A popular classification with British anaesthetists has been described by Mushin.¹ Those ventilators that by their design produce a pressure sufficient only to ventilate normal or mildly abnormal lungs are classified as pressure generators, i.e. the tidal volume delivered to the patient is limited by the pressures generated. Those ventilators that develop pressures sufficiently high enough to deliver a desired flow even to grossly abnormal lungs are deemed flow generators. However, as most other electromechanical devices in common usage are described in terms of power, the author prefers the first classification.

Efficiency of ventilators

This may be defined as the ratio of the intended tidal volume (as determined by the settings on the ventilator) over the actual delivered tidal volume. For example, when a ventilator acts on a bellows containing patient gas at atmospheric pressure, the gas undergoes a degree of compression in order to raise the pressure sufficiently to provide an inspiratory flow. Part of the bellows travel is taken up in compressing the gas. If the bellows travel is calibrated for volume, it becomes apparent that the tidal volume actually delivered is less than that indicated on the bellows scale. The greater the pressure required to ventilate a patient’s lungs, the greater will be the amount of gas lost in compression. This type of ventilator is regarded as relatively inefficient, as the discrepancy between anticipated and delivered tidal volumes may be as great as 25% in patients with significant pathological lung conditions.

Furthermore, the effective inspiratory time is shortened as, initially, time is lost in compressing the gas to the required pressure. Inefficient ventilators (which include most anaesthetic ventilators that supply circle systems) may well require validation of the delivered tidal volume, using a spirometer or capnograph. With the advent of more sophisticated measurement of flow and electronic feedback to the ventilator, the compliance of this type of system, and, therefore, the compression volume, can now be calculated and automatic adjustment made for most of the apparent ‘lost’ volume.

More efficient ventilators utilize respiratory gas already under a pressure greater than that required to ventilate a patient’s lungs, so that the gas is already compressed prior to being released and, therefore, none is lost in a ‘compression volume’. It is important to grasp this concept, as there may be a marked difference in the anticipated performance of ventilators.

lnspiratory characteristics of ventilators

Ventilators may produce a variety of pressure waveforms and inspiratory flow characteristics depending on the method of generation of respirable gas pressure and the resistance to flow that the gas meets during delivery of the intended tidal volume.

Low-powered ventilators

Low-powered ventilators deliver gas at modest pressure. This pressure is normally constant (Fig. 9.2A) and will produce an inspiratory flow rate of gas that is greatest in early inspiration, when the pressure differential between the ventilator and the lung is wide, but that slows during inflation of the lung as the pressures approximate (Fig. 9.2B).

Figure 9.2 Inspiratory characteristics of low-powered ventilators. A. Constant pressure (weighted bellows); B. Flow pattern in normal (F1) and abnormal (F2) lungs.

High-powered ventilators

High-powered ventilators function by delivering a sufficiently high driving gas pressure to overcome most abnormal resistance without significantly altering the flow from the ventilator, which remains largely unaltered from the intended settings.

Inspiratory characteristics will depend on a number of factors. The high driving pressure from a pipeline source or heavy weighted/spring-loaded storage bellows requires some form of flow restriction to prevent too rapid a rise or an excessive pressure transmitted to the patient’s lungs that could produce barotrauma. This may take the form of a fixed orifice restrictor. Here the flow will be constant (pipeline supply or weighted bellows) or gradually decreasing (spring loaded bellows) as the tension in the spring reduces with emptying of the bellows (Fig. 9.3A). However, practically in the case of the latter the reduction is insignificant and flow is virtually constant.

Figure 9.3 Inspiratory characteristics of high-powered ventilators. A. Constant high-pressure generation (P₁) well in excess of that required to ventilate abnormal lungs (heavy weight or pipeline gas supply) with fixed-performance flow restrictor. High-pressure generation produced by a bellows compressed by powerful springs provides a gradually declining pressure as the bellows empties (P₂ above). However, this is insufficient to affect the performance of the ventilator, which develops pressures and flows as if it were constant high-pressure generation. Flow patterns generated by P₁ and P₂ are hence similar in a given lung scenario. B. Constant high-pressure generation using a varying orifice flow-restrictor that is electronically controlled to provide different inspiratory flow patterns from the same ventilator (solid line = constant flow; dotted line = increasing flow.). P₃, pressure rise downstream of restrictor as a result of flow to normal lungs; P₄ as above but to abnormal lungs; P₅, inspiratory safety valve release pressure.

In more sophisticated ventilators, the inspiratory flow valve acts as a variable flow restrictor (Fig. 9.3B). These are able to respond to user-programmed inspiratory flow patterns.

Ventilators may be designed to force their bellows to be compressed either mechanically, via a linkage from a suitable power source, or pneumatically, by placing the bellows in a gas-tight container into which a pressurized gas source is fed (bag-in-bottle arrangement). The bellows in this type of ventilator normally fills with gas at near atmospheric pressures, so that when it is compressed, the pressure developed rises as it overcomes the resistive properties of the lungs. The resultant pressure and flow waveforms are dependent on the type of mechanical linkage (e.g. rotating cam/linear motor) or the type of pneumatic drive producing any of the waveforms seen in Fig. 9.3. Although these ventilators are classified as high powered, they do not require flow restrictors as there is no initial very high-pressure source present. However, they do need overpressure relief valves to protect against high pressures that might develop unexpectedly.

