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Terms in this set (50)

B. 1:9

The splitting ratio would be 1:9 for a sevoflurane vaporizer set to 2.1%.
Variable bypass vaporizers are used to administer sevoflurane, isoflurane, halothane, and enflurane. Variable bypass means the fresh gas flow through the vaporizer is split into two streams: one that flows through the vaporizing chamber and one that flows through the bypass chamber. The two streams are reunited at the vaporizer outlet where the fresh gas flow continues through the breathing circuit to the patient.
The vaporizing chamber contains liquid volatile agent as well as agent vapor. The vapor is at a concentration determine by its saturated vapor pressure (which is the partial pressure of the vapor when it is at equilibrium between liquid and vapor states). Saturated vapor pressures for sevoflurane, isoflurane, halothane, and enflurane are 160, 240, 243, and 170 mm Hg, respectively. This means that a vaporizing chamber filled with sevoflurane will have a vapor pressure of 160 mm Hg or a concentration of 21% (concentration = saturated vapor pressure/atmospheric pressure X 100% = 160 mm Hg/760 mm Hg X 100% = 21%). That is much higher than the concentration needed for anesthesia (about 2%). No matter what flow of gas passes through the vaporizing chamber, the concentration of the gas leaving the vaporizing chamber will be 21% sevoflurane. This gas stream must be diluted to safe anesthetic levels by the fresh gas flow passing through the bypass chamber. The ratio of gas flow through the vaporizing chamber to the gas flow through the bypass chamber is referred to as a splitting ratio. The concentration dial adjusts the splitting ratio.
For sevoflurane, in order to get a final concentration of say 2.1%, one-tenth (2.1%/21% = 1/10) of the total fresh gas flow must go through the vaporizing chamber. So if the total fresh gas flow is set to 1 L/min and the dial setting is at 2.1%, 100 mL/min of FGF will flow through the vaporizing chamber and the remaining 900 mL/min will flow through the bypass chamber.
A useful equation for calculating splitting ratios:
Flow through vaporizing chamber = Total FGF X [(Dial setting/100)/(Sat VP/Atm P)]
For the scenario presented in this question, choose any FGF rate:
Flow through vaporizing chamber = 1 L/min X [(2.1/100)/(160 mm Hg/760 mm Hg)]
Flow through vaporizing chamber = 1 L/min X 0.1 = 0.1 L/min = 100 mL/min
This means that for every 1 L of gas, 100 mL will be diverted in the vaporizing chamber and 900 mL will go through the bypass chamber. Thus the ratio is 100 mL:900 mL or 1:9.
B. Exchanging the ETT to an 8.0 ETT

Exchanging the endotracheal tube from a 6.5 to 8.0 ETT will decrease resistance to airflow the most allowing for the delivery of greater tidal volumes.
The extreme Trendelenburg of an obese patient can cause high pressure to be placed on the thoracic cavity from the shifting of organs and fat. This decreases FRC and makes ventilation difficult. Decreasing resistance to airflow will likely help in the ventilation of this patient.
The Hagen-Poiseuille Equation (or Poiseuilles Law) states that volume flow of a fluid through a rigid tube is directly proportional to the pressure gradient of the tube and to the tube radius (to a power of 4) and indirectly proportional to the length of the tube and viscosity of the fluid flowing through it.
Flow (V) = (?P?r4)/(8?l)?P = pressure gradientr = radius of tube? = viscosity of fluidl = length of tube
Changing the radius of the tube will almost always have the greatest effect on flow. In this case, the flow will more than double. Since the 8.0 ETT is about 1.23 times the size of the 6.5 ETT. When raised to the fourth power, this size difference results in a flow 2.3 times the original flow.
In larger terms, if the radius is doubled, the resulting flow is 16 times the original flow (24 = 16). Halving the length of the tube will only double the flow.
Cutting only a small amount off the length of the tube (choice A) will not cause nearly the change in flow that switching the tube can.
Administering paralytic reversal (choice C) will most likely decrease tidal volumes as the respiratory muscles lose their relaxation.
A Heliox mixture (choice D) is extremely helpful for turbulent flow as is present in upper airway obstruction. Since the patient is intubated, heliox is unlikely to help in this case. Note that Poiseuilles Law is only applicable to laminar flow of fluids. Laminar flow depends mainly on the viscosity of fluid whereas turbulent flow depends on the density. Helium and oxygen have very similar viscosities. Thus, heliox mixtures will not do much to change the laminar flow of gas. However, heliox is much less dense than pure oxygen and will greatly decrease turbulence.
B. 38 mL/min

