What is mechanical ventilation and when is it indicated?
Invasive mechanical ventilation delivers positive-pressure breaths through an endotracheal or tracheostomy tube. It does not treat the disease — it buys time and rests the respiratory muscles while the cause is treated. Non-invasive ventilation (NIV) delivers the same positive pressure through a tight-fitting mask.
StatPearls (Invasive Mechanical Ventilation) groups the indications into four categories:
- Airway compromise — the patient cannot protect or maintain the airway (e.g. coma, airway oedema, burns).
- Hypoventilation — hypercapnic (type 2) failure, e.g. COPD, neuromuscular weakness, drug overdose.
- Hypoxaemic respiratory failure — type 1 failure, e.g. pneumonia, pulmonary oedema, ARDS.
- Increased ventilatory demand — e.g. severe metabolic acidosis or sepsis that the patient cannot sustain.
What is the difference between volume control and pressure control?
This is the first decision on any ventilator. In volume-controlled ventilation the machine always delivers the same volume, and the pressure it generates depends on lung compliance. In pressure-controlled ventilation the machine always delivers the same pressure, and the tidal volume depends on compliance (StatPearls — Ventilator Management).
| Feature | Volume control | Pressure control |
|---|---|---|
| Set (independent) | Tidal volume | Inspiratory (driving) pressure |
| Varies (dependent) | Airway pressure | Tidal volume, breath to breath |
| If compliance falls (stiff lung, pneumothorax, secretions) | Pressures rise — watch the high-pressure alarm | Tidal volume falls — watch minute ventilation and CO2 |
| Main advantage | Guaranteed tidal volume and minute ventilation | Limits peak pressure; decelerating flow |
| Main risk | Barotrauma if pressures are not monitored | Hypoventilation if compliance worsens |
How do CMV, assist-control and SIMV differ?
These modes differ in who triggers the breath and how many breaths are fully supported.
| Mode | Who triggers | Breaths above the set rate | Typical use / problem |
|---|---|---|---|
| CMV (controlled mandatory ventilation) | Machine only — patient triggering is not possible | None — the patient is passive | Deeply sedated or paralysed patients, e.g. under general anaesthesia |
| AC (assist-control) | Machine (timer) or patient effort | Every triggered breath gets the full set breath | Most widely used ICU mode; risk of hyperventilation, breath stacking, auto-PEEP, respiratory alkalosis |
| SIMV (synchronised intermittent mandatory ventilation) | Machine, synchronised with patient effort | Spontaneous breaths get no full breath — volume depends on patient effort and compliance (pressure support can be added) | Listed among weaning modes; usually combined with pressure support for the spontaneous breaths |
AC is the most commonly used mode in many intensive care units because it is easy to use and has only four main settings — rate, tidal volume, FiO2 and PEEP (StatPearls). Its weakness is the anxious or tachypnoeic patient: every breath receives a full tidal volume, so minute ventilation rises and the patient can become alkalotic or develop air trapping.
What are pressure support, CPAP and PEEP?
| Term | What it is | Key point |
|---|---|---|
| PSV (pressure support ventilation) | Each patient-triggered breath is augmented by a set inspiratory pressure; no set rate or tidal volume | Used for weaning; tidal volume is unreliable → risk of CO2 retention if drive is poor |
| CPAP (continuous positive airway pressure) | One continuous pressure throughout the cycle; the patient does all the breathing | Corresponds to PEEP alone — improves oxygenation, does not assist ventilation |
| PEEP (positive end-expiratory pressure) | Pressure left in the airways at the end of expiration | Raises FRC, prevents alveolar collapse, reduces atelectrauma; usual starting level 5 cmH2O |
| Auto-PEEP (intrinsic PEEP) | Trapped gas because expiration is incomplete before the next breath | Seen in asthma/COPD and fast rates; raises intrathoracic pressure → hypotension |
What is BiPAP / non-invasive ventilation and when is it used?
Non-invasive positive pressure ventilation comes in two main forms. BiPAP (bilevel positive airway pressure) sets an inspiratory pressure (IPAP) and a lower expiratory pressure (EPAP); the difference between them is the pressure support that boosts tidal volume and improves ventilation. CPAP has a single continuous pressure that corresponds to PEEP and does not add pressure support.
| Aspect | Detail |
|---|---|
| Use it when | Acute hypercapnic respiratory acidosis (pH ≤ 7.35 with PaCO2 ≥ 45 mmHg), severe dyspnoea with accessory muscle use, persistent hypoxaemia on oxygen |
| Starting settings | IPAP about 8–10 cmH2O and EPAP about 4–5 cmH2O, titrated to response |
| Avoid when | Respiratory or cardiac arrest, haemodynamic instability, inability to fit the mask, excessive secretions, high aspiration risk, inability to protect the airway, uncooperative patient |
| Why it matters | Decreases mortality and the need for intubation in COPD exacerbations |
What are the key initial ventilator settings?
