Pressure of molecules push up & want to come out of the container
If you have infinity (open), you have no pressure
Each anesthetic agent has its own vapor pressure (for example: Sevo has vapor pressure of 170 mm Hg is pushing up WHILE 760 mmHg is pushing down) ? the % of sevo gas in the vaporizer is 22 %
* in other words, it means WHAT % of A MIXTURE is the GAS!
? temperature ? ? molecule movement ? ? vapor pressure
Then this Gas ? enter the LUNGS? then passes through the ALVEOLAR MEMBRANE into BLOOD ? Left side of heart ? then out to Rich Muscle Group (which = Brain and liver, kidneys) ? then muscles, skin, fat, & connective tissues
“Delivering a higher FI than the FA actually desired for the patient; similar to IV bolus and thus speed the induction of anesthesia”
For ex: Vapor pressure of Sevo = 170 mm Hg ? 170/760 = 22%
If 170 mm Hg is 23% of 760 mm Hg
then, _____ mm Hg is 2% of 760 mm Hg. (15 mm Hg)
15 mmHg of sevo (it will slowly get diluted to the patient)
If you want to overpressurize the gas –>you ? sevo to 8% = 60 mm Hg
What are the only true gas?
(while the other inhaled anesthetics are vapors of volatile liquid – for simplicity- all of them are referred to as gases!)
(Isomer = 2 (or more) compounds that contain the same number of atoms of the same elements but differ in structural arrangement and properties)
= Fick’s Law!!!
3 directly proportional with diffusion
1) concentration gradient (or pressure gradient)
2) Surface area (? surace area ? ? diffusion) Lungs surface area = a size of the tennis court
3) Solubility (tissue solubility – how soluble is it in there? Halothane has bld:gas partition coeffiction of 2.4 – highest solubility) ? solubility ? ?diffusion
2 inversely proportional with diffusion
1) membrane thickness (? thickness ? ? diffusion)
2) Molecular weight/size ( The bigger the molecule the slower it diffuses! The smaller the molecule the faster it diffuses)
Think semipermeable membrane as a filter! You need to have a push to get across that semipermeable membrane. Eventually partial pressure on both side almost equal, and there will be no longer enough push from the higher side ? It can’t equilibrate!
(Passive Diffusion: with semipermeable membrane!)
(what are they?)
2. Muscle (50% of body mass – take about 19-20 % of CO)
3. Fat (19% of body mas – 6% of CO)
4. Vessel poor group (bone/ligament/hair) (1%
Where the Blood goes = where the gas goes
N?O 0.46 (highest due to concentration effect – when we give N?O – we give 70%- a large %-high pressure gradient ? FASTER rate of rise FA/FI )
Des 0.42 (when we give Des. we give 4-6% therefore SLOWER rate of rise FA/FI)
Most rapid with least soluble anesthetics (, desflurane, sevo)
Therefore, Iso and Halothane: SLOW to go in the brain and SLOW to go out of the brain
. All volatile anesthetics except Des- all have Fat:Blood solubility of 40:1
. Des Fat: Blood solubility 20:1 (just half of solubility)
. All volatile anesthetic has Muscle: blood solubility of about 3:1 (except: des & 2:1)
The Brain (or Vessel Rich group) = equilibrate very quick (it get 75% of CO)
But the other 20% of CO goes mostly to muscle group. (This will take 4-6 hours for the inhaled anesthetic to equilibrate (base on its solubility ability) ..So, this is Why we are worry for longer case!
Fat group takes about 6% of CO ? it even take longer to equilibrate (about 12+ hours) to equlibrate between fat and blood
So, if I’m going to deliver anesthetic for 12 hours – do I want to go use an anesthetic that is going to saturate the fat twice as much as the other one? (Fat:blood for Des 20:1 and Sevo 40:1) –I shouldn’t use Sevo b/c it saturates in Fat twice as much ? longer Wake up
Case <2 hours - concern about blood & brain Case 4-6 hours - concern about blood, brain, and muscle Case >6 hours – concern about blood, brain, muscle, and fat compartment (longer cases – use Desflurance -NOT SEVO b/c it has more Fat:Blood solubility)
What are tissues of undesired effects?
Tissues of undesired effects are heart/kidney/liver/GI Tract (except the Brain tissues in the VRG)
FD is …
What are factors affecting F(D)?
Factors affect F(D)
. flowmeter settings of carrier gas (O?, air, N?O)
. Vaporizer %
The fractional concentration of anesthetic leaving the circuit is designated as FI
What are the factors that effecting FI?
