Guide — Mechanical Ventilation
Noninvasive Ventilation Basics
Noninvasive ventilation can prevent an intubation in the right patient and harm the wrong one. This guide covers what CPAP and bilevel actually do, who benefits, who should never get a mask, and the gas at 1–2 hours that decides whether NIV is working.
10 min read · Mechanical Ventilation
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
Noninvasive ventilation (NIV) is positive-pressure support delivered through a mask instead of an artificial airway. Done well, it buys a patient time to recover without the risks of intubation; done to the wrong patient, it delays a tube that was always coming. Two situations carry the strongest evidence: the COPD exacerbation with respiratory acidosis, and cardiogenic pulmonary edema.
Patient selection is the whole game. Pick the right candidate, set sensible pressures, manage the mask, and reassess on a clock — those four steps are most of NIV.
Key Concepts — CPAP vs Bilevel
| Modality | What It Delivers | Primary Effect | Typical Use |
|---|---|---|---|
| CPAP | One continuous pressure (5–10 cmH₂O) | Splints alveoli open, raises functional residual capacity, and offloads the heart (preload and afterload). | Cardiogenic pulmonary edema; OSA |
| Bilevel (BiPAP) | IPAP + EPAP; the gap is pressure support | Pressure support augments each breath to improve ventilation and CO₂ clearance; EPAP supports oxygenation. | COPD exacerbation with hypercapnia |
The one idea to keep straight: the difference between IPAP and EPAP isthe pressure support, and pressure support is what moves CO₂. CPAP has a single pressure and therefore no pressure support, so it does little for ventilation — it earns its keep by recruiting alveoli and unloading the heart. When the problem is hypercapnia, you need the bilevel gap.
Assessment & Findings — Patient Selection
Before a mask goes on, the patient has to clear two gates: a real indication, and no contraindication. The strongest-evidence indications first.
| Indication | Notes | Evidence |
|---|---|---|
| COPD exacerbation with respiratory acidosis | pH ≤ 7.35 and PaCO₂ > 45; benefit clearest at pH 7.25–7.35 | Strongest |
| Cardiogenic pulmonary edema | CPAP or bilevel; rapid preload and afterload relief | Strongest |
| Immunocompromised respiratory failure | Avoids intubation-related infection risk | Strong |
| Post-extubation prevention (high-risk) | Applied prophylactically, not as rescue of established failure | Strong |
| Obesity hypoventilation syndrome | Chronic hypercapnia; bilevel | Supportive |
| Contraindication | Why It Matters |
|---|---|
| Respiratory or cardiac arrest, apnea | NIV cannot ventilate an apneic or pulseless patient — intubate. |
| Cannot protect the airway or clear secretions | A mask offers no airway protection; aspiration risk is high. |
| Active vomiting or upper GI bleed | High risk of aspiration under positive pressure. |
| Facial trauma or recent upper-airway/GI surgery | Mask cannot seal; pressure threatens fresh surgical sites. |
| Hemodynamic instability | Positive pressure can deepen hypotension; the patient needs a definitive airway. |
| Severely depressed consciousness | Cannot protect the airway (relative — monitored CO₂ narcosis in COPD may be an exception per protocol). |
| Undrained pneumothorax | Positive pressure can enlarge it into a tension pneumothorax. |
RT Priorities & Interventions
- Start bilevel sensibly. IPAP 8–12 / EPAP 4–5 cmH₂O is a typical start. Titrate IPAP up in 2-cmH₂O steps toward a tidal volume around 6–8 mL/kg and a falling PaCO₂; raise EPAP for oxygenation and to stent open obstructed airways.
- Start CPAP for edema.5–10 cmH₂O, commonly 8–12 in cardiogenic pulmonary edema. Titrate FiO₂ to the saturation target — 88–92% in COPD.
- Get the mask right. A good seal without crushing pressure is most of the work: fit the interface, pad the bridge of the nose, manage leak, and check skin frequently.
- Coach and protect the airway. Talk the patient through the first minutes — anxiety and asynchrony sink NIV. Keep suction ready and the head of the bed up for aspiration risk.