| Setting | Usual starting point | Notes |
|---|---|---|
| Tidal volume (VT) | 6–8 mL/kg predicted (ideal) body weight | Scaled to height and sex, not actual weight; 4–8 mL/kg PBW when lung protection is needed |
| Respiratory rate | About 12–16 breaths/min | Adjust to PaCO2 and pH |
| FiO2 | Often start at 100%, then titrate down | Use the minimum FiO2 that keeps SpO2 about 90–96% |
| PEEP | 5 cmH2O | Raise in hypoxaemic failure such as ARDS |
| Plateau pressure | Keep below 30 cmH2O | Measured with an inspiratory pause; reflects alveolar pressure |
Peak pressure is the highest pressure during inspiration and rises with airway resistance (bronchospasm, kinked or blocked tube) as well as stiff lungs. Plateau pressure is measured during an inspiratory pause when flow is zero, so it reflects the alveoli. A high peak with a normal plateau means a resistance problem; a high peak with a high plateau means a compliance problem (stiff lung, pneumothorax, ARDS). Driving pressure = plateau pressure − PEEP.
What is lung-protective ventilation in ARDS?
The landmark ARDS Network trial (NEJM, 2000) randomised 861 patients with acute lung injury/ARDS to a traditional tidal volume of 12 mL/kg predicted body weight with plateau pressure up to 50 cmH2O, or a lower tidal volume of 6 mL/kg PBW with plateau pressure ≤ 30 cmH2O. The trial was stopped early because mortality was lower with low tidal volumes — 31.0% vs 39.8% — and ventilator-free days were greater.
PBW (men) = 50 + 0.91 × (height in cm − 152.4) | PBW (women) = 45.5 + 0.91 × (height in cm − 152.4)
Tidal volume in ARDS = 6 mL × PBW. Lung size follows height and sex, not body weight — so an obese patient does not get a bigger tidal volume.
- Tidal volume 6 mL/kg PBW (range 4–8), not actual body weight.
- Plateau pressure ≤ 30 cmH2O to avoid volutrauma from overdistension.
- Driving pressure (plateau − PEEP): StatPearls advises reducing tidal volume towards 4 mL/kg if it exceeds 14.
- PEEP to keep alveoli open and reduce atelectrauma.
- Prone positioning for severe ARDS: in the PROSEVA trial (P/F < 150 mmHg on FiO2 ≥ 0.6), sessions of at least 16 hours cut 28-day mortality from 32.8% to 16.0%.
What are the complications of positive-pressure ventilation?
- Ventilator-induced lung injury — barotrauma (pneumothorax, pneumomediastinum), volutrauma (overdistension) and atelectrauma (cyclic collapse and reopening). Lung-protective settings prevent all three.
- Haemodynamic effects — positive pressure raises intrathoracic pressure, decreasing right and left ventricular preload (and LV afterload), which can drop the blood pressure.
- Auto-PEEP / air trapping — air trapped in the alveoli raises intrathoracic pressure; it is a risk in obstructive disease and when the patient breathes fast on assist-control (breath stacking).
- Ventilator-associated events and pneumonia — a deterioration in respiratory status after a period of stability on the ventilator.
How is a patient weaned from the ventilator?
Weaning starts with a daily screen for readiness, then a spontaneous breathing trial (SBT). StatPearls (Ventilator Weaning) lists the readiness questions:
- Has the disease that led to ventilation resolved or improved?
- Haemodynamically stable — no shock, no vasopressor requirement, no significant arrhythmia.
- Oxygenation adequate — FiO2 below 50% and/or a low PEEP requirement.
- Awake and communicative enough to protect the airway.
| Item | Detail |
|---|---|
| How | Minimal support: pressure support about 5–8 and/or a small PEEP, or a T-piece / zero-support trial |
| Duration | At least 30 minutes; tolerating 30–120 min gives about an 80% chance of successful extubation |
| Pass criteria | Respiratory rate < 35, no significant rise or fall in blood pressure, SpO2 > 90% |
| RSBI (Tobin index) | Respiratory rate ÷ tidal volume (in litres); < 105 predicts success, > 105 predicts failure |
What are the common exam traps on ventilator modes?
- Volume control: pressure is the dependent variable. Pressure control: tidal volume is the dependent variable.
- AC vs SIMV: AC gives every patient-triggered breath the full set breath; SIMV gives only the set number of mandatory breaths.
- CMV: the patient cannot trigger the ventilator at all.
- PSV: patient-triggered, pressure-augmented, no set rate — a weaning mode.
- CPAP = PEEP alone; BiPAP = IPAP + EPAP (pressure support).
- ARDS: tidal volume from predicted body weight (height and sex), not actual weight; 6 mL/kg; plateau ≤ 30.
- High peak, normal plateau = airway resistance problem; high peak and high plateau = compliance problem.
- RSBI cut-off is 105 (breaths/min/L).