At 1 L/min total fresh gas flow, an isoflurane vaporizer set to 1.2% would allow 38 mL/min of gas flow through the vaporizing chamber.
Variable bypass vaporizers are used to administer sevoflurane, isoflurane, halothane, and enflurane. Variable bypass means the fresh gas flow through the vaporizer is split into two streams: one that flows through the vaporizing chamber (picking up anesthetic gas) and one that flows through the bypass chamber. The two streams are reunited at the vaporizer outlet where the fresh gas flow (now carrying anesthetic gas) continues through the breathing circuit to the patient.
The vaporizing chamber contains liquid volatile agent as well as agent vapor. The vapor is at a concentration determined by its saturated vapor pressure (which is the partial pressure of the vapor when it is at equilibrium between liquid and vapor states). Saturated vapor pressures for sevoflurane, isoflurane, halothane, and enflurane are 160, 240, 243, and 170 mm Hg, respectively. This means that a vaporizing chamber filled with sevoflurane will have a vapor pressure of 160 mm Hg or a concentration of 21% (concentration = saturated vapor pressure/atmospheric pressure X 100% = 160 mm Hg/760 mm Hg X 100% = 21%). That is much higher than the concentration needed for anesthesia (about 2%). No matter what flow of gas passes through the vaporizing chamber, the concentration of the gas leaving the vaporizing chamber will be 21% sevoflurane. This gas stream must be diluted to safe anesthetic levels by the fresh gas flow passing through the bypass chamber. The ratio of gas flow through the vaporizing chamber to the gas flow through the bypass chamber is referred to as a splitting ratio. The concentration dial adjusts the splitting ratio.
For sevoflurane, in order to get a final concentration of say 2.1%, one-tenth (2.1%/21% = 1/10) of the total fresh gas flow must go through the vaporizing chamber. So if the total fresh gas flow is set to 1 L/min and the dial setting is at 2.1%, 100 mL/min of FGF will flow through the vaporizing chamber and the remaining 900 mL/min will flow through the bypass chamber.
A useful equation for calculating splitting ratios:Flow through vaporizing chamber = Total FGF X [(Dial setting/100)/(Sat VP/Atm P)]
For the scenario presented in this question:Flow through vaporizing chamber = 1 L/min X [(1.2/100)/(240 mm Hg/760 mm Hg)]Flow through vaporizing chamber = 1 L/min X 0.038 = 0.038 L/min = 38 mL/min
A. The surgeon complains that the patient is moving after incision

