Tips for Anaesthesia Attachments

During your anaesthetic attachment, take advantage of the time with the anaesthetist to:

• identify the different types and sizes of

facemasks;

oropharyngeal airways;

supraglottic airways, for example, laryngeal mask, i-gel;

laryngoscopes;

tracheal tubes;

• discuss the principles of how the anaesthesia machine delivers a safe mixture of gases to the patient;
  
• understand the principles of mechanical ventilation;
  
• understand the principles and limitations of monitoring, in particular

non-invasive blood pressure;

ECG;

pulse oximetry;

capnography.

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Anaesthesia is a very practical specialty and, to practise safely, anaesthetists must be familiar with the equipment used. This ranges from the simple to the technical and its complexity is increasing relentlessly. The following is an overview of the equipment and monitoring currently in use. No excuse is made for including very simple devices; these are often the most valuable but if used wrongly may endanger the patient’s safety.

Airway Equipment

These are designed to fit closely to the contours of the face and a gas-tight fit is achieved by an air-filled cuff around the edge. Traditionally these devices were made from black rubber and were reusable – the BOC anatomical facemask is an example – and required disinfection between each patient. Increasingly they are now single use and are made from transparent plastics, allowing visualization of vomit, making them popular for use during resuscitation (Fig. 2.1).

Figure 2.1 Plastic, disposable facemask.

Simple Adjuncts

The oropharyngeal (Guedel) airway, and to a lesser extent the nasopharyngeal airway, are often used to help maintain the airway immediately after the induction of anaesthesia. However, their use does not guarantee a patent airway.

Oropharyngeal Airway

These are curved plastic tubes, flattened in cross-section and flanged at the oral end (Fig. 2.2). They lie over the tongue, and prevent it from falling back into the pharynx. They are manufactured in a variety of sizes and suitable for all patients, from neonates to large adults. The commonest sizes are 2–4, for small to large adults, respectively. The size required is estimated by comparing the airway length with the vertical distance between the patient’s incisor teeth and the angle of the jaw.

Figure 2.2 Oropharyngeal and nasopharyngeal airways.

Nasopharyngeal Airway

These are round, malleable plastic tubes, bevelled at the pharyngeal end and flanged at the nasal end (Fig. 2.2). They lie along the floor of the nose and curve round into the pharynx. They are sized according to their internal diameter in millimetres, and their length increases with the diameter. They are not commonly used in children, and sizes 6–8 mm in diameter are suitable for small to large adults, respectively. The correct size is estimated by made by comparing the airway diameter with that of the external nares.

Supraglottic Devices

In recent years there has been an increase in the number of different types of these airway devices available. They are all variations on a similar theme with various modifications to try and improve their suitability for wider applications.

The Laryngeal Mask Airway (LMA)

This was the original supraglottic airway device and, as its name suggests, it consists of a ‘mask’ that sits over the laryngeal opening. This is attached to a tube that protrudes from the mouth and connects directly to the anaesthetic breathing system. Around the perimeter of the mask is an inflatable cuff that helps to stabilize it and creates a seal around the laryngeal inlet. The LMA is suitable for use in all patients, from neonates to adults, as it is produced in a variety of sizes. The most commonly used in female and male adults are sizes 3, 4 and 5. They were originally designed for use in spontaneously breathing patients but it is possible to ventilate patients via the LMA. When doing this care must be taken to avoid high inflation pressures, otherwise leakage occurs past the cuff, reducing ventilation and potentially causing gastric inflation. The original LMA (or classic LMA) is a reusable device requiring sterilization between each patient, but recent concerns about the possible risk of prion disease transmission have resulted in increasing use of disposable versions (Fig. 2.3a).

Figure 2.3 Supraglottic airway devices. (a) Disposable LMA, (b) LMA Pro-Seal™, (c) i-gel™.

There have been a number of modifications to the LMA:

• A version with a more flexible and reinforced tube. This is useful in maxillo-facial or ear, nose and throat surgery as it allows the tube part to be flexed and directed out of the surgeon’s way without kinking and occlusion of the lumen.
  
