Limit Variables in Mechanical Ventilation

Created on Tue, 06/16/2015 - 17:36
Last updated on Tue, 06/16/2015 - 17:36

Limit variables

The "control" variable is definitely going to be achieved; limits are set to make sure all the other variables don't go out of control in the process of achieving the control variable.

Limit variables don't stop the inspiratory phase; they merely limit the flow so that the limits aren't breached.

…Thus, time cant be a limited variable: a time-limit, when reached, would abort inspiration! It could be a "time-cycled" breath.

Each breath can have ALL of the limit variables set.
One of those limit variables is going to be the control variable; for example you cant have a volume control mode without a volume limit (otherwise, what are you controlling?)

 Flow - limited

flow limit
When the flow is limited, it will not exceed the specified parameter until the cycling variable is met.

In this example, the mode is

  • flow-limited
  • volume controlled
  • time-cycled

Until the volume objective is reached, the flow will be maintained.
The guaranteed controlled volume will be delivered.
The flow wont exceed the specified threshold.

Volume-limited

volume limit
When volume is limited, and the volume limit is met, inspiration will not cycle to expiration, but there wont be any further flow.

This mode is

  • Volume-limited
  • Pressure-controlled
  • Time-cycled

When the volume limit is reached, the flow stops, but the pressure is maintained at the controlled level (because the time of the cycle has not yet elapsed).

Pressure-limited

pressure limit

When pressure is limited, the pressure limit will be maintained if it is reached before the end of inspiration.

This mode is

  • pressure limited
  • volume-controlled
  • time-cycled 

In the second breath, the volume control objective was not reached because the pressure limit kicked in. This happens when lung compliance is poor.

 

References

Most of this information comes from only two textbooks. With "Basic Assessment and Support in Intensive Care" by Gomersall et al (as well as whatever I picked up during the BASIC course) as a foundation, I built using the humongous and canonical "Principles and Practice of Mechanical Ventilation" by Tobins et al – the 1442 page 2nd edition.