Variable-Bypass Vaporizers and the Desflurane Vaporizer

J. Jeff Andrews, M.D.

Department of Anesthesiology
The University of Texas Medical Branch, Galveston, Texas


Introduction

Through the years, vaporizers have evolved from rudimentary ether inhalers to copper kettles to the present day temperature-compensated, variable-bypass vaporizers. With the introduction of the new inhaled anesthetic, desflurane, an even more sophisticated, electrically heated, pressurized, electronically controlled vaporizer has been introduced. Both variable-bypass vaporizers and the new Tec 6 desflurane vaporizer are discussed below.


Variable-Bypass Vaporizers

Variable-bypass vaporizers are used to deliver halothane, enflurane, isoflurane, and sevoflurane but not desflurane. Examples of variable-bypass vaporizers include the Ohmeda Tec 4, the Ohmeda Tec 5, and the Dräger Vapor 19.1. They are classified as variable-bypass, flow-over, temperature-compensated, agent-specific, out-of-circuit vaporizers. (1) Variable-bypass refers to the method for regulating output concentration. As gas flow enters the vaporizer's inlet, the setting of the concentration control dial determines the ratio of flow that goes through the bypass chamber and through the vaporizing chamber. The gas channeled to the vaporizing chamber flows over the liquid anesthetic and becomes saturated with vapor. Thus, flow-over refers to the method of vaporization. The Tec 4, the Tec 5, and the Vapor 19.1 are classified as temperature-compensated because they are equipped with an automatic temperature-compensating device that helps maintain a constant vaporizer output over a wide range of temperatures. These vaporizers are agent-specific and out-of-circuit because they are designed to accommodate a single agent and to be located outside the breathing circuit.

Figure 1 demonstrates the operating principles of variable-bypass vaporizers. Flow from the flowmeters enters the inlet of the vaporizer. More than 80% of the flow passes straight through the bypass chamber to the vaporizer outlet, and this accounts for the name "bypass chamber." Less than 20% of the flow from the flowmeters is diverted through the vaporizing chamber. Depending on the temperature and vapor pressure of the particular inhaled anesthetic, the flow through the vaporizing chamber entrains a specific flow of inhaled anesthetic. All three flows, that is, flow through the bypass chamber, flow through the vaporizing chamber, and flow of entrained anesthetic, exit the vaporizer at the outlet. The final concentration of inhaled anesthetic is the ratio of the flow of the inhaled anesthetic to the total gas flow. (1, 2)

The vapor pressure of an inhaled anesthetic depends on the ambient temperature. For example, at 20C the vapor pressure of isoflurane is 238 mmHg, whereas at 35C the vapor pressure almost doubles (450 mmHg). Variable-bypass vaporizers have an internal mechanism to compensate for different ambient temperatures, and the temperature-compensating valve of the Ohmeda Tec 4 is shown in Figure 2.3 At high temperatures, such as those commonly used in pediatric or burn operating rooms, the vapor pressure inside the vaporizing chamber is high. To compensate for this increased vapor pressure, the bimetallic strip of the temperature-compensating valve leans to the right. This allows more flow to pass through the bypass chamber and less flow to pass through the vaporizing chamber. The net effect is a constant vaporizer output. In a cold operating room environment, the vapor pressure inside the vaporizing chamber decreases. To compensate for this decrease in vapor pressure, the bimetallic strip swings to the left, causing more flow to pass through the vaporizing chamber and less to pass through the bypass chamber. The net effect is a constant vaporizer output.


Electrically Heated, Pressurized Vaporizers

Controlled vaporization of desflurane requires an electrically heated, pressurized vaporizer because of desflurane's unique physical properties. (4) Desflurane's vapor pressure is three to four times that of contemporary inhaled anesthetics, (5) and it boils at 22.8C, (6) which is near room temperature. Desflurane is moderately potent, with minimum alveolar anesthetic concentration (MAC) values of 6% to 7%. (7) Desflurane is potentially valuable because it has a low blood gas solubility coefficient of 0.45 at 37C, (8) and recovery from anesthesia is more rapid than with other potent inhaled anesthetics. (9)

To achieve controlled vaporization of desflurane, Ohmeda has introduced the Tec 6 vaporizer, which is electrically heated and pressurized. (10) The physical appearance and operation of the Tec 6 are similar to contemporary vaporizers, but some aspects of the internal design and operating principles are radically different. A simplified schematic of the Tec 6 is shown in Figure 2. There are two independent gas circuits, the fresh gas circuit (gray) and the vapor circuit (white). The fresh gas from the flowmeters enters at the fresh gas inlet, passes through a fixed restrictor (R1), and exits at the vaporizer gas outlet. The vapor circuit originates at the desflurane sump, which is electrically heated and thermostatically controlled to 39C, a temperature well above desflurane's boiling point. The heated sump assembly serves as a reservoir of desflurane vapor. At 39C, the vapor pressure in the sump is 1500 mmHg absolute, or approximately 2 atmospheres absolute. Just downstream from the sump is the shut-off valve. After the vaporizer warms up, the shut-off valve fully opens when the concentration control valve is turned to the on position. A pressure-regulating valve downstream from the shut-off valve downregulates the pressure to approximately 1.1 atmospheres absolute (74 mmHg gauge) at a fresh gas flowrate of 10 L/min. The operator controls desflurane output by adjusting the concentration control valve (R2), which is a variable restrictor. (4)

The vapor flow through R2 joins the fresh gas flow through R1 at a point downstream from the restrictors. Until this point, the two circuits are physically divorced. They are interfaced pneumatically and electronically, however, through differential pressure transducers, a control electronics system, and a pressure-regulating valve. When a constant fresh gas flowrate encounters the fixed restrictor, R1, a specific back pressure, proportional to the fresh gas flowrate, pushes against the diaphragm of the control differential pressure transducer. The differential pressure transducer conveys the pressure difference between the fresh gas circuit and the vapor circuit to the control electronics system. The control electronics system regulates the pressure-regulating valve so that the pressure in the vapor circuit equals the pressure in the fresh gas circuit. This equalized pressure supplying R1 and R2 is the working pressure, and the working pressure is constant at a fixed fresh gas flowrate. If the operator increases the fresh gas flowrate, more back pressure is exerted upon the diaphragm of the control pressure transducer, and the working pressure of the vaporizer increases. (4)

Table 1 shows the approximate correlation between fresh gas flowrate and working pressure for a typical vaporizer. At a fresh gas flowrate of 1 L/min, the working pressure is 10 millibars, or 7.4 mmHg gauge. At a fresh gas flowrate of 10 L/min, the working pressure is 100 millibars, or 74 mmHg gauge. Therefore, there is a linear relationship between fresh gas flowrate and working pressure. When the fresh gas flowrate is increased tenfold, the working pressure increases tenfold. (4)

The following two specific examples help demonstrate the operating principles of the Tec6.

Example A:
Constant fresh gas flowrate of 1 L/min with an increase in the dial setting.

With a fresh gas flowrate of 1 L/min, the working pressure of the vaporizer is 7.4 mmHg.  That is, the pressure supplying R1 and R2 is 7.4 mmHg.  As the operator increases the dial setting, the opening at R2 becomes larger, allowing more vapor to pass through R2.  Specific vapor flow values at different dial settings are shown in Table 2.

Example B:
Constant dial setting of 6% with an increase in fresh gas flow from 1 to 10 L/min.

At a fresh gas flowrate of 1 L/min, the working pressure is 7.4 mmHg, and at a dial setting of 6% the vapor flowrate through R2 is 64 ml/min (Tables 1 and 2).  With a tenfold increase in the fresh gas flowrate, there is a concomitant tenfold increase in the working pressure to 74 mmHg.  The ratio of resistances of R2 to R1 is constant at a fixed dial setting of 6%.  Because R2 is supplied by ten times more pressure, the vapor flowrate through R2 increases tenfold to 640 ml/min.  Vaporizer output is constant because both the fresh gas flow and the vapor flow increase proportionally.

Because desflurane's vapor pressure is near one atmosphere, misfilling contemporary vaporizers with desflurane can theoretically cause desflurane overdose and hypoxemia. (5) Ohmeda has introduced a unique, anesthetic-specific filling system to minimize occurrence of this potential hazard. The agent-specific filler cap of the desflurane bottle prevents its use with traditional vaporizers. The filling system also minimizes spillage of liquid or vapor anesthetic by maintaining a "closed system" during the filling process. Each desflurane bottle has a spring-loaded filler cap with an O-ring on the tip. The spring seals the bottle until it is engaged in the filler port of the vaporizer. Thus, this anesthetic-specific filling system interlocks the vaporizer and the dispensing bottle, preventing loss of anesthetic to the atmosphere.(10)

Major vaporizer faults cause the shut-off valve located just downstream from the desflurane sump (Figure 2) to close, producing a "no-output" situation. The valve is closed and a no-output alarm is activated immediately if any of the following conditions occur: the anesthetic level decreases to below 20 cc; the vaporizer is tilted; a power failure occurs; or there is a disparity exceeding a specified tolerance between the pressure in the vapor circuit and the pressure in the fresh gas circuit. (10)


Summary

The Tec 6 vaporizer is an electrically heated, thermostatically controlled, constant-temperature, pressurized, electromechanically coupled, dual circuit, gas/vapor blender. The pressure in the vapor circuit is electronically regulated to equal the pressure in the fresh gas circuit. At a constant fresh gas flowrate, the operator regulates vapor flow using a conventional concentration control dial. When the fresh gas flowrate increases, the working pressure increases proportionally. At a specific dial setting at different fresh gas flowrates, vaporizer output is constant because the amount of flow through each circuit is proportional. (4)


References

  1. Dorsch JA, Dorsch SE. Vaporizers. In: Dorsch JA, Dorsch SE, eds. Understanding anesthesia equipment. 2nd ed. Baltimore, MD: Williams & Wilkins, 1984:77.

  2. Schreiber P. Anaesthetic equipment: Performance, classification, and safety. New York NY: Springer-Verlag, 1972.
  3. Tec 4 Continuous Flow Vaporizer. Operator's manual. Steeton, England: Ohmeda, The BOC Group, Inc., 1987.
  4. Andrews JJ, Johnston RV, Jr. The new Tec 6 desflurane vaporizer. Anesth Analg 1993;76:1338.
  5. Andrews JJ, Johnston RV,Jr., Kramer GC. Consequences of misfilling contemporary vaporizers with desflurane. Can J Anaesth 1993;40:71.
  6. Jones RM. Desflurane and sevoflurane: Inhalation anaesthetics for this decade? Br J Anaesth 1990;65:527-536.
  7. Rampil IJ, Lockhart SH, Zwass MS,et al. Clinical characteristics of desflurane in surgical patients: minimum alveolar concentration. Anesthesiology 1991;74:429.
  8. Eger EI, II. Partition coefficients of I-653 in human blood, saline, and olive oil. Anesth Analg 1987;66:971.
  9. Taylor RH, Lerman J. Induction, maintenance and recovery characteristics of desflurane in infants and children. Can J Anaesth 1992;39:6.
  10. Tec 6 Vaporizer: Operation and maintenance manual. Steeton, England: Ohmeda, 1992.
  11. Andrews JJ. Anesthesia systems. In: Barash PG, Cullen BF, eds. Clinical anesthesia. Philadelphia: J.B.Lippincott, 1989:505.

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