---start anesth.lec.03.17.98---- anesthesia 3/17/98 Klide - anesthesia machine course organizer is dr soma - 221 2265 is his phone number; email is soma@vet.upenn.edu regarding exam schedule - there's only one on April 7th and the final. that other exam doesn't exist. the use of appropriate equipment is important in all endeavors, including anesthesia. so we are going to learn about anesthesia equipment now. how do you deliver inhaled anesthesia to patients? carefully. in the system there is a source of fresh gases - oxygen, sometimes nitrous oxide, and a pressure gauge to read the pressure, and a pressure reduction valve to reduce the tank pressure to a safe working pressure. gas goes through a flow meter you can control in L/min, and then through a vaporizer, to vaporize volatile liquid inhaled anesthetics. then the mixture of gases goes through a breathing system containing many parts. this system is attached to the patient. . excess gas goes out through a scavenger. most machines also have an oxygen flush device which lets you put a high flow of oxygen right into the circuit without picking up any anesthetic. oxygen tank is green. there are safety systems built in and one is color coding the gases. all the oxygen stuff is green in the US. he's pointing out pressure gauges, flow meters, vaporizers, breathing circuit (circle system) on a machine on the slide. slide: other machines with same parts in different configurations. you should be able to figure out one machine based on what you know about them in general. oxygen and nitrous come as compressed gas in cylinders. there are small portable cylinders and there are also large tanks. in this hospital, we need portable cylinders to move between floors and so forth. the small ones are E cylinders - they are mounted on the machine. there is a knob to turn it on and off. there is an orifince on the side through which gas exits and goes into the anesthesia machine. there are two small holes below that orifice which are part of a safety system called the pin indexing system. two holes are spaced differently for different gases. looking at the opposite side, you see a depression where a screw fits to hold cylinder onto machine, and what looks like a screw, which will melt if the cylinder is in a fire, to prevent an explosion (it allows gas to leak out). yoke - attached to machine - the neck of cylinder goes in here. there's a large screw to hold the cylinder in that hole, and there is a washer and fitting to attach to orifice gas comes out. there are two pins below that fitting which will fit into the two holes beneath the orifice to prevent you from putting the wrong kind of gas onto the yoke. pressure gauge - reads pressure in the cylinder. the pressure reduction valve is behind it. the pressure in the cylinder is very high and unsafe for direct use. it must be reduced to a lower working pressure. when gases are compressed, they will sometimes liquefy. it depends on the ambient temperature and the critical temperature for that substance. the critical temp is that above which liquefaction can't occur irrespective of the magnitude of pressure applied. critical temperature of oxygen is -100 centigrade, so compressed oxygen won't liquefy. nitrous oxide's critical temp is 36 centigrade - so it will liquefy at room temperature when it's compressed. this is important for understanding the pressure gauge's ability to determine gas left in cylinder. for oxygen, all the oxygen in the cylinder is gas. unless it's cooled beneath the critical temperature, but ours aren't. a small E cylinder when full is at about 2200 psi, and contains 625 L of O2. because all the oxygen in there is gas, as the gas is used, the pressure falls proportionately. if pressure is 1100 psi (1/2 of starting pressure), you have about 312.5 L left. so for oxygen, the pressure gauge indicates the quantity left in the tank. this is important. you want to check pressure gauge before you leave prep room to make sure you have enough to last to your destination. nitrous oxide is different. it will liquefy at room temp. in a full tank it's mostly liquid. as you use it, more of it becomes gas, and pressure stays relatively constant, until you use up all the liquid, and then pressure falls as you use it up. so pressure isn't a good indicator of what'sleft. p 4 - poem about cylinders. they are dangerous. be careful. if it falls down and the neck breaks off, it will jet away like a rocket or a balloon, and can go through the wall. you never leave them free standing,always must be supported or fastened in some way. you can also have a central supply of gas, and outlets in the wall to acquire the gas. there are different types of connectors. the central part of the fitting on the wall is where the gas comes out. there are notches around it to make sure you attach the oxygen tube to the oxygen fitting, and so forth. each fixture has a different notching system. so, how do you lower the pressure? the pressure reduction valve does this. then the gas goes to the flow meter. you have a float in there.with a T float you read the top of it. it will have a scale on the side in L or mL. some machines have a bank of color coded flow meters - blue for nitrous, green for oxygen, orange for cyclopropane which no one uses because it tends to explode. we're going to discuss vaporizers now. all the inhaled anesthetics but nitrous are liquids at room temp and have to be vaporized. in a mixture of gases, each gas contributes a partial pressure to the total pressure. the atmosphere has nitrogen, oxygen, Co2, etc. 1 atm is about 760 mmHg. the atmosphere is about 21% O2 and 79% N2. if you multiply the total pressure of the mix by the percent of gas in the mix, you get the partial pressure of the gas. 760 mmHg x 20% O2 = 160 mmHg partial pressure of O2 in atmosphere you should be able to get concentration from partial pressure and partial pressure from concentration if requested...