British Journal of Anaesthesia 1994; 72: 470-473

EQUIPMENT

The desflurane Tec 6 vaporizer

STEPHEN G. GRAHAM

SUMMARY

A late development model of the Tec 6 vaporizer forthe administration of desflurane vapour has beenevaluated. It is heated electrically and has bothelectronic monitors of vaporizer function andalarms. The new filling system is a significantimprovement over previous Tec filling systems. Thevaporizer requires a warm-up period before it maybe used, but when activated it provides an outputthat is approximately linear between 1 and 18%vapour concentrations, at flow rates between200mlmin'1 and 10 litre min~1. (Br. J. Anaesth.1994; 72: 470-^73)

KEY WORDSAnaesthetics, volatile, desflurane. Equipment: vaporizers.

using a flowmeter in a manner analogous to that ofthe copper kettle vaporizer.

DESCRIPTION

The Tec 6 is different in appearance from its pre-decessors (fig. 1). It is similar in width (11.0 cm) andheight (24.5 cm) but substantially deeper in section(25.0 cm). It weighs 9.2 kg and is pale grey in colourexcept for the dial assembly, front panel display andthe filling system, which are black. Each side bears agrille to allow free airflow around the electricalcomponents. The Selectatec mounting system,mechanical interlocks and dial release systems areidentical to those on the Tec 5 and similar to the Tec 4vaporizer. The Tec 6 requires a mains electricitysupply and will be available in both 220-240-V and

Desflurane is a fluorinated methylethyl ether. It hasfeatures that suggest it may be a significant advancecompared with previous agents: (a) a low blood-gaspartition coefficient (0.42) suggests a rapid onset ofand recovery from anaesthesia, and more immediatecontrol of anaesthetic depth. This also suggestsgreater efficiency if the drug were to be used in alow-flow circle system; (b) minimal metabolism; and(c) greater stability to heat, light and soda lime.

Although synthesized at about the same time asenflurane and isoflurane, the development of des-flurane has been delayed by difficulties in synthesisand by two physical characteristics that detract fromthe above advantages. Desflurane is considerably lesspotent that isoflurane, with a MAC of approximately6.0%, which would suggest a greater anaestheticconsumption rate and therefore greater cost; it alsohas a high vapour pressure (88.5 kPa at 20 °C) and aboiling point of 23.5 °C [1]. It is this latter problemthat is the subject of this evaluation. Desflurane is anextremely volatile liquid in the conditions prevailingmainly in U.K. operating theatres, but in manyclimatic regions (or in neonatal anaesthesia), theambient temperature may exceed the boiling pointand the liquid changes to a gas. To combat thisproblem Ohmeda have developed the Tec 6 vapor-izer to deliver desflurane under control. This reportevaluates a late development model, which differsslightly from the eventual production device. It mustbe noted that all published studies have so far used adifferent system for delivering desflurane vapour,that of direct metering vaporization, a system wherethe anaesthetic vapour flow is measured directly

FIG. 1. The Ohmeda Tec 6 vaporizer.

STEPHEN G. GRAHAM, M.B., CH.B., F.R.C.A., University Depart-ment of Anaesthesia, Leicester Royal Infirmary, University Close,Leicester LEI 5WW. Accepted for Publication: October 7, 1993.

THE DESFLURANE TEC 6 VAPORIZER 471

Fresh gas inflow(Selectatec mount)

Concnselection dial

Carrier gas/desflurane outlet

Pressuretransducers

Electricalheating —element

Vaporizingchamber

Desfluraneliquid

iquid level sensor

FIG. 2. The Ohmeda Tec 6 vaporizer for administration of desflurane. The agent is heated in the vaporizing chamberto provide vapour, which is metered through the pressure regulation valve to provide pressure to balance thatgenerated by the fresh gas flow passing through the fixed restrictor (reproduced, with permission, copyright held by

the BOC group).

100-120-V a.c. formats. It has been constructed toconform with IEC 601-1 and 601-2-13 regulationsapplying to a class 1 medical device.

PRINCIPLE OF OPERATION

Desflurane liquid is held within the vaporizingchamber (fig. 2) and is heated to 39 °C by two 100-Welectrical heater elements. Two further heaterelements warm the upper part of the vaporizer toprevent condensation. When operating temperaturehas been reached, the upper part of the vaporizingchamber contains desflurane vapour under pressureand solenoid-operated locks on the concentrationdial and vaporizing chamber outflow are released.Fresh gas from the flowmeters enters the vaporizerand passes through a fixed flow-restrictor, togenerate back-pressure, sensed by two independentpressure sensors. Desflurane vapour passes througha shut-off valve and a pressure regulating valve to theconcentration selection dial. The pressure of thevapour is detected also by these pressure sensors andby control of the pressure-regulating valve thepressure may be matched to the back-pressuregenerated by the fresh gas flows. This allows onesingle dial setting to deliver fixed concentrations ofdesflurane vapour on the output side of the vaporizerat varying fresh gas flows; higher flowmeter flowsproduce a larger measured back-pressure, matchedby a larger pressure of desflurane vapour, with aresultant greater flow of desflurane vapour from thevaporizer matching the increase in fresh gas flow.The concentration selector has a dial release bar on

