The respirable gas inlet

This inlet has a number of components (Fig. 8.3):

Figure 8.3 The respirable gas inlet of a self-inflating bag (see text). A. One way flap valve; B. 5 mm O₂ nipple; C. wide bore inlet; D. reservoir; E. overpressure relief valve; F. air entrainment valve.

• A one-way flap valve (A). This is fitted to the inlet of the self-inflating bag. When the bag is squeezed, the gas pressure inside the bag rises and causes the flap valve to close. This prevents the escape of gas back through the inlet. When the bag is released, its self-inflating characteristic causes fresh gas from the respirable gas inlet to be indrawn. This may be air, oxygen or a mixture of both.

• A small bore nipple (B). This is mounted on the inlet, to allow admixture of oxygen.

• A wide bore inlet (C). This supplies the bulk of the gas entering the bag and is usually air, unless oxygen is added, as above. In the latter situation, the final concentration of oxygen delivered is a function of the amount of added oxygen and its dilution with air in the self-inflating bag.

• A reservoir system (D). The inlet (C) may be fitted with a reservoir system. This feature is now widely used in almost all manual resuscitators. Its purpose is to store the oxygen fed into the system from the nipple (B). When the minute volume of oxygen supplied is greater than the volume given to the patient, the bag (D) will expand and will provide all the gas for ventilation (i.e. 100% oxygen). The reservoir must be fitted with an overflow valve (E) to prevent overfilling from too high a flow of oxygen and an entrainment valve (F) to allow ingress of air for when oxygen is not available or when lower concentrations of oxygen are required. Tables 8.1 and 8.2 show typical oxygen concentrations that are delivered under a variety of conditions, with two popular makes of resuscitators. The differences reflect the size of the relevant reservoirs used.

Table 8.1 Oxygen concentrations (%) in a Laerdal resuscitator

Table 8.2 Oxygen concentrations (%) in the Ambu system with reservoir

The non-rebreathing valve

This valve is housed at the opposite end of the bag to the gas entrainment system described above. It has a number of components that ensure that during the inspiratory phase, gas flows out of the bag and only into the patient port. When the patient exhales, the valve also ensures that this exhaled gas escapes through the expiratory port without mixing with the fresh gas stored in the bag. Functionally, most non-rebreathing valves are similar, although there are some differences in their efficiency (see below). They all also have suitable dimensions on the expiratory side to allow addition of a mechanical PEEP valve.

Although primarily used in resuscitators, these valves may also be incorporated into anaesthetic breathing systems (see Chapter 27).

There have been many designs since the original Ambu manual resuscitator valve, most of which are no longer produced or in use. These have been discarded due to a combination of their relative inefficiencies, tendency to jam particularly when wet and inability to be made economically as single-use items. There are now only two popular designs

Ambu single shutter valve (Figs 8.4A, B and C)

This valve is the version used in all current Ambu products. Compared to previous models, it has been simplified and now incorporates a single, multi-function shutter only. The valve shown here is reusable and can be dismantled for cleaning and sterilizing. A disposable version is incorporated into the SPUR II (single patient use resuscitator) system (Fig. 8.5B). The part of the valve body containing the expiratory pathway in both types may be unscrewed and a spring loaded PEEP valve substituted.

Figure 8.4 A. Ambu single-shutter valve. B. A SPUR II (single patient use resuscitator) system. C. Working principles: upper part, the valve is pushed onto the expiratory port and occludes it so that gas can enter only the patient port; lower part, exhaled gas pushes the valve against the inspiratory port, occluding it so that the gas can escape only through the expiratory port.

Figure 8.5 A. The Laerdal valve; B. working principles.

When this valve is used for controlled ventilation, manual compression of the self-inflating bag pushes gas against the concave aspect of the shutter causing it to move and occlude the expiratory port. The same movement opens the patient port to allow ingress of the gas.

At the beginning of the exhalation phase, exhaled gas impinges on the convex aspect of the shutter, causing it to move in the opposite direction, so that it opens the expiratory pathway as well as occluding the inspiratory port.

With spontaneous respiration, as inspiratory resistance (0.7 kPa at 10 l min⁻¹) through the valve is less than expiratory resistance (0.8 kPa at 10 l min⁻¹), gas will be drawn preferentially from the bag. Initial movement of gas will also cause the shutter valve to occlude the expiratory path so that the valve behaves in a similar manner to controlled ventilation.

The guide stem and flexible shutter are clearly visible through the transparent valve body and their ‘to and fro’ movement is an indicator of correct function.

The self-inflating bag supplied with this valve is made from silicone. The single use version has been upgraded from the original Spur system by the addition of extra ribbing to increase its recoil so as to allow a faster rate to be applied. The gas inlet is fitted with the two pressure relief valves as described before.

This high-efficiency non-rebreathing valve (Fig. 8.5) is made in three sizes (adult, child, infant). The valve itself has three components, a duck-billed inspiratory/expiratory valve, a valve body housing inspiratory and expiratory ports and a non-return flap valve sited in the expiratory port. Originally designed by Laerdal, it is now used by many manufacturers for their resuscitators. It may be made for single use only, in which case the valve housing is sealed and cannot be opened. The reusable version made of autoclavable materials may be dismantled for cleaning and sterilizing. However, care must be taken to reassemble all the components correctly as there have been reports of misassembly:

• Inspiratory phase. The central duck-billed portion of the main valve opens when the attached self-inflating bag is squeezed, or when a patient inhales through it. Almost simultaneously the outer disc-shaped portion of the valve is pushed against the apertures in the valve body, thus sealing the expiratory pathway.

• Expiratory phase. Positive expiratory pressure from the elastic recoil of the patient’s lungs causes the duckbilled section of the valve to close, thus preventing rebreathing it into the bag. Escaping gas also lifts the flaps on a non-return valve in the expiratory port. This is a supplementary valve that prevents air from being aspirated into the expiratory port during spontaneous respiration.

• Other features. All the sizes have a 23 mm external diameter expiratory port and a 22 mm external diameter/15 mm internal diameter inspiratory port so as to minimize the possibility of misconnection. The child and infant models have overpressure safety valves built into the inspiratory port of the valve and which are set to blow off at a pressure of 45 cm H₂O. If these pressures ever need to be exceeded, the safety valve can be overridden by finger pressure or a lock clip over the valve.

The self-inflating bag supplied with Laerdal resuscitators has thickened ribs of silicone rubber that provide the rigidity for the self-inflating action. These bags can be supplied with a supplementary reservoir bag (larger than that for the Ambu) for the supply of high oxygen concentrations to patients (Table 8.1). The entrainment port of the reservoir bags is fitted with a housing containing two valves. One is for air entrainment when little or no additional oxygen is used. The other is for the relief of any high-pressure build-up in the system from an excess of oxygen flow (see above).