** evaporation occurs in liquids. high energy molecules near surface sometimes escape into gas above the liquid. there's a property of liquids called "vapor pressure" which is a special partial pressure - the partial pressure of the vapor at equilibrium with the liquid. the saturation concentration is the maximum concentration of a vapor acheivable above a liquid at a given temperature. if you have a container that's closed, molecules leaving the surface of the liquid accumulate in the space between surface of liquid and lid of container. as concentration of molecules in that space increases, some will return to liquid. at some point an equilibrium is reached between molecules leaving and going back. at that point, the concentration of vapor is the saturation concentration, and the partial pressure of that vapor is the vapor pressure ** you need to know and understand those three things. the vapor pressure of a liquid varies with the temperature. as temperature goes up, vapor pressure goes up. as temp goes down, vapor pressure goes down - for any liquid. therefore, saturation concentration also increases and decreases as temperature increases and decreases. two things about vapor pressure - it changes with temperature, and that all anesthetics have a different vapor pressure. remember this. if you warm a liquid until its vapor pressure equals atmospheric pressure, it will boil. boiling occurs when vapor pressure equals atmospheric pressure. gas == vapor, but they are created differently. gas is created above the boiling point, and vapor is created below the boiling point. this is why we talk about vapors for volatile anesthetics, because they occur below the boiling point. although, desflurane can boil at room temp, so... you have to convert b/w vapor pressure and saturation concentration ** halothane: partial pressure 243 mmHg at 20 C. so, divide that by atmospheric pressure - 243/760 = 32% (not 23%) saturation concentration = VP/ATM VP = SC * ATM there's a table in the handout showing the vapor pressure and saturation concentrations of three anesthetics at different temps - as you increase temps, these items increase. try to remember vapor pressure and saturation concentration of these substances at 20 C for the next few years. ** MAC: minimum alveolar concentration at which 50% of individuals will respond to painful stimulus. the only way to determine mac is to really do it in a group of subjects. if you have a new anesthetic called X and you anesthetise the whole class to a 5% alveolar concentration and apply a painful stimulus, and none of us respond, and then we lighten it to 1%, all of us respond, and if you keep going back and forth you eventually find a concentration at which half of us respond, and that is the MAC value. mac produces light anesthesia - all patients lie still if not stimulated and half respond to stimulus. 1.3 MAC is your ED 95 at which 95% of patients will be at a surgical plane. Lambda is your blood:gas solubility coefficient, which relates how soluble the anesthetic is in blood vs air. relates to recovery rate and induction rate. if you take a container with some blood and some air in it, and you inject halothane or iso into the container, it will mix with air, dissolve into blood, and at equilibrium there will be some concentration of it in air and some other concentration in blood. the ratio b/w the concentrations is the blood:gas solubility coefficient, and that number helps to predict the rate of induction and recovery - the less soluble the agent,the faster the induction and recovery. metofane's lambda is 10, isoflurane's is 1.4. p 8 of handout - illustration of a vaporizer. starting on left in upper left, it's turned off. oxygen will flow in from left side, across the top, out the vaporizer, never entering the anesthetic chamber. the middle picture shows the knob halfway on. oxygen comes in, and part of it goes over the top, but part of it goes down a tube, crosses over the liquid, picks up vapor, comes up, and mixes with the pure oxygen, producing a final concentration on the other side. the third picture is all the way on. the oxygen comes in and all of it goes down and passes over the surface of the anesthetic to pick up vapor. this is the simplest type of vaporizer, and can be used with some agents, and with difficulty with other agents. for methoxyflurane, the inspired concentration required to induce anesthesia is very close to the saturation concentration - about 3%. the maximum you can acheive in