Ventilators are used to provide controlled ventilation to maintain oxygenation and removal of carbon dioxide. Many of them have the facilities to provide multiple ventilatory modes that can lead to some confusion. They can be used in the operating theatre, intensive care unit, during transport of critically ill patients and also at home (e.g. for patients requiring nocturnal respiratory assistance).
Positive pressure ventilators are overwhelmingly used in current clinical practice where a positive pressure within the breathing system is created driving the gas into the patienťs lungs. Negative pressure ventilators mimic the normal physiology by generating a negative intrathoracic pressure allowing gas flow into the lungs but their use in current practice is limited.
In this chapter, we will describe the basic functions of the ventilators, attempt to classify them and define the characteristics of the ideal ventilator. A selection of the commonly used ventilators in current practice are described in more detail. Only positive pressure ventilators are described.
The basic variables in any ventilator are:
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Tidal volume (mL/breath), which can be delivered either as a fixed volume (e.g. 500 mL) with variable peak airway pressures or a fixed pressure (e.g. 12–15 cm H 2 O) with variable volumes.
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Respiratory rate (breaths/min) can be set either by the ventilator (mandatory breaths) or by the patient (triggered breaths). These options allow different ventilatory modes:
- ◦
Control mode when the ventilator does all the work of breathing, delivering the set and mandatory breaths only.
- ◦
Spontaneous breathing mode when the patient does all the work of breathing, triggering all the breaths. The ventilator may provide supplementary oxygen or continuous positive airway pressure (CPAP) (see Chapter 13 ).
- ◦
Support mode when patient triggers the ventilator to deliver a breath. Here, the patient does some of the work of breathing but the ventilator offers support and does most of it.
- ◦
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Minute volume is the volume of gas inhaled or exhaled from patienťs lungs per minute . It is the product of tidal volume and respiratory rate. In some of the advanced modes of ventilation on modern intensive care ventilators, it is possible to set the minute volume directly and the ventilator will deliver variable tidal volumes and breathing rate depending on the lung mechanics.
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I:E ratio is the ratio of inspiratory time to the expiratory time. The operator in mandatory ventilation can set this either directly or indirectly through adjusting inspiratory time or respiratory rate. In spontaneous mode, the patient determines the I : E ratio. This can be very important to set appropriately in patients with acute respiratory distress syndrome (ARDS) or acute severe asthma.
The four common ventilatory modes used are described in the box below.
Using the variables, tidal volume (fixed volume or fixed pressure) and respiratory rate (patient or ventilator), four ventilator modes can be derived:
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Volume Control (VC): the desired tidal volume is delivered regardless of the measured peak airway pressure. The latter is dependent on the lung compliance.
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Pressure Control (PC): the desired positive pressure is reached delivering a compliance-dependent variable volume.
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Pressure Support (PS): the ventilator augments the patienťs spontaneous breaths. The ventilator functions as a demand flow system that support the patienťs spontaneous breathing efforts with a set pressure support. The pressure support setting defines the applied pressure. The patient determines the breath timing.
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All of the above can be incorporated into one mode in modern ventilators.
Classification of ventilators
There are many ways of classifying ventilators ( Table 8.1 ).
- 1.
Power source: ventilators can be:
- a.
Electrically powered: use standard mains electrical output.
- b.
Pneumatically powered: use high pressure gas input to power the gas.
- c.
Pneumatically powered microprocessor-controlled: both above power sources are required; electrical power is required to power a microprocessor within the ventilator that adds further control options to the gas flow such as pressure waveform.
- a.
- 2.
Pressure generation: as described above, a ventilator can either be:
- a.
Positive pressure: gas is pushed into the lungs, generating positive pressure within the lungs to cause chest expansion. Airway pressure is higher than atmospheric pressure so exhalation occurs due to this pressure gradient as well as the elastic recoil of the lungs and chest wall.
- b.
Negative pressure:
- i.
Tank ventilator or ‘iron lung’: gas is pumped out of the airtight tank to generate a vacuum around the body, decreasing intrapulmonary pressure and leading to chest expansion. As the vacuum is released, elastic recoil of the chest leads to expiration.
- ii.
Cuirass ventilator: an upper body shell or cuirass generates negative pressure only around the chest. This technique has been improved by the ability to generate two pressures, biphasic cuirass ventilation. This allows control of expiration as well as inspiration, and therefore the I : E ratio and respiratory rate.
- i.
- a.
- 3.
Control system:
- a.