the back, which is compressed when initially settinga desired concentration. The dial is graduated inincrements of 1 % between 0 and 10%, and inincrements of 2 % between 10 and 18 %. An interimstop is located at 12%, which requires depression ofthe dial release bar to bypass, in a manner similar tothat used on the 7 % Enfluratec 3 vaporizer.

OPERATION

Display and alarms

The display on the front of the vaporizer (fig. 3)provides information on operational status andquantity of desflurane in the vaporization chamber.During warm-up the display and alarms cyclethrough a check sequence, after which an amberlight emitting diode (LED) adjacent to the vapor-izing chamber content display is illuminated, thischanging to green when the vaporizer is ready for use(it is necessary for the unit to be locked to aSelectatec mount before it is ready for use). Turningthe concentration dial locks the unit to the backbar.The content indicator is a vertically mounted liquidcrystal display comprising a series of narrow hori-zontal bars, to simulate a depth of liquid, and isdriven by the output of a capacitative level indicatormounted vertically in the vaporizing chamber. Tothe right of this an arrow indicates the level at whichthe 450-ml vaporization chamber will accept a full240-ml bottle of desflurane. When only one bar isvisible on this display, an amber LED is illuminated,accompanied by an audible alarm. This indicatesthat the main part of the vaporization chamber is

472 BRITISH JOURNAL OF ANAESTHESIA

USE ONLY DESFLURANE

OPERATIONAL

NO OUTPUT

LOW AGENT

WARM-UP

ALARMBATTERY LOW

FIG. 3. The new version of the Tec 6 display panel (reproduced,with permission, copyright held by the BOC group). This may

be modified further on the clinical release vaporizers.

empty, leaving 50 ml in the lower part. The audiblealarm may be cancelled by the operator if desired. Ifthe vaporization chamber is not refilled at this stage,the remaining liquid is used up, and if the residualvolume decreases to less than 20 ml another alarmsounds, accompanied by a red LED. This com-bination of alarms indicates that the unit is non-functional and that no vapour is being delivered.The alarm may only be silenced by turning thevaporizer to the standby position without whichaction, of course, a backbar equipped with Tec 4 or 5vaporizers does not allow the use of an alternativeanaesthetic vaporizer. This alarm pattern follows anatural progression; a green LED indicates normaloperation, an amber LED suggests a warning thatthe operating limits have been approached and a redLED indicates that the unit is being operated beyondits safety limits, and has shut down. In this evaluationmodel, there is only a small difference in colourbetween the amber and green LED, which causedconfusion at the co-located "wanning up/ready"indicator. All other LED are alarms and hence onlyamber or red, which are readily distinguishable.During the period of this evaluation, Ohmedachanged the design of the display (fig. 3) and theseLED are now clearly separate and distinguishable.The warm-up cycle ends with an audible beep andcan be checked easily by attempting to turn theconcentration dial. If warm-up is not completed,such attempts are unsuccessful.

Causes of vaporizer shutdown

The solenoid-operated locks within the vaporizerrequire electrical power to remain in the openposition and power failure results in a closing ofthese locks. Faults detected by the internal tem-perature and pressure monitors result in rapidshutdown and a red LED signal. A battery isprovided, but it only powers the failure alarms; a"low battery" amber LED indicates the need forbattery replacement and the batteries are expected tolast more than 12 months in normal use.

Previous Tec vaporizers have been shown to alteroutput if tilted [2]. The Tec 6 incorporates a tiltdetector, which shuts off the vaporizer if the tiltexceeds approximately 10°. As the vaporizer isavailable only in Selectatec or Drager formats, tiltingoccurs only if the backbar fitting is distortedsignificantly. Since the solenoid-operated valves onlyopen when the vaporizer is wanned up and lockedonto the backbar, and turning the vaporizer on locksit onto the backbar, there seems little possibility ofinverting the Tec 6 with the concentration dial setabove zero.

Allowing the vaporizer to empty also results inshutdown as detailed above.

