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 measured-flow 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)

• ³⁰The 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^oC. 

    • 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).

  • ³⁰Using 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%.

  • ³⁰So, 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. 

  • ³⁰It 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 O₂ 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 measured-flow 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 O₂ flowing into the vaporizer and 5000 ml/min fresh gas inflow), other halothane (Fluothane) concentrations can be easily produced, e.g.100 ml/min O₂ 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^oC 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).

  • ³⁰With 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, O₂ 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 O₂ 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))

• ⁴¹Another approach to calculation using measured-flow 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^oC (P_a = 238 mmHg) and Total gas flow = 5 Liters:

    • 1% = V (ml/min) X (238/(760-238)) / 5 X 10³ 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 available---this calculation is:

    • For isoflurane at 20^oC: 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^oC = 243 mmHg): Vaporizer Flow_hal is about Vapor Concentration X Total Flow X 20

    • Enflurane (Vapor pressure at 20^oC ~ 190 mmHg):Vaporizer Flow_enf is about Vapor Concentration X Total Flow X 30

    • Sevoflurane (Vapor pressure at 20^oC  = 157 mmHg):Vaporizer Flow_sev is about Vapor Concentration X Total Flow X 40

  • These approximations are for 20^oC; 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^oC calculation.

⁴¹Using 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."

• ³⁰Eisenkraft, 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, 1001-1063.

• ³¹Susay, SR, Smith, MA, Lockwood, GG:  The saturated vapor pressure of desflurane at various temperatures.  Anesth Analg 83:864-866, 1996.

• ³²Kain, ML, Nunn JF:  Fresh gas econoimics of the Magill circuit, Anesthesiology 29:964, 1968.

• ³³Mapelson, WW:  The elimination of rebreathing in various semi-closed anaesthetic systems, Br. J. Anaesth 26: 323, 1964.

• ³⁴Miller, DM, Miller, JC: Enclosed afferent reservior breathing systems, Br. J. Anaesth 60: 469, 1988

• ³⁵Jackson-Rees, G: Anaesthesia in the newborn, Br. Med. J. 2: 1419, 1950.

• ³⁶Bain, JA, Spoerel, WE:  A streamlined anaesthetic system. Can. Anaesth Soc J 19: 426, 1972.

• ³⁷Milner, Q "Anaesthetic Breathing Systems", Update in Anesthesia, issue 7, article 4 (1997) http://www.nda.ox.ac.uk/wfsa/html/u07/u07_012.htm

• ³⁸Sykes, MK: Rebreathing circuits: A review. Br. J. Anaesth 40: 666, 1968.

• ³⁹Pethick SL: Letter to the editor. Can. Anaesth Soc J 22: 115, 1975.

• ⁴⁰Eisenkraft, JB: Vaporizers and vaporization of volatile anesthetics. In Eisenkraft JB, editor, Progress in Anesthesiology, vol 2, San Antonio,1988, Dannemiller Memorial Educational Foundation)

• ⁴¹Field Medicine, Walter Reed Army Medical Center (http://www.wramc.amedd.army.mil/fieldmed/885atutor/Theory.htm)