With each breath the ventilator provides for a patient, we have a certain amount of flow that rushes into the airway and lungs. The speed of that flow depends on a few independent variables that will change depending on the mode of ventilation you use. For example, if I am in a volume mode and tell my ventilator I wish to send 500ml’s of air into the lungs twelve times a minute, I would have a five second total breathe cycle time (60 seconds divided by 12 breaths). That five seconds has to be divided into inhalation and exhalation. So suppose I want to give that 500ml breath over 1 second, how fast would that flow need to be?
500ml’s x 60 seconds / 1 sec (I-time) = 30 lpm
This means that in order to put 500ml’s into the lungs in one second, my vent would need to flow at 30 lpm. This formula can be modified by changing the I-time. For example, if I needed to deliver the breath over 0.5 seconds, I would divide by 0.5 at the end (500 x 60 / 0.5 = 60). Half the inspiratory time gives me double the flow rate. As you can see from this formula, the IFR can also be increased by dialing in a larger tidal volume. However in an era of lung protection strategies, it is not suggested that this be your first move. To increase an IFR from 30lpm to 40lpm in a patient with an I time of 1 second, you would need to increase your Vt by almost 200ml's. This may or may not be within normal ranges for that patients IBW.
So why is this important?
Imagine if there was a governor on how fast you were able to inhale. Any disturbance to our inspiratory flow rate makes us very uncomfortable. This is likely the reason a patient with dyspnea who is tachypneic tears off the non-rebreather (NRB) we insist “will help." A NRB will deliver flow at 15 lpm. This oxygen delivery device is designed to not entrain atmospheric air easily. So the only flow you get from a “true” NRB, is what’s provided from the oxygen source and maybe what leaks around the edges. In order for that flow rate to be comfortable we would need to require no more than 250 ml’s over 1 second.
We know this is commonly not the case when a NRB is applied. Because of this, we have seen increased emphasis on oxygen delivery devices that do not restrict inspiratory flow rate (oxy-mask, nasal cannula, simple mask, etc.).
How do I know if I should increase the IFR on a mechanically ventilated patient?
There are a few signs that your mechanically ventilated patient wants more air and they want it now! It’s widely accepted that a adequate IFR can be calculated by multiplying your minute volume (Ve) by three. Here are two additional hints that your patient might be demanding more flow..
1. Obvious Agitation/Asynchrony
The first thing you will probably notice is the patient appears to be agitated. If you have addressed pain, hypoxia, and some of the other checklist items mentioned in Podcast # 3 (The Agitated Intubated Patient), consider increasing the IFR.
2. Terminal Flow Spike’s
You can learn a lot about your patients comfort and needs by watching the flow graph on a ventilator. If you notice that at the end of the flow cycle there is a sudden peak in flow, this could mean your patient wants more flow. Depending on what mode you are in and how the breath was triggered will determine how to potentially fix this request for more flow.
Each breath triggered by the patient or the ventilator will result in a full tidal volume (Vt) breath at whatever your Vt is set at. In order to increase the IFR, you will simply decrease your inspiratory time. To give 500ml’s over 0.5 seconds, my flow will need to essentially double to 60 lpm. One might make the mistake of thinking that because the patient triggered a breath towards the end of inhalation that they may need to increase the inspiratory time. Increasing the time over which the preset Vt is delivered will lower your IFR and very likely make the situation worse.
When the patient takes a breath on their own they will receive pressure support at a pre-set amount. This pressure should help them overcome any dead space and restriction from the ET tube. The amount of volume that is inhaled will depend on the patient’s strength and the set pressure support. The IFR will essentially be controlled by the patient with the option of set flow termination. It is generally accepted to aim for their spontaneous Vte to be < 75% of the ventilator set Vte. You will not see the terminal flow spike in this mode because they are in relative control of their IFR and inspiratory time.
The nomenclature changes somewhat when we switch to a pressure mode of ventilation. The amount of time it takes to reach our plateau is now called our "Rise Time". The rise time profiles are typically set from 1-9, with 1 being 0.1 second(100ms) and 9 being 1.0 second (1000ms). Your I time dial in pressure mode will now control the amount of time you remain at that plateau. This means that in order to increase your IFR in a pressure driven mode, you will need to decrease your rise time.
Similar to Volume SIMV, when a patient takes a breath in a Pressure SIMV they will have control over their IFR. Your set pressure support will once again help them overcome the dead space and resistance of the ET tube. We do however have the ability to determine when the breath should switch from inhalation to exhalation. This setting is called flow termination. Essentially what it does is measure the point of maximum peak flow, and switches to exhalation at a set percentage of that peak. For example if my patient I used earlier with a peak flow of 60 lpm, and I set my flow termination to 25%, I wouldn't switch to exhalation until the flow slowed down to 15 lpm. The flow decreasing tells me that the breath has been delivered and the patient isn't actively pulling flow. There is a direct correlation between volume and flow termination. It only makes sense that if I end my breath early, I will have less volume delivered.
It's a good idea to take individual aspects of ventilation and really meditate on their application. I encourage you to hook up a non invasive mask to the ventilator and experiment with what these flow rates feel like on yourself!
I look forward to any comments or suggestions from the FOAMed community!