Time-Triggered: it's the machines' choice; this is MANDATORY ventilation
When time runs out, the machine takes a breath for you, and you don't get a choice.
The patient is not involved in the triggering process.
This triggering mode is typically used when the patient is heavily sedated or paralysed; they can not be expected to make any respiratory effort.
Flow-Triggered: it's the patients' choice
The circuit has a constant flow in and out of the T-piece. It measures that flow; while you aren't breathing, flow out of patient equals flow into patient.
When you try to take a breath, you suck some flow off the circuit- and the machine detects that as an inequality of in-flow and out-flow. This triggers a breath.
This is good mode of triggering for sedated patients, as it requires minimal effort. The disadvantage, is that its TOO easy, and sometimes a non-respiratory effort will trigger the ventilator (eg. the heart beat or a hiccup).
Pressure-Triggered: it's the patients' choice, provided they can summon the effort it requires
The patient must generate a sufficient negative pressure before the machine will give them a breath.
This is hard work- while trying to generate a negative pressure, you are fighting the whole ventilator, sucking on a two metre hose with humidifiers and god knows what else installed in-line.
Needless to say, you wouldn't use this on some sort of crap-lung ARDS patient, or someone with congestive heart failure.
This mode of triggering is used to wean patients off ventilator by strengthening their own respiratory muscles. It can be used intermittently in people who have had respiratory muscle weakness due to critical illness polyneuropathy.
Volume-Triggered: it's still the patients choice, and harder still.
This is very similar to pressure-triggered ventilation, except the ventilator watches for a specific inspiratory volume, instead of a negative pressure or a flow deflection.
This is also hard work, as you now have to draw 50ml of air out of a non-compliant machine.
The amount of pressure this requires is variable, and depends on the ventilator tubing length, amount of water rained-out into it, how moist your humidifier is, how small your endotracheal tube is, and so forth.