise, compliance fell (for example pneumothorax, worsening ARDS, or mainstem intubation).^[12]
15. Airway resistance. Raw = (PIP − Pplat) ÷ inspiratory flow in L/s, measured on constant (square-wave) flow.^[12]
16. Static compliance. C_st = V_T ÷ (Pplat − PEEP). Track the trend. A falling C_st means stiffer lungs or chest wall.^[13]
17. Driving pressure. Pplat − PEEP. In a large ARDS analysis, lower driving pressure was linked to better survival, even within "protective" settings.^[14]
18. Fixing PaCO₂ with minute ventilation. Assuming dead space is unchanged, new rate (or V_E) ≈ current rate × current PaCO₂ ÷ desired PaCO₂. On volume control, change the rate first when V_T is already at a lung-protective size.^[13]
19. Oxygenation knobs vs ventilation knobs. FiO₂ and PEEP (mean airway pressure) change oxygenation. Rate and V_T change CO₂.^[13]
20. Auto-PEEP. Detect it with an end-expiratory hold or flow that does not return to zero. Fix it by giving more time to exhale: lower rate, shorter inspiratory time or higher flow, smaller V_T, and treat bronchospasm.^[13]
21. Lung-protective first. The NBRC paper is explicit: limit V_T and plateau pressure first. Normalizing blood gases is secondary.^[10]
22. NIV in COPD exacerbation. Bilevel NIV is strongly recommended for acute hypercapnic respiratory failure with pH ≤ 7.35. To lower PaCO₂, raise IPAP (more pressure support). To improve oxygenation, raise EPAP or FiO₂.^[16]
23. Weaning screen: RSBI. f ÷ V_T in liters during a brief spontaneous trial. Below about 105 predicts a successful wean.^[15]
24. Cuff pressure. Keep an adult ETT cuff at about 20 to 30 cm H₂O. Too low risks aspiration and leaks. Too high risks tracheal injury.^[17]
25. Where this sits on the exam. Blood gas analysis is listed under Patient Data in the 2027 outline. Ventilator initiation and changes fall under Initiation and Modification of Interventions, which is 50 of the 100 Breadth items.^[1]

Prefer ventilator items? Start free ventilator practice.

Study content only, not clinical advice. Follow your institution's protocols and the licensed provider's orders in practice.

Sources

1. NBRC: The Respiratory Therapy Examination Detailed Content Outline, effective January 2027 (PDF)
2. StatPearls: Arterial Blood Gas
3. StatPearls: Respiratory Acidosis
4. Merck Manual: Winter's Formula for Expected PCO2
5. StatPearls: Anion Gap and Non-Anion Gap Metabolic Acidosis
6. StatPearls: Alveolar Gas Equation
7. StatPearls: Carbon Monoxide Toxicity
8. Baird G. Preanalytical considerations in blood gas analysis. Biochem Med 2013
9. ARDS Definition Task Force. The Berlin Definition. JAMA 2012
10. NBRC: Setting the Tidal Volume in Adults Receiving Mechanical Ventilation (Bocklage & Balk, 2017, PDF)
11. ATS/ESICM/SCCM 2017 Guideline: Mechanical Ventilation in Adult ARDS (PDF)
12. StatPearls: Physiology, Airflow Resistance
13. StatPearls: Ventilator Management
14. Amato MBP et al. Driving Pressure and Survival in ARDS. NEJM 2015
15. Yang KL, Tobin MJ. Indexes predicting the outcome of weaning trials. NEJM 1991
16. ERS/ATS Clinical Practice Guideline: Noninvasive ventilation for acute respiratory failure (2017)
17. StatPearls: Ventilator Safety

Exam facts on this page come from nbrc.org and were last checked on October 30, 2026. The NBRC can change its policies at any time, so confirm details on nbrc.org before you apply or test. Written by Noel Navarro, a practicing respiratory therapist who built TidalPrep.