Assuming a rebreathing system, no ventilator, and just an oxygen tank (no nitrogen, etc):
When your patient is hooked up the only gas available to him.her is oxygen. That oxygen has 2 functions, for profusion of tissues (to keep the pet alive) and to carry your anesthetic gas.
How do you know if your O2 flow rate is too low?
There is no other gas in the system other than oxygen and the exhaled CO2 of your patient. So, with O2 flow rates that are too low you'd see:
- difficulty stabilizing anesthesia level because below 0.5L/min most vaporizors cannot reliably deliver anesthetic gas at the set percentage
- the reservoir bag would not be inflated
- the inhaled or IN CO2 would be above 0
- ETCO2 would steadily increase as CO2 builds up in the circle faster than the O2 can replace it
- SpO2 would begin to decrease
- mucous membrane color would turn cyanotic
- patient death
How do you know if your O2 flow rate is too high?
- you're going through oxygen tanks too quickly
- your reservoir bag is fully inflated
- you're going through your anesthetic gas too quickly
- your patient gets cold despite your warming efforts
Clearly a balance is needed, and you want to error on the side of too high of a flow rate. Most clinics use a default O2 flow rate of 1L/min for anything under 100lbs and that works well. Others calculate the rate based on 30ml/kg with a minimum of 500ml/min based on their vaporizor's limit for minimum O2 flow.
Common myths:
- Increasing O2 flow rate will decrease ETCO2.
- If the patient is not inhaling any CO2 than the cause of a high ETCO2 is inadequate ventilation. This means the rate or quality of breaths is the issue. Increasing the amount of oxygen available won't change ETCO2 in this case. Manual ventilation should be used to decrease the ETCO2 and anesthesia depth should be evaluated to see if the cause of inadequate ventilation is excessive anesthesia depth.
- Increasing the O2 flow rate (alone) will change anesthesia depth.
- While it is true that turning the oxygen flow rate up or down will also effect the amount of anesthetic gas in the system, the amount of anesthetic gas breathed in by the patient will remain the same unless you also change your vaporizer setting. This is because the amount of gas present is a percentage of the oxygen being delivered so it is flowing out of the system faster or slower too. It's like turning a hose on in a box with a hole in it designed to keep the box 2% full at all times, then turning the hose up and having the hole get bigger at the same time -you're just using more water, but the box stays 2% full.
- NOTE: It is true that increasing the oxygen flow rate for a short period of time while making changes in the anesthetic gas percentage will help the change take place faster since it will push the gas out of the system faster.
Cautions:
- If your patient is not adequately ventilating and you start giving him manual breaths, along with an increase of oxygen he/she will also be getting more anesthetic gas (because your oxygen is carrying a certain percentage of anesthetic gas, so more O2 means more gas). If the reason for inadequate ventilation was excessive depth, manually breathing for the patient could make the problem worse, so be sure to turn the vaporizer down and flush the system prior to PPV.