. breathing-circuit volume ( the bigger = more dilution)
. circuit absorption (not so much an issue with newer machine)
. The fractional concentration of anesthetic present in the alveoli is FA
. Ventilation (the faster/deeper the pt. breaths = the fast he/she loses conciousness = quicker equilibration of AI between alveolar, blood, brain)
. the concentration effect and second gas effect
(3 factors affect uptake are: solubility in the blood, alveolar blood flow (essentially = to CO), and the partial pressure difference between alveolar gas and venous blood)
concentration effect (same as overpressurizing)
Concentration effect: Giving higher FI of an inhaled anesthetic, the more rapid the rate of increase of the FA/FI
(see top part of the graph! 70% of N?O produces a more rapid increase FI/FA ratio than 10% of N?O
Concentration Effect-The total volume in the lung is decreased by the amount of gas taken up by the blood, this reduction concentrates the remaining gas in the lung.
(second gas effect and concentration effect are the essentially the same thing, just looking at a different view
Look at the lower part of the graph!
The FA/FI ratio for halothane ? more rapidly when administered with 70% N?O than 10% N?O
Uptake of large volumes of the first gas (Usually ) increases the rate of rise of a second gas given concomitantly.
. Alveolar Ventilation (VA)
. Characteristics of breathing circuit
. Hypoventilation ? rate of rise of FA (b/c it delivers less anesthetic to lungs ? ? rate of rise of FA/FI)
. Mechanical hyperventilation that ? venous return ? rise of FA (? venous return ? ? CO)
. Anesthetic solubility in rubber or plastic absorbs anesthetic gas slowing rise of FA.
. High gas in flow from anesthesia machine speeds rise of FA.
2. Alveolar to Venous Partial Pressure differences
– PA-PV = reflects tissue uptake of inhaled anesthetics
– Concentration Effect-The total volume in the lung is decreased by the amount of gas taken up by the blood, this reduction concentrates the remaining gas in the lung.
– Second Gas Effect- Uptake of large volumes of the first gas (Usually N2O) increases the rate of rise of a second gas given concomitantly.
Ventilation delivers anesthetic concentrations to the alveoli
1. So, hyperventilation delivers more anesthtic to lungs ? ______ (? or ? rate of rise of FA/FI
2. Hypoventilation delivers less anesthetic to lungs ? ____ (? or ?) rate of rise of FA/FI
3. ? FRC (functional residual capacity) ___ (? or ?) rate of rise of FA/FI (owing to dilution.
4. ? FRC ___ (? or ?) rate of rise of FA/FI
5. Ventilation/Perfusion mismatch tends to ____ (? or ?) rate of rise of FA/FI (high concentration of anesthetic gas toward 1 lung – ? concentration)
? CO ? ? FA (slow induction b/c uptake ? slowing the rise of alveolar partial pressure)
– Left to right = no change in FA ? therefore, no change in induction
– Right to left (bypass the lungs – not picking up anesthetic) = slower rise of arterial concentration of anesthetic and slower induction
SLOW train = pick up more anesthetic at time ? 1 CO? more concentration to the brain
Because PA = P blood = PCNS
if I know alveolar concentration is (PA) – it’s the same in the PCNS
We want FA/FI close to 1 (equilibrium between alveolar/blood/CNS)
#1 factor that affect PA is DILUTION
If a crying baby for ex: use 8% – overpressurizing – going to sleep with sevo in 4 breaths!)
30 y.o lady with egg allergy and can’t give propofol ? have her breath 5min on face mask to denitrogenate and fill up her lungs with O? – then start cranking up to the sevo 1%- then 2% – 3% “take another deep breath (just let her breath normal)-next thing you know- you are bagging/breathing for her
If you give the pt a volatile anesthetic – you are going to deal with hypotension (b/c volatile anesthetics are potent vasodilators)
Sevo has vapor pressure of 170
Know how to calculate partial pressure!
So, Sevo has vapor pressure of 170
So, if you have your machine calibrated at 760 mmHg and giving anesthetic at higher elevation ? you OVERDOSE your patient if you don’t recalibrate !
Opposite – machine from Colorado to Jacksonville – we are going to under-dose the patient
transcutaneous loss (minimal)
The most important elimination factor is EXHALATION
. High gas flows (turn your flow up to clear anesthetic out of the tubing)
. low anesthetic circuit volume
. low absorption by anesthetic circuit
. low solubility
. ? ventilation
(Ventilate your patient to get the anesthetic gas out 1st – don’t worry to wake the patient up –they will breath when the gas is out!)
. Longer anesthetic times
. Low gas flows
(there always dilution ?there always concentration gradient)
What is the end-target organ of inhalation anesthesia?
Which anesthetic has greatest uptake, why?
B/C Halothane has the lowest blood:gas solubility 0.42
1 time constant = 1/2 min = 63% of oxygen
2 time constant = 1 min = 86% (replacing 86% of oxygen in alveolar)
3 time constant = 1.5 min = 95%
4 time constant = 2 min = 98%
It takes at least 2 min to oxygenate the patient !
To prevent changing Sevo – ? fresh gas flow- prevent the pt to re-breath