low fresh gas flow and tidal volume

[lakersbaby]

Sorry this is a basic concept about the vent but when you have a set tidal volume how much of the fresh gas flow contributes to the tidal volume being delivered to the patient on the ventilator. When attendings say low gas flow is fine and put the total flow at 1L/min I see a pretty significant drop in tidal volume delivered but reading it says most ventilators can deliver set TV regardless of FGF rate. Thanks

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[deleted126335]

Depends on I/E ratio. i.e., 3 l/min fresh gas flow with 1:2 I/E... one third of each minute is spent in inspiration. Therefore 1 l/min is from the fresh gas flow. Double the flows to 6/min minute ventilation will increase by one liter per minute. Still remember it being on my written boards 20 something years ago. This is not an issue with newer, electronic machines.

 

[pjl]

100% dependent on the machine you use. If you have any really old ones there is a big difference, predicted by a handy equation. Any of the newer machines should have no difference even with really low flows below 1L/min (set at .3 or .4).

 

[pgg]

As mentioned, modern machines don't have this problem, but modern exams sure do (or did just a few years ago).

If the resp rate is set to 10, each breath takes 6 seconds. An I:E of 1:2 means 2 seconds per breath is inspiration, and 4 seconds is expiration. If your fresh gas flow is 1 L/min, that's about 17 mL of fresh gas second, and old machines dump that straight into the patient side of the circuit during inspiration, so 34 mL of gas would be added to each breath. Set TV to 500, deliver 534. If your fresh gas flow is 10 L/min, 170 mL/second adds 340 mL per breath. Set TV to 500, deliver 840.

Take home message is that fresh gas flows on old dumb machines can alter delivered tidal volumes by a clinically significant amount.

 

[pgg]

When attendings say low gas flow is fine and put the total flow at 1L/min I see a pretty significant drop in tidal volume delivered but reading it says most ventilators can deliver set TV regardless of FGF rate.

How old is the machine?

Also, with low flows, even small circuit leaks may affect the machine's ability to deliver and/or measure tidal volumes accurately. Obviously if the FGF is so low that the reservoir bag completely deflates, that'll affect TV too.

 

[Idiopathic]

Sorry this is a basic concept about the vent but when you have a set tidal volume how much of the fresh gas flow contributes to the tidal volume being delivered to the patient on the ventilator. When attendings say low gas flow is fine and put the total flow at 1L/min I see a pretty significant drop in tidal volume delivered but reading it says most ventilators can deliver set TV regardless of FGF rate. Thanks

changes in FGF should never decrease your tidal volume, .if anything it should increase the tidal volume in the manner described above

 

[norwood]

changes in FGF should never decrease your tidal volume, .if anything it should increase the tidal volume in the manner described above

I think what the OP is describing is setting a tidal volume while the gas flows are still at some high number, say 10 L/min, on an uncompensated oldschool machine, and then cutting the flows to 1 L/min. This will drop the tidal volume. OP's attending is correct that 1 L/min is a perfectly fine gas flow (and not even particularly low flow), but if his tidal volume is dropping, he needs to adjust the settings to get back to where he wants to be.

Wonder if his machine also makes him manually adjust the tidal volume when he increases or lowers the rate - hey, OP, what kind of machine are you using?

 

[kazuma]

As mentioned above, most modern Drager and GE machines have "fresh gas flow decoupling." We still have a few older machines in remote locations that don't decouple, so it's still something to be cognizant of at some institutions.