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.

Figure 9.5 A. Bird VIP neonatal ventilator; B. Silicone diaphragm of Bird VIP acting as a variable flow valve; C. Bird VIP Piston from flow valve that moves the diaphragm. D. Seechrist infant ventilator; E. schematic diagram of the ‘mechanical thumb’. A, patient connection; B, expiratory valve; C, deflated pneumatic valve; D, gas supply tube to pneumatic valve, which is now inflated.

The exhalation valve may be electronically controlled (see Solenoids and Variable flow control valves, below) and by varying the degree of occlusion of the FGF is able to produce different types of inspiratory waveform (Bird VIP, Figs 9.5A, B and C). Some designs use gas jets in the opposite direction to the fresh gas in place of the valve for this purpose (SLE 2000).

Minute volume dividers

A more economical method of using a continuous source of pressurized gas is to feed it into a ventilator system (Fig. 9.6) to be collected by a reservoir R, which is continually pressurized by a spring, a weight or its own elastic recoil. Two valves, V₁ and V₂, are linked together and operated by a bi-stable mechanism. When V₁ opens, V₂ closes and causes the reservoir to discharge gas to the patient, i.e. this is the inspiratory phase. When V₁ closes, V₂ opens and expiration is permitted, allowing the reservoir bag to refill in preparation for the next breath.

Figure 9.6 The principle of minute volume dividers (see text).

All of the driving gas that is supplied is delivered to the patient. If, for example, the fresh gas flow delivered to the ventilator is 10 l min⁻¹, this is delivered to the patient as the minute volume. However, it is divided into a number of inspiratory volumes or ‘breaths’, depending on the settings of the volume and rate mechanisms of the ventilator, for example, 10 breaths of 1 litre, 20 of 0.5 or 25 of 0.4 and so on. These ventilators are referred to as minute volume dividers as they merely divide up the intended minute volume supplied by the driving gas. Ventilators based on this principle were the most common type used in the UK from the 1960s until the late 1990s. Setting an anticipated flow on the flowmeter bank and a vapour concentration on a vaporizer allowed a predictable flow of anaesthetic agent and gas mixture in the absence of gas and agent monitoring which was not readily available. The introduction of the latter into anaesthetic practice and the increase in cost of the more recently introduced volatile anaesthetics has resulted in the virtual obsolescence of this type of ventilator. The most common type used was the series of ventilators designed by Dr Roger Manley that were detailed in previous editions of this text.

Servo 900 Series ventilator

The Servo 900 Series ventilator is a sophisticated multi-mode ventilator (see Chapter 10, Figs 10.19A and B). Many of its functions are more relevant to an intensive care environment; however, when used to ventilate anaesthetized patients it is most frequently used as a pneumatically driven, electronically controlled minute volume divider.

Fresh gas from the anaesthetic machine is fed into the low-pressure entry port sited on the side of the ventilator and is stored in a spring-loaded 2 litre bellows. The spring load can be varied with the front panel key to a maximum working pressure of 11.8 kPa/120 cm H₂O, although a much lower pressure of 5.88 kPa/60–65 cm H₂O is normally used. If the bellows is overfilled, excess gas is vented through a pressure-relief valve linked to this bellows. This pressurized gas supplies the inspiratory flow to the patient; hence in this mode, the FGF from the anaesthetic machine should be set slightly (12–15%) in excess of ventilatory parameters set up on the ventilator so that the bellows remain optimally filled.

Alternatively, high-pressure gas (420 kPa in the UK) from a blender (nitrous oxide/oxygen) and a special high-pressure vaporizer may be fed into the bellows via the high-pressure inlet port, which is also sited on the side of the ventilator. Prior to entering the bellows, the gas passes through a demand valve that ensures that when any gas is removed from the reservoir bag it is immediately refilled from the blender and vaporizer. This extends the role of the ventilator from that of a simple minute volume divider to that of a machine that can respond to the extra gas demand caused by a patient breathing spontaneously either with or between controlled tidal volumes delivered by the ventilator (SIMV models 900C and E). It also allows a patient’s respiratory efforts to be assisted by a variable amount when the ‘pressure support’ mode is selected (Servo 900C and D).

Bag squeezers

Most patients who require automatic ventilation during the course of an anaesthetic are connected to a circle or Mapleson D breathing system that employs a ‘bag squeezer’ type of ventilator. It relieves the anaesthetist of having to squeeze the reservoir bag and, apart from freeing him or her to do other things, offers the advantages of producing more regular ventilation, with controllable tidal volume and pressure.

