Anesthesia Equipment Session 3 Department of Anesthesia, Intensive Care and Pain Management Anesthesia Residents’ Academic Program (ARAP) Anesthesia Equipment Session 3 Raafat Abdelazim

Vaporizers Raafat Abdelazim

1846 Raafat Abdelazim

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Variable bypass, flow-over vaporizer Raafat Abdelazim

Critical temperature (gas, vapor, liquid) Vapor Pressure Boiling Point Gas Concentration Partial Pressure Volumes Percent Heat of Vaporization Specific Heat Thermal Conductivity Raafat Abdelazim

Vaporizer Basic Design Raafat Abdelazim

 Concentration  Concentration Zero (off) Raafat Abdelazim

Splitting Ratio Raafat Abdelazim

Basic Design  3 Problems Problem of high fresh gas flow Raafat Abdelazim

Solution:  surface area of contact a) wicks Due to capillary action, the anesthetic agent rises into the wicks. This dramatically  the surface area of anesthetic agent exposed to the fresh gas entering the vaporization chamber and thereby improves the efficiency of vaporization Raafat Abdelazim

Solution:  surface area of contact b) bubbles Some of the fresh gas flow is bubbled through a disk made out of a special material that is very porous Raafat Abdelazim

Problem of temperature Basic Design  3 Problems 1. High FGF Problem of temperature For vaporization to occur, the anesthetic molecules have to "escape" from the liquid state and become vapor. This process  the 'energy' left in the remaining liquid   temperature   vaporization Raafat Abdelazim

2 Solutions: a) “Giving heat” The metal helps to minimize the temperature drop by two ways Raafat Abdelazim

Metal is a very good conductor of heat and therefore is able to efficiently transfer heat from the surrounding air into the anesthetic agent Raafat Abdelazim

Metal acts like a 'heat store' Metal acts like a 'heat store'. It 'absorbs' heat (green arrows) till its temperature equals the temperature of the surrounding air Raafat Abdelazim

When the anesthetic agent starts to cool, the metal now 'donates' heat (yellow arrows), helping to minimize the temperature drop Raafat Abdelazim

When you turn the vaporizer off before your next case, the metal will continue to "absorb" heat from the surroundings and its temperature will rise, ready to donate heat when you turn the vaporizer on again Raafat Abdelazim

So in summary, the metal provides heat to minimize the temperature drop by two ways. One way is by 'donating' heat to the fluid (yellow arrows) and the other way is by conducting heat (red arrows) from the surrounding air Raafat Abdelazim

2 Solutions: b) Giving more flow When the temperature of the liquid agent , the output concentration of the vaporizer . A way of compensating for that problem is to  the flow of gas via the vaporizing chamber (altering the splitting ratio). This is accomplished by an automatic temperature compensating valve that influences how much flow goes via the vaporizing chamber. Raafat Abdelazim

A metal rod (shown in black below) shortens as the temperature . This valve uses the physical property that substances (e.g. metals and liquids) become smaller when the temperature . A metal rod (shown in black below) shortens as the temperature . Similarly, a liquid filled in collapsing bellows (shown in green below) becomes smaller in volume when cooled to a lower temperature. Raafat Abdelazim

This property is used in the design of automatic temperature compensating valves in vaporizers. In the design that uses a metal rod, the rod offers some resistance to flow into the vaporizing chamber. As the vaporizer cools, the rod becomes shorter, making the valve move away from the opening. This reduces the resistance to flow and thus more flow occurs into the vaporizing chamber. Raafat Abdelazim

away and thereby increase flow. Some vaporizers use the expansion or contraction property of a special liquid inside bellows (shown in green) to control the valve. As the temperature falls, the liquid in the bellows contracts into a smaller volume. This makes the bellows shrink, pulling the valve away and thereby increase flow. Raafat Abdelazim

Another method uses a "bi metallic" strip. Different metals expand and contract to differing extents when exposed to temperature changes. In the example below, the "green" metal expands and contracts more than the "red" metal. Raafat Abdelazim

