Ch 15 Conventional Anaesthetic Equipment

CHAPTER 15

CONVENTIONAL ANAESTHETIC EQUIPMENT

Outline: The anaesthetic machine • Gas source • Flow meter • Vaporisers • Delivery System or circuit The Magill circuit The Bain circuit The Circle System Checking the anaesthetic machine Other features of the anaesthetic machine • alarm devices • oxygen flush valves • scavenging devices

211

THE ANAESTHETIC MACHINE

Fig 15.1 A continuous flow (Boyle’s) anaesthetic machine The introduction of gaseous anaesthetics and the need to provide more accurate control of anaesthesia over long periods of time led to the development of the anaesthetic machine. At first glance the anaesthetic machine appears to be a formidable and complicated bit of machinery but if it is considered in its four components it becomes much simpler to understand. The four basic components or modules are the following: • The gas source – either piped gas or supplied in cylinders • The flowmeter • The vaporisers • The gas delivery systems − The T-piece − The Magill circuit (semi closed) − The Bain circuit

212

−

The circle absorber

213

THE GAS SOURCE Piped gas Piped gases are stored in a “bank”, remote from the operating room. The gases are piped into the operating room and connected to the anaesthetic machine via hoses with special connections to ensure that the nitrous oxide cannot be connected to the oxygen inlet and vice versa. Cylinders are fitted directly on to the anaesthetic machine by means of yokes. The pressure within the cylinders: The cylinders contain gases under a very high pressure. Oxygen is compressed at a pressure of about 147 bar (2000 psi). Nitrous oxide is compressed at a pressure of about 44 bar (600 psi). Construction of the cylinders: The cylinders have to be constructed according to definite specifications. They are made entirely out of steel that meets certain chemical and physical requirements or out of a chrome molybdenum mixture that is 20% lighter than steel. Each cylinder is designed to contain a gas under a specified pressure, so the cylinder must possess a minimum strength. Each cylinder has within it a valve which will seal the contents and also provide an exit for the gases and a means of filling the cylinders. Cylinder sizes: The size of the cylinder is designated by a letter stamped on the outside of the cylinder. Depending on the country of manufacture these letters will vary. Labelling and marking: All the cylinders used in the operating room have certain markings on them, stamped on the top end of the cylinder. They include: • The specification number, then the service pressure in psi. • A letter designating cylinder size. • The date the cylinder was tested (done at least every 5 years). Storing the cylinders • Store cylinders in a definitely assigned location, specific for cylinders. No other items should be stored at that site. • Storage rooms should be dry, fireproof and well ventilated. Cylinders should be protected from the following hazards: − Excessive rises in temperature − Highly flammable substances, e.g. gasoline, kerosene − Sparks or flames − Radiators − Corrosive agents

214

•

Store the used and unused cylinders separately.

215

Colour code for cylinders: There is an international standard for colour coding gas cylinders but not all countries follow it. The anaesthetist must know the appropriate colour coding used in the country of origin of the cylinders being supplied. Rules for handling cylinders • • • • • • • • • • • •

Do not allow oil, grease or other flammable substances to come into contact with cylinders, valves, regulators, gauges, hoses etc. Do not use oil or grease for lubrication. Do not handle cylinders or apparatus with oily hands or gloves. Keep connections to cylinders, regulators etc, always securely fixed to prevent gas leaks. Always make sure the hose is in good condition. Do not allow sparks or flame from any source to come into contact with cylinders or other anaesthetic equipment. Gauges and regulators are designated for use with specific gases. Do not use them for any other gases. Keep the cylinder valve fully open while the cylinder is in use. Before you connect a cylinder to a machine, make sure paper wrappings are completely removed and the cylinder label is completely visible. Never attempt to repair, alter or repaint a cylinder. Keep cylinder valves closed whenever the gas is not being used. Have defective or damaged equipment repaired by the manufacturer. Do not allow compressed gases to be handled by anyone who is not properly instructed and experienced. Handle cylinders and associated equipment gently. Avoid dropping cylinders or even knocking them.

Pin index system: An ever-present hazard in anaesthesia is the danger of attaching a cylinder to the yoke meant for a different gas. This is eliminated by the pin index system. The system consists of two pins projecting from the yoke in the anaesthetic machine, designed to fit into matching holes in the body of the cylinder valve. For any one gas there is only one combination of pins and holes. Unless the correct cylinder valve is attached to the correct yoke these pins and holes will not match and the cylinder will not fit. Therefore it is not possible with this system to fit a nitrous oxide cylinder to an oxygen yoke or vice versa. Reducing valves: These reduce the pressure of the gases in the cylinders. As mentioned earlier, the pressure in the oxygen cylinder is about 147 bar (2000 psi). This pressure must be reduced to about 4 bar (60 psi) if the gas delivered to the patient is going to be controllable and safe.