• The LMA Pro-Seal™ (Fig. 2.3b). This has an additional posterior cuff to improve the seal between mask and larynx, and reduce leak when the patient is ventilated. It also has a secondary tube to allow drainage of gastric contents.
  
• The i-gel™ (Fig. 2.3c). This is the latest development which uses a solid, highly malleable, gel-like material contoured to fit the perilaryngeal anatomy in place of the traditional inflatable cuff. It is single use.
  
• The intubating LMA (Fig. 2.4). As the name suggests, this device is used as a conduit to perform tracheal intubation without the need for laryngoscopy (see below).

Figure 2.4 Intubating Laryngeal Mask Airway (ILMA®).

The Intubating LMA (ILMA)

This is a modification of the LMA in which the mask part is almost unchanged, but a shorter, wider metal tube with a 90° bend in it with a handle replaces the flexible tube (Fig. 2.4). It is inserted using a similar technique as for a standard LMA, but by holding the handle rather than using one’s index finger as a guide. A specially designed reinforced, cuffed, tracheal tube can then be inserted, which will almost always pass into the trachea, due to the shape and position of the ILMA. Once it has been confirmed that the tube lies in the trachea, the ILMA can either be left in place or removed. This device has proved to be very popular in cases where direct laryngoscopy does not give a good view of the larynx and tracheal intubation fails. The most recent development is the C-Trach®, in which the larynx is viewed from the mask aperture using/via a fibre optic cable attached to a small monitor positioned at the proximal end of the device (Fig. 2.5).

Figure 2.5 C-Trach®: an ILM with integrated fibre optics to allow an indirect view of the larynx.

Tracheal Tubes

These are manufactured from plastic (PVC), are single use to eliminate cross-infection, and are sized according to their internal diameter. They are available in a range of sizes at 0.5 mm diameter intervals making them suitable for use in all patients from neonates to adults, and are long enough to be used orally or nasally. A standard 15 mm connector is provided to allow connection to the breathing system.

The tracheal tubes used during adult anaesthesia have an inflatable cuff to prevent leakage of anaesthetic gases back past the tube when positive pressure ventilation is used, and also to prevent aspiration of any foreign material into the lungs. The cuff is inflated by injecting air via a pilot tube, at the distal end of which is a one-way valve to prevent deflation and a small ‘balloon’ to indicate when the cuff is inflated. A wide variety of specialized tubes have been developed, examples of which are shown in Fig. 2.6a–d.

• Reinforced tubes: used to prevent kinking and subsequent obstruction as a result of the positioning of the patient’s head.
  
• Preformed tubes: used during surgery on the head and neck, and are designed to take the connections away from the surgical field.
  
• Double lumen tubes: effectively two tubes welded together side-by-side, with one tube extending distally beyond the other. They are used during thoracic surgery, and allow one lung to be deflated whilst ventilation is maintained via the bronchial portion in the opposite lung.
  
• Uncuffed tubes: used in children up to approximately 8 years of age as the narrowing in the subglottic region provides a natural seal. (Specialized cuffed tubes for children below this age are used in some paediatric units.)

Figure 2.6 Tracheal tubes: (a) standard, (b) preformed (RAE tube), (c) reinforced tube, (d) double lumen tube.

Laryngoscopes

Direct

These are the traditional laryngoscopes, designed to allow direct visualization of the larynx to facilitate the insertion of a tracheal tube. They consist of a blade with a light at the tip, attached to a handle that contains the batteries for the light. The most popular type in use is the curved blade designed by, and named after, Sir Robert Macintosh (Fig. 2.7a). Different sized blades are available. There have been many developments in the design of this device, and one of the most successful is the McCoy blade (Figs 2.7b and c). This has a flexible tip operated by a lever adjacent to the handle that increases the elevation of the epiglottis to improve the view of the larynx. Occasionally a straight-bladed laryngoscope may be used, such as the Magill blade.

Figure 2.7 Laryngoscopes: (a) Macintosh, (b) McCoy, (c) McCoy with tip flexed, (d) McGrath.

