Flow, Volume, Pressure, Resistance and Compliance
Everything about mechanical ventilation can be discussed in terms of flow, volume, pressure, resistance and compliance.
The ventilator is just a slightly more complex leaf blower.
Once you remove the fancy stuff, a ventilator is merely a turbine which delivers FLOW.
Yes, flow is volume delivered over time. It is measured in litres per minute (L/min.).
Everything else in the delivery of ventilation (all the pressures and volumes) is controlled by the adjustments in flow.
The relationship of flow and volume
At a basic level, one would like to use this leaf blower to deliver a volume to one's patient. This, after all, constitutes "ventilation".
Working from the above equation, volume is flow multiplied by time.
One can envision some sort of very basic Leonardo da Vinci-style ventilator, a basic bellows where the rate of flow is fixed and where the volume is controlled by adjusting the time.
The relationship of flow and pressure
Pressure is what is generated when flow is directed into a closed system like your lung.
"Resistance" in this setting is due to the resistance of the endotracheal tube and the patients own airways, as well as the resistance of the chest wall and the lung itself against being distended (the reverse of compliance).
Airway resistance is really the pressure difference which drives flow, divided by the volumetric rate of flow.
That driving pressure difference is the gradient between "outside" pressure and "inside" pressure (or, in a ventilated patient, the inspiratory pressure and the alveolar pressure).
Of course, resistance is only a meaningful concept while there is flow. No flow means there is nothing to resist!
This is the total sum of resistance in the "patient circuit" - from the tubes connecting the patient to the ventilator, to the bronchi, the chest wall, the lung parenchyma, the distended abdomen, the bronchopleural fistula... This is the net product of all these factors.
By looking at a pressure waveform which forms with the delivery of a known volume one can begin to separate these multiple factors (at least into a "static" and a "dynamic" group, meaning those which depend on flow and those which do not).
This is the change in volume produced by a given pressure.
Compliance in this setting is the total lung compliance (i.e. change in volume divided by change in pleural pressure).
Seeing as every other equation here is related back to flow, one can rearrange this one to also contain flow.
One can calculate compliance by dviding the tidal volume by the difference between PEEP and plateau pressure.
Thus at a PEEP of 10, with a plateau pressure of 30 and a tidal volume of 400ml, the patient has a compliance of 400 / (30-10), which is 20ml/cm.
What is the point of all this?
One is able to control the flow, time, volume and pressure on a ventilator.
If you specify an inspiratory time, you can only really change one other variable- either flow, volume or pressure. The others will be dependent on lung compliance and lung resistance. They are all related.