Filling system

Desflurane will be available in brown glass bottles,labelled Suprane,. containing desflurane 240 ml,which are plastic coated to protect against pressureeffects in the event of damage to the bottle. There isno need for preservative in desflurane and the inertnature of the compound helps minimize the need forfrequent cleaning of the vaporizer. The bottles willbe complete with a bayonet fitment. This is inserted,against a spring, into the filling port on the left frontof the vaporizer and the bottle rotated upwards. Thisopens a channel into the vaporizer sump and thebayonet fitment is locked in place while the fillingtakes place. To release the bottle, it is rotated back tothe starting position and the spring forces the bottlefree. While this positive action is desirable, thepossibility exists that the unwary may be surprised,with the bottle being dropped as a consequence. Thecoating on the bottle protects against the risk ofsplintering in the event of damage to a warm,pressurized bottle of desflurane (accidental anddeliberate breakage of a number of bottles duringthis evaluation has suggested that the plastic coatingis likely to be an effective protection, even if thebottles are pressurized at the time of damage). Thevaporizer can be filled readily while in operation. Adrainage facility is provided on the base of thevaporizer for service use only and cannot be activatedaccidentally.

Performance

The vaporizer has been evaluated in both benchtests and clinical use. The content of a full vaporizeris such that even at moderate fresh gas flow rates of2-3 litre min"1 and delivering concentrations of3-6% v/v, it is unlikely that refilling should berequired in a single operating session. If required,the vaporizer may be filled whilst in use; from

THE DESFLURANE TEC 6 VAPORIZER 473

the fill-up mark a full bottle could be emptied intothe vaporizer in 125-130 s. This time is reduced ifthe vaporizer is cold or not being used and are-designed filling system reportedly allows a bottleto be emptied into the vaporizer in about 70 s[Ohmeda, personal communication]. During testingwith a fresh gas flow of 5 litre min~' and delivering aconcentration of 3 %, the output of the vaporizer wasunchanged during filling. A warm-up period isnecessary before use of the vaporizer and thisobviously depends on the liquid capacity and initialtemperature of the vaporizer at that time. Never-theless, the warm-up time for a full vaporizer inthese tests was 190-220 s, this being reduced to125-140 s when it contains only 200 ml, and thisshould not prove unnecessarily restrictive in prac-tice, although if the ambient temperature wasextremely low a prolonged warm-up time (perhaps10 min) may be required [Ohmeda, personal com-munication]. Similarly, if the vaporizer should needto be moved into the operating theatre from theanaesthetic room with the patient, the warm-up timeon reconnection of the electrical supply is reduced toabout 20 s and this is largely the result of theelectronic self-checking which occurs whenever thevaporizer is powered up.

The Tec 6 is a plenum vaporizer, but despite theinclusion of flow-restrictors, operation of the vapor-izer has no observable effect on the flows indicatedon the flowmeter of the anaesthetic machine. Theoutputs of the vaporizer at high, medium and lowfresh gas flows and using oxygen or 67 % nitrousoxide in oxygen were measured using a DatexCapnomac, updated to measure desflurane, andprecalibrated with gas standards. The output deteri-orates a little at the limits of fresh gas flow, but theinclusion of the electrical heating prevents furtherdeterioration of this performance with time. At allpoints, the concentration of desflurane delivered at aselected dial setting was smaller with a nitrousoxide-oxygen mixture compared with oxygen. As

the back-pressure generated by a gas flow through aresistance depends on the physical characteristics ofthat gas, this is not surprising. The differences werenot great and should have little effect in clinicalpractice.

In conclusion, the Tec 6 is a robust, but admittedlyrather heavy, vaporizer, requiring a Selectatecmount, and care should be taken with older modifi-cations of anaesthetic machines with limited rigidity.It has a new filling system, which represents asubstantial improvement over previous keyed fillingdevices and is still being improved. Its output isapproximately linear over the full operating range of0-18% v/v and with flow rates of 500 ml min"1 to10 litre min1. Although heated electrically it has arapid warm-up time and can be moved betweenanaesthetic room and operating theatre with littleextra delay. The requirement for an electrical deviceon the backbar may initially disturb many anaesthet-ists, who may fear power failure stopping delivery ofanaesthesia or electrical safety, but this has notprevented the successful introduction of electricallydriven ventilators and monitoring systems onto themodern anaesthetic machine. The inclusion ofinternal monitoring circuitry allows for greaterprecision and consistency of vapour delivery andthese seem likely to be incorporated into futuregenerations of vaporizer.

ACKNOWLEDGEMENTS

I wish to acknowledge the help of Ohmeda (a Division of BOCHealth Care) for the loan of the Tec 6 vaporizer and permission toreproduce artwork presented here.

REFERENCES1. Jones RM. Desflurane and sevoflurane: inhalational anaes-

thetics for this decade? British Journal of Anaesthesia 1990;65: 527-536.

2. Scott DM. Performance of BOC Ohmeda Tec 3 and Tec 4vaporizers following tipping. Anaesthesia and Intensive Care1991; 19: 441^43.