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Chemistry/Physics
Week 3, Class Outline

7/21

  1. Behavior of gases and vapors
    1. Review of Dalton’s Law of partial pressures – the total pres. exerted is made of the sum of all gases which is exerts pressure in a container. Need to know vapor pressures of each agent. Total pres. will = 760mmHg
    2. Concentration of gas in mixture – If you were to deliver a specific concentration of gas say a total of 2.5Liters of flow with 2liters of room air and 0.5Liters of O2, how much total O2 % is being delivered? There is 21% of O2 in room air so there is 21% of one liter thus 210ml. Since there are 2L of r/a then it is 210 plus 210 added to the 500ml of O2 in .5L of flow for a total of 920ml. Divide 920ml by the total flow of 2.5L and this will yield 36.8% total O2 flow.
    3. Pressure gradients and gas diffusion – gas diffuses from areas of higher concentration to areas of lower conc.
    4. Partial pressure differences in oxygen – If you have 100%O2 at STP and you added halothane. The partial pressure of halothane (or its vapor pressure) will exert a pressure on the O2 and the sum of these pressures will = atmospheric pressure (sea level is 760) First determine the partial pressure of halothane, its vapor pres. is 243, divide this by 760 = .32 or 32%. Remaining % is O2 so its % is 68. 0.68x760 = 516mmHg of O2 remaining in flask. Can also simply subtract the vapor pressure of halothane from the atmospheric pressure to get the partial pressure of O2 since Dalton’s Law of partial pressures applies here. For each 1-liter change in O2 flow there is a 4% change in O2 flow via a simple mask.
    5. Rate of diffusion – is related to partial pres. gradients and r/t square root of molecular weight.
    6. Anesthetic gas coefficient/solubility coefficients – Solubility will determine the amt of anesthetic that will dissolve into the blood. It is looked at in terms of blood/gas partition coefficients. An agent with a high solubility is taken up more into the blood and will be released to the tissues slower while an agent with a low solubility is not taken up as well by the blood but will go into the tissues faster. The significance of the coefficients is how fast anesthetics go into the patient. Desflurane is 0.42. A very small amt. is dissolved in blood but will be taken up into the tissues quickly. Potency is high, but solubility is low. Halothane with a b/g C of 2.4 needs to saturate the blood 1st before it can be given off to tissues. It takes a higher level before it can be given to tissues.

      Blood/Gas Partition Coefficients

      1. Desflurane 0.42
      2. Nitrous 0.47
      3. Sevoflurane 0.69
      4. Isoflurane 1.4
      5. Enflurane 1.8
      6. Halothane 2.4

      The oil/gas part. Coef. Relate to the uptake within the brain

    7. Pulmonary blood flow and uptake and distribution - The uptake of anesthetic agents = the solubility(lambda) x cardiac output (Q) x differences in alveolar to venous partial pressures divided by barometric pres.   Uptake=lamdaxQx Pa-Pv/Pb
    8. Partial pres. of anesthetic in the lungs will be directly proportional to amt. of anesthetic in brain. The alveolar to venous partial pres. difference will be a factor in the rate of uptake. The pres. gradient moves from high to low. The more present in alveoli will have more to build up in brain. I.e. To increase the amt of anesthesia delivered increase ventilation – handbag and faster or increase the volume% of gas delivered. Measure the end tidal flows of gases to assure the brain’s tension as it should be close to exhaled value which should = brain’s value. I.e. Desflurane is measured as %inhaled and exhaled. If 8% is inhaled then exhaled will be about 6.4%. The amt. in the brain should be 6.4%. It is much more important to know the anesthetic value on exhalation as the depth of anesthesia is directly r/t amt. of agent in brain for inhalation agents only. The alveolar tension of anesthetic gases is directly r/t pulmonary ventilation. Another factor of distribution is the cardiac output. The better the cardiac flow the better the pulm flow and the better the uptake and distribution of agent.
    9. Anesthetic gas absorption by tissues - The anesthetic of vapor diffusing into blood is directly r/t minute volume (ventilation) and the potential agent taken up by tissue.
    10. MAC – Minimum alveolar concentration – relative measure of potency of an agent. MAC of each agent never changes. There can be a requirement change but actual MAC #’s never change. MAC is the required % of agent so that 50% of people don’t move to noxious stimuli. The following will influence MAC requirements: age, wt, sex of patient, amt. of CNS depression and the individual agent. MAC values are:
      1. Desflurane 6.0
      2. Nitrous 104
      3. Sevoflurane 2.0
      4. Isoflurane 1.15
      5. Enflurane 1.68
      6. Halothane 0.75

