---start anesth.lec.03.19.98-----

anesth
3/19/98
Klide

anesthesia 
equipment

Good morning. There are some handouts for the later lecture in 
the back of the room (EKG handout)

we talked before about VIC and VOC 
and what happens when you change O2 flows. see p 27 of handout. + means 
increase, - means decrease. 

with VOC, there is no change in vaporizer 
output with changes in ventilation. with VIC, there are marked changes in 
vaporizer output with changes in ventilation.

with VOC when minute 
volume is increased, inspired concentration may go down a little bit. 
with VIC, when minute volume is changed, inspired concentration goes up a 
lot. 

with VOC,when fresh gas flow eg O2 flow is decreased, inspired 
concentration is decreased.
with VIC, oppsite. fresh gas decreased, 
inspired concentration increases. in both cases you save money

VOC have 
to be highly efficient - have to acheive saturation concentration to work 
properly. you do know vaporizer output

VIC should NOT be efficient=- if 
too efficient will deliver dangerous concentration. output of vaporizer 
isn't known. you can increase or decrease it but you don't know what it 
is.

disadvantages of VIC: output varies with ventilation. also, can be 
accumulation of water in the vaporizer (exhaled gas from patient has 
water vapor in it at a warm temperature, and as gas goes through the 
circuit it cools, and vapor condenses. this can happen in the vaporizer. 
then water mixes with volatile anesthetic, and if you're using it with a 
wick, wick can get wet with water and get less efficient. also you have 
to actually think about what you are doing with the VIC

advantages of 
VIC: used to be cheap, but that's not true anymore really so forget it. 
also, can use with different agents unlike VOC which are agent specific.


disadvantage of VOC: initial cost (high), may be difficult to produce 
adequate inspired concentration with low flow.
advantages of VIC: you 
know delivered concentration (but do you care??), output doesn't vary 
with ventilation.

SAFETY
most of the safety systems built in are related 
to oxygen and nitrous oxide. if a nitrous oxide source was accidentally 
connected to the O2 side of the anesthesia machine, what would happen? 
when you turned on the oxygen flow meter and thought you were giving 
oxygen, you'd give 100% nitrous and the patient would turn blue and die. 
so there are efforts to prevent this.

different parts are color coded - 
here, O2 is green, and nitrous is blue.
in most of the world, O2 is 
white.

CGA standard - compressed gas association standard - large 
cylinders are connected through the pressure reduction valve. the 
fittings are threaded. the threads are varied by threads per inch, pitch 
of threads,and direction of threads. for each gas, threading system is 
diffferent to prevent you from putting the wrong gas on. 

pin index 
safety system: on the cylinder there are holes at different distances and 
pins are on the machine and this is specific for each gas.

diameter 
index safety system - male and female screw type fittings on the hoses, 
threaded differently for different gases

O2 fail safe system: if you 
have O2 and nitrous connected and you are using both gases, what happens 
of O2 tank empties and you don't know about it? you deliver 100% nitrous 
and kill the patient. to prevent this, modern machines have a fail safe 
system that is of one or two types. it senses the pressure on the O2 
side,and if it falls to a minimum amount, machine will turn off the 
nitrous. so you give no gas to the patient, which is generally better 
than 100% nitrous. you would notice this b/c the breathing bag would 
collapse, patient couldn't inhale, you could rapidly fix it. that's a 
complete failsafe system. the proportional failsafe - machine knows what 
flow of nitrous you've chosen, and doesn't let you choose an O2 flow less 
than 30%, so you can't deliver a hypoxic mixture to the animal.


vaporizer filling: you have to pay attention and put the right stuff in, 
or you can use one of the special fittings on the bottles of volatile 
liquid that fits only onto the right vaporizer.

Closed Circuit 
Anesthesia

there are many ways for getting from one place to another. 
you can sail on a small sailboat, a cruise ship, or a windsurfing board 
with various levels of comfort.

same in anesthesia. some patients are 
small and some are large, and some are mean and uncooperative. we need to 
restrain them. consider the goal of induction. ultimately with inhaled 
anesthetic. we need to initially restrain them either moderately or 
markedly depending on temperament. we need to intubate and connect them 
to the machine. 

preoperative meds: we talked about this before, we'll 
do it again. today in generalities. we'll get more info later.

most 
animals will be apprehensive, even friendly animals are this way in 
hospital environment. they may be cooperative but still apprehensive. 
this will cause increases in HR and BP due to sympathetic stimulation. 
also there are increased circulating catecholamines. drugs that interact 
with catecholamines to produce ventricular arrhythmias include xylazine, 
thiopental, propofol, and halothane (all sensitize myocardium to catechol 
effects).

