TUBE TYPES

Polyvinyl chloride or silastic tubes are suitable for long-term intubation. There is little difference in the safety of uncuffed and cuffed tubes provided correct sizes are used and cuff pressures are regulated. The cuff should be inflated with the minimum volume of air necessary to maintain a seal, and checked every 4–8 hours. Recording of pressure in the cuff is desirable. Nonetheless, cuffs reduce effective tube diameter, which would otherwise be available for gas flow. High-volume low-pressure cuffed tubes may be used after puberty when the cricoid region of the trachea has enlarged and is no longer a narrow region at special risk. In circumstances of poor lung compliance, high airway resistance or foreseeable difficulties changing a tube (e.g. as in facial burns), it may be preferable to use a cuffed tube from the outset.

The curve of the tube should be of the Magill type – curved but neither preformed nor of variable diameter. Preformed tubes (e.g. Rae and Childs) make suction difficult and automatically stipulate length in relation to diameter and circumference. Tubes of non-uniform diameter (e.g. Cole and Oxford) may damage the vocal cords, and do not prevent endobronchial intubation.

TUBE SIZE

A selection of sizes, one larger and one smaller than the anticipated size, should be available at intubation. Tube sizes according to body weight and age are given in Table 104.1 and 104.2. A rough guide for internal diameter size in the child is age (in years)/4 + 4 mm.

Table 104.1 Orotracheal and nasotracheal tube lengths for premature neonates

Table 104.2 Endotracheal tube sizes (internal diameter) and orotracheal and nasotracheal depths for infants and children

Correct selection of size is necessary to avoid pressure-induced ischaemia/ulceration of tracheal mucosa and to limit aspiration around the tube. The correct size should allow not only a small leak on application of moderate inflation pressure (25 cmH₂O, 2.5 kPa) but also enable adequate pulmonary inflation and expiration. When lung compliance is poor, it is permissible to use a cuffless tube without a leak or a cuffed tube to achieve lung inflation.

PLACEMENT

Oral or nasal tubes may be used. Orally placed tubes are preferred in acute resuscitation and when only a short duration of intubation is required, e.g. for anaesthesia during insertion of a central venous cannula. Nasal placement of tubes is preferred in the setting where spontaneous or mechanical ventilation is required for prolonged periods.

Nasal tubes are more comfortable and manageable for the awake patient and are more easily secured to the face, but pose risks, which include sinus infection and necrosis of the alae nasi. Oral tubes are difficult to fix, uncomfortable and are easily occluded by biting by conscious/semiconscious patients.

TUBE LENGTH (DEPTH)

The endotracheal tube tip should be located in the mid-trachea so that the risks of accidental extubation and endobronchial intubation are minimised. Correct positioning of the tube should always be checked by auscultation in the axillae, by chest X-ray immediately after intubation and daily thereafter.¹ At intubation, visualisation of the depth of the tube passed through the cords should be a guide only, since the tube advances when the head is released from extension as the laryngoscope is removed.²

The lengths (depths) at which orotracheal and nasotracheal tubes can be chosen initially on the basis of weight or gestation in premature neonates are given in Table 104.1, and on the basis of age for infants and children in Table 104.2. The lengths are at the lips for oral tubes and at the exit from the nose for nasal tubes. For oral tubes, a guide to correct depth for 2 years and over is given by the formula: (age in years/2 + 12 cm), while for nasal tubes the formula is: (age in years/2 + 15 cm). The depth of insertion of any tube should be documented or marked on the tube.

FLOW-INFLATED BAGS

These are exemplified by Jackson-Rees modified Ayre’s T-piece. Gas flow must exceed 220 ml/kg per min for children and 3 l/min for infants to prevent rebreathing during manual ventilation. It is possible to control precisely the concentration of inspired oxygen, which is an advantage, especially for the premature neonate at risk of oxygen-induced retrolental fibroplasia. However, considerable experience is necessary to provide adequate ventilation without barotrauma in the intubated infant. The circuit has neither a valve nor any pressure-relief device. A pressure gauge should be incorporated in the circuit to help prevent barotrauma.

SELF-INFLATED BAGS

These bags are designed to give positive-pressure ventilation attached to either a mask or endotracheal/tracheostomy tube. Rebreathing is prevented by one-way duck-bill valves, spring disk/ball valves or diaphragm/leaf valves.

The Laerdal bag series (infant, child, adult) typify these devices. A pressure-relief valve (infant and child size) opens at 35 cmH₂O (3.5 kPa). A pressure monitor can be incorporated in the circuit. Supplemental oxygen is added to the ventilation bag, with or without attachment of a reservoir bag, whose movement may serve as a visual monitor of tidal volume during spontaneous ventilation when intubated. However, the valve may offer too much resistance for spontaneous ventilation and they should not be used to provide supplemental oxygen to a spontaneously breathing patient via a mask placed loosely over the face.

With Laerdal and Partner bags, negligible amounts of oxygen (0.1–0.3 l/min) issue from the patient valve when 5–15 l/min of oxygen is introduced into bags unconnected to patients.⁴ The valve is unlikely to open unless the mask is sealed well on the face. The delivered oxygen concentration is dependent on the flow rate of oxygen, use of the reservoir bag, and the state of the pressure relief valve (whether open or closed).

Other self-inflated bags include the Combibag, Ambu, AirViva and Hudson RCI.

PHASE VARIABLES

The manner of initiation of inspiration (triggering), sustaining inspiration (limit variables), initiation of expiration (cycling) and sustaining expiration are the phase variables. Initiation of inspiration, or triggering, may be dictated by machine or patient. Usual triggers are the elapse of time or generation of pressure or flow by the patient. Flow triggering during spontaneous ventilation is more energy efficient and better tolerated than other variables, particularly when the flow sensor is located at the endotracheal tube. Mandatory breaths are all those triggered by a preset frequency or minimum minute ventilation, or terminated by a preset frequency or tidal volume. Spontaneous breaths are those initiated by the patient and terminated by lung mechanics or ventilatory drive. The quantification of the inspiratory phase is by time, pressure or volume. Initiation of expiration (termination of inspiration) or cycling is achieved by elapse of time, pressure, volume or flow. Expiration is usually sustained by pressure alone.

Modes of ventilation may be described on the following generic basis. It is derived from the controller variables. It should not be assumed that a manufacturer’s mode terminology accurately defines its functional attributes or is intuitively obvious. Terminology is not necessarily interchangeable between different manufacturers. Although some new modes are genuine inventions, another purpose appears to be differentiation of product from competition. The capabilities of any ventilator can be defined by the following: