Anesthesia Pharmacology: Physics and Anesthesiology
Measured Flow Vaporizers
from: Field Medicine, Walter Reed Army Medical Center
Before we consider the commonly used variable bypass vaporizer, we will first consider measured flow vaporizers, as exemplified by the Ohmeda 885A field machine shown above.
The diagram below emphasizes the major characteristics of the measuredflow vaporizer: Figure below adapted from reference 30 and from Eisenkraft, JB: Vaporizers and vaporization of volatile anesthetics. In Eisenkraft JB, editor, Progress in Anesthesiology, vol 2, San Antonio,1988, Dannemiller Memorial Educational Foundation) (reference 40)

^{ 30}The example above is based on a fresh gas flow rate into the circuit of 5L or 5000 mls.
If the requirement were 1% (vol/vol) a volatile agent, then the vaporizer would have to generate 50 mls of halothane (Fluothane) vapor per minute (1% x 5000 mls).
We know that in the vaporizer part of the circuit, halothane (Fluothane) would represent 32% of the atmosphere, a number calculated earlier and derived from the ratio of 243 mm Hg/760 mm Hg, where 243 mm Hg represents the saturated vapor pressure of halothane (Fluothane) at 20^{o}C.
Again focusing on the vaporizer part of the circuit, we know that 32% of the atmosphere will be halothane (Fluothane) vapor and we also know that this percentage corresponds to 50 ml halothane (Fluothane).
^{ 30}Using the relationship developed earlier, SVP_{vapor} (mm Hg)/total pressure (mm Hg) and = agent vapor (x ml)/(carrier gas (y ml) + agent vapor (x ml) and substituting appropriately we would have 243/760 = 50/(y+50) where y would be the carrier gas, which must be 68% of the total given that halothane (Fluothane) takes up 32%.
^{ 30}So, 0.32 = 50/(y+50) or 0.32/50 = 1/(y+50) or y = 106 mls of carrier gas flow, meaning that a total of 156 mls of gas is flowing to the vaporizer part of the circuit and since the fresh gas flow going into the the whole circuit is 5000 mls, this analysis indicates that the vaporizer output of 156 mls will be diluted by 4844 mls fresh gas bypassing the vaporizer. This dilution will create the 1% (vol/vol) specified since corresponds to 50 mls diluted into 5000 mls.
^{ 30}It is suggested that usually, clinically, the flow rate to the measured flow vaporizer is typically set at 100 mls/min using 5L/min fresh gas on the main flowmeter which gives a somewhat less than 1% halothane (Fluothane) concentration. Previously, we back calculated to get the 106 mls O_{2} flowing into the vaporizer by starting out with 50 ml of halothane vapor, 5000 mls/min total flow and the 1% requirement for final halothane concentration. However, if we keep the total flow at 5000 mls/min, but for convenience set the measuredflow at 100mls/min instead of 106mls/min. As a consequence, the actual halothane (Fluothane) concentration would be 0.91% (47/5147).
By integer multiplication or division of these two flow parameters (100 ml/min O_{2} flowing into the vaporizer and 5000 ml/min fresh gas inflow), other halothane (Fluothane) concentrations can be easily produced, e.g.100 ml/min O_{2} flowing into the vaporizer and 2500 ml/min fresh gas inflow would yield about 2% halothane (Fluothane).
As usual, it is critical to insure sufficient diluent gas to prevent lethal halothane (Fluothane) concentrations to be delivered to the patient.
The similarity in vapor pressures at 20^{o}C between halothane (Fluothane) and isoflurane (Forane) allow gas flows calculated for one to achieve a certain volume% would apply to the other. The circumstance is different, however for an agent with a differing vapor pressure such as enflurane (Ethrane).