216

Gauges: Oxygen is entirely in its gaseous form in the cylinder, whereas nitrous oxide is primarily in the liquid state. The gauge in the oxygen cylinder therefore registers the contents in the cylinder fairly accurately. The gauge in the nitrous oxide cylinder, however, will tend to record the vapour pressure of the gas above the liquid in the cylinder. As long as there is any liquid in the cylinder of nitrous oxide the gauge will register full and will only fall when there is only gas left in the cylinder. THE FLOWMETER The gases pass from the reducing valve, via pressure tubing, to the flowmeter calibrated for each gas. The flowmeters record the volume of gas flowing to the patient per minute. There are various designs for the flowmeters. We will describe those used in the Boyle's machine. The flowmeter in the Boyle’s machine is referred to as a "Rotameter". It consists of a vertical glass tube tapered at the lower end. Rotating a knob at the base of the machine permits the entry of gases into the flowmeter. In the glass tube is an indicator or a bobbin. The height of the float in the tube indicates the flow of gases through the flowmeter. The flow should be read at the top of the bobbin.

Fig 15.2 Oxygen rotameter The indicator/bobbin has a rim on the top with grooves cut obliquely on it. The stream of gases entering the flowmeter impinges on these grooves and causes the float to rotate. It is essential that this float should rotate all the time. Sometimes grease and dust may cause the float to stick. The flowmeter cannot then be relied on to give an accurate reading. Since the glass tube tapers with the narrow end at the bottom the markings on the flowmeter are not equally spaced. The markings at the lower end are further apart and the markings at the upper are closer together. Two other makes of anaesthetic machine are Ohmeda and Drager.

217

THE VAPORISER

Fig 15.3 Vaporiser design From the flowmeters the gases pass in the direction of the vaporisers. The vaporiser enables volatile agents to be introduced into the gaseous mixture. These volatile agents are liquids at room temperature and do not need to be stored under pressure. The function of the vaporiser is to vaporise this liquid. The ideal vaporiser should yield an unchanging (constant) concentration despite variations in gas flow, temperature and the amount of liquid anaesthetic contained in the vaporiser. Boyle's bottle The simplest type of vaporiser for an anaesthetic machine is the Boyle's bottle. The volume of gas diverted into the Boyle's bottle can be controlled by adjusting a lever. When this volume is at its maximum the vaporisation of the liquid can be further increased by depressing a "cowl" which brings the gases entering the Boyle's bottle closer to the liquid anaesthetic. The output of the vaporiser is not constant. As the liquid vaporises and its temperature falls then the evaporation of the liquid falls too and the concentration delivered to the patient drops with time. Hence this vaporiser is not temperature compensated. This vaporiser is also not calibrated, so we cannot know how much is given to the patient with each position of the lever.

218

The Boyle's bottle can be used for ether and trilene. It can also be used for halothane but this is not recommended. Halothane is a very potent anaesthetic and should be administered very carefully in a specially designed vaporiser so that the concentration being administered to the patient can be known at all times.

Fig 15.4 A Boyle’s bottle suitable for vaporising Ether

Fluotec vaporiser - Mark II The Fluotec vaporiser provides most of the features desirable in a good vaporiser. • It is temperature compensated. • It provides a constant concentration at various gas flows (4 to 15L/min). • Its efficiency does not depend on how much liquid halothane is in the vaporiser. • The control dial allows concentrations to be altered from 0 to 4% in 0.5% gradations.

219

Fig 15.5 A Fluotec mark II Vaporiser

The Fluotec can be fitted on the back bar of the Boyle's machine. Rotation of a spindle increases the flow of gases diverted through the vaporising chamber. The dial must be pulled forward before it can be turned on. It is important to remember that with controlled ventilation and low gas flows the concentration of halothane delivered may be greater than that shown on the dial. Use of more than 1% halothane with IPPV is best avoided if possible. Fluotec Vaporiser - Mark III, IV, V This delivers concentrations ranging from 0 to 5%. The concentration dial locks when it is turned to the OFF position. It must be unlocked before it can be turned to the ON position. A special filling device ensures that only halothane is used in the Fluotec Mark III vaporiser. There are many variations on the ‘TEC’ concept made by different manufacturers. They are all temperature and flow compensated.