It is likely that the surgeon with complain about patient movement with incision due to inadequate volatile agent concentration.
In this scenario, you are administering sevoflurane with a vaporizer calibrated for isoflurane. Sevoflurane has a lower vapor pressure (160 mm Hg) than isoflurane (240 mm Hg). Therefore, in a variable bypass vaporizer, less fresh gas flow must be diverted into the vaporizer chamber to reach 2% isoflurane than must be diverted to reach 2% sevoflurane. Therefore, the actual concentration of sevoflurane delivered by an isoflurane vaporizer will be LESS than the dial setting because LESS fresh gas flow is diverted into the chamber. In this case, the concentration of sevoflurane being delivered to the patient is about 1.3% (see the calculation below). This is significantly below the minimum alveolar concentration required to keep a patient asleep during surgical incision.
In summary, if you fill a vaporizer with an agent having a LOWER vapor pressure, then the delivered concentration will be LOWER than the dial setting.
In contrast, if you fill a vaporizer with an agent having a HIGHER vapor pressure, then the delivered concentration will be HIGHER than the setting.
Recall this useful equation for calculating splitting ratios:
Flow through vaporizing chamber = Total FGF X [(agent concentration/100)/(Sat VP/Atm P)]
This equation can be utilized to calculate the actual concentration in one of these scenarios.
For a vaporizer calibrated for isoflurane, a dial setting at 2% with a FGF at 5 L/min would divert 0.317 L/min through the vaporizing chamber:
Flow through vaporizing chamber ISO = 5 L/min X [(2/100)/(240/760)] = 0.317 L/min
So 0.317 L/min of gas flow will flow through the vaporizer, but the vaporizer contains sevoflurane, not isoflurane.
Thus, the vapor pressure will be different. Rearranging the equation to solve for agent concentration gives the following equation:
Agent concentration = (vaporizing chamber flow/total FGF) X (Sat VP/Atm P) X 100%
Agent concentration = (0.317 L/min/5 L/min) X (160/760) X 100% = 1.3% sevoflurane
That was a lot of work. Luckily there is a shortcut. Forget about calculating the splitting ratio. Here is the simplified calculation:
Actual Agent B concentration = Dial setting X (Agent B VP/Agent A VP)
Agent A is the volatile anesthetic for which the vaporizer is calibrated, whereas Agent B is the volatile anesthetic that was used during the incorrect filling.
So, in the above scenario, the simplified calculation would be:
Actual sevo conc = 2% X (160 mm Hg/240 mm Hg) = 1.3% sevo
C. Remove endotracheal tube

During a flash fire of the airway, immediate removal of the endotracheal tube or airway device is indicated.
Electrocautery or laser surgical devices may ignite combustible materials in the presence of oxygen. The risk is increased in superoxygenated environments and thus it is preferable to reduce oxygen concentrations to as low as the patient will tolerate, preferably close to room air concentrations (or at least below 0.30 FiO2). Other ways to reduce risk includes to withhold ventilation during cautery and to use an foil-wrapped tubes during operations where a laser may be in operation or filling the endotracheal tube cuff with saline. If the tube does ignite, the first action is to remove the endotracheal tube and ventilate the patient using a mask or via the tracheostomy if one can be rapidly placed. After a fire has occured, switching to room air concentration will not snuff out the fire and is an inappropriate and unbeneficial action to take. Water should not be poured into the oropharynx or down the endotracheal tube, which should be removed immediately. Turning off mechanical ventilation will be done after removal of the airway which should be immediate to remove a burning fuel source. Turning all airway gases off may sound like it will deprive the fire of oxygen or combustion supporting gas, but the amount of oxygen and gas already present and available in the lungs and airway tracts is sufficient enough to continue burning for some time and thus the airway itself has to be removed first.
B. Surfactant decreases surface tension, reducing the pressure within small alveoli

Laplaces Law has applications in respiratory and cardiovascular physiology. In the context of alveoli, Laplaces Law can be written as: Pressure = (2 X surface tension)/radius. When comparing two bubbles of different sizes but with equal surface tension, the smaller bubble will have a greater pressure. Thus, alveoli that are smaller would tend to empty into larger alveoli. However, this does not occur due to a coating of surfactant within alveoli. The surfactant works by decreasing surface tension such that, as an alveolus gets smaller, the surfactant becomes more concentrated, decreasing surface tension further. This ensures equal pressure within all alveoli regardless of size. Prematurely delivered babies may have difficulty breathing due to underdeveloped lungs and a lack of surfactant which makes inhalation especially difficult. Think about blowing up a balloon. At first it takes a lot of pressure to begin inflation, but as the balloon gets larger, less pressure is needed to continue inflation and it becomes easier to blow up. A premature baby must overcome this initial resistance to inflation every breath. This is why "preemies" may spend their first few months on a ventilator.
Laplaces Law can also be used to explain the mechanism of COPD caused by emphysema. In this disease, the alveolar walls are destroyed creating larger airways that are much easier to inflate because they are like halfway inflated balloons. However, the overinflated alveoli lack sufficient surface tension and elastic recoil to readily deflate. (Recall that exhalation is a passive process in a healthy individual.) Thus, exhalation becomes more difficult and lung volumes tend to increase due to air trapping. Chest x-rays of these patients show very large lungs.
The other choices are incorrect as surfactant actually reduces elastic recoil and alveolar closing. Surface tension caused by the interaction of water molecules is the driving force behind elastic recoil. While surfactant indirectly assists with oxygen exchange by helping to keep alveoli open and available for the diffusion process, surfactant is not directly involved in the gas exchange process. Serous fluid, not surfactant, decreases friction in the pleural cavity. Finally, although it is true that type 2 alveolar cells produce surfactant, this choice does not answer the question that is asked.
C. Second gas effect