In a bimetallic strip, 2 metals with very different degrees of thermal expansion (different coefficients of thermal expansion) are fixed together. In the example below, when the temperature drops, the "green" metal contracts much more than the "red“ metal. Because they are fixed together, they cannot contract independently. Instead, the "green" metal "tries" to drag the "red" metal and causes the bimetallic strip to bend. Raafat Abdelazim

In the vaporizer, the bimetallic strip is fixed in such a way that it offers a resistance to flow entering the vaporizing chamber. When the temperature of the vaporizing chamber drops, the bimetallic bends and moves away. This reduces the resistance to flow and thus more flow occurs into the vaporizing chamber. Raafat Abdelazim

Basic Design  3 Problems 1. High FGF 2. Temperature The “Pumping Effect” Positive pressure ventilation (PPV)  intermittent P changes. During PP: P  During expiration: P  These P changes can be transmitted back into the vaporizer and can affect the concentration of anesthetic agent delivered. The effect of changing P affecting the output of the vaporizer is called the "pumping effect". Raafat Abdelazim

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When the bag is squeezed (PPV), pressure is transmitted back into the vaporizer. This "back P" opposes the flow of the fresh gas in both the "by pass" channel and the vaporizing chamber. The fresh gas entering the vaporizer tries to move forward and gets compressed both in the 'by pass' channel and the vaporizing chamber. However, the vaporizing chamber volume is much larger than the 'by pass' channel volume, and thus, more fresh gas gets compressed into it than into the 'by pass' channel. Raafat Abdelazim

This extra fresh gas that enters the vaporizing chamber collects anesthetic vapor. Raafat Abdelazim

When the PP is suddenly released (expiration), the previously compressed gas now suddenly expands in all directions. Raafat Abdelazim

Some of the rapidly expanding gas (containing vapor) enter the inlet of the vaporizer and cross over into the 'by pass' channel. Raafat Abdelazim

Normally, a vaporizer 'by pass' channel does not have vapor Normally, a vaporizer 'by pass' channel does not have vapor. So this vapor due the 'pumping effect' is additional. When this 'by pass' vapor flows across to the exit of the vaporizer, it meets the vapor from the vaporizing chamber. The addition of the 'by pass‘ vapor to the vapor from the vaporizing chamber  the final concentration of anesthetic delivered. i.e. The 'pumping effect'  the delivered concentration of anesthetic agent. Raafat Abdelazim

Vaporizer designers have various tricks to  the 'pumping effect' LONG INLET TUBE The vaporizer inlet tube can be made longer. When the 'back pressure' is suddenly released during expiration, the extra gas in the vaporizing chamber will suddenly expand. However, the extra gas containing vapor expands into the long inlet tube and doesn't reach the 'by pass' channel. Raafat Abdelazim

INCREASED RESISTANCE The vaporizer can be designed to have a high internal resistance to flow. This high resistance "resists" changes to flow caused by the intermittent 'back pressure' of PPV. Raafat Abdelazim

ONE WAY VALVE A 'one way' valve (also called unidirectional valve) can be put between the vaporizer outlet and the ventilator / breathing system. The one way valve allows gases to flow forwards Raafat Abdelazim

This valve prevents flow from occurring in the reverse direction This valve prevents flow from occurring in the reverse direction. This  the transmission of 'back pressure' to the vaporizer Raafat Abdelazim

Ignore at this stage of learning Concentration Calibration Variable Bypass Vaporizers Electronic Vaporizers Vaporization Methods Flow-over Injection Temperature Compensation Mechanical Thermocompensation Supplied Heat Computerized Thermocompensation Effects of Altered Barometric Pressure Effects of Intermittent Back Pressure Pumping Effect Pressurizing Effect Interplay between Pressurizing and Pumping Effects Effects of Rebreathing Concentration-calibrated Vaporizers Ignore at this stage of learning Raafat Abdelazim

TEC 6 vaporize Raafat Abdelazim

Datex-Ohmeda Aladin Cassette Vaporizer Raafat Abdelazim

Datex-Ohmeda Aladin Cassette Vaporizer Raafat Abdelazim