The driving gas from the ventilator does not ventilate the patient directly. The bellows (or bag) is squeezed pneumatically by enclosing it in a gas-tight Perspex canister and feeding the driving gas (under pressure) into the space between the bellows and canister (Figs 9.7A and B). The enclosed bellows is often referred to as a ‘bag in bottle’, which, although somewhat archaic (the original designs were just this), is perfectly descriptive. Fig. 9.7A shows a rising bellows arrangement. The bellows is filled by patient gas when in use and attached to a breathing system. For the rising bellows, the claimed advantage is that should it develop a leak, the bellows would collapse without any mixing of patient and drive gas. Its detractors claim that the pressure required to fill the bellows adds expiratory resistance and may prevent complete exhalation. Proponents claim that it provides a degree of positive end expiratory pressure (PEEP), which may be beneficial. The arrangement is very popular with many manufacturers of anaesthetic workstations (see below and Chapter 4).

Figure 9.7 Various types of ‘bag squeezer’ ventilators: A. rising bellows arrangement; B. descending bellows arrangement. C. pneumatic piston with mechanical linkage; D. pneumatic piston E. cam driven linkage from an electric motor; and F. screw threaded piston (worm drive) powered by electric motor.

Fig. 9.7B shows a descending bellows arrangement. The patient gas is sucked into the bellows by a weight placed in the base. The alleged advantage is the absence of expiratory resistance. This arrangement also allowed the drive unit to be placed above the bellows in the free-standing versions. It could be placed on the lower shelf of an anaesthetic machine with the controls easily to hand. However, a tear in the wall would not result in a bellows collapse and driving gas would be able to enter the bellows and dilute patient gas. Also, the bellows is normally full of air prior to connection to a breathing system and needs to be purged prior to use. This design is no longer popular. The Penlon Nuffield 400 series ventilator was the most common example used in the UK, but is no longer produced (see previous editions).

The bellows may be both mechanically inflated and deflated by a lever attached to a gas-powered piston (Fig. 9.7C). This removes any potential risk of drive and patient gasses mixing. The piston may be driven by a smaller quantity of gas than in the methods described above. This could be important in situations where cylinders only are available. As the expiratory phase is controlled by the travel of the piston a second reservoir is required should exhalation be faster or slower than the ventilator so as to avoid respiratory embarrassment. This method is no longer popular with manufacturers. The most common models distributed in the UK were the Manley Servovent and the Oxford Mark 2 ventilators (see previous editions).

The bellows may also be squeezed mechanically, by means of a motor and suitable gears and levers. Fig. 9.7E shows a cam and piston arrangement. The movement of the cam produces a sinusoidal inspiratory pressure rise. In the expiratory phase the bellows may be re-expanded by the pull of the piston or if the piston rod is decoupled during this phase (‘lost motion drive’) by springs. This is no longer a common method.

Fig. 9.7F shows a bellows whose travel is produced by the linear travel of a worm gear driven by an electric motor. The speed of the motor can be altered both in inspiration and in exhalation to produce a variety of flows. This is the method used in the Dräger E series of ventilators (see later).

The bellows may be removed and a suitable length of wide-bore breathing hose substituted (Fig. 9.7D). In this arrangement, the ventilator may push the driving gas directly into the breathing system. This gas and patient gas are not physically separated, but the length of hose ensures that the driving gas does not enter the patient part of the breathing system (see Chapter 5). The Penlon Nuffield 200 series has probably been the most popular ventilator in the UK that uses this method.

Advances in ventilator designs

Most of the recent advances in ventilator design have been due to sophisticated electronics. Two key features need some preliminary explanation in order to understand how this new generation of ventilators function and perform. One of these is the electronic flow valve. This has become the major component in the driving gas pathway and has reduced greatly the number of working parts in the ventilator. The other, is the programmable microprocessor that controls the operation of this valve.

Electronic flow valves

There are three types of valve in common use.

Proportional (flow) valves (Fig. 9.8A) When an electric current is fed around a wire coil (solenoid) E in the valve, the magnetic field produced displaces a ferromagnetic core D that acts as a piston and opens a small valve C normally held shut by a spring F. The aperture exposed can be varied depending on the size of the current used, hence the correct term ‘proportional flow valve’. This type of valve is used in high-pressure gas pathways. The valve aperture is very small (1.5 mm²) and the movement of the valve is also small so that: (a) the flow can be rapidly varied between 1 and 120 L min⁻¹ and (b) the size of the solenoid may be compact enough to fit into the equipment. The response time is usually less than 100 ms.

Figure 9.8 Electronic flow valves. A. Proportional flow valve; A, gas input; B, gas output; C, valve; D, ferromagnetic core; E, solenoid coil. F, spring that holds the valve shut in the resting state. B. Solenoid driven on /off valve instead of a spring magnet G attracts the core D and holds the valve shut. C. A low-pressure proportional flow valve. The valve C and valve orifice is wide. A delicate spring H holds the valve open.

Proportional valves may be used to control the whole of the inspiratory phase of a ventilator (including the size and duration of the tidal volume), the ventilation rate and the shape of the pressure waveform by varying the size of the valve aperture. Valves may also be used in parallel, each for an individual gas so that the unit behaves as a blender and can vary the gas concentrations in the mixture as well as the flow.