this simple vaporizer is also about 3%. therefore you can use this kind of vaporizer, because you can acheive almost the right concentration for induction, and you can then decrease it to produce varying depths of anesthesia. for halothane, a reasonable concentration to induce is about 3%, but the saturatoin concentration at room temp is 32% which is much much higher. if a patient got 30% halothane,induction would be very fast and probably lethal. because the vaporizer described above is inefficient, it doesn't acheive the saturation concentration, but it's still not optimal. to increase efficiency of vaporizer: one way to increase concentration of anesthetic coming out of vaporizer is to increase area of contact between gas and liquid. in our vaporizer the area is small, but if we made it larger...you can do this using cotton wicks placed into the liquid, which draw up liquid by capillary action, increasing surface area, increasing concentration of anesthetic - this is used to get the concentration of metofane up to three % if you take alcohol at rm temp and pour it on your hand it feels cold because it evaporates, losing high energy molecules and cooling. a graph of ether evaporation shows that as ether evaporates, temperature falls, saturation concentration falls, and therefore the gas coming out contains a reduced amount of ether. you need to prevent this cooling effect. you could do this by putting the jar of ether into a container of water which is warm, which will slow the fall in temperature of the ether by adding heat to the ether as it tries to cool. now the temp still drops, but not as much as before. the glass jar isn't a good container because glass is an insulator, inhibiting heat transfer from water to ether. it would better if container were metal. so, the next advance in vaporizers was the "copper kettle" made of a lot of copper, which is a good heat conductor.it conducts heat from the mass of the machine into the anesthetic. the anesthetic tries to cool, removes energy from the metal, and stays at a constant temperature. inside the vaporizer is a metal disk with a lot of holes in it. the gas coming in is broken into a large number of small bubbles which does what the wicks did in the other model - increases surface contact area, increasing efficiency, so you do acheive the saturation concentration. ok, now he is drawing something on the board. you have an oxygen flow meter to control oxygen going into the copper kettle. oxygen and anesthetic will come out - concentration will be saturation concentration. another oxygen flow meter will send oxygen into a mixing chamber, to mix oxygen with the gas mixture, to produce your final concentration of anesthetic in oxygen going out of the machine into the patient. see p 11 of handout. concentration of agent x volume = amount of agent decide what total flow of gas you want to deliver and what concentration of anesthetic you want to deliver. then you figure out what the flow from the kettle needs to be to produce the amount of anesthetic that mixes in the chamber to produce the desired final concentration. you finally need to calculate the flow of oxygen through kettle to mix with bypass oxygen to produce final concentration. we'll do that after lunch. ---break--- again - there is an oxygen flow meter controlling flow to the copper kettle containing the volatile liquid anesthetic. there is a separate meter controlling the bypass oxygen flow. the numbers for flow and vaporizer settings we'll talk about thursday - how you decide what flow and setting to use. for now, we'll just get some examples. in the notes it says pounds, but we're going to use kgs. the 50 lb dog is a 25 kg dog. 50 ml/lb is 100 ml/kg, 5 ml/lb is 10 ml/kg. for this 25 kg dog we want to give 2500 mL/min at 3% concentration of halothane. see p 12 of handout. how much halothane is in the 2500 mL? 3% x 2500 is 75 ml/min of halothane vapor to mix with the oxygen to produce 3% halothane in the gas delivered to the patient. rearranging things: V = A/C (volume = amount divided by concentration) 250 mL = 75/0.33% (remember, concentration of halothane exiting kettle is about 33% at room temperature) so flow in has to be 175 ml/min of oxygen. it will pick up 75 ml of halothane vapor and then you'll have 250 ml/min of oxygen plus halothane mixture. remember, the vaporizer produces 33% halothane. you add this to your total of 2500 coming out - now you have 2750 ml/min coming out and a total concentration of 2.75% or something. or, you could set your other flow meter to 2250 mL/min, and then you'd have a total of 2500 ml/min of gas, containing 3% halothane. copper kettle was common for decades and some are still around. they still use some at NBC. the next advancement was the production of agent specific vaporizers. each type of vaporizer was created for use with one specific drug. these were also made of copper and were quite heavy. this was to prevent cooling. they contain a thermometer and a concentration selection knob. there's a fill aperture and a view window to see how full it is. what happens inside it? oxygen flows in through a single aperture. the flow is divided by a smart person inside the vaporizer who does all the math. some