Anaesthesia working station vaporizers types

Vaporizer

1. V A P O R IZ E R S

2. Objecti v e s : The need of vaporizers The physics behind the scenes Classification of vaporizers Types of vaporizers in use Hazards Filling systems Safety Features

3. Definition: It is a device that changes a liquid anesthetic into the vapor and adds clinically useful amount of this vapor to the Fresh Gas Flow(FGF).

4. Why do we need vaporizers? Most inhalational anaesthetics agents exist as liquid form at room temperature and atmospheric pressure. Vaporisers convert this liquid into vapor form. They add a precisely determined concentration of this vapor into anaesthesia circuit over a wide range of temperature , pressure and carrier gas flow rate. Reliable,accurate,adjustable delivery of inhaled anesthetic agent.

5. Physics of Vaporisers Vapor Pressure Molecule of a volatile liquid inside a closed container break away from the surface and form a vapor. The partial pressure that can be exerted by a gas at a given temperature is its vapor pressure. These molecules bombard the walls of the container, creating a pressure. After attaining the equilibrium the vapor pressure is called S aturated Vapor P ressure Saturated Vapor pressure is directly propotional to the temperature.

6. Gas Concentration Partial pressure (Dalton’s Law ) The part of the total pressure due to any one gas in the mixture is called the partial pressure of that gas. The total pressure of the mixture is the sum of the partial pressures of the constituent gases. Depends only on the temperature of the agent Volume percent Volume of a gas in relation to a total of 100 units of volume for the total gas mixture. Partial Pressure/Total pressure= Volume percen t (v/v%) Vaporiser dials are marked in v/v%

7. Properties of Common Anaesthetic agents Agents Boiling point (degree C,760 mm Hg) Saturated Vapor Pressure (torr ,20 degree C ) MAC % Halothane 50.2 243 0.75 Isoflurane 48.5 238 1.15 Desflurane 22.8 669 6.4 Sevoflurane 58.6 157 2 Vol% in VC 32 % 87% 31% 21%

8. Boiling Point A liquid’s boiling point is the temperature at which its vapor pressure is equal to the atmospheric pressure. It is directly dependent on the atmospheric pressure. Below this temp. tendency to form bubble is instantly crushed by greater atmospheric pressure. Critical Temperature The temperature above which a substance cannot be liquefied. No matter how much pressure applied.

9. Specific heat The quantity of heat required to raise the temperature of 1 gm of the substance by 1 o C Reflects heat capacity Vapourisers are made of material with high specific heat so the temp. change is gradual

10. Latent Heat of Vaporisation The number of calories necessary to convert 1g/1ml of liquid into a vapor . The temperature of agent drops as vaporisation proceeds & heat has to be supplied by the surroundings. The cooling of agent will further decrease vaporisation and subsequently lesser concentration of agent is delivered to the patient, unless external heat is supplied.

11. Thermostabilisation Thermal Conductivity Reflets heat conduction i . e how fast the substance transfers heat. Thermostabilization is achieved by constructing a vaporiser of a metal with high thermal conductivity so as to minimise temp change when vaporiser is in use. (Cu> Al > Brass> Steel> Glass) Wicks should be in contact with metal part to replace the heat of vaporisation quickly.

12. Dorsch & dorsch classification Regulation of output concentration Variable bypass:Ether bottle , TEC Measured flow: copper kettle Electronic Method of vaporization Flow over: 1) with wick-TEC, OMV 2)without wick - Goldman bottle Bubble through: copper kettle Injection: Desflurane Location of the vaporizer In circuit: VIC - EMO, Goldman bottle. Out of circuit :VOC - TEC

13. Temperature compensation Thermocompensated - use of thermostats : Tec 4,5,7 Non temperature compensated: Goldman Thermobuffered : EMO Electronic: Tec 6, Aladdin cassette Specificity : Agent specific- TEC Multi agents - Goldman bottle Resistance : Plenum: TEC Draw over: Goldman bottle, EMO

14. Concentration Calibrated (Variable Bypass) A lso called direct-reading, dial-controlled, automatic plenum, percentage-type, and tec-type vaporizers Designed to be located between flowmeters and common gas flow outlet. Total fresh gas passes through the vaporiser. The flow is split into two parts by the variable resistance proportionating valve.

15. Splitting ratio depends on the resistance of variable orifice at inlet/outlet and on temperature of agent/carrier gases. Major part flows through bypass chamber and smaller part flows through vaporiser chamber. Agent concentration is controlled by a dial calibrated in volumes percent

16. Variable bypass ON 9 : 2 4 A M 16 O F F Splitting ratio

17. Desired anaesthetic percentage Halothane 243 mm Hg Isoflurane 238 mmHg Sevoflurane 160 mm Hg 1% 46:1 45:1 25:1 2% 23:1 22:1 12:1 3% 15:1 14:1 8:1 Splitting ratios : It is determined by the conc. set on dial.