the animals may be in pain. in processing them for anesthesia 
you must move them and you may need analgesics. 

induction can be 
acheived by IM, IV, or inhalation administration of drugs. usually IV 
administration is used, so you need venous access - a needle or catheter. 
you need animal to be cooperative for this. so you give tranquilizers, 
sedatives, and/or opiates in various combinations.

if we take a dog, 
give it hydromorphone, place an IV cath, give thiopental, intubate, what 
would happen if you just wait. what would the dog do over time? it would 
wake up in some amount of time. our goal, then, is a transition from the 
IV anesthetic to the inhaled anesthetic. you give the IV, intubate, 
connect to the machine, give inhaled anesthetic - goal is to get animal 
anesthetized with inhalant before it wakes up from IV. we need to 
consider how to achieve this. we know we can give some quantity of 
inhaled anesthetic based on amount and flow of anesthetic. you have your 
machine, and you can control two things - the vaporizer setting and the 
oxygen flow.

for any chosen O2 flow, there is an appropriate vaporizer 
setting to have the patient at the right depth of anesthesia. 

consider 
the O2 flow. the minimum O2 flow we can deliver is the metabolic oxygen 
requirement. this varies with size - from 10 ml/kg/min for small animals 
to 2 ml/kg/min for large animals. this is the minimum amount you can 
deliver per minute to your patient. it is what they need.

the maximum 
flow, if using the circle system, is the respiratory minute volume, which 
would make the circle non rebreathing. higher than that would be 
pointless, as nothing would change in the circuit.

so your range may be 
from 5 ml/kg/min up to 200 ml/kg/min - very wide range.

you also know 
now that depending on the flow you choose, you will influence the 
inspired concentration of inhaled anesthetic by the patient. in all 
discussions, consider most commonly used circuit and vaporizer system - 
circle system with VOC. what will the effect of O2 flow be on inspired 
concentration for VOC as you adjust flow up or down? when flow is high, 
inspired concentration is higher than when flow is low. one of the 
limiting factors is that the commercial vaporizers have some maximum 
concentration setting for the anesthetic - for iso and halo, max is 5%. 
as it happens, if you try to start induction with inhaled anesthetic at 
low flows, with the vaporizer at 5% (maximum), the patient will wake up 
from injectable anesth before falling asleep from the inhaled anesthetic.


so, during induction with VOC, your flow must be high enough to produce a 
high enough inspired concentration to get the animal asleep at a 
reasonable time.

what  gas is in the patient's lungs prior to 
anesthesia? air. 
what is air? oxygen and nitrogen
what is the gas in the 
circle system likely to be? air.
you induce, intubate, connect to 
machine, and both patient and machine are full of air. you want to get 
anesthetic into the patient. you do this by providing a certain inspired 
concentration. you want to get the air out of the circuit and replace it 
with your fresh gas (O2 and anesthetci). as animal inhales and exhales, 
fresh gas mixes with exhaled gas and old gas is flushed out popoff valve. 
so, over time, the concentration of nitrogen being exhaled will decrease. 
as fresh gas flow (FGF) is smaller, nitrogen is flushed out of the system 
a little more slowly. in a hurry, use higher flow, not in a hurry, use 
lower flow. if you use high flows, though, most of your O2 and anesthetic 
is wasted and causes pollution, so be judicious. you have some concern 
for time and money, so you choose some intermediate flow rate that gets 
the nitrogen out fast enough without wasting too much stuff.

there seems 
to be a fire in the building right now...we're evacuating.
ok, we're 
back. some welders in the next room set some cardboard on fire. this 
wouldn't happen at Cornell, damnit.

so, considering the flows to flush 
the nitrogen out of the circuit - a moderate flow of 1/2 the minute 
volume or less is reasonable. what other factors will affect inspired 
concentration?

the circle itself is made of many parts which are rubber 
or plastic. the inhaled anesthetics are soluble to varying degrees in 
rubber and plastic. there is also a lot of sodalime which the anesthetics 
are soluble in. some amount of anesthetic going into circle are taken up 
by the plastic and chemical, lowering the inspired concentration. some 
agents like methoxyflurane are more soluble than other agents like 
halothane,causing a big difference in the dialed concentration you need 
to provide an adequate inspired concentration.

what else? the patient. 
what happens in the patient? it inhales the gas, removes some of the 
gases and then exhales. so some amount of anesthetic is taken up, and the 
exhaled gas contains the leftover amount, less than what was inhaled. the 
difference b/w inhaled and exhaled anesthetic amount varies with time. at 
first, the patient has no anesthetic in itself, so it inhales and takes 
up proportionately more anesthetic which is distributed to various 
tissues. so exhaled gas is very low in anesthetic, early on. the exhaled 
gas mixes with fresh gas, lowering inspired concentration. over time, 
animal "fills up" with anesthetic, removing less and less from the 
inspired gas. for now, the important part is that it removes a lot early, 
and a lot less later.