On/off valves (Fig. 9.8B) These are constructed in a similar fashion to a proportional flow valve. However, they function only as on/off valves. They may be used to switch a flow on or off. However, they can be pulsed on and off rapidly to produce a desired flow. Similarly, they may be used in parallel to blend gasses to a desired concentration. The valve is held shut by a magnet that attracts the ferromagnetic core inside the solenoid. When a steady current is fed around the solenoid, the magnetic field induced in the core opposes the fixed magnet and the valve opens. When the current flow ceases the magnet attracts the core and the valve shuts usually with the help of the incoming gas flow.

Low-pressure proportional flow valves This type of valve (Fig. 9.8C) (also Fig. 4.30) is held open by a delicate spring H. The valve and valve aperture are large to allow a high gas flow at low pressure. When the solenoid is activated, the valve (C) may be partially or completely closed. These devices are used mainly in low-pressure gas pathways as expiratory and PEEP valves.

Microprocessors

These devices control the electrical signal to the proportional flow valve. They can be programmed to provide a wide variety of ventilatory parameters and modes and they allow further enhancement of these to occur via reprogramming. Most ventilators now operate on the same principle. The differences seen are in the user display, quality of components and programming variables.

A few examples of ‘bag squeezer’ ventilators in common use in the UK are described below. The descriptions are not meant to be exhaustive but to point out the general working principles and some interesting features. User manuals for the individual ventilators will provide a greater depth of knowledge where required.

Spacelabs Healthcare (Blease) 700/900 series

This ventilator (Fig. 9.9A) is an electrolically controlled, pneumatically driven ‘bag squeezer’ with an ascending bellows arrangement. The driving gas is controlled by a rapid response proportional flow valve (Fig. 4.28) via a microprocessor to provide a wide range of ventilatory parameters. A line diagram of the pneumatics is shown in Fig. 9.9B.

Figure 9.9 A. Blease 900. B. Working principles of Blease 900. A, drive gas input; B, gas filter and water trap; C, on/off switch; D, pressure regulator; E electronically operated proportional inspiratory flow valve; F, pneumotachograph; G, electronic expiratory/multifunction valve; H, mechanical overpressure relief valve; J, driving gas exhaust; K bellows assembly; L, pneumatic valve; M, pathway to breathing system; N, patient gas exhalation port; P ventilator control unit; Q, fresh gas flow sensor; R, patient gas pressure and flow sensor.

Inspiratory phase In volume control mode (VC), driving gas from a pressurized source A (270–760 kPa (36–101 psi)) is passed through a filter B, a regulator D (set at 259 kPa (34.5 psi)) and fed through an electronic proportional flow control valve E. The driving gas passes to a pneumotachograph F and to an airtight canister K that contains a rising bellows arrangement. As it enters the canister it compresses the bellows and forces the patient gas within into the breathing system. A miniature bellows valve (L) in the canister is also activated by the driving gas and seals off the expiratory port (N) so that all the intended tidal volume in the main bellows reaches the patient.

The control of the driving gas through the inspiratory flow valve is via a microprocessor in the ventilator control unit (P). It receives information from a number of sources: the pneumotachograph F, the intended ventilation settings on the front panel of the ventilator, a second microprocessor for cross checking, and from two other flow sensors (Q & R not shown in diagram). The first of these is fitted to the anaesthetic machine and measures the FGF. The second is fitted to the breathing system and measures the inspired flow to the patient. Alterations in the FGF and changes in the lung elastance or airways resistance which would normally alter the delivered volume are recognized by the second microprocessor which applies a correction to the output of the main processor P to restore the intended tidal volume.

Expiratory phase At the end of the inspiratory phase, the expiratory valve G opens and the canister gas is vented to atmosphere (usually through the scavenging system on the workstation). The expiratory valve is a low-pressure proportional flow type (see above) with a light weight metal disc covering the expiratory port (Fig. 4.30). It is held in the closed position by a weak spring and a push rod connected to a solenoid arrangement. During normal exhalation, the pushrod is retracted by the solenoid to allow relatively unhindered expiratory flow. If PEEP is required the solenoid advances the pushrod so that it partially closes the valve to maintain the desired pressure.

When the drive gas pathway to atmosphere is opened in this way, the pressure in the canister falls. This allows the patient to exhale back into the bellows and when the latter is full, it forces the pneumatic valve L open, so that any excess gas is vented to atmosphere through the expiratory pathway. Both phases are time-cycled by the microprocessor.

During this phase, the bellows behaves as a reservoir bag, allowing spontaneous respiration to take place if desired.

Additional modes

In ‘precision’ pressure control mode (PC), the inspiratory valve opens normally and when the pressure in the breathing system reaches the pre-set level, the expiratory valve starts to oscillate between open and closed so as to maintain this. PEEP is produced in a similar fashion when required.

