Research Brief 4234 – Mechanical Breathing
Machines
1. The development of the breathing machine
The breathing machine – or artificial ventilator –
is described by Young and Sykes (1994) as a mechanical device
that replaces the function of the inspiratory (breathing-in)
muscles. In reality it is far more sophisticated than the
description alludes to. It has a primary use in medical care
and in particular for patients with breathing difficulties,
respiratory complaints and those that require anaesthetising
for operation.
The history of air resuscitation differs according to which
author one chooses to believe but the earliest recording writes
Pilbeam (1986) is from the early 16th century when Paracelcus
used a fire bellows in connection to a tube inserted into
the patients’ mouth. It is unclear as to who is regarded
as the inventor of the ventilator not requiring a human power
source. Goerig et al (1987) credit George Fell with the invention
of the artificial breathing machine. Rendell-Baker and Pettis
(1987) then write that Draeger developed the first commercially
available artificial ventilators. Experimentation continued
until a polio epidemic in Copenhagen in 1952 in which 1400
university students were employed in shifts to manually operate
these artificial ventilators. This scare triggered a great
surge in research for an automatic breathing machine.
In 1967 the first mechanical and automatic positive pressure
ventilation machine was introduced. Positive pressure acts
to increase the pressure in the airways. Negative pressure
ventilators reduce pressure around the thorax using rigid
chambers that enclose the thorax or the whole body neck down.
These are usually known as tank respirators or “iron-lungs”.
They are outdated in modern times though simply due to their
size and the lack of access for the patient. Their use is
minimal in contemporary healthcare and for this reason the
remainder of this piece will focus upon positive pressure
ventilators.
2. Biological theory and requirements
Although the biological theory behind artificial ventilators
is complex the following section will show a simplified process
of human breathing. According to Kestner (1981), inspiration
is caused by a pressure gradient between the atmosphere and
the alveoli due to contraction of the inspiration muscles.
This pressure gradient generates airflow into the lung until
the point when the alveolar pressure is equal to atmospheric
pressure. Expiration (breathing-out) is a passive process
and as the muscles relax an opposite pressure gradient is
created which causes an airflow to leave the lungs. This is
known as spontaneous breathing. But what happens when this
process cannot take place?
What happens is that a mechanical ventilator is used to,
in effect, do the breathing for the patient. Al-Shaikh and
Stacey (2001) define properties of the ideal breathing system
as;
· Simple and safe to use.
· Delivers the intended inspired gas mixture.
· Permits spontaneous, manual and controlled ventilation
in all age groups.
· Efficient, requiring low fresh gas flow rates.
· Sturdy, compact and lightweight in design.
· Permits the easy removal of waste exhaled gases.
· Easy to maintain with minimal running costs.
So, let us now move on to investigate the different types
of ventilator that there are currently in use.
3. Ventilation Classifications
3.1 Standard features
Most ventilators use standard components in reservoir air
bags, adjustable pressure limiting (APL) valves, and tubing.
The APL valve is a one-way valve that allows exhaled air and
excess fresh gas to leave whilst disallowing the entry of
atmospheric gases. The valve opens and releases gas when it
reaches a certain pressure and this defined pressure can be
altered. The main task of the reservoir bag is to accommodate
fresh gas intake and to limit pressure build up within the
system. The use of tubing should be straightforward.
3.2 Classification
The most common classification used to differentiate between
systems using the previous components is the Mapleson (1962)
classification. This classification is based upon two characteristics;
a. The method by which gas is driven into
the lung – flow or pressure
b. The cycle between inspiration and expiration
– timed or thresholded
The following diagram shows the different classifications
of ventilatory modes. It should be noted that these are not
breathing machines per se – they are the physical modes
of artificial breathing.
Figure 1. Classification of Positive Pressure Ventilatory
Modes
(Aitkenhead and Smith, 1996)
Note: The arrow indicates fresh gas supply
Referring to the above diagram, according to Al-Shaikh and
Stacey (2001) modes B and C are used for recovery and emergency
whilst modes A,D and E are used for anaesthesia. Mode A, also
known as the Magill system, works by the following method;
As the patient exhales the gas is pushed through the tubing
towards the reservoir bag which is filled continuously with
a flow of fresh gas. The pressure build up causes the APL
valve to open and expel the alveolar gas. At this point the
patient should have inspirated once more thus receiving a
mixture of the fresh gas and rebreathed gas. Mode D is similar
but utilises a coaxial tube to move breathed and fresh gas
without mixing them. Modes B and C operate similarly but uses
greater use of fresh gas input to avoid rebreathing which
is important for recovery situations. Modes E and F are used
primarily in paediatrics as resistance to expiration of the
mode is minimal which is obviously important when treating
this group.