18. Measured flow All the fresh gas does not flow through the vapourizer Vaporiser heats the agent above its boiling point and this separately heated and pressurised vapor stream is precisely injected into the FGF. Increasing the FGF dilutes the output.

19. Example : copper kettle, desflurane tec6, drager diva

20. Amount of liquid used by the Vaporizer Ehrenwerth and Eisenkraft formula: ml of liquid used per hour = 3 x fresh gas flow (L/minute) x dial setting (v/V%) This formula is based on the fact that typically 1 ml of liquid volatile agent yields about 200ml of vapor If FGF is 2L/min and the dialed concentration is 2%: ml of liquid used per hour is = 12mL

21. Method of Vaporisation Flow over Stream of carrier gas passes over agent Increasing the area of gas-liquid interface enhances efficiency of vaporisation. done by using baffles or spiral tracks to lengthen the gas pathway Wicks are also used

22. Bubble Through The carrier gas is bubbled through the volatile liquid and picks up anesthetic vapour . Example : Boyle’s Bottle Injection The vapor or inhalational liquid is injected directly into the fresh gas flow. Example : Desflurane Tec 6 and Drager Zeus Maquet { Flow i }

23. THERMOCOMPENSATION MECHANICAL : Metal rod in metal jacket Bimetallic strips- GE Ohmeda Tec type COMPUTERISED /ELECTRONIC: Aladin Cassette,Drager Diva,Maquet

24. 9 : 2 4 A M 16 When the vaporizer is warm c ool c ool w a r m M e c h a n i c a l t h e r mo compensation

25. RESISTANCE Typical plenum vapourizer offers a flow resistance of 20cm H Unsuitable for use in breathing circuit as resistance to flow is high Mounted on back of machine Gases have to be driven under pressure through it.  

26. Low resistance to gas flow situated within the breathing circuit useful in situations where pressurized gas is unavailable LOW RESISTANCE

27. FACTORS AFFECTING VAPOURIZER OUTPUT Gas flow rate : Output decreases at very low flows and very high flows,especially with high dial settings Temparature Carrier gas composition Intermittent back pressure : pumping and pressurizing effect

28. Effects of intermittent back pressure Pumping effect Concentration delivered is higher during controlled/assisted ventilation More pronounced with less agent, low gas flow, low dial setting.

29. Mechanism: Positive pressure at outlet during assisted or controlled ventilation Increased gas pressure inside vap o rising chamber More gas pickup more anaesthetic agent Increased concentration at outlet Prevention: Keep vapor chamber small, long spiral tube to lead to vapor chamber .

30. Effects of intermittent back pressure Pressurising effect Output of some vaporizers is lower than that at free flow to atmosphere. More with high flow : same no. of vapour molecules ,more no. of carrier gas molecules.

31. Hazards Incorrect Agent – Prevented by agent specific filling devices, colour coding Tipping – Prevented by draining the vaporizer before being removed Overfilling – Low level filling port & liquid level indicating glass Leaks – Prevented by odor of the agent, agent monitoring Contaminants in the Vaporizing Chamber Physical Damage No Vapor Output – Lack of output will cause awareness

32. Filling Systems There are various different methods to fill the vaporizers - Funnel Fill System Keyed Filling System Quik-Fill System Easy-Fill System Desflurane Filling Systems Color Codes Yellow – sevoflurane Purple – isoflurane Orange – enflurane Red – halothane,

33. Funnel Fill Vaporizers may be filled by a conventional funnel-fill mechanism, in which the liquid anaesthetic is simply poured into a funnel in the vaporizer. Complication is filling with wrong agent

34. Keyed fill In this system, an agent-specific filler tube is used one end of which fits into a slot on the vaporizer, and the other end fits into a collar on the bottle of anaesthetic. The fitting on the vaporizer and the collar on the bottle are specific to each agent.

35. Quik fill The bottle has a permanently attached, agent-specific filling device that has three ridges that fit into slots in the filler. Only for Sevoflurane

36. Easy Fill A colour coded bottle adaptor is attached to bottle and then fitted into the vaporizer.

37. Interlock Devices These are vaporizer exclusion system which prevent more than one vaporizer from being turned on at the same time. When unlocked, the dial moved a pin moves the extension rods which projects out and the concentration control dials of the neighboring vaporizers become inoperational .