The ventilator also supports a feature termed ‘advanced’ pressure support ventilation (PSV). This can be a very useful tool when used for spontaneous respiration, in overcoming a reduced inspiratory effort as well as the higher inspiratory resistance of circle breathing systems. A flow transducer in the patient circuit allows the device to sense the initiation of a spontaneous breath and to assist this by activating the electronic proportional inspiratory flow valve E. The degree of support can be adjusted by the user by altering the value of the support pressure. A flow transducer rather than a pressure one is used to sense inspiratory effort, as it functions independently of any PEEP applied or any transient pressures applied to the system. A potential disadvantage of flow sensing can be seen at low functional residual capacity (FRC) where low compliance requires greater respiratory effort to generate a given flow than at a higher FRC. In practice the system is more efficient when a degree of PEEP is also applied. This raises the FRC and places the lungs on a better part of the compliance curve where a similar inspiratory effort produces a higher patient inspiratory gas flow and, therefore, an earlier response.

The ventilator also supports most of the combination ventilatory modes, such as pressure-controlled SIMV (SIMV-PC), volume-controlled SIMV (SIMV-VC) and either of these with pressure-supported spontaneous breathing, e.g. SIMV-PC+PSV. An inspiratory pause can be dialled in, and there is the return of the ‘sigh’ option. The latter in default mode delivers a breath larger than tidal volume by 10% every 10th breath, although both values may be altered by the user. The ventilator may be used to ventilate paediatric patients using the appropriate flow sensor and ventilator settings.

Ventilator controls The front of the ventilator casing houses a flat panel TFT touch screen, similar to that found on a modern computer (Fig. 9.9A). Also, there is a rotary knob (Trak wheel) on the bottom left of this. Adjustable parameters may be selected by touching the appropriate box and using the up/down arrows to select the desired value. Alternatively the Trak wheel may be rotated to select a parameter. When pushed inwards (clicking) it highlights that value which may then be changed by further rotation. This value is confirmed by again ‘clicking’ on the wheel.

Many other features, including back-up ventilation in case of apnoea, and safety systems are available. These are not described here, but can be found in the manufacturer’s user manual.

The ventilator design can, therefore, be classified as a high-powered, time-cycled ‘bag squeezer’ ventilator.

GE Healthcare 7900 Smartvent

This is another example of a pneumatically driven microprocessor controlled bag squeezer (Fig. 9.10). The operating principles are similar (although with minor variations) to that described above in that high-pressure driving gas is passed through an electronically controlled proportional flow valve to externally manipulate a bellows arrangement that contains patient gas. In this device the same proportional valve is used to deliver a bias flow to produce PEEP during the expiratory phase by acting on the passive expiratory valve (Fig. 9.10 B). The electronic control is handled by a programmable microprocessor to deliver a wide selection of respiratory modes. As with most anaesthetic ventilators of this type, these modes are similar to those found on a typical ITU device. Hence, there is VCV and PCV, both with SIMV facility and the ability to allow and/or pressure support a spontaneous breath. The GE version of pressure support ventilation (PSV Pro) may also be used to augment intended spontaneous respiration. This mode also has a back-up for unexpected apnoea. If the patient does not take a breath within the pre-set apnoea delay time, the apnoea alarm will activate and the ventilator will automatically switch to the back-up mode, which is SIMV-PC.

Figure 9.10 GE Smartvent 7900 ventilator.

A, ventilator control panel as part of the anaesthetic machine user interface (Aisys, GE Healthcare, shown here with US gas colouring). Image courtesy of GE Healthcare. B, working principles: (A) drive gas input filter; (B) Gas Inlet Valve – solenoid, opens when ventilator is on, closes under fault conditions such as system overpressure; (C) drive gas pressure regulator (output at 170 kPa); (D) flow control (proportional) valve; (E) mechanical overpressure safety valve (110 cm H₂O); (F) bellows; (G) drive gas check valve (3.5 cm H₂O bias); (H) exhaust valve manifold; (J) bellows pressure relief ‘pop off valve’; (K) connection to circle system; (L) patient and drive gas exhaust to scavenging; (N) control bleed to atmosphere; (P) piloting pressure to exhaust valve.

With the advent of fast response proportional valves, all these modes may be used in paediatric as well as adult patients. Different manufacturers often add subtle changes to various modes and also use slightly differing nomenclature for similar modes to that of their competitors. For example, the 7900 has a ‘volume guarantee’ mode in PCV. Here, the clinician sets an intended tidal volume, half of which is delivered in the first breath using volume controlled mode. The pressure generated by this breath is then used to calculate the level of pressure control for subsequent breaths which are gradually stepped up with the aim of achieving the set tidal volume within seven breaths. Hence all breaths after the first are delivered using pressure control to quickly reach the inspiratory pressure needed to achieve that the desired tidal volume at the set breath rate and I:E ratio. This information is constantly updated to ‘guarantee’ the desired tidal volume hence the mode being called PCV-VG.

The evolution of the anaesthetic workstation has now resulted in the separation of the ventilator ‘engine’ from the user interface. The latter is now usually placed next to the patient monitoring display so that the user has an ergonomic view of both. As a result the traditional concept of a stand-alone ventilator is disappearing in this context, although this ventilator engine is still in use in stand-alone devices on some older machines. Nevertheless, the user controls and display are similar across the range.