The second gas effect is the phenomenon that is exploited during pediatric mask inductions.
Nitrous oxide diffuses much more rapidly into the blood from the lungs than volatile anesthetic agents. This quick diffusion results in a decreased alveolar concentration of nitrous oxide in the lungs and increased concentrations of the remaining gases. Concentrating the remaining gases including the volatile agent speeds induction. Use of nitrous oxide also causes an increase in alveolar ventilation. Alveolar ventilation is inherently driven by gas diffusion across the alveolar membrane. As oxygen is dissolved into the blood, more air is pulled into alveoli to replace the deficit. Nitrous oxide diffuses into the blood very quickly (1 L/min) thus causing a large deficit and increasing flow of new gas into the alveoli.The second gas effect speeds both induction and emergence by this mechanism.
Choice A is incorrect as diffusion hypoxia refers to the dilution of oxygen by the rapid diffusion of nitrous oxide out of the blood during emergence.
Choice B is only partly correct as the concentration effect refers to the increased uptake of a single gas by using higher concentrations than target blood levels. The second gas effect can be considered a special case of the concentration effect. Thus, choice C is the better answer.
Choice D is incorrect as the Fick principle (not to be confused with Ficks Law of Diffusion) is applied to the calculation of oxygen consumption.
Choice E is incorrect as the Bernoulli principle deals with the relationship between fluid flow velocity and pressure.
C. Hemoglobin

Increasing hemoglobin will by far have the highest effect on oxygen content (CaO2) in the blood over any of the other choices.
Blood carries oxygen two ways: dissolved and bound to hemoglobin. The formula for calculating oxygen content reflects this physiology:
CaO2 = ( PO2 X .0031 ) + ( [Hb] X SaO2 X 1.39)
The formula sums the oxygen that is dissolved in the blood to the oxygen carried by hemoglobin. The first part of the equation, regarding dissolved oxygen, follows Henrys Law. Henrys Law states that the concentration of a gas in solution is directly proportional to the partial pressure of gas. The solubility coefficient determines how much of the gas is dissolved in solution. For oxygen in the blood, the solubility coefficient is 0.003 mL/dL/mm Hg. Thus, to determine the amount of dissolved oxygen, multiply the solubility coefficient by the partial pressure of oxygen (PO2 X .003). To find the amount of oxygen bound to hemoglobin, multiply the concentration of hemoglobin by the oxygen saturation and multiply by the oxygen carrying capacity of hemoglobin ([Hb] X SaO2 X 1.39). Theoretically, one gram of hemoglobin can carry up to 1.39 mL of oxygen. Some sources say that this theoretical carrying capacity overestimates the actual amount of oxygen that is carried by hemoglobin and cite constants as low as 1.34. Most anesthesia references use 1.39, however, so this review will do the same
Looking at the equation, it is clear that increasing partial pressure of oxygen will not change oxygen content greatly. PO2 is only multiplied by .003, showing a very slight increase even if greatly elevated partial pressures exist.
Increasing hemoglobin can greatly increase oxygen content of the blood, however, due to its higher carrying capacity of 1.39.
This question assumes a normal oxygen saturation. However, there is a limit to which saturation can be increased (100%, or 1.0). FiO2 will cause increases in PO2, which, again, does not greatly increase oxygen content. Increasing iron content could theoretically increase hemoglobin levels especially in iron-deficient anemic disease states. However, it would not have the same quick impact of giving a unit of packed red blood cells.