Closed and open loops systems: in closed loop systems, microprocessors allow feedback loops between the control variable (such as tidal volume) as set by the operator and the measured control variable (exhaled tidal volume). If the two differ, for example due to a leak, the ventilator can adjust to achieve the desired expired tidal volume by increasing the volume delivered. Open loop systems deliver ventilation as set by the operator but do not measure or adjust.
- b.
Pneumatic circuit: the means of delivering gas flow from a high-pressure gas source to the patient can be either:
- i.
Internal
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Single: the gas from the high pressure source flows directly to the patient (e.g. modern intensive care unit [ICU] ventilators)
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Double: the power source causes gas flow to compress a chamber such as bellows or ‘bag-in-a-chamber’. The gas in the chamber is then delivered to the patient.
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- ii.
External: tubing goes from the ventilator to the patient.
- i.
- a.
- 4.
Suitability for use in theatre and/or intensive care.
- 5.
Suitability for paediatric practice .
- 6.
Drive mechanism: the internal hardware that converts electrical power or gas pressure into a breath to the patient.
- a.
Flow devices: compressors, driven by pistons, rotating blades, diaphragms or bellows, move atmospheric pressure gas into a higher pressure storage chamber which is then delivered as a breath. Blowers generate high flows of gas as the direct ventilator output.
- b.
Volume displacement devices: the volume of gas to be delivered to the patient is displaced by a moving part such as a piston or spring loaded bellows.
- a.
- 7.
Output control mechanism: valves regulate gas flow to and from the patient.
- a.
Proportional solenoid valve: opens in very small increments dependent upon flow required.
- b.
Digital on/off valves: a collection of valves, whereby each one is either fully open or closed. Each valve produces a certain flow by controlling the opening/closing of a specifically sized orifice.
- a.
Exam tip: A basic understanding of the classification of ventilators is important.
| Power source | Electrically powered Pneumatically powered Pneumatically powered microprocessor-controlled |
| Pressure generation | Positive pressure Negative pressure |
| Control system | Open and closed loops systems Pneumatic circuit: Internal: single or double External |
| Suitability for use | Operating theatre Intensive care unit Both |
| Paediatric practice | Yes/no |
| Drive mechanisms | Flow devices Volume displacement devices |
| Output control mechanism | Proportional solenoid valve Digital on/off valves |
Characteristics of the ideal ventilator
- 1.
The ventilator should be simple, portable, robust and economical to purchase and use. If compressed gas is used to drive the ventilator, a significant wastage of the compressed gas is expected. Some ventilators use a Venturi to drive the bellows, to reduce the use of compressed oxygen.
- 2.
It should be versatile and supply tidal volumes up to 1500 mL with a respiratory rate of up to 60 breaths/min and variable I : E ratio. It can be used with different breathing systems. It can deliver any gas or vapour mixture. The addition of positive end expiratory pressure (PEEP) should be possible.
- 3.
It should monitor the airway pressure, inspired and exhaled minute and tidal volume, respiratory rate and inspired oxygen concentration.
- 4.
There should be facilities to provide humidification. Drugs can be nebulized through it.
- 5.
Disconnection, high airway pressure and power failure alarms should be present.
- 6.
There should be the facility to provide other ventilatory modes, e.g. synchronized intermittent mandatory ventilation (SIMV), continuous positive airway pressure (CPAP) and pressure support.
- 7.
It should be easy to clean and sterilize.
Some of the commonly used ventilators are described as follows.
Manley MP3 ventilator
The Manley MP3 ventilator (Spacelabs Healthcare, Hertford, UK) is a minute volume divider (time-cycled pressure generator). All the fresh gas flow (FGF) (the minute volume) is delivered to the patient divided into readily set tidal volumes ( Fig. 8.1 ). It is rarely used in current practice.
Components
- 1.
Rubber tubing delivers the FGF from the anaesthetic machine to the ventilator.
- 2.
The machine has two sets of bellows. A smaller time-cycling bellows receives the FGF directly from the gas source and then empties into the main bellows.
- 3.
It has three unidirectional valves.
- 4.
An adjustable pressure limiting (APL) valve with tubing and a reservoir bag is used during spontaneous or manually controlled ventilation.
- 5.
The ventilator has a pressure gauge (up to 100 cm H 2 O), inspiratory time dial, tidal volume adjuster (up to 1000 mL), two knobs to change the mode of ventilation from and to controlled and spontaneous (or manually controlled) ventilation. The inflation pressure is adjusted by sliding the weight to an appropriate position along its rail. The expiratory block is easily removed for autoclaving.
