Ventilator Modes
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Direct answer
Name any ventilator mode with three answers: who triggers the breath, what limits it, and what cycles it off — and the mode list stops being a maze. Assist-control gives every breath, machine-timed or patient-triggered, the full set support; SIMV delivers synchronised mandatory breaths with unsupported or pressure-supported breaths between; pressure support (PSV) makes every breath the patient's own, triggered by effort and cycled off when inspiratory flow decays to about a quarter of its peak; pressure control caps the driving pressure and lets volume float with compliance; PRVC chases a volume target using pressure-control breaths adjusted breath by breath. Mode selection is secondary to two habits: lung-protective settings wherever the mode sits, and a daily assessment of whether the patient can breathe without the machine.
What you must remember
- Assist-control (AC): a guaranteed minimum minute ventilation — every triggered breath also receives the full set tidal volume; the risk is a tachypnoeic patient stacking full breaths into hyperventilation and auto-PEEP.
- SIMV: mandatory breaths synchronised to patient effort within a timing window; breaths in between carry only the set pressure support (or nothing) — trials showed gradual SIMV weaning is no faster than daily spontaneous breathing trials.
- PSV: patient-triggered, pressure-limited, flow-cycled (at roughly 25% of peak inspiratory flow); the comfort mode, because the patient sets rate, timing and depth.
- A spontaneous breathing trial is pressure support 5-8 cm H2O with PEEP no more than 5, or a T-piece, for 30-120 minutes — the readiness test before extubation.
- Pressure control (PCV): time-triggered, pressure-limited, time-cycled; produces a decelerating flow profile and a dependent tidal volume — watch volumes as compliance changes, benefit when airway pressure must be capped.
- PRVC: pressure-control breaths with the pressure level auto-adjusted each breath to achieve a set volume — decelerating flow comfort plus a volume guarantee.
- CPAP: no ventilator breaths at all, continuous positive pressure through spontaneous breathing — 5-15 cm H2O in obstructive sleep apnoea, 5-10 in cardiogenic pulmonary oedema.
- Mode-by-scenario heuristics: absent drive (anaesthesia, brain injury with hyperventilation targets) — volume AC; ARDS — lung-protective volume control or PRVC; acute asthma — low rate, long expiration, permissive hypercapnia; weaning — daily SBT rather than stepwise SIMV.
- PSV still needs backup: without an apnoea backup rate set, a patient who stops triggering hypoventilates silently.
One patient, four days of modes
Night one after emergency laparotomy: paralysed and sedated on volume AC — 420 mL at 16 per minute, PEEP 5, FiO2 0.5 — no drive to trigger anything. Day two he wakes and triggers above the set rate; each triggered breath also receives the full 420 mL, so minute ventilation and pressures climb — the assist-control signature problem. Transition to PSV 12 with PEEP 5: he now triggers every breath, the ventilator boosts each, and flow cycling ends them as demand falls.
Day three, the extubation question: a spontaneous breathing trial — PS 7, PEEP 5, 60 minutes — watching rate under 35, heart rate under 140, saturation above 90%, no agitation. He passes, is extubated onto high-flow oxygen or a mask. Had he failed, the analysis reverses into causes: load (bronchospasm, secretions), capacity (weakness, cardiac failure), drive (sedation, encephalopathy) — a mode change cannot fix any of these by itself.
Where students slip
Three slips recur. First, treating PSV as a guarantee rather than an assist: it is a support mode, not a ventilation mode, unless backup ventilation is armed — the apnoea alarm without backup is a monitor, not a therapy. Second, misreading "fighting the ventilator" on assist-control as under-sedation when it is flow starvation: the set inspiratory flow, say 40 L/min, is below the dyspnoeic patient's demand — increase flow or move to a pressure mode before reaching for sedation. Third, defending SIMV as the weaning mode on tradition: trials favour the daily spontaneous breathing trial, with SIMV reserved for specific transitions such as muscle reconditioning. None of this changes the floor under every mode — 6 mL/kg predicted weight, plateau under 30.
Frequently asked questions
How does assist-control differ from SIMV?
In AC every breath — set or triggered — receives full support; in SIMV only the synchronised mandatory breaths do, and the spontaneous breaths between receive only the set pressure support.
How does a pressure support breath end?
By flow cycling — inspiration terminates when inspiratory flow falls to about 25% of its peak, matching the patient's own neural timing.
What settings constitute a spontaneous breathing trial?
Pressure support 5-8 cm H2O with PEEP 5 or less (or a T-piece) for 30-120 minutes, with defined pass criteria for rate, saturation, heart rate and mental state.
Why does tidal volume vary in pressure control?
Volume becomes the dependent variable — it changes with compliance and resistance; a worsening volume signals a worsening lung or airway.
What is PRVC?
Pressure-regulated volume control: pressure-control breaths whose pressure is automatically adjusted breath-to-breath to deliver a set tidal volume.
Which mode suits a patient with no respiratory drive?
Assist-control (continuous mandatory ventilation), because trigger-based modes presume patient effort.