Conclusion
Volume and pressure have a direct relationship. Whichever parameter you adjust will affect the opposite value. When all other settings are kept the same and the lungs are the same, the following statement are true:
In Volume Control |
In Pressure Control |
| As the volume set is increased, the pressure increases. | As the set pressure is increased, the volume will increase. |
| If the Pressure reached in the lungs is too high (>30-35cmH20), dropping the volume will help. | If the volume in the lungs is too high, dropping the pressure will help. |
| If more volume is needed, increase the volume and expect the pressures to climb. | If more volume is needed, increasing the pressure will increase the volume delivered. |
Inspiratory time and flow can also impact the pressure and volumes experienced by the lungs. When all other settings are kept the same, the following statements are true:
In Volume Control (constant volume to be delivered) |
In Pressure Control (constant pressure and/or same target volume) |
| If the flow is increased, the time to deliver the breath is shorter. | If the inspiratory time is increased, the volume delivered will increase. |
| If the flow is decreased, the time to deliver the breath is longer. | If the inspiratory time is decreased, the volume delivered will decrease. |
| If the flow is increased and the inspiratory time is shorter, the pressure needed to hit the same volume will be higher. | If the inspiratory time is increased, the set pressure can be decreased to hit the same volume. |
| If the flow is decreased and the inspiratory time is longer, the pressure needed to hit the same volume will be lower. | If the inspiratory time is decreased, the set pressure will need to be increased to hit the same volume. |
And if the settings on the ventilator do not change, but the lung compliance or resistance in the lungs changes:
In Volume Control (no change to settings) |
In Pressure Control (no change to settings) |
|---|---|
| If the compliance of the lungs increases (more stretchy), the pressure used to hit the volume decreases. | If the compliance of the lungs increases (more stretchy), the volume delivered with the set pressure will increase. |
| If the compliance of the lungs decreases (less stretchy), the pressure used to hit the volume increases. | If the compliance of the lungs decreases (less stretchy), the volume delivered with the set pressure will increase. |
| If the resistance of the lungs increases, the pressure used to hit the volume increases. | If the resistance of the lungs increases, the volume delivered with the set pressure will decrease. |
| If the resistance of the lungs decreases, the pressure used to hit the volume decreases. | If the resistance of the lungs decreases, the volume delivered with the set pressure will increase. |
