Volume and pressure are the two major control variables in mechanical ventilation. One may have volume-controlled or pressure-controlled ventilation. The word "control" here implies that the person setting the ventilator is most interested in that specific variable, and wants to control that variable exclusively (i.e. without letting the respiratory system make any decisions about it).
Thus, when one controls volume, one disregards the patient's idea of what their tidal volume should be. It's not up to you, one says to the patient. I will tell you what your tidal volume is. Whatever pressure happens to be generated in the respiratory circuit by such a volume will be determined by the lung compliance of the patient.
Volume control ventilation
The above-described manner of controlling volume is best depicted as a ventilator graph.
Observe: this is a Volume-Controlled mode.
Volume is controlled: There is a set volume; once you reach it, that’s it. The ventilator then either exhales (like in volume-cycled ventilation) or holds an inspiratory pause.
Time is also a controlled variable here. There is a set respiratory cycle time; the inspiration must be completed within a predetermined period, lets say 1 second.
Flow remains constant until the volume is delivered. Flow is defined as volume over time. Thus the flow curve does not change if the volume is the controlled variable, and there is a set respiratory cycle time.
Pressure fluctuates depending on lung resistance, because the flow remains the same
( seeing as pressure = flow x resistance)
Pressure control ventilation
In this case, one abandons any idea of fixed tidal volumes, because oe is for whatever reason disinterested in maintaining a steady minute volume ventilation. Who cares about the CO2, you might say to yourself. In pressure-controlled ventilation, the tidal volume is left up to the lung compliance to decide. All you are concerned with is the pressure.
Pressure is controlled: There is a set pressure; once you reach it, it is maintained for the duration of the inspiratory phase. The waveform is square, or nearly so.
Time is again a controlled variable here. There is a set respiratory cycle time; the inspiration must be completed within a predetermined period.
Flow starts high (to reach the desired pressure) and decreases on a slope, to maintain that pressure (as the lungs fill with air, they stretch and become less compliant, and so less flow is required to maintain the same pressure).
When lung compliance is poor, less flow is required to maintain the same high pressure.
Volume fluctuates depending on flow: seeing as volume is flow times time, and with the time being a set value, volume will decrease if flow decreases remains the same
( seeing as pressure = flow × resistance)
This is a Pressure-Controlled mode.