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Pressure control vs volume control
One guarantees the breath and lets the pressure move. The other guarantees the pressure and lets the breath move. Everything else follows from that.
The difference in one line
| Volume control | Pressure control | |
|---|---|---|
| You set | Tidal volume, rate, flow, inspiratory time, PEEP, FiO2 | Inspiratory pressure, rate, inspiratory time, PEEP, FiO2 |
| The lung decides | Airway pressure | Tidal volume |
| Breath ends when | The set volume has been delivered | The set time has elapsed |
| Flow pattern | Usually square, as set | Decelerating, as the lung fills and the gradient falls |
The same problem, two appearances
A patient whose compliance falls overnight shows it as rising pressures in volume control and as falling tidal volumes in pressure control. Same lung, same deterioration, two different alarms.
This is the practical reason the two views are never independent: in volume control, falling compliance raises pressure for the same breath; in pressure control, it shrinks the breath for the same pressure. Whichever mode you are in, you are looking at one half of the same measurement and should ask for the other.
What pressure control buys
- A hard ceiling on inspiratory pressure, which is reassuring when the worry is barotrauma.
- A decelerating flow pattern, which many patients find more comfortable and which can improve gas distribution in heterogeneous lung.
- Direct control of inspiratory time, independent of flow.
The cost is that tidal volume is no longer guaranteed. In a deteriorating lung it falls exactly when the patient can least afford it, and the minute ventilation goes with it.
What volume control buys
- A guaranteed minute ventilation, which matters when the CO2 is the problem, and matters enormously in severe metabolic acidosis.
- A plateau pressure that is directly interpretable, and therefore a driving pressure you can act on.
- A clean separation of resistance from compliance using the peak-to-plateau gap.
The cost is that pressure is uncapped and will rise to whatever the lung demands, so the alarms have to be set and believed.
Which to choose
No mode is right or wrong in general. Volume control is the reasonable default for most medical ICU patients, because lung-protective ventilation is specified in volume per predicted body weight and because the plateau and driving pressure fall out of it directly.
Pressure control earns its place when pressure is the thing you most want controlled, and in patients whose own effort is substantial enough that a fixed flow fights them.
What matters far more than the choice is what you do after it: measure the plateau, work out the driving pressure, look at the flow trace for trapping, and set the tidal volume from predicted body weight rather than actual weight. Those are the same in both modes, and they are the parts that change outcomes.
One thing that is not a mode problem
If a patient is fighting the ventilator, changing the mode is rarely the answer and is often how an hour gets lost. Check the tube, listen to the chest, look at the saturation, then look at the flow trace for trapping and the pressure trace for flow starvation. Dyssynchrony is usually a settings mismatch or a physiological emergency, not evidence that the mode is wrong.
Nobody ever mastered a ventilator by reading about one.
Put the same simulated patient on both modes, then drop their compliance and watch where the change shows up. The Ventilator Workbook puts a live ventilator in front of you with a simulated patient behind it, so you can break things that cannot be harmed. Three modules and time on the simulator are free, and no card is needed.