Pulmonology Topic

What the ventilator actually controls.

The ventilator is a machine you can read. Five cards on what the screen is telling you: what a mode controls, reading the waveforms, asynchrony, trapped air, and the high-pressure alarm.

Reviewed October 2026 · verify against current guidelines

Ventilation · 1 of 5

What the ventilator actually controls.

A mode is not a setting. It is three answers — what starts the breath, what the machine holds constant during it, and what ends it. Every mode name on the dial is one combination of those three.

The three questions — read a mode across the row, not by its name

Mode	Triggered by	Holds constant	Ended by
Volume A/C	Patient or clock	Flow and volume	Volume delivered
Pressure A/C	Patient or clock	Pressure	The clock
PRVC	Patient or clock	Pressure, aiming at a volume	The clock
Pressure support	Patient only	Pressure	Flow falling to ~25%

You can guarantee one, never both — compliance decides the other

Volume control — volume is fixed	A stiffening lung is paid for in pressure. The tidal volume holds and the peak climbs, so the injury announces itself on the alarm.
Pressure control — pressure is fixed	A stiffening lung is paid for in volume. The pressure holds and the tidal volume falls away quietly, with no alarm unless you set one.

The settings behind the mode

Trigger sensitivity. About −1 to −2 cmH₂O, or a flow trigger of 2–5 L/min. Too tight and the patient works to open the valve; too loose and the circuit self-triggers.
Inspiratory time. Sets the I:E ratio, normally about 1:2. Shorten expiration and the lung does not finish emptying.
Rise time and flow. How fast the breath is delivered. Too slow in a hungry patient looks like air hunger on the pressure trace.

Two traps — assist-control does not mean the machine does all the work; it means every breath gets full support, and a patient triggering 35 times a minute is working hard inside a mode that looks controlled. And no mode is protective by itself — protection is the tidal volume and the driving pressure, whichever mode delivers them.

Ventilation · 2 of 5

Reading the waveforms.

Three traces, three questions. Pressure tells you what the breath cost. Flow tells you whether the lung finished emptying. Volume tells you whether what went in came back.

Two volume-control breaths — the second is held at end-inspiration

Pressure	Peak — flow plus lung. Plateau — the lung alone.
Flow	Back to zero before the next breath.
Volume	What went in has come back out.

Two holds, two answers — both need a passive patient

Inspiratory hold → plateau	Two seconds with the breath held in. The gap between peak and plateau is airway resistance; the plateau itself is what the alveolus felt.
Expiratory hold → total PEEP	Three to five seconds at end-expiration. Anything above the PEEP you set is trapped air the lung could not release.

Peak minus plateau is the whole diagnosis. A widening gap is a resistance problem — secretions, bronchospasm, a kinked or bitten tube; the lung itself has not changed. Both rising together is a compliance problem — edema, consolidation, pneumothorax, a distended abdomen, a stiff chest wall.

Two traps — in pressure control there is no plateau to measure, because pressure is what the machine is already holding — switch to a volume breath if you need the number. And an effort during either hold invalidates it: the trace wobbles, and the value you read is the patient's, not the lung's.

Ventilation · 3 of 5

The breath the patient didn't want.

Two pumps have to agree on when the breath starts, how big it is, and when it ends. Disagreement is common, it is visible on the traces before it is visible in the patient, and two of the four types injure the lung.

Four patterns on the traces — flow trace, except where noted

Double trigger	Two breaths stacked, so the delivered volume doubles. Lengthen the inspiratory time or raise the set volume.
Ineffective effort	The patient asks and gets nothing. Usually trapped air or too tight a trigger. Fix the trapping first.
Flow starvation	On the pressure trace. Scooped out, because demand exceeds the set flow. Raise the flow or the pressure support.
Reverse trigger	The machine breath provokes the effort. Seen in deep sedation, and deepening it further makes it worse.

The two that do damage. Double triggering stacks volume: a protective 6 mL/kg breath delivered twice with no expiration between is a 12 mL/kg breath, and the plateau you carefully set is not the pressure the alveolus saw. Reverse triggering loads the diaphragm eccentrically: the muscle contracts while lengthening, which is how a muscle tears, and regional gas shifts within the lung even when the tidal volume looks right.

Order of fixes

Settle the cause before the settings: pain, fever, delirium, trapped air, a blocked tube. Then the mode and the flow. Paralysis is the last option, not the first, and sedation alone will often make reverse triggering worse rather than better.

Two traps — the displayed respiratory rate counts machine breaths, so a patient making ineffective efforts is breathing faster than the number says — count the chest, not the screen. And “fighting the ventilator” is a description, not a diagnosis: the trace tells you which of the four it is, and each has a different fix.

Ventilation · 4 of 5

When the lung won't empty.

Expiration is passive, and it takes time. If the next breath arrives before the lung has finished emptying, the leftover volume stays — and its pressure never appears on the PEEP you set.

The flow trace tells you first — expiratory flow, below the zero line

Emptied	Flow reaches zero and stays there. There is a flat gap before the next breath.
Trapping	The next breath cuts expiration off mid-flow. Every breath leaves a little more behind.

Measure it, then unload it — an expiratory hold on a passive patient

Total minus set	Auto-PEEP is the difference. Hold expiration 3 to 5 seconds and read what the pressure settles at.
Give it time	Lower the rate, shorten the inspiratory time, accept the rising CO₂. Expiration needs the seconds more than the lung needs the breath.
Widen the exit	Bronchodilators, suction, the largest tube available. And treat the effort itself — a struggling patient traps more.

Why a trapped lung arrests. The trapped volume raises the pressure inside the chest for the whole cycle, so venous return falls. The blood pressure drops, and the reflex is to bag faster — which traps more. Disconnect the circuit for 20 to 30 seconds. If the pressure recovers, it was trapping. If it does not, look for a pneumothorax instead.

Two traps — an expiratory hold reads zero in a patient who is actively breathing, so a normal number does not exclude trapping — trust the flow trace. And in a patient triggering every breath, adding external PEEP up to about 80% of the measured auto-PEEP makes triggering easier rather than worse, because the effort no longer has to overcome the trapped pressure first.

Ventilation · 5 of 5

When the pressure alarm sounds.

Take the patient off the ventilator and bag them. That single move separates the machine from the patient, tells you how hard the lung is to inflate, and buys the time to work through the rest.

Back on the ventilator, one question splits the differential — inspiratory hold: peak, then plateau

Gap widens — resistance	Secretions, bronchospasm. A kinked or bitten tube. A clot in the lumen. Too narrow a tube.
Both rise — compliance	Pneumothorax, trapped air. Edema, consolidation. A distended abdomen. A stiff chest wall.

The crashing patient — the four that kill, in the order you can exclude them

Displaced	Tube out, or down a bronchus. Check depth, listen, confirm with capnography.
Obstructed	Secretions, a clot, a bitten tube. Pass a suction catheter — it either goes or it does not.
Pneumothorax	Absent sliding on ultrasound. Under tension it needs decompressing before imaging.
Equipment	Circuit, valve, water in the tubing. Bagging has already ruled this in or out.

The bag tells you two things at once. Hard to squeeze: the problem is in the patient or the tube — suction, then look for the pneumothorax and the trapped air. Easy to squeeze: the problem was the ventilator or the circuit, and the patient is now safe while somebody checks it.

Two traps — raising the alarm limit is not a treatment, and a high-pressure alarm that stops sounding because the limit moved is a lung still being injured in silence. And the alarm that matters more is the low one — low pressure or low exhaled volume means a disconnection or a leak, and it is the alarm people mute first.

Educational reference only — not medical advice. Verify against local protocol.

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