DreamMachine

10+ Year Member
Jul 15, 2006
373
3
Status
Attending Physician
Basic Question:
When you increase the fresh gas flow above the minute ventilation where does that extra volume of gas go?
 
Last edited:

drrosenrosen

Anesthesiologist/Pain Physician
10+ Year Member
Nov 16, 2005
547
178
Humboldt, CA
Status
Attending Physician
DreamMachine said:
Basic Question:
When you increase the fresh gas flow above the minute ventilation where does that extra volume of gas go?
scavenge
 

ReefTiger

Junior Member
10+ Year Member
5+ Year Member
Nov 30, 2005
89
0
Status
Attending Physician
Whenever you increase FGF you actually increase minute volume. I don't know the formula off hand, but I have been told that newer machines have a mechanism to compensate for this increased flow and keep tidal volumes the same. As far as I have understood it, there is some gas that goes to power the ventilator bellows and that can vent out the scavenging system as well. This is the only way I can figure gas not passing through the lungs is taken out of the circuit.

Anyone else with a better explanation?
 
About the Ads

seattledoc

Senior Member
10+ Year Member
15+ Year Member
Aug 11, 2003
177
0
49
seattle
Visit site
Status
The excess gas simply goes through the inspiratory limb, past the Y-piece connected to the ETT, and into the expiratory limb. From the expiratory limb, the scavenger removes excess gas, approximately equal to the FGF delivered.

As for how the FGF changes tidal volume, it depends on the liters per minute of FGF, the breaths per minute and the I:E ratio. e.g. if the FGF is 6 liters per minute, that equals 100 mls per second. Lets say the vent is giving 10 breaths per minute and the I:E ratio is 1:2. Every "breath cycle lasts 6 seconds and of that six seconds, 2 seconds is spent in inspiration phase. In 2 seconds, the vent delivers an additional 200 mls (remember 100mls per second FGF). So if you've got 600 mls tidal volume set on the vent, the patient will be getting 800mls.
You're right, the newer vents (narkomed 6000 comes to mind) have a mechanism that keeps the FGF from changing the set tidal volume. It works OK but can still cause problems with low tidal volume settings and high FGF (i.e. at the end of a pediatric case)
 

militarymd

SDN Angel
10+ Year Member
15+ Year Member
Dec 17, 2003
5,889
18
Visit site
Status
Attending Physician
DreamMachine said:
Thanks for replying, very informative.

Another basic question:
If some of it goes through the Y piece without entering the patients lungs, how does it not effect the end tidal concentration readings?
During exhalation, the Inspiratory one way valve closes, so that no fresh gas is entering the inspiratory limb.
 
About the Ads