38. Features of modern vaporizer Variable bypass : Fresh gas splits into bypass gas and carrier gas Flow over : Carrier gas flows over the surface of the liquid volatile agent in the vaporizing chamber Temperature compensated: Equipped with automatic devices that ensure steady vaporizer output over a wide range of ambient temperatures Agent-specific: Only calibrated for a single gas, usually with keyed fillers

39. Problems of Desflurane Desflurane is much more volatile than all the other inhalational agents. Its boiling point is low -- only 22.8°C, so most of it gets evaporated at normal room temperatures Vapor pressure of desflurane at 20°C is 664 mm Hg. At 1 atmosphere and 20°C , 100mL/min flow passing through vaporizing chamber would carry 735 mL/min of desflurane versus 29, 46 and 47 mL/min of enflurane, Isoflurane and halothane respectively Under these conditions to produce 1% of desflurane we need 73 L/min Fresh Gas Flow as compared to 5 L/min for other anaesthetics, to pass through vaporizer

40. Tec 6 Only for desflurane . It is described as a gas/vapor blender than as a vaporizer 425 ml capacity, 20 bars of approx 20 ml each. 1 bottle contains 240 ml desflurane Initially it takes 5-10 mins to reach optimal temp. Desflurane is heated to 39 °C (102 °F) and pressure maintained at 2 atm. FGF does not enter vaporization chamber, instead Desflurane vapor enters the path of FGF

41. T ec-6 Dial calibration is from 1% to 18% Thermocompensation by mechanical means is impossible, external heating is needed. Flow rate from 0.2 to 10 l/min N o effect of tilting. Can be filled while in use

42. 1. FGF/vaporizer inlet; 2. Pressure sensor/FGF; 3. Fixed resistor; 4. Heating element; 5. Desflurane sump assembly; 6. Sump shut-off valve; 7. Pressure regulating valve; 8. Working pressure sensor; 9. Variable resistor—concentration control dial; 10. Differential pressure transducer; 11. Vaporizer outlet.

43. Aladin vaporiser Used for all agents Cassette color coded for agent & magnetically coded for machine Halothane, enflurane and isoflurane- Keyed fill/easy fill. Capacity- 250 ml Flow at output is controlled by CPU. CPU and valve at inlet of vapor chamber prevent against back pressure fluctuations Metal plates increase heat capacity , conductivity and vaporising surface area- improve temp stabilisation and vaporisation Accuracy- decreases at high gas flows, temp <20% or FGF>8l/min

44. Take home points : 1. fundamental principles remain same 2.SVP IS THE MOST IMPORTANT DETERMINING FACTOR IN VAPOURISER CONSTRUCTION 3. ALL VAPOURISERS NEED THERMOCOMPENSATION AND MOST OF THEM HAVE MECHANICAL THERMOCOMPENSATION 4.INBUILT SAFETY FEATURES IN FILLING AND USING VAPOURISERS TO DECREASE THE HUMAN ERROR FACTOR.

45. THANK YOU

46. Types of Vaporisers in use Tec-5 for halothane, enflurane, isoflurane and sevoflurane Temperature compensated Highest accuracy is a FGF less than 5l/min and dial <3%. Temperature range: 15-35°C- less output at < 15°C and unpredictability high at >35°C. Pumping effect - increase in output. Carrier gas composition- at low flow- output is low with air/ Nitrous than with oxygen. At high flow- output is slightly increased.

47. TEC 1 Introduced by cyprane company in 1956 as FLUOTEC 1 for halothane Also called Mark1 Problem of proportioning valve sticking there is a risk of overdose.

48. TEC-2 Variable bypass Flow over with wick Temperature compensated Plenum type, concentration poorly calibrated and agent specific. Disadvantages : 1) H alothane preservative gets deposited inside causing operating spindle to stick. Backs pressure forcing saturated vapor back into bypass chamber. H igh concentration at low flows.

49. TEC 3 Models - fluotec mark 3 P e n t e c m a r k 2 Variable bypass Flow over with wicks Automatic thermocompensation Agent specific Disadvantages - dial rotation problems- 1) rotation beyond off position 2) rotated by 180 degree 3) Leak from dial setting

50. TEC 4 It was remoddled TEC 3 Vapouriser designed for ‘out of circuit ‘use in continuous flow technique of inhalation anaesthesia with built in temperature compensated and pressure compensated capabilities. Disadvantages- 1) difficulty in operation one handed. 2) yearly service interval.

51. 1. Flow path when vaporizer “off”; 2. Fresh gas flow when vaporizer “on”; 3. Bypass; 4. Thermostat; 5. Vaporizing chamber flow; 6. Helical channel; 7. Porous Teflon® wicks with steel wire spiral; 8. Wick skirt; 9. Liquid agent; 10. Vaporizing chamber outlet through rotary valve; 11. Concentration control dial

52. Saftey Features of Vaporizers • Clear color-coding indicator on the vaporizer and agent bottle • Agent specific filling systems with sealed bottles • Agent level indicators • Mounting systems with interlock to prevent simultaneous use of two vaporizers • Filling port is low to avoid overfilling • Electronic vaporizers have audiovisual alarm systems which detect malfunction—tilting, low agent and low temperature. • Agent monitoring allows detection of misfilling , overdose and low output. The use of this monitor prevents incidences of accidental awareness.