Ventilator Basics

As stated earlier in SIMV the patient may breathe in-between the scheduled breaths, but instead of receiving a full preset tidal volume as in assist-control, a patient-triggered breath does not result in a machine-delivered mandatory breath.They receive a tidal volume that is 100% dependent on their own power, which will vary depending on the patient's current respiratory status, respiratory strength, sedation, and pain level. The patient gets whatever volume he or she can pull through the ventilator circuit. This is of worry and could put the patient at risk for increased respiratory muscle fatigue. Because of this It is advisable to add a pressure support breath to an SIMV mode. Pressure support is just like BiPAP but with an intubated patient. This feature is exclusive to the SIMV mode, is not available in assist-control modes, and is only utilized during spontaneous patient-triggered breaths. This pressure support breath acts as a “boost” breath to overcome added airway resistance within the circuit and the respiratory tract. Eric Bauer's Ventilator Management textbook has a good analogy regarding pressure support, “If your pressure support is inadequate, think of this as you sucking a very thick milkshake out of a straw. As you add pressure support, this thins the milkshake, and the thinning milkshake becomes easier to suck and drink out of the cup.” As you add pressure support, you make that breath easier for them to take with their potentially weak respiratory effort.

Step-by-Step Guide

1. Set Pressure Support: Set the PS level to a value above the PEEP. This is the pressure that will be delivered to support the patient's spontaneous breaths
   1. 10-20 cm H2O
      1. Minimum PS of 5 is necessary to overcome ETT resistance
2. Monitor and Adjust Tidal Volume: Observe the patient's spontaneous tidal volume to ensure it is in the target range of 6-8 mL/kg of ideal body weight
3. Adjust Pressure Support:
   1. Increase PS if the tidal volume is low or the patient is showing increased work of breathing
   2. Decrease PS if the tidal volume is high or the respiratory rate is on the lower end of normal
   3. Additional indicators that the patient may require more support from the ventilator are the heart rate and blood pressure. If there is an increase by 20% from the initial baseline when the change is made, this may indicate the patient is not tolerating the change