      There are different levels of MAC: MAC awake is the % conc. on inhalation agent where pt can open eyes to command. MAC intubating is the alveolar conc. required to intubate and not have them move. This is usually about 1.5x normal MAC. MAC block sympathetic response to stimuli is the MAC required to block the symp response. It is also 1.5x normal MAC. The additive affect of another anesthetic agent with respect to nitrous will decrease the MAC requirement of the other agent. Nitrous at 70% will decrease the MAC requirement by 40%.

    11. Influences of absorption
      1. Inspired tension – tension is = to partial pres. Thus the alveolar tension is r/t pulm. Ventilation. Increasing the inspired conc. or the alveolar ventilation increases the rate of rise of the alveolar conc.
      2. Tidal exchange – The tidal volume is the amt. of air exchanged with every breath (doesn’t include the FRC). If increase the rate and depth of ventilation this will increase the amt. of anesthetic delivered.
      3. Functional Residual Air Volume – or capacity (FRC) an increase in size of the FRC will dilute the tidal volume and thus will slow the effects of the anesthetic agent. Conversely a decrease in FRC will speed up the effects. I.e. Pedi and obese patients have a lower FRC so will have a faster uptake of drugs based just on this.
      4. Diffusional surface area – The area of lungs that can exchange gas and can be affected by shunt since areas are ventilated but not perfused or in bibasilar atelectasis will decrease the uptake of anesthetic since there is less lung available for distribution.
      5. Blood solubility – Ostwald solubility Coefficient is the ratio of agent in the liquid phase to the ratio of agent in its gas phase. Can use the #’s of coef to predict induction and emergence into and from anesthesia. Can tell the speed of induction and how fast the anesthesia levels will change. The more soluble in blood, the more dissolves in the blood therefore the slower increase in alveolar tension and therefore the slower increase in brain uptake. Blood sucks up more and leaves less in alveoli so alveolar conc. is less. I.e. Sevoflurane is less soluble and doesn’t need to saturate in blood before effects are seen. Would see a quicker emergence then also. This is directly r/t blood/gas coefficients. Halothane will take longer to go to sleep and longer to wake up.
      6. Blood brain barrier – this area is more of a factor in IV anesthetics. Inhalation agents do cross the b/b barrier while some IV meds do cross and some don’t. I.e., Why would robinul by better to increase HR than atropine. You wouldn’t get the central effects of an increased HR in robinul while you would with atropine. The end tidal anesthetic tells us what the brains level of anesthesia is.
      7. Tissue solubility – r/t whatever is in the other tissue other than in the brain.
      8. Tissue blood flow – travels from the lungs to the vessel rich areas first then to muscle groups, fat groups. Fat also draws anesthetic from well-perfused tissues by intertissue diffusion.
    12. Elimination of volatile agents - is the converse for induction. We breathe off all the agents that we are given and the time is determined by the blood/gas coefficients
    13. Diffusion hypoxia – seen as a saturation decrease in patients on emergence from nitrous as an inhalation agent. Since nitrous has a low B/G coef, the large volumes of N2O diffuse from the blood to the alveoli upon termination of the agent. In the 5-10 minutes after the N2O has been turned off the nitrous can diffuse into the alveoli and displace O2. Thus the patient is now at risk for tissue hypoxia. CO2 can also by displaced by the nitrous and the ventilation drive can be wiped out especially in pts with opoids on board or in pts with COPD. The way to avoid this is to always give 100%O2 for 5-10min after the termination of nitrous.
    14. Denitrogenation of blood and tissues - prevent entrapment of air. Get a good seal with the O2 mask and breathe in 100%O2 for 5-10min. It will wash out the nitrogen and build up a reserve or FRC so if pt is apneic he can draw O2 from the FRC and maintain his O2sat. This is very important for geriatrics, obese, peds, full stomach (pregnant after 14 wks, reflux disease, just ate - due to an incompetent esophageal sphincter to protect the airway)
    15. Gas volumes and closed spaces – deals with nitrous delivery. Avoid nitrous in bowel surgery, eye surgery or any surgery where there may be a closed space of air since nitrous will diffuse in and increase the size of the space. It will also diffuse into the ETT cuff.