Control panel (Fig. 9.10) This is housed in a flat panel display surrounded by various keys and a single rotary control that the manufacturer calls a command or ‘comm’ wheel. The display is divided into sections. The lower part of the screen has boxes with displayed values for intended tidal volume, rate, inspiratory/expiratory ratio, maximum pressure limit and PEEP. Below each of these is a ‘soft key’, which, when pressed, highlights that parameter. The latter may be adjusted to a new value by turning the comm wheel. The upper section of the display shows the measured ventilatory parameters and a graph of airways pressure. This information is retrieved from the twin flow transducers placed in the circle breathing system.

Above the comm wheel is a menu panel with dedicated buttons (hard keys) for selecting start/end case, checkout, main menu, help and alarms set up and silence.

The bellows unit This unit houses the manufacturer’s circle absorber, a conventional rising bellows arrangement, inspiratory and expiratory flow transducers and an APL valve. The latter has a mandatory overpressure relief valve incorporated for safety during spontaneous breathing (see Chapter 4).

Interesting features When used in VCV or SIMV mode, the inspiratory flow transducer in the breathing system measures delivered tidal volume. It passes this information back to the microprocessor, which updates every six breaths and adjusts the ventilation to ensure that the values set on the ventilator and the measured values match. This system compensates for changes in both lung compliance and FGF from the anaesthetic machine. It even has a user selectable Heliox mode that compensates for the change in gas density and viscosity that would otherwise alter the information from the flow transducers.

Paediatric mode The ventilatory parameters available (tidal volume 20–1500 ml and rates of 4–100 breaths) and the compensation features mentioned above are sufficient to allow it to be used in infants and neonates. Most anaesthetists, however, use the ventilator in pressure control mode for such applications. Here, it may be beneficial to use narrow-bore breathing hose (15 mm). The ventilator design can, therefore, be classified as a high-powered, time-cycled ‘bag squeezer’ ventilator.

Dräger anaesthetic ventilators (E models)

The bag squeezer ventilators fitted currently to some of the anaesthetic workstations marketed by Dräger (Primus, Fabius and Cicero) are electrically powered and controlled (see also Chapter 4). These ventilators are made up from three modules, a control module, the ventilator module and the circle breathing system.

Control module (Fig. 9.11A) The ventilatory parameters available depend on the configuration of the control module in the model used. The basic model, the Fabius CE allows volume-controlled ventilation only. The top of the range Primus has the facility for both VCV and PCV, either as stand-alone features or with the provision for spontaneous breathing with and without pressure support in these modes. It also allows a purely spontaneous respiration mode with triggered pressure support. The control unit shown is a mid-range model, the Fabius GS. Like the ventilator described above, it has a rotary control (bottom right) to alter selected variables. On the left of the unit are the keys that select the mode of ventilation. The middle of the unit has a thin film transistor (TFT) screen that displays all the relevant information. To the right of this are two banks of keys to select the alarms, menu set-up, home, alarm silence and standby. The home key restores the screen to the default after any submenu called up is no longer required and the standby key stops the ventilator and keeps any ventilatory parameters selected for use again.

Figure 9.11 A. Dräger E ventilator as fitted to the Fabius GS: (1) ventilation mode hard keys; (2) user interface display screen; (3) set up key; (4) alarm limits hard key; (5) home key; (6) alarm silence key; (7) alarm status indicators; (8) standby mode key; (9) power on indicator; (10) rotary knob; (11) soft keys (functions); (12) lamp switch. B. Working principles: (1) electric motor; (2) rod with screw thread; (3) piston; (4) cylinder; (5) rolling rubber seal; (6) incremental encoder; (7) sensor; (8) light barrier; (9) ventilator bellows. C. inspiratory phase: (1) inspiratory flow transducer; (2) expiratory flow transducer; (3) ventilator; (4) fresh gas; (5) fresh gas decoupling; (6) valve controlling P_max/PEEP; (7) absorber; (8) scavenging; (9) APL valve; (10) reservoir bag. D. early expiratory phase. E. late expiratory phase.

(Fig. 9.11A reproduced with permission from Dräger Medical UK.)

Ventilator module (Fig. 9.11B) Unlike the ventilators described above, this device does not require a pressurized source of gas as its power source. The ventilator has an electric motor (1) with a hollow spindle. The inside of the spindle has a screw thread. A rod (2) with a matching thread passes through the spindle. When the electric motor spins, the spindle rotates and the action of the two threads, which are interlocked, causes the rod to move through the spindle. This movement is referred to as either a recirculating ball screw or a worm drive. One end of the rod is connected to a piston (3) that moves backwards and forwards inside a cylinder (4), depending upon the direction and duration of current flow in the electric motor. The head of the piston is fitted with a rolling neoprene seal (5) so that on the downstroke it is capable of producing a sub-atmospheric pressure to the bellows that sits above it. The position of the piston rod at any one time is sensed by a high-resolution incremental encoder (6) and allows precise volume (0.03 ml) delivery. The encoder consists of a metal disc that has 1024 perforations around the edge. When the electric motor is working, this disc spins between two arms of a sensor (7) that counts the passage of the perforations and then calculates the linear movement of the piston rod. At the bottom of the cylinder there is a light barrier to detect the lower stop position of the piston.

