Volume Control Specific Settings: Tidal Volume and Flow

In volume control, the clinician who is setting the ventilator will dial in the set volume for each breath. Remember, in volume control we will set the volume, but in pressure control, the volume will be based on the pressure and Itime that is set by the clinician.

We have learned how to find a safe range for tidal volumes for our patients, but what number do you start with? It is safe to start with any number within the 6-8ml/kg range. For people that are new to ventilation, starting at 8 ml/kg of IBW is the best recommendation for adult ventilation strategies. Why is this? Because most patients who are being intubated have compromised ventilation, and their CO₂ levels are not being maintained with changes to how they are breathing. Giving the patient the largest “safe” breath you can will ensure the patient is breathing in as much oxygen as possible and exhaling as much CO₂ as possible per breath. You can always come lower on your tidal volume later after follow-up bloodwork. Start with 8 ml/kg and then reassess after that.

In Volume Control, you also usually set the max flow of air going into the lungs. Some ventilators will ask for you to set an Itime instead (discussed below) of a flow, but the classic versions of volume control have a max inspiratory flow setting. For users who are not experienced with ventilators, the default for an adult is usually a flow of 65 liters per minute (lpm) with a decelerating flow pattern. A decelerating flow pattern means that the flow peaks at initiation and then slows down as the lungs fill. This setting will work for the overwhelming majority of adult patients.

The only time where you might consider increasing the flow above 65 lpm is if you have a patient who is triggering breaths and gasping or pulling the air in beyond what the ventilator is supplying. Usually, when this happens, the ventilator alarms are activated. You can try increasing the flow to 70 or 75 lpm until you satisfy their air flow needs. The max flow you should ever set is 80 lpm, but this setting would be used for a very small number of patients. They will be spontaneously triggering breaths, and if you are struggling, you may consider trialing a spontaneous mode (to be discussed in the next chapter) or more sedation if small increases in flow do not satisfy the patient’s need.

In the beginning, leave the flow at the default of 65 lpm with a decelerating flow pattern and do not change this unless you really think the patient is pulling air quicker than the ventilator. When in doubt, sedate your patient and continue to ventilate with the advised flows of 65 lpm.

A word of caution: The higher the flows used, the higher the pressure you will be pushing into the lungs. Increasing the flows can cause large spikes in pressure that can cause damage to the lungs (barotrauma). This is why flows should only be adjusted if the patient is definitely outstripping the ventilator’s delivered flows. Watch your peak pressures and keep them below 35 cmH₂0 (PIP <35cm H20).