Mechanism of action
- 1.
The FGF drives the ventilator.
- 2.
During inspiration, the smaller bellows receives the FGF, while the main bellows delivers its contents to the patient. The inspiratory time dial controls the extent of filling of the smaller bellows before it empties into the main bellows.
- 3.
During expiration, the smaller bellows delivers its contents to the main bellows until the predetermined tidal volume is reached to start inspiration again.
- 4.
Using the ventilator in the spontaneous (manual) ventilation mode changes it to a Mapleson D breathing system.
Problems in practice and safety features
- 1.
The ventilator ceases to cycle and function when the FGF is disconnected. This allows rapid detection of gas supply failure.
- 2.
Ventilating patients with poor pulmonary compliance is not easily achieved.
- 3.
It generates back pressure in the back bar as it cycles.
- 4.
The emergency oxygen flush in the anaesthetic machine should not be activated while ventilating a patient with the Manley.
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It is a minute volume divider.
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Consists of two sets of bellows, three unidirectional valves, an APL valve and a reservoir bag.
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Acts as a Mapleson D breathing system during spontaneous ventilation.
Penlon Anaesthesia Nuffield Ventilator Series 200
This is an intermittent blower ventilator. It is small, compact, versatile and easy to use with patients of different sizes, ages and lung compliances. It can be used with different breathing systems ( Fig. 8.2 ). It is a volume-preset, time-cycled, flow generator in adult use. In paediatric use, it is a pressure-preset, time-cycled, flow generator.
Components
- 1.
The control module consists of an airway pressure gauge (cm H 2 O), inspiratory and expiratory time dials (seconds), inspiratory flow rate dial (L/s) and an on/off switch. Underneath the control module there are connections for the driving gas supply and the valve block. Tubing connects the valve block to the airway pressure gauge.
- 2.
The valve block has three ports:
- a.
a port for tubing to connect to the breathing system reservoir bag mount
- b.
an exhaust port which can be connected to the scavenging system
- c.
a pressure relief valve which opens at 60 cm H 2 O.
- a.
- 3.
The valve block can be changed to a paediatric (Newton) valve.
Mechanism of action
- 1.
The ventilator is powered by a driving gas independent from the FGF. The commonly used driving gas is oxygen (at about 400 kPa) supplied from the compressed oxygen outlets on the anaesthetic machine. The driving gas should not reach the patient as it dilutes the FGF, lightening the depth of anaesthesia.
- 2.
It can be used with different breathing systems such as Bain, Humphrey ADE, T-piece and the circle. In the Bain and circle systems, the reservoir bag is replaced by the tubing delivering the driving gas from the ventilator. The APL valve of the breathing system must be fully closed during ventilation.
- 3.
The inspiratory and expiratory times can be adjusted to the desired I : E ratio. The tidal volume is determined by adjusting the inspiratory time and inspiratory flow rate controls. The inflation pressure is adjusted by the inspiratory flow rate control.
- 4.
With its standard valve, the ventilator acts as a time-cycled flow generator to deliver a minimal tidal volume of 50 mL. When the valve is changed to a paediatric (Newton) valve, the ventilator changes to a time-cycled pressure generator capable of delivering tidal volumes between 10 and 300 mL. This makes it capable of ventilating premature babies and neonates. It is recommended that the Newton valve is used for children of less than 20 kg body weight.
- 5.
A PEEP valve may be fitted to the exhaust port.
Problems in practice and safety features
- 1.
The ventilator continues to cycle despite breathing system disconnection without an alarm.
- 2.
Requires high flows of driving gas.
- •
An intermittent blower with a pressure gauge, inspiratory and expiratory time and flow controls.
- •
Powered by a driving gas.
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Can be used for both adults and paediatric patients.
- •
Can be used with different breathing systems.
Bag in bottle ventilator
Modern anaesthetic machines often incorporate a bag in bottle ventilator.
Components
- 1.
A driving unit consisting of:
- a.
a chamber ( Fig. 8.3 ) with a tidal volume range of 0–1500 mL (a paediatric version with a range of 0–400 mL exists)
Fig. 8.3
Bag in bottle AV800 ventilator.
(Courtesy Penlon Ltd, Abingdon, UK [ http://www.penlon.com ].)
- b.
an ascending bellows accommodating the FGF.
- a.
- 2.
A control unit with a variety of controls, displays and alarms: the tidal volume, respiratory rate (6–40 breaths/min), I : E ratio, airway pressure and power supply ( Figs 8.3 and 8.4 ).
