hi this is Michael Altos we're continuing our discussion of uptake and delivery of innovational anesthetics this is recording part two now we're going to talk about how we deliver inhaled anesthetic to the patient our goal of course is to get the anesthetic molecules in the brain and achieve a concentration of them in the brain remember that everything equilibrates and at equilibration at equilibrium the partial pressure is the same throughout the entire system that means the partial pressure of agent in the CNS should be the same as the partial pressure in the blood which should be same the same as the partial pressure in the alveoli and there's rapid transfer of gases from the olive to the blood to the CNS so at equilibrium if you know PA the partial pressure in the alveoli then you know pcns how much partial pressure there is in the brain again just a quick comment about nomenclature people don't usually say ah yeah how much anesthesia were we using 7.6 millimeters of mercury of isoflorine that's not how people talk we don't talk about partial pressures of isofluorane we usually talk about fractional concentration so we say one percent ISO well one percent of what one percent of atmospheric pressure one percent of 760 which is 7.6 millimeters of mercury since f is proportional to P if there's one percent ISO in the alveoli there's also one percent isoflorine in the CNS okay once again physiologically these anesthetic gases work based on partial pressure in the brain not fractional concentration so if you would perform anesthesia up on Mount Everest you couldn't achieve the same anesthesia with one percent isoflorine that you achieved down at sea level the patient's still getting one percent isoflorine but one percent of what a very thin air and it's not the one percent that the brain needs to have anesthesia it's the 7.6 millimeters of mercury and our vaporizers are actually calibrated to deliver a partial pressure not a fractional concentration even though the dial says one percent it's really calibrated to deliver a partial pressure so your civil vaporizer actually only delivers one percent when you're when you're at atmospheric pressure of 760 at sea level but it will always deliver approximately the right partial pressure at any altitude and this is true for all of our vaporizers except Des fluorine foreign term that we need to understand is solubility solubility is how much gas can dissolve in a given solution and we use this letter Lambda to indicate the solubility coefficient some people call it the partition coefficient and usually when we show this Lambda we're talking about BG the blood gas partition coefficient this is the ratio of two concentrations the concentration of anesthetic dissolved in the blood and the anesthetic agent that's in the gas phase remember that at equilibrium partial pressures are the same but concentrations are not and that's what this solubility coefficient tells us when the solubility coefficient is high greater than one it means there's more agent in the blood and less in the gas phase we're talking about a soluble agent that's more soluble in Blood and you need to dissolve more gas in that blood in order to generate a certain partial pressure because we want to achieve equilibrium a lower solubility when the coefficient is less than one means that you have less agent in the blood and more agent in the gas phase so you don't need to put very much agent into the blood before it becomes saturated and you start generating a partial pressure and then you can have equilibri equilibration of the partial pressures we're going to go over this concept a few times okay remember that there are other partition coefficients we're just talking about the coefficient that describes movement between the gas in the alveoli and the blood in the pulmonary circulation but there are partition coefficients that describe every movement from one compartment to another from blood to gas from brain to blood from muscle to blood from fat to blood anytime we describe movement of gas from one environment to another we can use a partition coefficient to describe how it moves this table shows the partition coefficients for different inhalational agents moving in different environments the ones that you're most likely to be asked to memorize are the blood gas coefficients but I've also highlighted the fat blood coefficients because often when we talk about an agent building up during a long anesthetic we're thinking about how much does it build up in the fat all right we now have enough terms that we can start describing the process of anesthetizing a patient and that process is called induction induction is the process of getting an aesthetic agent from the anesthesia machine into the patient's brain so the patient goes to sleep and it would be nice if we could just inject it right into their brain but in fact we have this complicated system of giving it into an anesthesia machine and then into their lungs and into their blood and distributing it throughout their body until it gets to their brain our goal is to achieve a steady state of anesthetic partial pressures throughout the whole system we have a vaporizer which continuously adds agent to the fresh gas flow at a fixed concentration the fresh gas mixes with the circuit gas in your bag and your tubing canister and your piping and the concentration of agent starts getting diluted and over time the compartments all equilibrate and the concentration of agent in the circuit Rises so here we have our anesthesia machine our breathing circuits and now we're going to attach this system to the patient's lungs we have fi it's the fractional concentration of inspired agent the agent leaving the circuit and if we get this circuit all primed up let's say with two percent xivofluorine so there's two percent sibo fluorine throughout this entire system then we have the fa the alveolar fractional concentration this is Agent in the lungs and at the moment we connect this to the lungs F A should be zero because there's no agent in the lungs yet and so if we look at the ratio of f a to fi that's also zero f a is zero and over time we expect these to all equilibrate equilibrium happens when F A and f i are the same or we could say f a divided by fi equals one and a fast induction would be F A over f i reaching 1 very quickly remember that F A is proportional to PA the partial pressure of gas in the lung which at equilibrium is equal to the partial pressure in the blood which is equal to partial pressure in the CNS so when F A equals f i and we're at equilibrium then we know that whatever is being put into the lungs is the same partial pressure is what's in the brain now that's all very nice but unfortunately every time we start putting some agent into the lungs the blood comes and takes it away so it's not that easy for f a to reach fi because every time we put some agent in the lungs the pulmonary circulation takes it away the agent passes from the alveoli into the blood and this movement is dictated by our partition coefficient the blood gas solubility coefficient soluble agents that have a high partition coefficient rapidly leave the lungs and non-soluble agents leave the lungs slowly let's look at some examples for nitrous oxide the partition coefficient is 0.47 and we can interpret this to mean one milliliter of blood contains only about 0.47 as much nitrous as one milliliter of air at equilibrium or we could say blood dissolves nitrous oxide 47 as well as air does but either way what we're saying is that nitrous oxide is not very soluble in Blood and it would rather be in the gas phase On The Other Extreme we have halothane whose partition coefficient is 2.4 we can say a milliliter of blood contains 2.4 times as much halothane as a milliliter of air or blood dissolves halothane 2.4 times better than air does this is a soluble inhalational anesthetic the process that we are describing is called uptake uptake is simply moving agent from the lungs into the blood but we don't just have blood flow the blood delivers anesthetic agent to the entire body the muscles the fat and of course the brain which is where it's going to work and as the blood goes to all these different tissues agent passes out of the blood into each of those tissues with movement that's dictated by a different partition coefficient the tissue blood partition coefficient and when the agent moves from the blood to the tissue now the blood when it comes back to the lungs has room to absorb more agent from the lungs we're going to stop here review the concepts that we've discussed so far make sure you have them really solid and please ask me if you have any questions and we'll continue with this discussion in the next recording