Fig. 20.2A shows a piston pump, which is capable of creating high vacuum but, in transportable models, has a relatively low displacement (i.e. can only sustain low flow rates). Fig. 20.2B shows a diaphragm pump, which is a variation of the piston pump. It is mechanically simpler but is also frequently much noisier. One reason for the increased noise is that often, rather than using a conventional rotating electric motor, a much simpler large electromagnet displaces the diaphragm. Fig. 20.2C shows a form of rotary pump that can produce a high vacuum without conventional one-way valves. Fig. 20.2D shows a rotary pump capable of producing very high flows, as would be required in a dental surgery. It works in the same way as a vacuum cleaner and has the disadvantage of being extremely noisy. Fig. 20.2E shows a pump that works on the principle of the Archimedean screw. This type of pump can produce a high vacuum for a comparatively small size of machine. Fig. 20.2F shows the principle of a Venturi pump which makes use of the Bernoulli effect. Compressed fluid (gas or liquid), passing through a narrow orifice, creates a region of negative pressure beyond that orifice, which can be used to entrain adjacent air/debris. The main disadvantage of this simple affair is that it uses and is thus wasteful of large volumes of driving fluid, which is usually oxygen from cylinders. However, it does have the virtue of being extremely portable (Fig. 20.3). Finally, Fig. 20.2G shows a simple bellows mechanism with a pair of one-way valves, as would be used in manually operated suction apparatus.

Figure 20.3 Working principles of a suction device using a Venturi injector.

The pump may be:

• permanently sited as in pipeline suction systems

• transportable, usually powered by mains electricity and supported on castor wheels; or

• truly portable, powered by battery, a cylinder of gas or by human energy (hand or foot operated).

There are international standards for each type of suction apparatus and these are listed at the end of this chapter.

Internal connections

The vacuum source is connected to the filter and collection vessel by tubing that is as rigid as possible to avoid collapse due to the vacuum. In pipeline systems the tubing can be made from metal or hard plastics, but the internal tubing in transportable units has to be firm but flexible, as does the final connection to the collection vessel in all types.

Filter

This is fitted between the collection vessel and the vacuum source. It is to prevent contamination reaching the pipeline or pump and being expelled into the atmosphere. Filters should remain dry and must be replaced at the intervals recommended by the manufacturer, otherwise there is risk of reducing suction efficiency and of the filter becoming an infection risk itself.

Collection vessel

The collection vessel stores the aspirate prior to its disposal. It needs to be of sufficient volume to hold enough aspirate so that it does not need emptying too often. However, the larger its volume, the greater the displacement that is necessary from the pump, in order to reduce the pressure in the vessel before aspiration can occur efficiently. It is important that the inlet to the collection vessel has an internal diameter sufficiently large to admit the largest particles expected in the particular application of the apparatus (see Further Reading).

To minimize risks of contamination to theatre staff, all suction apparatus, except portable emergency units, should make use of a disposable collection system (Fig. 20.4A). This usually consists of a rigid outer transparent container with volume markings on its side and an inner (transparent) disposable plastic sleeve with an integral lid. The aspirate is stored in the inner sleeve and when full is capped off and discarded. As it is disposed of with other clinical waste, a gelling agent should be added to the vessel so as to solidify the contents to prevent accidental spillage. Thus, staff should never come into contact with any aspirate.

Figure 20.4 A. Disposable collection vessel system. Note the outer container with volume markings and the inner sleeve fitted with a lid. B. A disposable collection system with multiple collection vessels.

Suction tubing to disposal

There must be a suitable length of suction tubing, the patient end of which is usually fitted with a detachable rigid suction hand-piece. This tubing is a compromise between rigidity, to avoid collapse when the vacuum is applied, and flexibility, for ease of use. With portable hand-held apparatus, the hand-piece may be connected directly to the collection vessel for ease of use.

Efficiency

The efficiency of suction apparatus depends upon:

• the degree of vacuum (sub-atmospheric pressure) that can be produced by the pump, with particular regard to the time taken to achieve it

• the displacement, i.e. the volume of air at atmospheric pressure that the pump is able to move in unit time

• the internal resistance of the suction apparatus as a whole. This is related not only to the length and diameter of tubing and other components, but also to the tubing and other accessories between the apparatus and the liquid being aspirated

• the viscosity of the matter being aspirated.

Different pump designs may have differing displacements and high-vacuum capabilities and, therefore, may be selected for specific tasks. For example, liposuction requires high vacuum to dislodge fat globules that have a high viscosity but a small displacement because only a relatively small volume and no air are removed. In contrast, dental suction requires high displacement to remove large amounts of water spray, air and dental debris, but does not require a high vacuum. This type of apparatus must also have a low internal resistance and be connected to a relatively wide-bore hand-piece and tubing to maximize the rate of removal of debris.

Other components of suction apparatus

Vacuum control valve or regulator

This may be fitted between the vacuum source and the collection vessel. A vacuum control valve is a bleed valve that when opened, admits air, thereby reducing the degree of vacuum. A vacuum regulator operates on a similar principle to a pressure regulator and so no energy is wasted from the system. Regulators are always used with pipeline systems, whereas control valves are common with transportable electrically driven units.