^{ 30}With enflurane (Ethrane), the vaporizing chamber would exhibit 23% (saturated vapor pressure:175 mm Hg/760 mm Hg) and, by analogy with our calculations for halothane (Fluothane) above, O_{2} flow would represent 77% with respect to the vaporizer atmosphere. In this example, if the objective is one percent enflurane (Ethrane) at a 3000 ml/min (3L/min) flow rates, then 30ml/min of enflurane (Ethrane) vapor must be generated (1% x 3000ml/min). The O_{2} flow to the vaporizer would be calculated using the equation introduced earlier (SVP_{vapor} (mm Hg)/total pressure (mm Hg) and = agent vapor (x ml)/(carrier gas (y ml) + agent vapor (x ml)) or specifically 175/760 = 30/(30+y), whereupon solving, y = 100ml/min carrier gas. The enflurane (Ethrane) case is represented in the figure below (adapted from reference 30 and from Eisenkraft, JB: Vaporizers and vaporization of volatile anesthetics. In Eisenkraft JB, editor, Progress in Anesthesiology, vol 2, San Antonio,1988, Dannemiller Memorial Educational Foundation) (reference 40))
^{41}Another approach to calculation using measuredflow vaporizers is as follows:
Vapor Concentration = (Vaporizer Flow (ml/min) X (P_{a}/(P_{b}P_{a})) X 100)/Total Gas Flow (L/min) X 1000(ml/L) OR
Vapor Concentration = Vaporizer Flow (ml/min) X (P_{a}/(P_{b}P_{a})) Total Gas Flow X 10
Where P_{a} = Anesthetic vapor pressure at ambient temperature; P_{b} = Barometric pressure = Barometric pressure
Sample calculation for the case of a desired 1% Isoflurane concentration at 20^{o}C (P_{a} = 238 mmHg) and Total gas flow = 5 Liters:
1% = V (ml/min) X (238/(760238)) / 5 X 10^{3} mls OR
1 = V (ml/min) X .46 / (50) OR
V (ml/min)= 1 X 50 / .46 OR
V= 109 ml/min
So for this case, the initial gas flow setting would be: Vaporizer flow = 109 ml/min and fresh gas flow = 4891 ml/min (Total gas flow  Vaporizer flow)
An approximation is also availablethis calculation is:
For isoflurane at 20^{o}C: Vapor Concentration = (Vaporizer Flow X .46) /(Total Flow X 10)
Solving for Vaporizer Flow: VF_{iso} = (Vapor Concentration X Total Flow X 10)/.46 OR
VF_{iso} = Vapor Concentration X Total Flow X 22 which is about VF_{iso} = Vapor Concentration X Total Flow X 2
Approximations for other agents:
Halothane (Vapor pressure at 20^{o}C = 243 mmHg): Vaporizer Flow_{hal} is about Vapor Concentration X Total Flow X 20
Enflurane (Vapor pressure at 20^{o}C ~ 190 mmHg):Vaporizer Flow_{enf }is about Vapor Concentration X Total Flow X 30
Sevoflurane (Vapor pressure at 20^{o}C = 157 mmHg):Vaporizer Flow_{sev }is about Vapor Concentration X Total Flow X 40
These approximations are for 20^{o}C; a Vernitrol Flow Calculator" (Whiz wheel) is available to facilitate calculations and is typically included with the vaporizer field unit. A Vernitrol wheel is shown below set up for isoflurane at 20^{o}C calculation.
^{ 41}Using the whiz wheel:
1. "Align the desired "% Concentration" scale value with the desired "Total Flow" scale value.
2.Align the movable hairline with the appropriate anesthetic agent and temperature on the "Liquid Temperature" scales.
3.The hairline them intersects the proper flow rate on the "Flow Thru Vernitrol Vaporizer" line."
Citations
^{ 30}Eisenkraft, J.B. "Anesthesia Delivery Systems", in Principles and Practice of Anesthesiology, 2nd edition, volume 1, (Longnecker, D.E., Tinker, J.H., and Morgan Jr, G.E., Mosby, St. Louis, 1998, 10011063.
^{ 31}Susay, SR, Smith, MA, Lockwood, GG: The saturated vapor pressure of desflurane at various temperatures. Anesth Analg 83:864866, 1996.
^{ 32}Kain, ML, Nunn JF: Fresh gas econoimics of the Magill circuit, Anesthesiology 29:964, 1968.
^{ 33}Mapelson, WW: The elimination of rebreathing in various semiclosed anaesthetic systems, Br. J. Anaesth 26: 323, 1964.
^{ 34}Miller, DM, Miller, JC: Enclosed afferent reservior breathing systems, Br. J. Anaesth 60: 469, 1988
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^{ 36}Bain, JA, Spoerel, WE: A streamlined anaesthetic system. Can. Anaesth Soc J 19: 426, 1972.
^{ 37}Milner, Q "Anaesthetic Breathing Systems", Update in Anesthesia, issue 7, article 4 (1997) http://www.nda.ox.ac.uk/wfsa/html/u07/u07_012.htm
^{ 38}Sykes, MK: Rebreathing circuits: A review. Br. J. Anaesth 40: 666, 1968.
^{ 39}Pethick SL: Letter to the editor. Can. Anaesth Soc J 22: 115, 1975.
^{ 40}Eisenkraft, JB: Vaporizers and vaporization of volatile anesthetics. In Eisenkraft JB, editor, Progress in Anesthesiology, vol 2, San Antonio,1988, Dannemiller Memorial Educational Foundation)
^{ 41}Field Medicine, Walter Reed Army Medical Center (http://www.wramc.amedd.army.mil/fieldmed/885atutor/Theory.htm)