Indirect

Recently, numerous devices have been developed that make use of advanced optics and electronics in order to overcome the difficulties when the larynx cannot be directly visualized using the laryngoscopes described above. The operator can visualize the larynx either by ‘looking through’ these devices or by having the image displayed on a separate screen. Some examples that highlight the different technologies used are included here:

• Videolaryngoscopes, for example the McGrath Scope (Fig. 2.7d). There are several of these devices available from different manufacturers. They are more like a conventional laryngoscope except that they have a small camera at the tip. This image is displayed on a small screen and allows a better view of the larynx. Also, some of them have modified shaped blades or tracheal tube guides to help with tube placement. These devices may also have a role to play in training as a supervisor can see what the student sees and offer advice and guidance to improve technique.
  
• Fibreoptic bronchoscope (Fig. 2.8). A narrow diameter flexible bronchoscope that transmits the image from the tip of the scope via thousands of small diameter glass fibres to an eye piece or a display monitor. The tip is manoeuvrable from the handle to help guide the scope in the right direction, and there is a suction channel to remove any secretions from the airways. An appropriate size and length tracheal tube is loaded onto the bronchoscope which is then inserted into either the nose or the mouth and advanced until it lies in the trachea. Once the tip of the bronchoscope is inside the trachea the tracheal tube is passed over the scope until it is seen to pass the tip of the scope and also lie in the trachea. Then the bronchoscope is removed and the tracheal tube cuff is inflated and it is connected to the breathing system. This procedure can be done with the patient awake, following suitable sedation and airway anaesthesia, or with the patient anaesthetised.
  
• Airtraq® (Fig. 2.9). This device uses a prism to aid with visualizing the larynx, and a slot on the side into which the tracheal tube is inserted that helps with placement of the tube.
  
• Optical stylets (Fig. 2.10). Very similar in principle to the flexible fibre optic bronchoscope except that they are rigid, and only suitable for oral use in patients under general anaesthesia.

Figure 2.8 Fibreoptic intubating bronchoscope. A tracheal tube has been mounted ready to advance into the trachea.

Figure 2.9 Airtraq®, a single-use device for intubation. Allows an indirect view of the larynx and has a guide to insert the tracheal tube.

Figure 2.10 Optical stylets. Bonfils (above), Shikani (below).

Gum Elastic Bougie

This is a 60 cm-long malleable introducer, with a slightly angled tip. Its construction allows it to be bent into a gentle curve before it is introduced so that it can be directed blindly behind the epiglottis into the trachea. It is then rigid enough to allow a tracheal tube to be passed over it.

The Safe Delivery of Anaesthesia

Most hospitals use a piped medical gas and vacuum system (PMGV) to distribute oxygen, nitrous oxide, medical air and vacuum. The pipelines’ outlets act as self-closing sockets, each specifically configured, coloured and labelled for one gas. Oxygen, nitrous oxide and air are delivered to the anaesthetic room at a pressure of 400 kilopascals (kPa) (4 bar, 60 pounds per square inch (psi)). The gases (and vacuum) reach the anaesthetic machine via flexible reinforced hoses, colour-coded throughout their length (oxygen – white, nitrous oxide – blue, vacuum – yellow). These attach to the wall outlet via a gas-specific probe and to the anaesthetic machine via a gas-specific nut and union. Cylinders are used as reserves in case of pipeline failure. The gas content has traditionally been indicated by the colour of the body and shoulder of the cylinder (Table 2.1), although the contents must always be confirmed by checking the attached label. However, recent legislation has proposed that all medical gas cylinders should have a white body with coloured shoulders (Table 2.1). This change will occur gradually, being complete by 2025. In the interim period, to limit errors, the content will be written on the body of all cylinders. All cylinders have a pin-index safety mechanism to prevent the connection of the wrong cylinder to the wrong terminal on the anaesthetic machine.

Table 2.1 Medical gas cylinder colours

Oxygen

Piped oxygen is supplied from a liquid oxygen reserve, where it is stored under pressure (7–10 bar, 1000 kPa) at approximately minus 160 °C in a vacuum-insulated evaporator (VIE), effectively a large thermos flask. Gaseous oxygen is removed from above the liquid, or at times of increased demand, by vaporizing liquid oxygen using heat from the environment. The gas is warmed to ambient air temperature en route from the VIE to the pipeline system. A reserve bank of cylinders of compressed oxygen is kept adjacent to the VIE in case the main system fails. A smaller cylinder is attached directly to the anaesthetic machine as an emergency reserve. The pressure in a full cylinder is of oxygen is 13 700 kPa (137 bar, 2000 psi) and this falls proportionately as the cylinder empties.