    16. Second Gas Effect and concentration effect - this always is in reference to N2O and another gas. The larger volume of one gas goes into the alveoli faster and entrains the second gas as well. This only lasts 5-10 minutes and then equilibrium is reached and you no longer see the effect. This would change the amt. of the second anesthetic needed but only for those 5-10min.
  2. Vapors, vaporizers, and Vaporization of volatile anesthetics
    1. Definitions: Vapors are the gaseous phase of an agent that is normally a liquid at room temp. Vapor pressure is the pres. exerted by a vapor and depends on the agent and the ambient room temp. The vapor pres. is in a container closed to the atmosphere is when the volume of liquid in the container where the molecules are mostly in the liquid phase with a small amt. of vapor or gas phase and the molecules of gas in the vapor exert a pressure on the liquid. Each agent has a different vapor pressure. Saturated vapor pressure is when the gaseous phase is above a liquid contains all the molecules it can hold at a given temp before it "rainsout".
    2. Measurement of vapor pressure: It is agent specific
      1. Desflurane 669mmHg
      2. Nitrous
      3. Sevoflurane 160
      4. Isoflurane 240
      5. Enflurane 172
      6. Halothane 244
    3. Temperature effects – Hypothermia will decrease the amt. of anesthetic needed.
    4. Definitions: Boiling Point: temp when vapor pres. = atmosphere and when all liquid goes into a vapor. 
      Latent Heat of Vaporization
      : Amt. of energy needed to convert from liquid to vapor phase. I.e. why a steam burn is worse than a boiling water burn. The amt. of energy is higher with steam and this is transferred to the burned area thus the burn is worse. 
      Specific Heat
      : the quantity of energy measured in kilocal required to raise the temp of substance by one degree C. I.e. Water is one kcal/one gram/per one-degree change in C.
    5. Vapor pressures of current agents – as above
    6. Regulating and calculating vaporizer output – The current vaporizers on the market are all agent specific and temp controlled. They regulate output by dilution. The amt. of vapor coming out is r/t flow in thru the vaporizer. I.e. Isoflurane of saturated med – open machine allow air to pass thru and it will pick up any vapor there. Amt. of airflow thru will determine amt. picked up. Dilute it down since a saturated vapor would be lethal to patient. Vaporizers measure an absolute pres. in mmHg in volume% which is a relative ratio of molecules where as partial pres. is actual value. This has to deal with the uptake and potency of anesthetic agents, which is directly r/t part pres. and indirectly r/t volume%. The dial on the vaporizers will give an approximation of what is being delivered not an exact amt.
    7. Temperature compensation and efficiency of vaporizers – vaporizers convert liquid to vapor and control the amt. delivered. They are made of materials with a high specific heat so temp changes are gradual within the system. They conduct heat thru liquid which is why copper is good and stainless steel is bad.
    8. Types of vaporizers
      1. Flow over which are the most accurate
      2. Bubble thru which are the least accurate. Examples are the Copper Kettle and the Vernitrol where the flow rate of the carrier gas is saturated by traversing the anesthetic liquid after passing thru a bubble-forming metal sieve. These have to be manually adjusted to the final concentration.
      3. Variable bypass which allow fresh gas flow to go over the vaporizer and only a small amt. or part of it goes thru and mixes with other gases farther down the circuit.
      4. Tec6 is a special one for desflurane since it has a very high vapor pressure at room tem. This vaporizer warms and pressurizes liquid desflurane to 1500mmHg. There is a variable resistance that is controlled by a differential pressure transducer.
    9. Carrier gas and vaporizer output – fresh gas flow through vaporizers would pick up whatever agent. This occurs in variable bypass ones where the carrier gas splits off the vaporizer and mixes with the vaporizer then the fresh gas flows will combine with the outflow of the vaporizer with the gas carrying the agent. Fresh gas flow is whatever gas is running through the machine while the carrier gas is whatever flow picks up the anesthetic agent and will dump it back into the total gas flow.
    10. Barometric pressure and vaporizer output – These differences vary inversely but this is more r/t older types of delivery systems that aren’t temp regulated or pres. controlled and you had to figure out vapor% on your own.

 

  B/G MAC
Volume %
Vapor Pressure
mm/Hg
Desflurane 0.42 6 669
N2O 0.47 104 51
Sevoflurane 0.68 2 160
Isoflurane 1.4 1.15 240
Enflurane 1.9 1.7 172
Halothane 2.4 0.75 244

 

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