Interesting features

• Inspiration (Fig. 9.11C). During the inspiratory phase the ventilator delivers the intended amount of volume to the patient. It does this by diverting the FGF from the anaesthetic machine via a decoupling valve (5) into the reservoir bag and not the patient. Furthermore, the delivered tidal volume from the ventilator enters the breathing system downstream of the absorber (which is isolated) and, therefore, minimizes the compression volume of the inspiratory pathway. The reservoir bag will be seen to expand with the addition of fresh gas, which may be a little unnerving for those not familiar with this system. There is a flow transducer in the inspiratory pathway that measures gas flow, which is then displayed on the control unit.

• Exhalation (Fig. 9.11D). The expiratory travel of the piston is fixed by the I/E ratio of the ventilator. The sub-atmospheric pressure created by the downstroke of the ventilator sucks in gas from both the reservoir bag and the exhalation volume from the patient. Towards the end of the exhalation phase, the reservoir bag fills and surplus gas is dumped through the scavenging port. Again the reservoir bag will be seen to move. In the top of the range machine, information from the expiratory flow transducer is passed to the microprocessor, which in turn causes the movement of the piston backstroke in the ventilator to match the expiratory flow.

Paediatric mode When these ventilators are switched on, the control unit software performs a leak and compliance test (except for the base model workstation). Compliance compensation, along with the low compliance of the bellows and breathing system, allows accurate delivery of small tidal volumes if required. More commonly, the ventilators may be used in the pressure support made that compensates for any small leak caused by an uncuffed endotracheal tube.

The ventilator design can, therefore, be classified as a high-powered, high-efficiency, time-cycled ‘bag squeezer’ ventilator.

Intermittent blowers

These ventilators are driven by a pressurized source of gasses or air, at a pressure of 250–400 kPa (37.5–60 psi). The driving gas pathway is very small with a low internal compliance making this type of device very efficient. The major component is an electronically timed and activated proportional flow valve or a pneumatically timed oscillator that divides the driving gas into tidal volumes the size and rate of which can be adjusted. Sophisticated ventilators such as those used in intensive care and anaesthetic workstations make use of a proportional flow valve (see above). Automatic resuscitators and more basic anaesthetic ventilators use the pneumatic oscillator principle (see below) as this is cheaper, does not require the same sophistication of operation and is powered by the driving gas requiring no electrical supply.

Pneumatic oscillator A typical example is seen in Fig. 9.12. The diagram is a very simplified version and does not attempt to show the detailed pneumatics that are essential for its function. Driving gas enters at point (1). It divides into three pathways. The main one passes to a cylinder that contains a shuttle (2), which travels between the ends of the cylinder. In the inspiratory phase (Fig. 9.12A), the driving gas passes into the cylinder and through a hole in the shuttle into the gas pathway (3) to the patient. The other two pathways supply two pneumatic timers (6) (inspiratory) and (7) (expiratory), each of which has a needle valve that regulates flow to the timer mechanism at the relevant end of the cylinder. When the flow causes sufficient build up of pressure in the inspiratory timer, the shuttle is forced to the opposite end of the cylinder (Fig. 9.12B) and in doing so causes three events:

Figure 9.12 A pneumatic oscillator. A. Inspiratory phase. B. Expiratory phase. (1) Driving gas; (2) shuttle; (3) gas pathway to patient; (4) inspiratory timer vent; (5) expiratory timer vent; (6) inspiratory timer; (7) expiratory timer.

1. It blocks off the flow of driving gas through the cylinder terminating the inspiratory flow.

2. It opens a vent (4) to open on the inspiratory side of the cylinder that allows the pressure in the inspiratory timer to be released.

3. It causes the shuttle to occlude the expiratory timer vent (5) so that a pressure can build up to reverse the direction of the shuttle and terminate the expiratory phase.

Working principles of pneumatically controlled intermittent blowers

A generic line diagram of a typical pneumatically powered and controlled ‘intermittent blower’ is shown in Fig. 9.13. High-pressure driving gas (300–400 kPa) is connected to the device at point (F). When the main pneumatics on/off switch (G) is turned on, gas flows through it to a regulator (H) that reduces the driving pressure to approximately 275 kPa. Gas flows on to the oscillator (J). The output from this passes through a variable flow restrictor and exits the device to be attached to a breathing system. The delivered tidal volume is a function of the inspiratory timer (K), which is calibrated in seconds, and flow restrictor (R), which is calibrated in l s⁻¹. The respiratory rate is determined by the cycle time: inspiratory time (adjusted at K) plus the expiratory time (adjusted at L).