he's drawing a chart of inspired concentration and 
vaporizer settings vs time.

goal is to acheive 1.3 mac for inspired 
concentration. that concentration should be the alveolar concentration. 
there are changes in the circuit that decrease inspired concentration, 
and so forth. if you dial 1.3 mac for halothane (1.2 %), and set the 
right flow, and look at the inspired concentration over time, you will 
get a curve starting at 0, rising, and approaching 1.3 % inspired. how 
long it takes depends on chosen flow and solubility of agent in the 
patient and the equipment. but for any agent, the itme it takes for the 
patient to have that inspired concentration would be unacceptably long. 
so what do you do to overcome this problem? instead of dialing 1.3 MAC, 
dial 2 MAC. now what happens? this is 2% for halothane. you get a 
similarly shaped curve that reaches 1.3 mac much  more quickly. if you 
dial 3%, you reach 1.3 mac still more rapidly. so that is how you 
overcome the differences b/w inspired concentration and dial setting - 
start with a setting at a higher concentration of MAC than you want to 
acheive. now, if you leave the vaporizer setting that high, you will 
overanesthetize your patient, and possibly kill it. so the goal is to 
pick a MAC multiple that will get the animal to 1.3 mac at a reasonable 
amount of time (not too fast, not too slow), and when the animal reaches 
the appropriate depth of anesthesia, you will change the vaporizer 
setting and/or the flow, to provide a different inspired concentratino to 
maintain the animal. 

again, start at fixed flow - say 100 ml/kg/min, 
and dial 3% halothane - the animal goes to sleep quickly, you determine 
when it is deep enough, and then you decrease the inspired concentration 
- one of two ways. either turn the flow down, or turn the vaporizer down. 
there are reasons for choosing one or the other which we will discuss.


going back to the vaporizer discussion - we talked about using 3% 
halothane to induce dogs with halothane - and these are the reasons for 
that. to get the animal asleep in a reasonable amount of time.

look at 
methoxyflurane - it is more soluble in the plastic and in the patient. if 
you set the vaporizer for 1.3 mac for metofane, it would take a day to 
anesthetize it. even at 3 or 4 times mac for metofane, it would take a 
long time to reach the right depth of anesthesia in the patient, because 
so much is taken up that the exhaled gas has very little of it. so you 
have to use a much higher multiple of mac, about 10 times mac, to induce 
a patient with methoxyflurane. 1.3 mac is about 0.3 % for metofane. you 
need about 3% for induction. so even though mac for metofane and 
halothane are quite different, the vaporizer settings for induction are 
the same. this is because of solubility differences in the patient and 
the equipment. 

again: MAC for metofane is 0.2 something, and 1.3 mac is 
about 0.3%. metofane is really soluble in the patient and in the 
equipment. if you put the vaporizer on 0.3% it would take til tomorrow to 
reach the alveolar concentration of 0.3%. the way to overcome that is to 
use a higher vaporizer setting than 1.3 mac. using 2 or 3 times metofane 
mac is STILL not good enough. you have to use about 10 times metofane mac 
(that turns out to be 3% - 10 x 0.3) to induce with metofane in a 
reasonable amount of time. halothane, on the other hand, is less soluble 
in the patient and the equipment. it doesn't take as long to reach an 
alveolar concentration of 1.3 mac. you only need to use about 3 times 
halothane mac - which turns out to also be 3%.

now, once you reach the 
anesthetic state required, you adjust for maintenance. you either 
decrease flow, or decrease vaporizer setting.

once the animal is 
induced, you can choose either method to reach the maintenance 
concentration. people choose their preferred method. most places use a 
semi-closed circle system with moderate flows, some flow between the 
minimum and maximum. the minimum is 5ml/kg/min so some might use 20 or 50 
ml/kg/min. everyone has a reason. 

here at penn, we use the minimum 
flows in a closed circle manner. why? good question. you need to know 
that not many people maintain anesthesia this way, though. 

