Mechanical Ventilation Basics
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Direct answer
Mechanical ventilation supports patients in respiratory failure — indicated for refractory hypoxaemia, ventilatory failure with respiratory acidosis (typically pH below 7.25 with rising carbon dioxide), or exhausted respiratory effort in severe asthma, COPD exacerbation, poisoning and neuromuscular weakness. Initial settings in a passive adult are volume-controlled assisted-control with tidal volume 6 to 8 mL per kg of ideal body weight, rate 12 to 16, PEEP 5 cm of water and FiO2 titrated to a saturation of roughly 94 to 98 per cent. Weaning hinges on daily readiness screening and a spontaneous breathing trial.
What you must remember
- Indications: failure of oxygenation, failure of ventilation (respiratory acidosis), or exhausted respiratory effort — plus airway protection in coma with a Glasgow Coma Scale of 8 or below.
- Modes: assisted-control ventilation delivers a minimum set rate with patient-triggered breaths (the default); synchronised intermittent mandatory ventilation gives set breaths synchronised with patient effort; pressure support ventilation assists spontaneous breaths and is the usual weaning mode.
- Initial adult settings: FiO2 1.0 then wean quickly, tidal volume 6 to 8 mL per kg ideal body weight (4 to 6 in ARDS), rate 12 to 16, PEEP 5, inspiratory time about 1 second; targets — SpO2 94 to 98 per cent (88 to 92 in chronic hypercapnia), plateau pressure below 30 cm of water.
- Complications: ventilator-associated pneumonia (head end elevated 30 to 45 degrees, oral care with chlorhexidine, subglottic suction tubes), barotrauma and volutrauma, auto-PEEP in obstructive disease, and muscle atrophy from oversedation.
- Weaning: daily screen — cause improving, awake, haemodynamically stable, PaO2/FiO2 above about 150 with low PEEP and FiO2; then a spontaneous breathing trial; the rapid shallow breathing index (rate divided by tidal volume in litres) below 105 predicts success.
- Post-extubation stridor: consider a cuff-leak check and prophylactic steroids in high-risk patients; non-invasive ventilation may bridge COPD patients who fail an initial trial.
From first settings to extubation day
Take a passive adult sedated for sepsis-related hypoxaemia. Choose volume-controlled assist-control — the default mode — because it guarantees every breath the full preset tidal volume. Calculate tidal volume on ideal body weight from height and sex, never actual weight: 6 to 8 mL per kg, dropping to 4 to 6 in ARDS, with plateau pressure held below 30 cm of water via an inspiratory hold. Set rate 12 to 16, PEEP 5, and start FiO2 at 1.0 with rapid weaning toward the saturation target. From day one, run the bundle that prevents harm: head end elevated 30 to 45 degrees, daily sedation holds with an extubation assessment each morning, oral chlorhexidine care, subglottic suction tubes, and vigilance for auto-PEEP in the obstructed. Each morning, screen readiness: When the screen passes, run a spontaneous breathing trial and calculate the rapid shallow breathing index — rate divided by tidal volume in litres, below 105 predicting success. Before pulling the tube in a high-risk patient, check the cuff leak and consider prophylactic steroids; after it, bilevel support can bridge the hypercapnic COPD patient.
How the exam frames ventilation
The questions cluster around numbers and defaults: the pH (below about 7.25) and GCS (8 or less) that justify intubation, assist-control as the mode for a new passive patient, 6 mL per kg in ARDS, plateau limit 30, head elevation for pneumonia prevention, RSBI below 105. Mode names are deliberately jumbled — assist-control guarantees a minimum rate with full support, SIMV gives synchronised mandatory breaths with spontaneous breathing between, pressure support is the weaning workhorse. The classic trap is tidal volume on actual weight: an obese patient sized to their measured kilograms is over-ventilated.
Frequently asked questions
What are the main indications for mechanical ventilation?
Refractory hypoxaemia, ventilatory failure with respiratory acidosis (pH under about 7.25), respiratory muscle fatigue, and airway protection in coma (GCS 8 or less).
What is the difference between AC and SIMV?
Assist-control delivers full preset support for every breath, machine or patient triggered; SIMV gives mandatory breaths synchronised with patient effort between which the patient breathes spontaneously.
What are typical initial ventilator settings?
Volume-controlled assist-control, tidal volume 6 to 8 mL per kg ideal body weight, rate 12 to 16, PEEP 5 cm of water, FiO2 titrated to SpO2 94 to 98 per cent.
What is the rapid shallow breathing index?
Respiratory rate divided by tidal volume in litres; a value below 105 during a spontaneous breathing trial predicts successful weaning.
How is ventilator-associated pneumonia prevented?
Head-end elevation of 30 to 45 degrees, daily sedation holds with extubation assessment, oral chlorhexidine care, subglottic secretion drainage and early mobilisation.
When is non-invasive ventilation preferred?
Conscious, cooperative patients with COPD exacerbation and respiratory acidosis, cardiogenic pulmonary oedema, or immunocompromised hypoxaemic failure — not in coma, aspiration risk or haemodynamic instability.