Figure 9.13 Schematic diagram of a pneumatically controlled intermittent blower (see text for details). F, high-pressure driving gas input (300–600 kPa); G, pneumatic on/off switch; H, pressure regulator; J, oscillator; K, variable pneumatic inspiratory timer; L, variable pneumatic expiratory timer; R, inspiratory flow restricter.

Classification of intermittent blowers

Intermittent blowers are used in four different ways (Fig. 9.14).

Figure 9.14 Classification of intermittent blowers. A. Basic resuscitator. B. Sophisticated resuscitator. C. Ventilator for intensive care. D. Anaesthetic ventilator for Mapleson D system. A, resuscitator/ventilator; B, patient valve; C, overpressure relief valve; D, patient pathway; E, expiratory pathway.

Basic resuscitators

The simplest design is used as a basic resuscitator (Fig. 9.14A). The working principles are shown in Fig. 9.15A. It has no separate on/off switch and no flow restrictor and a fixed expiratory timer. It has a single control (K) for tidal volume, cycling rate and I/E ratio, which is actually the variable inspiratory timer. Since the flow rate is constant, when the inspiratory time is lengthened, the tidal volume is increased, the cycling rate is reduced and the I/E ratio is prolonged, and vice versa.

Figure 9.15 A. Working principle of a basic resuscitator: F, driving gas; H, pressure regulator; J, oscillator; K, tidal volume and rate control. B. A basic resuscitator: the Pneupac adult/child resuscitator. The overpressure relief valve with its red cap is seen connected to the patient valve. C. Working principles of the Pneupac patient valve (inspiratory phase): P, piston; S, spring.

An example of this type is the Pneupac adult/child resuscitator (Fig. 9.15B), which, although no longer in production (since 2004), is still widely used.

The working principles of the patient valve are explained in Fig. 9.15C.

Sophisticated resuscitators

This basic model has been superseded by the Pneupac VR1 (Fig. 9.16), which now has an on/off switch, manual mode to comply with International Liaison Committee on Resuscitation (ILCOR) CPR guidelines, a demand function to allow spontaneous breathing through the device and variable flows across the frequency range to deliver gentler breaths. These devices can also be used in toxic environments (for example hazardous area response teams: HART). The working principle is very much as in Fig. 9.17A (see below), but with controls K, L and R combined in one single control for the user. This allows greater speed in the deployment of the device. The variable flows are also available when pushing the manual control button. The manual control is now a standard feature on most resuscitators to allow compliance with the changing CPR ventilation to chest compression ratios (currently 2/30). Also most basic resuscitators now allow direct connection of the patient valve to the resuscitator so that it can be used on top of the mask/tracheal tube (Fig. 9.16B). Alternatively, a length of wide bore tubing may be placed between the patient valve and resuscitator (see Fig. 9.17A).

Figure 9.16 (A). The Pneupac VR1 top view. (B). Side view showing the patient valve for connection to a facemask /endotracheal tube.

Photograph courtesy of Smiths Medical, UK.

Figure 9.17 A. A Pneupac Ventipac ventilator with the airways pressure line, Laerdal pattern non-rebreathing valve with a PEEP attachment. B. Working principles: F, driving gas input; G, on/off valve; H, pressure regulator; J, oscillator; K, inspiratory timer; L, expiratory timer; M, demand detector; N, demand valve and demand gas pathway; O, air mix selector switch; P, needle valve for airmix; Q, needle valve to supplement flow from F for no airmix; R, variable flow restrictor; S, air entrainment port with non-return valve; T, variable pressure relief valve; W, pilot line to demand detector; X, airways pressure display; Y, Laerdal pattern non-rebreathing valve.

More features may be added to a resuscitator to increase its scope. However, it then starts to resemble an ITU ventilator. An example is the Pneupac Ventipac (Fig. 9.17A). The increased sophistication may be seen in the line diagram Fig. 9.17B. The output from the oscillator (J) is passed via a variable flow restrictor (R) on to two coupled needle valves (P) and (Q), operated by a switch (O). If P (air mix mode) is selected, the driving gas is passed into a Venturi that entrains a fixed amount of ambient air from S (this port has a non-return valve) and the total flow is fed into the patient breathing system.

If Q (no air mix) is selected, both needle valves are activated. The bore of Q is such that its output matches the entrainment from S to supplement the flow from P so that no air is entrained and the delivered content is 100% driving gas (usually oxygen). The output of the ventilator is connected to the wide-bore hose of breathing system, the other end of which is attached to a light-weight low-resistance Laerdal pattern non-rebreathing valve. The device has separate controls for inspiratory time, expiratory time and inspiratory flow rate and so is able to provide greater ventilatory flexibility than the base model.