- Reassess at 1–2 hours with a gas. Improving pH, respiratory rate, and comfort means continue. No improvement — or a rising PaCO₂ and falling mental status — means escalate. NIV must never delay a needed intubation.
Common Pitfalls
- Using NIV to rescue established post-extubation failure. The evidence supports preventionin high-risk patients applied early — not rescuing a patient already failing after extubation, where it can delay reintubation and worsen outcomes.
- Ignoring a contraindication to buy time. A vomiting, obtunded, or hemodynamically unstable patient needs an airway, not a mask.
- Chasing oxygenation with IPAP. IPAP minus EPAP is pressure support, which drives ventilation and CO₂. EPAP (like CPAP) is the oxygenation lever — confusing the two leads to wrong titration.
- Tolerating a large leak. Leak destroys triggering and pressure delivery; fix the mask before turning up the numbers.
- Letting NIV run past its trial. If the 1–2 hour gas is not better, the decision is escalate — not “give it another few hours.”
Board Exam Pearls
- The classic NIV win is the COPD exacerbation with respiratory acidosis (pH 7.25–7.35, PaCO₂ > 45). It lowers intubation rates and mortality.
- Cardiogenic pulmonary edema responds to CPAP — the positive pressure cuts preload and afterload and recruits flooded alveoli.
- Bilevel = IPAP + EPAP; the difference (pressure support) does the ventilation. CPAP is a single pressure and does not actively ventilate.
- Absolute “do not use NIV”: apnea or arrest, cannot protect the airway, active vomiting or GI bleed, hemodynamic instability, and an undrained pneumothorax.
- Reassess with a gas at 1–2 hours. No improvement → intubate; do not let the mask delay the tube.
FAQ
What is the difference between CPAP and bilevel?
CPAP delivers one continuous pressure throughout the breath; it splints alveoli open and unloads the heart, which is why it works in cardiogenic pulmonary edema. Bilevel delivers a higher inspiratory pressure (IPAP) and a lower expiratory pressure (EPAP); the gap between them is pressure support, which augments each breath and clears CO₂. If the problem is ventilation and hypercapnia, you need the bilevel gap.
Which patients benefit most from NIV?
The strongest evidence is in two groups: COPD exacerbations with respiratory acidosis (pH 7.25–7.35, PaCO₂ > 45) and cardiogenic pulmonary edema. NIV also helps immunocompromised patients in respiratory failure and prevents reintubation in selected high-risk patients when applied early — as prevention, not as rescue of established failure.
When should I not use NIV?
Avoid it when the patient cannot tolerate or be safely supported by a mask: respiratory or cardiac arrest, apnea, inability to protect the airway or clear secretions, active vomiting or upper GI bleed, facial trauma or recent upper-airway surgery, hemodynamic instability, and an undrained pneumothorax. Severely depressed consciousness is a relative contraindication, though monitored CO₂ narcosis in COPD is sometimes an exception per protocol.
How soon should NIV start working?
Reassess within 1–2 hours, and include an arterial blood gas. Improving pH, a slowing respiratory rate, and a more comfortable patient mean it is working — continue. If the gas and the patient are not better, escalate to intubation. The danger of NIV is not the mask itself but using it to delay an intubation the patient needs.
Check the gas that decides NIV success
pH and PaCO₂ are what select a patient for bilevel and what tell you, an hour or two later, whether to continue or intubate. Practice reading the acidosis that starts NIV and the follow-up gas that ends it.
Open the ABG Interpreter →Related Resources
Sources
- Rochwerg B, Brochard L, Elliott MW, et al. Official ERS/ATS clinical practice guidelines: noninvasive ventilation for acute respiratory failure. Eur Respir J. 2017;50(2):1602426.
- Global Initiative for Chronic Obstructive Lung Disease. Global Strategy for Prevention, Diagnosis and Management of Chronic Obstructive Pulmonary Disease (current annual report). GOLD.
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Noninvasive ventilation chapter.