Synchronised Intermittent Mandatory Ventilation (SIMV)

Created on Tue, 06/16/2015 - 17:48
Last updated on Tue, 06/16/2015 - 17:51

Intermittent mandatory ventilation (IMV)

IMV intermittent madatory ventilation

    • Volume Controlled …but it can be pressure-controlled
    • Time-Triggered – and the pressure support breaths are flow-triggered
    • Volume- limited OR Pressure-limited, Flow-limited (but doesn’t have to be flow-limited)
    • Time-Cycled (but it can be volume-cycled if you don’t want inspiratory pauses)

     The key feature is, ITS NOT CONTINUOUSLY MANDATORY.

     

    The patient can decide on their own respiratory rate and they can take a number of pressure-supported breaths.  However, among those, the machine also gives a set number of mandatory breaths, to supplement the patients own efforts.

     

    You get little control over the minute volume. The patient may take as many pressure-supported breaths as he feels like, all you can do is guarantee a certain minute volume with volume-controlled IMV.

    This is slightly better for the partially sedated patient; they get to try and breathe a little.
    The problem is, the machine has its own rate, and it doesn’t care about what the patient wants. It just has a job to do; and if it has been programmed to deliver a volume-controlled time-cycled breath every 10 seconds, then its going to do just that, and if you're trying to breathe or cough or something- that’s just too bad, you're getting your 500ml of gas.

Synchronised intermittent mandatory ventilation (SIMV)


SIMV: Synchronised Intermittent Mandatory Ventilation

  • Volume Controlled …but it can be pressure-controlled
  • Time-Triggered AS WELL AS Flow-Triggered
  • Volume- limited OR Pressure-limited, Flow-limited (but doesn’t have to be flow-limited)
  • Time-Cycled (but it can be volume-cycled if you don’t want inspiratory pauses)

 The key feature is, ITS SYNCHRONISED.

The patient can decide on their own respiratory rate and they can take a number of pressure-supported breaths. However, among those, the machine also gives a set number of mandatory breaths.

If the patient makes an effort around the same time the next mandatory breath is due, the ventilator delivers to them an assist-control breath, with volume control.

You get little control over the minute volume. The patient may take as many pressure-supported breaths as he feels like, all you can do is guarantee a certain minute volume with volume-controlled IMV.

A wide-awake patient would still find this mode relatively comfortable, because of the way it synchronises breaths.

 

Applications of volume-controlled SIMV

This mode of ventilation has advantages in acute severe asthma. Oh's manual recommends it for this application because the square wave pattern of flow generates a short inspiratory time, which decreases the risk of dynamic hyperinflation. Pressure-controlled modes have the advantage of controllable peak pressures, but in asthma the high peak airway pressure is wholely due to the airway resistance, and the plateau pressure is typically modest.

 

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.

Jonson B, Beydon L, Brauer K et al. Mechanics of respiratory system in healthy anesthetized humans with emphasis on viscoelastic properties. J Appl Physiol 1993; 75 : 132–40.

Zavala, Elizabeth et al.Effect of Inverse I: E Ratio Ventilation on Pulmonary Gas Exchange in Acute Respiratory Distress Syndrome Anesthesiology: January 1998 - Volume 88 - Issue 1 - p 35–42