Understanding Inhalant Equilibrium and Time Constants
When the vaporizer is turned on to a certain percentage the patient does not immediately see that concentration of inhalant in their brain. For that inhalant concentration to be seen in the patient's brain the inhalant must diffuse across several partial pressure gradients (vaporizer to breathing circuit; breathing circuit to inspired air; inspired air to alveolar air; alveolar air to blood; blood to brain). For the inhalant to diffuse into the next "compartment" it must establish equilibrium, meaning that the partial pressure of the inhalant is equal between both compartments.
Inhalant uptake can be correlated to the anesthetic depth and there are numerous factors that play a role in determining the speed of uptake (and removal) of the inhalant within the body. Establishing equilibrium within the breathing circuit is the longest phase as once the inhalant reaches the alveoli, equilibrium is established relatively quickly when discussing the newer inhalants that are insoluble (isoflurane, sevoflurane, desflurane).
The time constant principle is used to help determine how long it is going to take before equilibrium is established within the circuit and therefore the patient's brain. The time constant is dependent on the overall circuit volume and the oxygen (O2) flow rate set on the flow meter.
Time Constant (min) = Circuit Volume (L) / O2 flow rate (L/min)
The O2 flow rate is the biggest contributing factor that will change the time constant. It takes at least 5-time constants to see a 99.9% change in the inhalant concentration within the breathing circuit.
Clinical Example
We have just induced a 20 kg patient, intubated and attached a dual wye hose rebreathing circuit to the endotracheal tube. The flow meter is turned on to 1 L/min and the ET tube cuff is properly inflated. The vaporizer is set to 2 % isoflurane. How long will it take for the patient to see this concentration?
Let's assume that the overall circuit volume is 4 L since this patient will be using a 2 L reservoir bag. Time Constant (min) = 4 L / 1 L/min The time constant is equal to 4 min. It takes 5-time constants to change the inhalant concentration in the breathing circuit. This means that it will take approximately 20 minutes (4 min x 5 = 20 min) for the patient to see the 2 % isoflurane in their brain if we allow them to spontaneously breathe on their own.
If the flow meter is increased to 2 L/min then the time that it takes to establish equilibrium will be decreased by half (e.g. 4 L / 2 L/min = 2 min; 2min x 5-time constants = 10 min). This is why it is recommended to use a higher O2 flow rate right after induction. It will help establish equilibrium much faster and the O2 flow rate can be turned down to maintenance rates.
Other factors that will enhance inhalant uptake include increasing the vaporizer dial setting and increasing alveolar ventilation by manually providing breaths to the patient. However, if you change the vaporizer dial setting but do not increase the O2 flow rate then it is still going to take a good amount of time to establish equilibrium to that new dial setting.
For example, using the above patient, if you increase the isoflurane vaporizer from 1 % to 3 % and have the flow meter set on 0.5 L/min it is going to take 40 minutes for the patient to actually see this change in the inhalant concentration (4 L / 0.5 L/min = 8 min; 8 min x 5 time constants = 40min) if they are spontaneously breathing. If you turn up the vaporizer AND increase the flow meter to 4 L/min then it would only take 5 minutes for this new concentration to be established in the circuit and patient's brain.
How is circuit volume determined?
We take our best educated guess and realize that it is just an estimate. Because this is the numerator part of the equation it plays only a small role in determining the time constant. The big player is the oxygen flow rate.
To determine an estimated circuit volume, I start with the calculated reservoir bag because that is a known number. Then I look at the CO2 canister. A liter fluid bag fits nicely in our canister, so I call it 1 L. You can set the CO2 canister value to whatever value you want....just keep it constant between calculations whenever you are using that anesthesia machine.
Same goes with the breathing hoses. Set each breathing circuit at a particular liter value. For example, for the expandable adult dual wye hose rebreathing circuits that are 60 inches long, I set the volume at 2 L. The regular length (40 inch) dual wye rebreathing circuits and the Universal F, I use 1 L. These are all just estimated volumes that I set....there is nothing special about these numbers.
There are also miscellaneous parts that take up volume like the one way valves, common gas outlet and fresh gas inlet. I tend to overestimate things, so I just say that these volumes are included in the circuit volumes.
If you are using a ventilator, you can add that volume as well by looking at how much the bellow housing will hold. But, when it comes down to it, we are just making an educated guess for the total circuit volume.
Whenever the patient is disconnected from the circuit it may affect equilibrium. If you induce a patient out in a prep area and then move them into the surgical suite and use a different machine, you will need to start the process of establishing equilibrium over with that new machine. If the patient is disconnected from the circuit and moved to a different position or moved to a gurney and then immediately reconnected to the circuit equilibrium will be disrupted but it should not take more than a few minutes to re-establish equilibrium.
Changing equilibrium
Once the circuit is in equilibrium, changing the flow meter setting or giving controlled breaths (manual or mechanical) will not change the inhalant concentration in the circuit or patient’s brain. The only way the inhalant concentration can be changed is if there is a change made to the vaporize dial setting.
Non-Rebreathing Circuits
Because of the low circuit volume and high O2 flow rate required for a non-rebreathing circuit, inhalant equilibrium will be established almost immediately. The high O2 flow rates are required for the circuit to properly flush the exhaled CO2 from the circuit. Therefore, the induction and maintenance oxygen flow rates are the same.