of the oxygen goes through and bypasses the anesthetic. some goes down to the liquid anesthetic area, picks up vapor, and mixes with bypass oxygen to make the correct concentration. there's a thermometer and a concentration scale and a scale of temperature on the concentration scale. when you cross the concentration line you want with the temperature you have, the vaporizer will produce the right concentration. so you choose a total oxygen flow - it goes into vaporizer, gets split, and comes back out at the right concentration. in order for this to work correctly, the saturation concentration must be acheived in the vaporizing chamber - must be very efficient- so there is a big cotton wick to increase surface area of contact b/w gas and liquid. these vaporizers were made in the early 60s and are used a lot these days. but this is still too much of a hassle. the next kind was smarter - two people inside. one to adjust gas flow, one to correct for temperature. you still have a single O2 flow in. it still splits like before - some goes to bypass, some picks up vapor, they mix and go out. inside is a valve controlled by a thermostat - the temperature compensating valve that opens and closes the valve as temperature changes, changing the proportion of oxygen that bypasses and proportion that doesn't. so you dial a flow that comes into the machine, and you dial a concentration, and that is all you do. these are still single agent vaporizers,built for a specific agent with a specific saturation concentration and vapor pressure. if you use a different agent with a different vapor pressure, it won't produce the correct concentration. sometimes the difference is small - with iso and halothane, vapor pressures are almost the same, so it's not a big deal - you could use halothane vaporizer for iso - but with other agents, it could be life threatening. if you think about halothane,the saturation concentration is 32%. what's a reasonable inspired halothane concentration? between 1 and 3% so therefore a very small amount of the oxygen is diverted to the chamber - most will bypass it. but if the vaporizer were made for methoxyflurane, which has a saturation concentration of about 3% but a similar reasonable inspired concentration of 1-3%, then most of the oxygen would go to the chamber, and very little would bypass it. so if you put halothane in a metofane vaporizer, it's bad. if you put metofane into a halothane vaporizer, the animal won't be anesthetized - oxygen is mostly diverted, won't pick up enough metofane to anesthetize patient. but halothane in a metofane vaporizer will produce lethal concentrations of halothane. so you must be careful. two ways to do this - just be careful and pay attention to what you are doing. the other option is to get a vaporizer with a special safety device which requires a locking mechanism to be used - your cap fitting must match the fill aperture. but that's a real pain and it leaks a lot. so now we have these simple agent specific vaporizers. how accurate are they? on pp 17, 18, and 19 are a bunch of graphs to look at accuracy. there are dial settings and actual concentrations plotted against temperature, time, and flow. as the temperature varies, the concentration remains equal to that which was chosen, so this seems to work. as time goes on, the concentration remains equal to the dial setting, so this works too. as flow varies, the concentration remains equal (varies a bit at high settings but that's not significant) to the dial settings. this graph is from a vaporizer manufacturer's literature. these people are driven by lawsuits,so they only went down to the flow used in an adult human, because they figured they would get sued otherwise. but for us, the flows in the low range are important! on page 18 are more graphs. as flow varies at low total gas flow, concentration remains essentially equal "close enough" to dialed setting. the upshot is that modern vaporizers are accurate over a wide range of flows. on page 19 is a graph for an older vaporizer. the Fluotec Mark II is still available on the open market and you can get them cheap or even free - but, the concentration of halothane is NOT equal to the dialed setting at below 4 L/min flow. as flow gets lower, at low concentration settings, the concentration drops, but at higher concentration settings, as flow gets lower, the concentration rises. what a headache! at intermediate dial settings, output may increase or decrease as flow rate drops below 4 L/min. the manufacturer put these graphs on a plastic card chained to the vaporizer. but it's really hard to use these vaporizers at below 4 L/min. now, halothane isn't totally stable in the bottle so it contains a preservative called thymal (?) which is sticky, like pine tar. as vaporizer is used, preservative accumulates - you can see this as a yellow or brown stain on the sight glass. in these older vaporizers, it causes the moving parts to stick, makes it hard to turn the knobs - eventually it seizes up and you have to get it cleaned and recalibrated, and that's about $300. Breathing Systems: two kinds - non-rebreathing and rebreathing. if you just took the hose from the vaporizer and stuck it into the animal's trachea, what