Tracheal and tracheostomy tubes and airways
Tracheal tubes
Features of tracheal tubes (Fig. 5.1)
1. The ‘size’ of a tracheal tube refers to its internal diameter which is marked on the outside of the tube in millimetres. Narrower tubes increase the resistance to gas flow, therefore the largest possible internal diameter should be used. This is especially important during spontaneous ventilation where the patient’s own respiratory effort must overcome the tube’s resistance. A size 4-mm tracheal tube has 16 times more resistance to gas flow than a size 8-mm tube. Usually, a size 8.5–9-mm internal diameter tube is selected for an average size adult male and a size 7.5–8-mm internal diameter tube for an average size adult female. Paediatric sizes are determined on the basis of age and weight (Table 5.1). Tracheal tubes have both internal diameter (ID) and outside diameter (OD) markings. There are various methods or formulae used to determine the size of paediatric tracheal tubes. A commonly used formula is:
2. The length (taken from the tip of the tube) is marked in centimetres on the outside of the tube. The tube can be cut down to size to suit the individual patient. If the tube is cut too long, there is a significant risk of it advancing into one of the main bronchi (usually the right one, see Fig. 5.2). Black intubation depth markers located 3 cm proximal to the cuff can be seen in some designs (Fig. 5.1). These assist the accurate placement of the tracheal tube tip within the trachea. The vocal cords should be at the black mark in tubes with one mark, or should be between marks if there are two such marks. However, these are only rough estimates and correct tracheal tube position depth should always be confirmed by auscultation.
The bevel
1. The bevel is left-facing and oval in shape in most tube designs. A left-facing bevel improves the view of the vocal cords during intubation.
2. Some designs have a side hole just above and opposite the bevel, called a Murphy eye. This enables ventilation to occur should the bevel become occluded by secretions, blood or the wall of the trachea (Fig. 5.3).
The cuff
Tracheal (oral or nasal) tubes can be either cuffed or uncuffed. The cuff, when inflated, provides an air-tight seal between the tube and the tracheal wall (Fig. 5.4). This air-tight seal protects the patient’s airway from aspiration and allows efficient ventilation during IPPV.
1. The cuff is connected to its pilot balloon which has a self-sealing valve for injecting air. The pilot balloon also indicates whether the cuff is inflated or not. After intubation, the cuff is inflated until no gas leak can be heard during intermittent positive pressure ventilation (IPPV).
2. The narrowest point in the adult’s airway is the glottis (which is hexagonal). In order to achieve an air-tight seal, cuffed tubes are used in adults.
3. The narrowest point in a child’s airway is the cricoid cartilage. Since this is essentially circular, a correctly sized uncuffed tube will fit well. Because of the narrow upper airway in children, post-extubation subglottic oedema can be a problem. In order to minimize the risk, the presence of a small leak around the tube at an airway pressure of 15 cm H2O is desirable.
High-pressure/low-volume cuffs
Low-pressure/high-volume cuffs
1. These exert minimal pressure on the tracheal wall as the pressure equilibrates over a wider area (Fig. 5.6). This allows the cuff to remain inflated for longer periods.
2. They are less capable of preventing the aspiration of vomitus or secretions. This is due to the possibility of wrinkles forming in the cuff.
The pressure in the cuff should be checked at frequent and regular intervals (Fig. 5.7 and 5.8). The pressure may increase mainly because of diffusion of nitrous oxide into the cuff. Expansion of the air inside the cuff due to the increase in its temperature from room to body temperature and the diffusion of oxygen from the anaesthetic mixture (about 33%) into the air (21%) in the cuff can also lead to increase in the intracuff pressure. An increase in pressure of about 10–12 mmHg is expected after 30 minutes of anaesthesia with 66% nitrous oxide. A more recent design cuff material (Soft Seal, Portex) allows minimum diffusion of nitrous oxide into the cuff with a pressure increase of 1–2 mmHg only. The pressure may decrease because of a leak in the cuff or pilot balloon’s valve.
Route of insertion
1. Tubes can be inserted orally or nasally (Fig. 5.9).
2. The indications for nasal intubation include:
a) surgery where access via the mouth is necessary, e.g. ENT or dental operations
b) long-term ventilated patients on intensive care units. Patients tolerate a nasal tube better, and cannot bite on the tube. However, long-term nasal intubation may cause sinus infection.
3. Nasal intubation is usually avoided, if possible, in children up to the age of 8–11 years. Hypertrophy of the adenoids in this age group increases the risk of profuse bleeding if nasal intubation is performed.
4. Ivory PVC nasotracheal tubes cause less trauma to the nasal mucosa.
Connectors
These connect the tracheal tubes to the breathing system (or catheter mount). There are various designs and modifications (Fig. 5.10). They are made of plastic or metal and should have an adequate internal diameter to reduce the resistance to gas flow.
Specially designed tracheal tubes
This anatomically L-shaped tracheal tube is used in anaesthesia for head and neck surgery because it is non-kinking (Fig. 5.11). The tube can be made of rubber or plastic and can be cuffed or uncuffed. The bevel is oval in shape and faces posteriorly and an introducing stylet is supplied to aid the insertion of the tube. Its thick wall adds to the tube’s external diameter making it wider for a given internal diameter. This is undesirable especially in paediatric anaesthesia.
