Non-invasive Ventilation
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Direct answer
Non-invasive ventilation (NIV) supports ventilation through a tight-fitting mask rather than an endotracheal tube, as continuous positive airway pressure (CPAP) for oxygenation problems such as cardiogenic pulmonary oedema, or bilevel positive airway pressure (BPAP) with separate inspiratory and expiratory settings for ventilatory failure. Its strongest evidence is in the acidotic exacerbation of chronic obstructive pulmonary disease — pH 7.25 to 7.35 with PaCO2 above 45 mmHg — where it reduces intubation and mortality. It succeeds only in selected, conscious patients and must be abandoned for intubation when a one- to two-hour trial fails.
What you must remember
- Modes and physiology: CPAP holds a single positive pressure throughout the cycle, recruiting alveoli and easing oxygenation (cardiogenic oedema, obstructive sleep apnoea); BPAP adds inspiratory pressure above the expiratory level, augmenting tidal volume and unloading the muscles to blow off carbon dioxide.
- Firm indications: chronic obstructive pulmonary disease exacerbation with respiratory acidosis (pH below 7.35 with PaCO2 above 45 mmHg) — intubation and mortality fall; cardiogenic pulmonary oedema (CPAP preferred); immunocompromised patients with type 1 failure, avoiding ventilator pneumonia; weaning and post-extubation support in hypercapnic COPD; and obesity-hypoventilation decompensation.
- Contraindications: respiratory or cardiac arrest, coma or inability to protect the airway, copious secretions or aspiration risk, untreated pneumothorax (drain first), facial deformity, burns or trauma preventing a mask seal, recent upper gastrointestinal surgery, haemodynamic instability or shock, and patient refusal — confusion is a relative contraindication warranting a short monitored trial.
- Practical setup: spontaneous bilevel mode, starting inspiratory pressure around 10 to 15 and expiratory around 4 to 5 cm of water, titrated to respiratory rate, comfort and blood gases, with oxygen bled into the circuit to the target saturation (88 to 92 per cent in chronic retainers).
- Trial and reassessment: judge response at one to two hours — improvement in pH, respiratory rate, heart rate and conscious level predicts success; deteriorating gases, rising rate or distress mandates intubation without delay; severe acidosis at baseline (pH below about 7.20) lowers success and should be managed where intubation is immediate.
- Complications and pitfalls: mask leak and pressure ulcers (dressings, alternating masks), gastric distension, aspiration, mucosal dryness (humidifier), eye irritation, claustrophobia, and the greatest hazard — persisting too long with a failing trial.
- Acute severe asthma and pneumonia with heavy secretions are not routine indications; NIV also serves selected do-not-intubate pathways with clear goals-of-care documentation.
A one-hour trial, watched closely
A 68-year-old smoker with an exacerbation arrives drowsy but rousable, pH 7.28, PaCO2 68, breathing at 28. He is conscious, protecting his airway, haemodynamically stable, with no pneumothorax — he clears the contraindication screen, so bilevel ventilation starts now rather than after he tires further. Fit the mask carefully, set inspiratory 10 to 15 with expiratory 4 to 5 cm of water, bleeding oxygen toward 88 to 92 per cent. Stay with him early: leaks, comfort and synchrony decide tolerance. At one to two hours, judge with numbers and signs together — a rising pH, falling respiratory rate and heart rate, clearer conscious level predict success. If the rate climbs, gases worsen or he becomes drowsier, abandon the trial and intubate without sentiment: persisting too long with a failing trial is the greatest hazard of the technique. If he improves, step the pressures down and think ahead to weaning and post-extubation support.
How the exam frames NIV
Two vignettes dominate: COPD exacerbation with gases asking for the intervention — bilevel for pH 7.25 to 7.35 — and florid pulmonary oedema asking for the mode, CPAP. The contraindication list is the favourite multi-select: Two errors recur: interchanging CPAP and BPAP, and misremembering the trigger as the carbon dioxide value or saturation, when it is the pH.
Frequently asked questions
Which is the strongest evidence-based indication?
The acidotic exacerbation of COPD — pH below 7.35 with PaCO2 above 45 mmHg — where bilevel ventilation reduces intubation and mortality.
What is the difference between CPAP and BPAP?
CPAP applies one continuous pressure for oxygenation, while BPAP adds an inspiratory pressure above the expiratory level to augment ventilation and clear carbon dioxide.
What are the main contraindications?
Arrest or coma, unprotected airway, copious secretions, haemodynamic instability, untreated pneumothorax, facial barrier to a mask seal and recent upper gastrointestinal surgery.
How is response to NIV judged?
By clinical improvement and repeat gases at one to two hours — a rising pH with falling respiratory rate predicts success; deterioration mandates intubation.
Which settings are typical starting values?
Inspiratory pressure 10 to 15 cm of water over an expiratory pressure of 4 to 5, titrated to rate, comfort and blood gases.
Why does the pneumothorax need drainage first?
Positive airway pressure enlarges a pneumothorax and risks tension physiology, so an intercostal drain precedes mask ventilation.