would happen? animal would blow up. you need to insert a device so that animal can breathe in and out or you can breathe for it. the difference b/w the two kinds of systems is obvious. in a non-rebreathing system, the patient doesn't rebreathe previously exhaled gas. in a rebreathing system, the patient does in fact rebreathe some previously exhaled gas. Rebreathing systems: two types - circle and to-and-fro. to-and-fro is old, not used anymore.the circle system is the most common system used in people and animals. the circle system is made of various parts. the terms used to describe the use of the circle are related to the oxygen flow being used in relation to the oxygen consumption of th epatient. if we deliver exactly the amount of oxygen the patient is consuming per minute, it is a "closed" circle system. if the flow is higher than the oxygen consumption of th epatient, it's a "semi-closed" circle system. the parts are the same, it's the relationship b/w flow and oxygen consumption that is changing. metabolic oxygen requirement is calculated as 10 x kg ^3/4 m./min it's related to surface area, not weight. in fact, drug doses should be given in relation to surface area of patient, not weight. we get away with using weight for many drugs, because the range of weights is relatively small and inaccuracy isn't that important. but for toxic drugs, they need really accurate dosing, and those drugs are dosed by surface area. 20 kg dog needs about 5 ml/kg/min - note that this varies with size 400 kg horse needs 2.2 ml/kg/min 1 kg kitten needs 10 ml/kg/min as animal gets smaller, oxygen consumption per unit wt increases, and dose of drug per unit wt increases - and as it gets larger, these things decrease. diagram on p 20 - parts of circle system gases, flow meter, vaporizer -> then you get to the circle system, a rebreathing system. now, what's in exhaled gas? CO2, O2, anesthetic, and H2O. it's ok to rebreathe the O2 (and it saves you money). the anesthetic is also ok to rebreathe and it also saves money. the water is also ok. But the CO2 isn't ok, so you have to remove it. in the circle system is a canister or two containing chemicals which remove the CO2 from the expired gas. just remember that the substance is a chemical, the reaction with CO2 produces water vapor and heat, and eventually the substance is used up and must be changed. you have to know when to change it. the chemicals used to absorb the CO2 are in a crystal structure that also encloses some water. some amount of water in the crystal is required to make it work well. there are two problems with it, though. if the soda lime gets dried out, it will react with inhaled anesthetic, producing some breakdown products - for desflurane, enflurane, and isoflurane, dry sodalime will produce CO which is bad bad bad. sevoflurane will also break down in dry sodalime to produce a nephrotoxin. so you have to have water in the sodalime - it can't be dry. the chemical reaction produces heat. when the circle is being used, if you feel the absorber canister,you can feel a band of heat on it which moves down to the bottom as more is used. that is how you can manually feel if you need more sodalime - if there is no band of heat, it's empty. but that isn't a good way to do it. now, they've added a pH indicator. as reaction occurs, and sodalime is depleted, dye changes color - usually from white to blue. so you see a band of blue moving down and when it is getting near the bottom, it is time to change. if you just had hoses and absorber,what would happen to exhaled gas? you wouldn't know which hose it was going through - no guarantee that inspired gas would have gone through the absorber. you have to have the two unidirectional valves - the inspiratory valve and the expiratory valve, forcing gas to move through in one direction only. these ensure that inspired gas is fresh or has come through absorber. what happens during respiratory cycle? oxygen is flowing in at some rate. the patient inhales and exhales at some rate. when you inhale, the flow rate of gas through the trachea isn't constant - it goes from zero to a peak and back down. but the flow in is constant. so there is a mismatch of flow rates during most of the cycle. what happens when animal is trying to inhale at a rate faster than the flow rate? it creates negative pressure on the circuit. and it can't exhale fast enough. so they put a rubber bag onto the circuit,to provide elasticity - the animal can breate in and out of it as fast or slow as it needs to. also if you have to ventilate the patient you can squeeze the bag, to ventilate the patient. so some flow of oxygen is coming in. patient is consuming oxygen at some rate. if flow in is more than that used, the volume in the system will be increasing - so bag would get bigger and bigger and eventually explode. so you need a pressure relief valve, also called a pop off valve. it opens when the pressure in the system rises, and allows excess gas to be released. what happens if you want to ventilate the patient mechanically by squeezing the bag? if pop off valve is open, gas will go out that valve when you squeeze - so if you want to ventilate, you have to close the pop off valve. chronic exposure