In either type the delivery of the intended tidal volume is assured owing to the power developed by the ventilator (unless the pressure relief valve opens). More sophisticated ventilators will provide an alarm signal if this occurs.

Great store has been placed on the ability of different flow waveforms to increase ventilatory efficiency in various clinical situations. However, in anaesthetic practice the claimed advantages are less demonstrable.

Classification of ventilators according to cycling

Intermittent automatic ventilation of the lungs consists of two phases: inspiratory and expiratory. A ventilator is said to cycle between the two phases.

Cycling mechanisms in ventilators

Gas flow to and from the patient from a ventilator (cycling) is usually controlled by a series of one-way valves that are operated and synchronized either:

• mechanically

• electronically, or

• pneumatically.

Examples of these will be described where appropriate in the section on individual ventilators.

Ventilation modes

The terminology used to describe the way in which a ventilator combines its power capability and cycling to deliver a tidal volume has previously been almost self-explanatory despite manufacturers coining their own names.

Originally, ventilators were used to ventilate apnoeic/paralyzed patients. Where a desired tidal volume and rate was delivered by a high-powered ventilator, this was usually referred to as controlled minute ventilation (CMV), volume-controlled ventilation (VCV) or volume ventilation (VV).

Low, or high-powered ventilators that have a pressure limit for the delivery of a tidal volume, were said to deliver pressure-controlled ventilation (PCV), pressure ventilation or pressure mode.

As ventilation strategies developed to include the ability to synchronize delivered tidal volume with the patient’s respiratory effort, the term synchronized intermittent mandatory ventilation (SIMV) became ubiquitous. A variety of approaches – many fundamentally similar – exist to the delivery of this type of ventilation by the different manufacturers, each, unfortunately, tending to use their own proprietary nomenclature (see Chapter 10).

In addition, a sophisticated ventilator may have a facility that supports spontaneous respiration by sensing an inspiratory breath and assisting it by adding extra gas from the device. This may be termed assisted spontaneous breathing (ASB) or pressure support ventilation (PSV). This may also be used in conjunction with SIMV. There are a number of other strategies used by manufacturers, which are explained in more detail in Chapter 10.

Ventilator controls (general principles)

The three variables involved in setting the ventilatory parameters on a ventilator are traditionally arranged in the simple equation below:

Armed with this equation, a user should be able to switch on and set up any ventilator in its basic mode for controlled ventilation. The equation can have only two user defined variables, the third being dependant upon the other two. Hence a ventilator may only have two of the above variables assigned to dials present on its control panel, i.e. it may have controls for tidal volume and rate, minute volume and rate, or minute volume and tidal volume.

The equation may be made slightly more complicated by the fact that tidal volume and rate are derived values (see below) and can be represented in a different form. For example, tidal volume is derived from the inspiratory flow delivered by the ventilator, and the time over which this is delivered. It may be expressed in the equation:

Hence a ventilator may have no tidal volume control dial, but will have a flow control and an inspiratory time control to perform the same function.

Rate is derived from the cycle time and expressed as the number of complete cycles per minute. It may be expressed in the equation:

For example, if the inspiratory time is 2 s and the expiratory time is 4 s then:

Classification of ventilators according to application

The miscellany of ventilator designs available and principles upon which they work is a result of (a) the wide spectrum of applications for which they are required and (b) efforts to harness the different power supplies that have been made available. However, there are four principal types of ventilator which are classified here according to their application in clinical practice:

1. ‘mechanical thumbs’

2. minute volume dividers

3. bag squeezers

4. intermittent blowers.

Mechanical thumbs

The most common source of pressurized gas is that found in cylinders and pipelines. This may be administered to a patient most easily as a continuous flow into the simplest of breathing systems, the T-piece (Fig. 9.4A). In Fig. 9.4B, the anaesthetist has occluded the open end of the T-piece with his thumb. The force of the fresh gas flow (FGF) inflates the patient’s lungs until the anaesthetist removes his thumb from the open end, which allows expiration to occur (Fig. 9.4C). By rhythmical application of the thumb to occlude the T-piece, intermittent positive pressure ventilation (IPPV) is achieved. The FGF has to be high enough to inflate the lungs during inspiration and, as it is not stored during exhalation, this method is wasteful of gas. Therefore, it is suitable only for use in neonatal anaesthesia. Furthermore, the advent of more efficient ventilators and gas monitoring has seen the usage of this type of ventilation decline.

Figure 9.4 The T-piece principle and the ‘mechanical thumb’ (see text).

However, in special care baby units the ‘mechanical thumb’ principle is still used in modern ventilators, albeit with greater sophistication.

In ventilators such as the Sechrist (Fig. 9.5D), the anaesthetist’s thumb is replaced by a pneumatically operated valve (Fig. 9.5E), the cycling of which is determined by the settings on the ventilator controls.