Mechanical Ventilation in the ICU
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Direct answer
Six millilitres per kilogram of predicted body weight, plateau pressure below 30 cm H2O and the lowest workable FiO2 form the backbone of safe mechanical ventilation, whatever the mode delivering them. Volume control with assist-control is the default initial setting, pressure support is the weaning workhorse, and liberation — not the machine — is the daily objective: a spontaneous awakening trial followed by a spontaneous breathing trial with pressure support of 5 to 8 cm H2O decides extubation in most patients.
What you must remember
- Predicted body weight: males 50 + 0.91 × (height in cm − 152.4); females 45.5 + 0.91 × (height in cm − 152.4) — tidal volumes are set on this, never on actual weight.
- Lung-protective defaults: tidal volume 6 mL/kg (4 to 8 range), plateau pressure below 30 cm H2O measured by inspiratory hold, driving pressure under 15 cm H2O, permissive hypercapnia accepted if pH allows.
- Plateau pressure reflects compliance; peak pressure reflects airway resistance — the difference between them localises the problem to the circuit, the tube or the bronchi.
- Modes worth naming: volume assist-control (constant volume, variable pressure), pressure control (constant pressure, variable volume), SIMV (now rarely a weaning choice), PRVC (volume-targeted, pressure-regulated), APRV for refractory oxygenation failure.
- Auto-PEEP shows on the flow-time curve as expiratory flow that never reaches zero before the next breath — treat with longer expiratory time, lower rate or shorter inspiratory time, not simply more PEEP.
- Weaning package: daily spontaneous awakening trial, then spontaneous breathing trial on pressure support 5 to 8 cm H2O or T-piece for 30 to 60 minutes; rapid shallow breathing index below 105 supports readiness but does not replace clinical judgement.
- Post-extubation: prophylactic non-invasive ventilation for hypercapnic COPD and other high-risk groups, high-flow nasal oxygen otherwise, with reintubation criteria fixed in advance.
Setting up, then reading, a ventilator
Begin with a 60-year-old ventilated for severe pneumonia: volume assist-control, rate 18, tidal volume 6 mL/kg predicted body weight, PEEP 8, FiO2 titrated from 1.0 downward to a saturation of 92 to 96 per cent. Thirty minutes later the ventilator alarms. Read it systematically. Airway pressures high? Perform an inspiratory hold — a plateau of 22 cm H2O with a peak of 45 means resistance: suction the tube, check for kinking, give bronchodilators, bite-block for a bitten tube. Plateau itself high? That is compliance — pneumothorax, atelectasis, pleural fluid, auto-PEEP or abdominal distension, and a bedside ultrasound or chest film sorts them. Now watch the flow curve: expiratory flow tracing a decay that fails to return to zero in a wheezing patient confirms auto-PEEP; drop the rate, shorten inspiration, and only then consider extrinsic PEEP to about 80 per cent of the intrinsic value to ease triggering. On day three, provided the FiO2 is 0.4 or less and PEEP 8 or less, interrupt sedation in the morning; if the patient wakes and breathes, run the spontaneous breathing trial and extubate before evening. Each step converts a viva answer into a reproducible sequence, which is exactly what examiners reward.
Where the trapdoors are
Candidates routinely quote RSBI 105 as law; it is a probability, and a tired, anxious or fluid-overloaded patient can fail the number yet succeed after diuresis and reassurance. SIMV is still offered as a weaning mode in some textbooks — evidence dismantled that years ago, so answer "assist-control for rest, pressure support for weaning." Mixing up peak and plateau pressure under pressure is the classic viva collapse: remember the inspiratory hold is the examination manoeuvre that separates resistance from compliance. Finally, dyssynchrony is a signal to hunt causes — pain, obstruction, auto-PEEP, anxiety — before it becomes an indication for deeper sedation or paralysis; sedating the problem away lengthens ventilation and, in ARDS, deeper sedation early is associated with worse outcomes.
Frequently asked questions
How is predicted body weight calculated for tidal volume?
Males: 50 + 0.91 × (height in cm − 152.4); females: 45.5 + 0.91 × (height in cm − 152.4); set 6 mL/kg of this weight.
How do you measure plateau pressure?
An end-inspiratory pause (inspiratory hold) of half a second at zero flow; values above 30 cm H2O demand volume or compliance correction.
What distinguishes peak from plateau pressure?
Peak includes airway resistance; plateau (measured at zero flow) reflects alveolar compliance — a wide gradient points to the tube or airways.
How is auto-PEEP detected and managed?
Expiratory flow that fails to reach zero on the flow-time curve; manage by increasing expiratory time — lower rate, shorter inspiration, bronchodilators.
What constitutes a spontaneous breathing trial?
30 to 60 minutes on pressure support 5 to 8 cm H2O or T-piece with an RSBI below 105, stable gas exchange and haemodynamics, intact mental status.
Which patients get prophylactic non-invasive ventilation after extubation?
Hypercapnic COPD and other high-risk groups — age above 65, cardiac failure, weak cough — reduce reintubation with post-extubation NIV.