to inhaled anesthetics is bad, so there is a scavenging system. it collects gas coming through popoff valve and carries it away. so the sodalime removes CO2, you have breathing tubes, Y piece, unidirectional valves, breathing bag, pop off valve, scavenging system, and there's also a pressure gauge that isn't in the picture,that measures pressure in the circuit - important for PPV. ---break----- so, circle system is most common breathing system. disadvantages: relatively complex - have to change sodalime - inspired concentration not known advantages: less pollution, decreased use of resources, easy to ventilate, cheaper, conserves heat and water vapor two potential problems with circle system - moving gas through circuit requires some effort, and in old circle systems, valves were very crude and required a lot of effort. the fear was that little patients would not adequately ventilate. so other types of circuits were created - nonrebreathing systems. so one concern was resistance to breathing, and this was true with old machines but probably isn't with modern machines. the second concern was dead space in the equipment -the volume of gas between the patient and the nearest source of fresh gas. the breathing hoses are full of fresh gas - but the Y piece contains dead space. standard tubing is about 3 or 4 feet long, but you may use longer tubing when anesthetizing patients for MRI or other circumstance where the machine can't be near the patient. a 30 foot long breathing tube doesn't change the dead space. the dead space is just some fraction of the Y piece. what happens as patients get very small? say you have a canary hooked up to this system we see the dog currently hooked up to. as the canary inhales and exhales, what happens? the tidal volume is so small, it will just inhale and exhale the same gas over and over. for very small patients this system is inadequate. so for some size of patient the circle becomes inadequate. Non-Rebreathing Systems: several types, classified by number of valves. in these systems the patient doesn't rebreathe exhaled gas. 0 valves - ayre's Y piece, bain's coaxial system 1 valve - Magill - exhalation 2 valves - Stephen-slater - inhalation and exhalation the coaxial system is most common now. you have two tubes - one inside the other. oxygen is connected to a smaller diameter inner tube, flows down almost to the patient. then, there is a space between the outer tube and the inner tube. when the fresh gas reaches the end of the inner tube, it goes out into the space between the tubes, out to the rebreathing bag, and out the scavenging system. when the patient breathes, it inhales gas from between the two tubes - this is a large volume, and there is no resistance to movement of gas into patient. b/c the inner tube comes right down to the patient, there's almost no dead space. when patient exhales, gas goes between the two tubes. suppose you turn off the oxygen flow. patient exhales into space between two tubes. then it goes to inhale - then it would inhale previously exhaled gas. to avoid that, you keep oxygen flowing continuously. turn oxygen back on - this pushes the gas that's between the two tubes away from the patient. the oxygen is flowing and flushing the space away from the patient continuously. the gas in the space between the two tubes up by the patient is therefore always fresh gas. to keep this true, oxygen must flow at a certain rate in relation to the size of the patient, to make sure it flushes the gas far enough away that patient doesn't inhale used gas. well, what's an adequate oxygen flow? depends on type of nonrebreathing system, and if animal is breathing spontaneously or being ventilated. he's chosen one flow for us to learn for all nonrebreathing systems regardless of type of ventilation - this is higher than it has to be for some systems, but will always work. this is 3x the animal's respiratory minute volume. that's the amount of gas moving through the trachea in one minute. that's the tidal volume times the respiratory rate. an estimate of the respiratory minute volume is about 200 ml/kg/min in a small animal. so you want to keep your flow at 3 times 200 ml/kg/min or about 600 ml/kg/min. some nonrebreathing systems in some circumstances can use lower flow rates, but this flow rate will always work. slide: bain's coaxial type nonrebreathing system. the disadvantages of these systems is that they are expensive to use - you waste a lot of anesthetic and oxygen using high flow rate. because you use more, you use more natural resources, energy, create more pollution, and for the patient there is a loss of water vapor and heat in the exhaled gas. resp tract gets dry, mucous gets thick, can see increased morbidity from breathing in the dry air. also, expired gas is warmer than room air, and inspired gas is dry and room temperature. advantages - you know the inspired concentration (may or may not be important). there's no soda lime to change. there's decreased resistance and decreased dead space, so that's the key for very small patients. there is dissension in the ranks re: when to use this system. most places use nonrebreathing systems in patients b/w 5-10 kg. here, we use them in patients less than 