In addition, should the patient attempt to breathe, a demand detector (M) senses the pressure in the breathing system via a pilot line (W) and triggers the demand valve (N). This takes its gas supply from the high-pressure gas inlet upstream from the pneumatic switch (G) and allows the demand valve to function even if the ventilator is switched off. If the latter is switched on, however, the demand valve operates in conjunction with the oscillator to integrate this signal and to extend the expiratory phase as a function of the spontaneous tidal volume up to a maximum time dictated by the frequency settings of the ventilator. Thus, if the patient demands a high flow for a short duration or low flow from a longer duration (i.e. similar tidal volumes), an equal expiratory time will be allowed before the next breath. The cumulative effect of successive spontaneous breaths by the patient causes the ventilator to become inhibited, although, in fact, this is only on a breath-by-breath basis. Inhibition starts at 150 ml and increases to a full inhibition of 450 ml. The level of spontaneous breathing required to fully inhibit the oscillator is fixed at that of the typical adult breathing spontaneously. This is taken as tidal volume of about 450 ml at 12 to 16 breaths per minute. Higher spontaneous ventilation rates can readily be taken and will result in complete inhibition of the ventilator. Lower rates will only give partial inhibition, but providing the demand flow is above 15 l min⁻¹ the ventilator will still interact with the patient and synchronize its ventilation pattern with the spontaneous breathing. The ventilator has a variable pressure relief valve (T). It also has a battery operated multi-functional pressure alarm and pressure gauge.

Intensive care ventilators

Fig. 9.14C shows a very basic line diagram of an intermittent blower used as typical intensive care ventilator. These are discussed in more detail in Chapter 10.

Ventilators for anaesthetic breathing systems

Fig. 9.14D illustrates the use of an intermittent blower with an anaesthetic breathing system. The patient valve may be placed adjacent to the ventilator and the output connected to a breathing system (Mapleson D or circle system) in place of the normal reservoir bag (see Chapter 5). It must be remembered that sufficient length of wide-bore hosing must be used between the ventilator and the breathing system to prevent any driving gas from diluting the anaesthetic intended for the patient. The ventilator that popularized this method in the UK is the Penlon Nuffield 200 series (Fig. 9.18).

Figure 9.18 Penlon Nuffield Series 200.

Jet ventilation

Conventionally, the lungs of a patient are normally ventilated by providing a seal to the upper airway, so that sufficient pressure may build up to provide movement of gas into the lungs. Alternatively, a high-pressure jet of gas may be directed into the airway without the need for a seal. The kinetic energy of the gas molecules is sufficient to overcome the elastic properties of the lungs and to cause them to expand. Furthermore, the speed of the molecules leaving the jet may act as a Venturi and entrain adjacent gas so as to increase the volume provided. The efficiency of the jet depends on a number of factors.

The jet is at its most efficient in terms of gas delivery and ability to entrain when its path is in a straight line. Sharp bends dramatically decrease both of these. The amount of gas delivered is also increased as the driving pressure is raised.

Jet ventilation may be very useful in situations where the airway is so narrowed that only a small gas delivery device may be passed or in situations where conventional airway management devices would impede the view of a surgeon or the conduct of an operation.

There are two ways in which a high-pressure jet of gas may be used to ventilate a patient.

Manually controlled jetting

For short procedures such as rigid bronchoscopy, a patient may be anaesthetized and then ventilated by applying an intermittent jet via the bronchoscope, using a manually operated injector device such as the Manujet (see Airway management devices, Chapter 6). The jet behaves as a Venturi causing air to be entrained by the driving gas. The latter is usually oxygen, at a pressure of up of 420 kPa (60 psi). Anaesthesia is normally maintained intravenously.

The tidal volumes, inspired oxygen concentrations and airway pressures are not easily measured. When the delivery system is attached to the operating laryngoscope and is supraglottic, barotrauma is very unlikely. If the jet is placed below the vocal cords, for example, by using a flexible catheter then caution must be exercised during the procedure. Any obstruction above the catheter will prevent exhalation and a dangerous build-up of pressure may result. If a manual method is used, a check should be made for a complete exhalation after each delivered tidal volume.

Automatically controlled jetting

Specially designed ventilators with the ability to cycle at high rates are available that can deliver jets of gas automatically, and monitor the airway pressure such that a breath is not delivered if the preceding expiratory pause pressure is above a user set limit, thus helping to prevent breath stacking and barotrauma.

The Acutronic ‘Monsoon’ jet ventilator is one example of this in Europe and the UK. The Monsoon has a built-in humidifier system for use in longer procedures and the intensive care unit. It is based around high-performance, electronically controlled solenoid flow valves. The basic model has a single valve capable of frequencies from 12 to 1600 cycles per minute. The Monsoon + (Fig. 9.19) has a second valve capable of 1–100 cyclesl min⁻¹ to allow either stand-alone ventilation or the superimposition of a second low-frequency, tidal-type ventilation. The ventilator has three main controls. One alters the rate, another controls the I/E ratio (by altering the percentage of the cycle that is inspiration), and the third operates a reducing valve that controls the output pressure of the driving gas. (In the advanced model, there are duplicate controls for the second jet.) The delivered tidal volume/minute volume depends on a combination of all three settings and all of these ventilatory parameters are displayed on a screen on the front panel of the device.