Closed 
Circuit Anesthesia.

flow requirement is th metaboli oxygen consumption.


exceptions: 
-early on in inductino, to flush circuit. 
use of vaporizer 
out of circuit during early anesthesia due to maximum setting on 
vaporizer dial.
-nitrous oxide - when you turn flows down, you can't use 
nitrous. if you use nitrous, you need a higher flow.
-size of patient - 
patient size vs amount of sodalime - consider a zebra - it is large 
compared to the amount of sodalime in the cannister, so that sodalime 
won't remove all the CO2. at higher flows, the exhaled gas is flushed out 
so you dno't need as much sodalime, so a patient that is really large 
would need a higher flow maybe.

arguments against closed circuit 
anesthesia and why they are invalid:

1. hypoxia - the flows we dial in 
closed circuit anesthesia are very low, at the lower limit of range of 
flow meter. usually, a measuring device is most inaccurate at the 
extremes of the device. this concerns some people. is that a valid 
concern? think about it. say a 10 kg dog is connected and the breathing 
bag is a 2 L bag (choose bag based on patient size). the bag is filled 
b/c we start at high flow. now, what gas is in the bag? oxygen, CO2, 
anesthetic. the gas before coming back to patient goes through sodalime, 
so that leaves O2 and anesthetic. we set the flow meter as inaccurate as 
possible - turn it off. will the animal become hypoxic? yes, it will use 
up the oxygen available at a rate equal to metabolic oxygen requirement - 
which is about 5 ml/kg/min, or 50 ml/min for this ten kg dog. the bag 
holds 2000 ml, so over time, every minute, the bag will get smaller by 50 
ml. the gas animal inhales will be oxygen, and lower and lower 
concentrations of anesthetic. so in 40 minutes, bag will collapse and dog 
will get hypoxic. this is if flow meter is *off*. so, if there is small 
error, the bag will get smaller with time, but it will take much longer 
to collapse, so the whole concern about inaccuracy in the flow meter is 
invalid.

2. vaporizer accuracy at low flows - for modern vaporizers, 
this isn't a concern, even at very low flows.

3. unknown inspired 
concentration - anyone using a circle system at flows less than the 
minute volume (everyone) doesn't know the inspired concentration. all 
they know when they use higher flows than we use is that inspired 
concentration is closer to vaporizer setting than ours is. does that 
matter? if you have an anesthesia machine with a vaporizer and a scale 
with no numbers, and a flow meter with no numbers, and you don't know the 
anesthetic, could you anesthetize a patient? yes. keep O2 high enough 
that it is adequate - so start as high as possible. monitor the patient, 
watch the bag. keep adjusting flow until bag stays the same size - that 
mark on the scale is the animal's O2 consumption. then for vaporizer 
setting, start very low and adjust up as needed. that might be awkward at 
first but eventually you'd know about where to set it. you could mark S 
for start and M for maintain. it would work fine. you don't know the 
numbers. so what?

4. people say it is difficult. well, dr klide thinks 
it is no more or less difficult than any other method. 

5. you have to 
monitor patient more closely. well, no. the patient doesn't know how you 
have achieved the inspired concentration! it just breathes. 
physiologically, it responds to the inspired concentration regardless of 
how you have acheived it. you still have to monitor appropriately as you 
do with any anesthetized patient, but there is no increased monitoring 
requirement with low flow anesthesia. 

Advantages of Low Flow:

to the 
wallet/world:

2. minimizes use of oxygen and anesthetic - usually the % 
of that used vs that delivered is very low, and the rest is wasted as it 
goes through popoff valve. with low flows, there is less waste.
3. saves 
time filling vaporizers because you use less anesthetic - therefore 
reduces human exposure during filling time
4. saves ordering and handling 
time
5. decreased use of natural resources - energy is used in the 
creation of oxygen tanks and anesthetics. 

to the patient:

1. conserves 
heat - more warm exhaled gas is retained in the circle, temp of inspired 
gas is higher at low flows, slowing heat loss
2. conserves water vapor - 
as flows are decreased, inspired concentration of water vapor increases, 
preserving normal respiratory function
3. keeps CO2 absorbent moist - if 
sodalime dries out, there are reactions which produce toxic breakdown 
products esp for desflurane, enflurane, and isoflurane which make CO and 
sevoflurane which makes a nephrotoxin.
4. provides physiological 
information - if you anesthetize an animal and it's on maintenance at low 
flow, and the bag inflates and deflates at constant rate over time, and 
then suddenly the bag starts getting bigger, that means that the animal 
has started using less oxygen - might indicate a drop in temperature 
(causes small decrease in O2 use), or might indicate accidental change in 
flow rate for mechanical reasons (bump into machine, etc), or might 
indicate a decrease in cardiac output (assess cardiovascular status!)


other advantages:

1. minimizes global pollution - ozone layer effects, 
acid rain, global warming. 2. also, it's much slicker.

Disadvantages:


None, but you have to change soda lime more often, which may really be an 
advantage because it reduces risks of making breakdown products.
may also 
be hard to find leaks.

Semi-closed system advantages: None. some say it 
will increase the income of the manufacturers of the anesthetics...


Disadvantages of semi-closed system - opposite of closed circuit 
advantages.

so for closed circuit anesthesia, you get better results at 
a lower cost.
----end---