2 lbs. resistance isn't a problem until patients are very very small with modern machines. for dead space issues, they change the hoses to smaller diameter hoses for small animals, because those hoses use a smaller Y piece. so almost all cats and dogs are on the circle system here. at other places, most cats are on a nonrebreathing system. Vaporizer location in the circle: you have gas, circuit, and patient. the precision, agent specific vaporizers always go outside of the circle. the inefficient glass jar type vaporizers we first talked about are inside the circle - see page 25. in an out of the circuit system, exhaled gas can't enter the input side of the vaporizer. with an in the circuit system, exhaled gas can get to the input side of the vaporizer. VOC: vaporizer out of circuit system with high flow: in the circle system, using O2 flow equal or above respiratory minute volume, it becomes nonrebreathing. that is, the animal will not inhale any previously exhaled gas. you wouldn't do this, but you need to know that number. so O2 flow over 200 ml/kg/min would create nonrebreathing circuit - inspired concentration is therefore same as delivery concentration (same as vaporizor setting) as in all nonrebreathing systems. (normally, you're going to choose a flow somewhere between metabolic oxygen requirement and the minute volume. you're not going to choose something over the minute volume). so now, your inspired concentration is equal to vaporizer setting, right? say you leave that the same, but decrease the O2 flow. will your inspired concentration change? well, now, not all the exhaled gas is flushed out - it mixes with fresh gas. it contains oxygen and anesthetic in lower amounts than the fresh gas. so, now your concentration isn't equal to the vaporizer setting anymore - it's a little lower. the lower the oxygen flow, the more exhaled gas remains in the circle, so inspired anesthetic concentration will decrease. as flow is turned down with vaporizer setting left the same, the inspired concentration will decrease. so at very low flow, vaporizer may be set to 5%, and inspired concentration may be 1%. at very high flows, inspired concentration is basically same as vaporizer setting. this is with an out of the circuit system, which is most common. so, we don't know the exact inspired concentration, but we do know the appropriate vaporizer setting to produce the needed concentration. VIC: vaporizer in circuit. when you start, there is no exhaled gas coming in, so concentration coming out would be what you set it at, but these don't have concentration dials anyway, but if they did, it would be right. now, leave the vaporizer setting the same, and decrease O2 flow. what happens to inspired anesthetic concentration? it will increase, perhaps markedly, because exhaled gas will contain some anesthetic,and that mix will go to the input side of the vaporizer. the vaporizer doesn't know the incoming gas has anesthetic in it, and in fact it assumes the incoming gas doesn't contain anesthetic. so it diverts too much to the chamber, so you end up with an increased concentration. the lower the O2 flow, the higher the inspired anesthetic concentration. with VIC, changes are opposite from VOC when you change O2 flow. your goal is to present patient with a specific concentration, which you can acheive by setting flow and vaporizer appropriately. consider mechanical ventilation in both circumstances. if you ventilate the patient, modern agent specific vaporizers are made such that the pressure changes which occur during ventilation do not change vaporizer output. PPV doesn't change output of VOC. VIC is opposite. when you start PPV, output of vaporizer markedly increases. if you switch from spontaneous ventilation to PPV you will have to markedly turn down the vaporizer concentration setting to prevent inappropriately deep anesthesia. again - changes in flow - what happens with nitrous oxide? VOC: nitrous is used as a gas. you dial a flow of N2O and a flow of oxygen. say you have a 10 kg dog and you set flow of nitrous to 5 L/min and O2 to 5 L/min. after they mix, the concentration of each is 50% in the mixed gas. flows are very high, so inspired concentration will equal the dialed setting - 50% each. now, we turn down the flow meters but keep proportions the same - let them be 500 mL/min each. concentrations are still 50% each. what happens to inspired concentrations? depends on uptake of gases. if uptake is exactly the same, concentration is the same. but it isn't. after the first few minutes, nitrous uptake is much less than O2 uptake. so, animal inhales a 50-50 mix, and exhales a mix with more nitrous and less oxygen, which mixes with fresh gas, changing concentration - so over time, inspired concentration of O2 is reduced, and inspired concentration of N2O goes up. now N2O has a MAC of about 200, so you need it to be at high concentrations anyway, allowing for low concentrations of oxygen. this means you could end up with a situation of hypoxia. to prevent this, you have to either measure inspired oxygen, or use high flows. question - when using nitrous are you also using volatile liquid? answer - usually yes. could use injectable anesthetic instead. ---end-----