Armoured tracheal tube
Armoured tracheal tubes are made of plastic or silicone rubber (Fig. 5.12). The walls of the armoured tube are thicker than ordinary tracheal tubes because they contain an embedded spiral of metal wire or tough nylon. They are used in anaesthesia for head and neck surgery. The spiral helps to prevent the kinking and occlusion of the tracheal tube when the head and/or neck is rotated or flexed so giving it strength and flexibility at the same time. An introducer stylet is used to aid intubation.
Polar and RAE tracheal tubes
The polar tube is a north- or south-facing preformed nasal cuffed or uncuffed tracheal tube (Fig. 5.13). It is used mainly during anaesthesia for maxillofacial surgery as it does not impede surgical access. Because of its design and shape, it lies over the nose and the forehead. It can be converted to an ordinary tracheal tube by cutting it at the scissors mark just proximal to the pilot tube and reconnecting the 15-mm connector. An oral version of the polar tube exists.
The tube can be temporarily straightened to insert a suction catheter.
Laser resistant tracheal tubes
These tubes are used in anaesthesia for laser surgery on the larynx or trachea (Fig. 5.14). They are designed to withstand the effect of carbon dioxide and potassium-titanyl-phosphate (KTP) laser beams, avoiding the risk of fire or damage to the tracheal tube. One design has a flexible stainless steel body. Reflected beams from the tube are defocused to reduce the accidental laser strikes to healthy tissues (Fig. 5.15). Other designs have a laser resistant metal foil wrapped around the tube for protection. The cuff is filled with methylene blue coloured saline. If the laser manages to damage the cuff, the colouring will help identify rupture and the saline will help prevent an airway fire.
Fig. 5.15 The reflected laser beam is defocused.
Tracheostomy tracheal tubes
These are curved plastic tubes usually inserted through the second, third and fourth tracheal cartilage rings (Fig. 5.18).
Components
1. An introducer used for insertion.
2. Wings attached to the proximal part of the tube to fix it in place with a ribbon or suture. Some designs have an adjustable flange to fit the variable thickness of the subcutaneous tissues (Fig. 5.19).
3. They can be cuffed or uncuffed. The former have a pilot balloon.
4. The proximal end can have a standard 15-mm connector.
5. The tip is usually cut square, rather than bevelled. This is to decrease the risk of obstruction by lying against the tracheal wall.
6. A more recent design with an additional suctioning lumen which opens just above the cuff exists. The cuff shape is designed to allow the secretions above it to be suctioned effectively through the suctioning lumen (Fig. 5.20).
7. Some tubes have an inner cannula. Secretions can collect and dry out on the inner lumen of the tube leading to obstruction. The internal cannula can be replaced instead of changing the complete tube in such cases. The cannula leads to a slight reduction of the internal diameter of the tube.
8. There are different sizes of tracheostomy tubes to fit neonates to adults.
9. Older uncuffed metal tracheostomy tubes made of a non-irritant and bactericidal silver are rarely used in current practice. Some designs have a one-way flap valve and a window at the angle of the tube to allow the patient to speak.
Tracheostomy tubes are used for the following
1. Long-term intermittent positive pressure ventilation.
2. Upper airway obstruction that cannot be bypassed with an oral/nasal tracheal tube.
3. Maintenance of an airway and to protect the lungs in patients with impaired pharyngeal or laryngeal reflexes and after major head and neck surgery (e.g. laryngectomy).
4. Long-term control of excessive bronchial secretions especially in patients with a reduced level of consciousness.
5. To facilitate weaning from a ventilator. This is due to a reduction in the sedation required, as the patients tolerate tracheostomy tubes better than tracheal tubes. Also, there is a reduction in the anatomical dead space.
Problems in practice and safety features
The complications can be divided into:
b) tube misplacement (e.g. into a main bronchus)
c) occlusion of tube by cuff herniation
d) occlusion of the tube tip against carina or tracheal wall
a) blockage of the tube by secretions which can be sudden or gradual; this is rare with adequate humidification and suction
c) overinflation of the cuff leads to ulceration and distension of the trachea
d) mucosal ulceration because of excessive cuff pressures, asymmetrical inflation of the cuff or tube migration.
The fenestrated tracheostomy tube (Fig. 5.21)
1. The fenestration (window) in the greater curvature channels air to the vocal cords allowing the patient to speak.
2. After deflation of the cuff, the patient can breathe around the cuff and through the fenestration as well as through the stoma. This reduces airway resistance and assists in weaning from tracheostomy in spontaneously breathing patients.
Laryngectomy (montandon) tube
This is a cuffed tube inserted through a tracheostomy to facilitate intermittent positive pressure ventilation during neck surgery (Fig. 5.22). It has the advantage of offering better surgical access by allowing the breathing system to be connected well away from the surgical field. Usually, it is replaced with a tracheostomy tube at the end of operation.