Nitrous Oxide

Piped nitrous oxide is supplied from several large cylinders joined together to form a bank and attached to a common manifold. There are usually two banks, one running with all cylinders turned on (duty bank), and a reserve. In addition, there is a small emergency supply. Smaller cylinders are attached directly to the anaesthetic machine. At room temperature, nitrous oxide is a liquid within the cylinder, and while any liquid remains the pressure within the cylinder remains constant 5400 kPa (54 bar, 800 psi). When all the liquid has evaporated, the cylinder contains only gas and as it empties, the pressure falls to zero.

Medical Air

This is supplied either by a compressor or in cylinders. A compressor delivers air to a central reservoir, where it is dried and filtered to achieve the desired quality before distribution. Air is supplied to the operating theatre at 400 kPa for anaesthetic use, and at 700 kPa to power medical tools.

Vacuum

The final part of the PMGV system is medical vacuum. Two pumps are connected to a system that must be capable of generating a vacuum of at least 50 kPa below atmospheric pressure. This is delivered to the anaesthetic rooms, operating theatres and other appropriate sites. At several stages between the outlets and the pumps there are drains and bacterial filters to prevent contamination by aspirated fluids.

The Anaesthetic Machine

Its main functions are to:

• reduce the high pressure gases from either the pipeline or cylinders to a pressure that is safe for onward delivery to the patient;
  
• control the flow of gases allowing a known, accurate, and adjustable composition to be delivered into the anaesthetic breathing system.

In addition to these functions, many modern anaesthetic machines contain integral monitoring equipment and ventilators.

Reduction of Pressure

Cylinders contain gases at very high pressures (see above) which can vary depending on the content or temperature of the cylinder. The gas from them first passes through reducing valves to ensure a constant supply of gas at 400 kPa is delivered to the flowmeters. As piped gases are already delivered at 400 kPa, no further pressure reduction is required.

Control of Flow of Gases

Traditionally, on most anaesthetic machines, this has been achieved by the use of flowmeters (‘rotameters’; Fig. 2.11):

• a specific, calibrated flowmeter is used for each gas;
  
• a needle valve controls the flow of gas through the flowmeter;
  
• where accurate, low flows are required, two tubes are used in series, the first has a smaller diameter and a narrow, low flow range (e.g. 0–0.5 L/min), the second is wider with a greater flow range (0.5–10 L/min);
  
• a rotating bobbin floats in the gas stream, its upper edge indicating the rate of gas flow;
  
• several flowmeters for different gases (oxygen, air and nitrous oxide), are mounted with oxygen to the left; the control for oxygen has a different knurled finish and is usually more prominent;
  
• flowmeters do not regulate pressure.

Figure 2.11 Oxygen, air and nitrous oxide flowmeters on an anaesthetic machine.

Anaesthetic machines have several safety features built into the gas delivery system:

• the oxygen and nitrous oxide controls are linked preventing less than 25% oxygen from being delivered;
  
• an emergency oxygen ‘flush’ device can be used to deliver pure oxygen at greater than 40 L/min into the breathing system;
  
• an audible alarm to warn of failure of oxygen delivery – this discontinues the nitrous oxide supply and if the patient is breathing spontaneously air can be entrained;
  
• a non-return valve to minimize the effects of back-pressure on the function of flowmeters and vaporizers.

Increasingly, on many modern anaesthetic machines, flowmeters have been replaced with electronic control of gas flow. The anaesthetist simply dials in the required flow and this is delivered into the anaesthetic system. The flow of gas is then displayed on a monitor screen either numerically or as an analogue representation of a flowmeter.

Related posts:

Anaesthetic Assessment and Preparation for Surgery Local and Regional Anaesthesia Drugs and Fluids Used During Anaesthesia The Practice of General Anaesthesia Special Circumstances Post-Anaesthesia Care