Vacuum gauge

The vacuum gauge is also placed between the vacuum source and the collection vessel. It is calibrated in mmHg or kPa or both scales. The purpose of the gauge is three-fold: to test the apparatus for efficiency and leaks, to allow for adjustment of the available vacuum during use and to warn of excessive suction being applied to tissues. Note that modern vacuum gauges indicate counter-clockwise.

Cut-off over-flow valve

This is part of the collection vessel assembly and prevents any aspirate from leaving the vessel and entering the vacuum source or controller. It usually consists of a float, which is lifted by the rising level of aspirate when the vessel is full.

In order to test both the collection vessel and especially its inlet and also the cut-off overflow valve, an international standard test vomit has been specified. It consists of food-grade xantham gum, water and 1 mm glass beads (see Further reading).

Foam prevention

Foaming can be a problem in the collection vessel. Rising foam may cause premature closure of the overflow cut-off valve or it may fail to activate the valve and pass beyond it, contaminating the filter or pump and causing failure. Foaming also makes it difficult to estimate the amount of aspirate in the vessel. It may be reduced by the addition of a small quantity of silicon-based emulsion or by silicon coating of the inside of the collection vessel.

Multiple collection vessels

Modern suction apparatus usually has two collection vessels with a manually operated valve, so that the vacuum source can be rapidly switched from a full vessel to a fresh one. Alternatively, where large volumes of aspirate are anticipated, a number of collection vessels are connected in series in a carousel with all the individual valves open. Here, aspirate overflows from the first vessel into the next and so on (Fig. 20.4B). When the accurate estimation of small volumes of aspirate is required, as in paediatric surgery, a small calibrated vessel may be used in addition to the main apparatus. This container is disposable and is usually close to the operative field.

The suction nozzle, catheter or hand-piece

The design of what is referred to in International Standards as the ‘applied part’ depends upon the application. The commonest examples of hand-held suction nozzles are shown in Fig. 20.5. The key requirement of any suction ‘applied part’ is that the smallest internal diameter is at the very tip. If there are smaller diameters between the tip and the collection vessel, then blockages are likely to occur. The shape of the tip should be smooth so as to prevent damage to delicate tissues. The practice of allowing the tip to be occluded by any tissue, to reduce the noise of suction when not actually aspirating, must be deprecated as it causes tissue damage. Similarly, the noise of suction when using transportable, electrically powered suction machines should be reduced by switching off the motor rather than by occluding the suction tubing, as this may overload the motor. Hand-held suction nozzles of the Yankauer type (Fig. 20.5) often have a hole on the handle for fine control of suction with a finger.

Figure 20.5 Two examples of hand-held Yankauer suction nozzles.

Bronchial suction catheters should have smooth tips to avoid damaging delicate tissues and usually have several holes around the tip.

To protect staff, closed suction catheter systems are now commonly used (Fig. 20.6). The catheter is enclosed in a flexible sheath, which is permanently attached to a special 15 mm-taper adapter, which is left in circuit between the catheter mount and the endotracheal or tracheostomy tube. These systems are replaced every 24 h or as specified by the manufacturer.

Figure 20.6 Closed bronchial suction system.

Local vacuum units

Piped vacuum systems are now installed in most hospitals. The ‘behind the wall’ equipment and terminal outlets are described in Chapter 1. Medical suction apparatus may be connected into this system, wherever there is a terminal outlet.

There are two main types of ‘local’ apparatus:

1. Free-standing floor units, mains electricity powered and often with two collection vessels used for surgical purposes in the operating theatre (Fig. 20.7). Small, ‘low-suction’, low displacement units are also available for bedside use for intracavitary and continuous wound drainage. Generating typically 5–50 mmHg sub-atmospheric pressure, these devices are usually mains and rechargeable battery-powered piston pumps.

2. Wall-mounted units are local suction controllers for connection to central piped vacuum source (see Chapter 1). These have a single collection vessel as shown in Fig. 20.8. There are two types of controller: conventional ‘high-suction’ and also ‘low-suction’. Low-suction controllers are deliberately limited to provide safe suction for intra-pleural drainage or nasogastric suction. Confusion between the two types of suction controller can have disastrous consequences, hence the resurgence of free-standing ‘low-suction’ units.

Figure 20.7 Free-standing medical suction unit.

Figure 20.8 A pipeline vacuum unit. On the right is the controller, which is plugged directly into a flush-fitting outlet (obscured in this picture). On the right is the reservoir jar.

Choice of suction apparatus

When selecting a suction apparatus for a particular purpose, the following points need to be considered:

• Must it be portable? If so, should it be hand/foot operated (Fig. 20.9)? If not, should it be powered by electricity (mains or battery) or is pipe-line vacuum available? Could it be powered from a gas cylinder using a Venturi injector?

• Is a high displacement needed?

• Is a high vacuum needed?

• What size should the collection vessel be?

Figure 20.9 Ambu ResCue hand-operated suction apparatus.

Courtesy of Ambu International Ltd.

It is important to ascertain, for an electrically driven suction machine, whether it is rated for continuous or intermittent use.

High-volume aspirators, as used in dental surgery, use a pump similar to that used in a vacuum cleaner (Fig. 20.10A). They usually have several suction tubes of different diameters for different applications (Fig. 20.10B). These tubes and their nozzles should never be obstructed, as a high flow of air is required to cool the motor, which would otherwise overheat. This type of high-flow suction is needed in dental surgery to aspirate the spray of water used to cool the tooth during high-speed drilling.