220

The Goldman Vaporiser This is a simple glass bowl which will hold up to 20 ml of liquid. The bowl is attached to a head. The gases are partly diverted into the vaporising chamber and the rest of the fresh gas flow travels through the bypass, depending on the position of the lever. There are three positions between the OFF and ON positions in the Mark I and Mark II models of the Goldman Vaporiser. The Mark III model has two settings between the OFF and ON positions. The Goldman vaporiser is simple, easy to clean and can be used with multiple agents, e.g. halothane, trilene. The Goldman vaporiser is not ideal but is relatively safe, as it cannot yield a vapour strength greater than 3% halothane. Further, the small vaporising chamber prevents a dangerously high concentration of vapour being delivered to the circuit when the vaporiser is first turned on.

Fig 15.6 The Goldman vaporiser

221

THE DELIVERY SYSTEM The commonly used delivery systems are: • The Ayres T-piece • The Magill’s circuit • The Bain circuit • The circle system The Ayres T–piece is discussed under Paediatric Anaesthesia in Chapter 20. The Classification of Breathing Systems

Fig 15.7 Mapleson's classification of breathing systems

222

THE MAGILL CIRCUIT This is the Mapleson A circuit. It is suitable for adults and children over the age of 3 years where spontaneous respiration is to be used. It consists of a bag and an expiratory valve arranged as in the diagram. For spontaneous respiration the fresh gas flow should be equal to the patient's minute volume. The circuit is not suitable for IPPV because of the danger of rebreathing but if the expiratory valve is replaced with a non-rebreathing valve then the Magill circuit is suitable for IPPV. The fresh gas flow must be equal to the minute volume. Non–rebreathing valves can carry their own hazards, e.g. sticking of the valve.

Fig 15.8 A Magill circuit

223

THE BAIN CIRCUIT The Bain (co-axial) circuit is a modification of the Mapleson D system, in which fresh gas enters at the patient end but without any bulky inlet port. An expiratory valve and reservoir bag is placed at the other end. The Bain circuit is 1.8 metres in length and is composed of an outer tube made of light weight conductive corrugated plastic tubing 22 mm in diameter and an inner tube 7 mm in diameter. The Bain circuit is connected to the anaesthetic machine using a special valve which allows fresh gas to be delivered to the patient through the small bore inner tube while exhaled gases pass through the outer tube to a reservoir bag and expiratory valve. A scavenging device is incorporated in the expiratory valve.

Fig 15.9 The Bain circuit Fresh gas flow and the Bain circuit The fresh gas flow in the adult patient requiring IPPV is 70 ml/kg. For patients less than 50 kg use a minimum flow of 3.5 L. For spontaneous respiration, 100 ml/kg body weight is required. The circuit must be checked before use in case the inner tube has become dislodged. (See also Chapter 20 Paediatrics) Advantages of the Bain circuit • It is a single tube from the machine to the patient. • There are no valves at the patient end so is less bulky and can be hidden under the drapes, e.g. in head and neck surgery. • It is light in weight. • It can be used for all ages. • It can be used for spontaneous or controlled ventilation. • It is easily sterilised. • A scavenging valve is available for it.

224

THE CIRCLE SYSTEM Principle: The circle system works on the principle that if carbon dioxide is absorbed from the anaesthetic circuit then the same oxygen and anaesthetic mixture can be used over and over again, with the addition of only minimal amounts of oxygen and anaesthetic agent. By means of one-way (uni-directional) valves the oxygen and anaesthetic mixture passes into the patient and then the expired gases pass through a canister containing soda lime. The carbon dioxide is absorbed and the oxygen and anaesthetic gases and vapours pass again to the patient. Somewhere between the bag and the patient these gases are joined by the fresh gas flow from the machine. The circle system was first designed as a closed circuit. The valve (pop-off valve close to the mask) was closed completely and a very minimal flow of oxygen between 250-400 ml was used. However this took for granted that there was absolutely no leak in the circuit. In practice the circle system is used as a "semi closed" system, i.e. there is a small leak at the pop-off valve and higher gas flows are used. A total flow rate of 4 L/min nitrous oxide and oxygen is used at the beginning of the anaesthetic, in order to flush out the system and achieve a steady state. After three minutes, a total flow rate of 3L/min is used, comprising 1 L/min of oxygen and 2 L/min of nitrous oxide (N2O).

Fig 15.10 The scheme of a circle system The Circle Absorber In the canister the carbon dioxide (CO2) produced by the patient is absorbed by either soda lime, a mixture of 4% sodium hydroxide and 94% calcium hydroxide with a small amount of silica added to make the mixture hard or baralyme, a mixture of 80% calcium hydroxide and 20% barium hydroxide. Baralyme is sufficiently hard to prevent crumbling and the formation of alkaline dust, which sometimes may occur with soda lime.

225

Important points about soda lime The chemicals in soda lime combine with carbon dioxide to form carbonates and water. Heat is released as a result of this reaction. The soda lime canister therefore feels warm. The granules of soda lime are of a special size so absorption is achieved with minimum resistance. incorporated into the soda lime and this changes colour is exhausted. The canisters used may have single chambers.

that the maximum An "indicator" is when the soda lime chambers or dual

The soda lime canister is filled to the brim. A single chamber canister of soda lime can be used for two hours continuously. After a canister of soda lime has been rested for several hours it can be used again for an hour and this can be repeated a few times. The soda lime must then be discarded. However, the exhaustion time of the canister depends on the size, the packing of the canister, the quality of the soda lime used, the fresh gas flow and the rate of carbon dioxide production. The colour change of the indicator and the temperature of the canister should not be a substitute for careful clinical observation of the patient for signs of carbon dioxide retention such as a rise in heart rate and blood pressure, a deepening of respiration if the patient is breathing spontaneously, sweating and arrhythmias. Advantages and disadvantages of the circle system Advantages • • • • •

226

The cost of anaesthesia is reduced because of the lower fresh gas flows required. Operating theatre pollution is reduced if a scavenging system is employed. Inhaled gases are warmed and humidified. The length of the corrugated tubing can be increased (e.g. in neurosurgery and surgery of the head and neck), giving maximum surgical exposure without interference by anaesthetic equipment. Bacterial filters, humidifiers and spirometers can be used in conjunction with the circle absorber.

Disadvantages • • • • • •

The circle absorber is composed of many parts that can be disarranged or may malfunction. Resistance is greater than with other systems. It is therefore not suitable for use in very young children. The dead space extends from the Y-piece or the junction of the inspiratory and expiratory tubes. This too makes it unsuitable for use in very young children. Cross infection may be a problem as some of the components are difficult to clean. The system is bulky. Trilene cannot be used with soda lime as it produces a toxic substance.

CHECKING THE BOYLE'S MACHINE • •

• • •

•

Check that adequate supplies of gas and oxygen are available before commencing an anaesthetic with the Boyle’s machine. Check that both the cylinders of oxygen and nitrous oxide are not empty. Ensure that you have a reserve cylinder of oxygen available for replacement. Turn on the flow meter by turning the knob at its base. Make sure the bobbin rotates and does not stick. Check that the liquid anaesthetic to be used is in the vaporiser. Make sure the soda lime is fresh (i.e. that the indicator has not changed colour). Make sure that the canister is filled to the brim. With the usual gas flows used, about 500g of soda lime in a small canister has to be changed at least every two hours. If a large canister with two compartments is used then the canister must be "reversed" when the indicator changes colour. It will last five times longer than the small canister. Check the machine and circuit for gas leaks. The expiratory valve is completely closed. Cover the opening at the patient's end of the Y-piece completely with the palm of your hand. The oxygen and nitrous oxide flow meters are adjusted to give a flow rate of about 4 L/min. The reservoir bag will fill and will not deflate when the bag is squeezed if there are no leaks in the system.

227

OTHER FEATURES OF ANAESTHETIC MACHINES •

Alarm devices are designed to initiate a signal when the oxygen pressure is low. Various designs are available. It is important that every anaesthetic machine be fitted with an oxygen alarm device. The alarm should be triggered by a low oxygen pressure and not by nitrous oxide flow. Alarm devices triggered by nitrous oxide flow will not function if the nitrous oxide and oxygen fail simultaneously or if there is prior failure of the nitrous oxide supply.

•

Oxygen flush valves are available on most machines. It is important to be familiar with the position of the oxygen flush valve on the machine you are using. The valve directs a very high flow of oxygen (at least 30L/min) to the outlet of the machine. The valve is useful if it is necessary to fill the reservoir bag quickly with 100% oxygen.

•

Scavenging devices have now become standard in operating theatres. They are discussed under the heading "Hazards in the Operating Theatre" (See Chapter 61).

228