4. The Mechanics and Physics
4.1 Power
The previous section detailed a simplified view of ventilatory
modes and typical components of the ventilation loop of a
breathing machine. Actual mechanical breathing machines are
more complex than this as they incorporate power and feedback
mechanisms. Most ventilators are either electrically or compressed-gas
powered which in turn drives the mechanism designed to ventilate.
A typical drive mechanism will consist of a wheel, rod and
piston – which delivers the gas.
Whilst the gas power transmission is fairly straightforward
modern-day ventilators are controlled by micro-processor servos.
There are essentially three types of control according to
Branson et al(1995) which are pressure, volume or flow controllers.
In order to fully understand the mechanics of an assisted
breathing process we must understand the biology and physics.
The process is based around the need to achieve the correct
pressure to cause a flow of gas to increase the volume of
the lungs.
4.2 Physical variables
Pressure, flow and volume are all variables in the process.
For example and in direct relation to the problem in hand,
the flow rate can be kept constant. A constant volume flow
rate is generated by the piston – by altering the stroke
length - and delivered to the lung. However, there is a problem.
There are two factors that affect the flow rate, and therefore
the pressure and volume. Firstly, resistance encountered as
the gas enters the throat – the airways are resistant
in terms of gas density and viscosity to name but a few. Secondly,
something called compliance. The relationship between the
gas volume and the pressure of alveolar gas is important.
It depends upon the elastic properties of the alveoli and
is described by compliance. Compliance is, writes Pilbeam
(1986), the change in volume corresponding to the change in
pressure accompanying the volume change. In relation to the
desired constant volume flow rate we will no longer have this
constant flow due to the effects of resistance and compliance.
Whilst a constant flow rate is generated-known as tidal flow
- it is not delivered. This pressure variance in the lungs
is dependent upon the volume flow rate. Something must be
done to compensate for this change and ensure that the flow
rate generated is also delivered.
4.3 Schematics
There are many machines that can do this on the market today
– such as Draeger . Using a flow controller, when resistance
and compliance change, pressure will change but the volume
will not. Piston ventilators compensate for this compliance
by measuring it and delivering an appropriately proportional
additional volume of flow to ensure that the desired tidal
flow is delivered to the lung. The diagram below is a typical
schematic;
Figure 2. Schematic of Piston Ventilation Circuit
Adapted from Draeger (2003)
Using the schematic as a guide and taking the ventilatory
pressure of the human to be zero – ie. the patient cannot
breathe spontaneously – it can be seen how the ventilator
functions. Fresh gas is sent into the system and the piston
acts as the force upon this gas to push it through the inspiratory
valve, which is calibrated to a set level, and into the lungs.
As pressure reaches a certain level the gas is expired and
forces its way through the expiration valve. Some of the gas
is absorbed at this point and the cycle begins again. It should
be noted that in order to compensate for compliance and resistance
the desired measurement is taken as seen on the diagram. This
value is then used to automatically calculate the desired
additional volume, for example, of fresh gas required to maintain
the tidal volume flow from generation to delivery. The standardised
process is shown in the diagram below;
Figure 3. Block diagram of control system
Branson et al (1995)
The control system may be mechanical, pneumatic, fluidic,
electric or electronic. Its basic function is to use sensors
to measure the error signal – caused by resistance and
compliance – and to automatically feedback and adjust
the input to achieve the desired output. As mentioned before
the control variable may be either pressure, volume or flow.
Time is sometimes used also .
5. Constant flow machines
One example of a constant flow controller on the market is
the Siemens Servo 900C (Branson et al 1995; Young and Sykes
1994; Ingelstedt et al 1972). As the name implies it uses
servo control in measuring flow in order to adjust the output
control valve accordingly to maintain a constant flow of air
as the load – compliance and resistance – change.
This particular machine uses two of both pressure and flow
transducers – one at inspiration and one at expiration.
The flow transducers are electronically integrated to measure
volume. These volume signals can read both inspired and expired
tidal volume and feedback to the main control point at which
the inspiration flow is altered in line with what is desired
in compensation. For example if constant flow is required
the inspiratory valve is opened until the desired flow is
reached. If the sensors find that this flow is decreasing
due to the effects outlined then the valve will automatically
be opened further to compensate and allow increased gas flow.
Alternatively to the schematic (figure 2) this particular
machine is powered by an internal compressor in the form of
bellows that is pressurised by a spring. The spring force
sets the working pressure of the ventilator.
6. Literary Summary
Breathing machines can be traced back hundreds of years from
the simple use of bellows right through to modern day advancements
such as the positive pressure automatic ventilator or even
further to the soda-lime ventilator described by Al-Shaikh
and Stacey (2001) as using soda-lime to absorb carbon dioxide
and so ensure the system uses as fresh gas as possible.
They are used to assist, control and maintain artificial
breathing where the human may either be unable to breathe
properly, or indeed, at all. The main applications are in
anaesthesia and for recovery and emergency situations.
Most machines are powered by either a piston or bellows which
is used to act as the force upon a separate supply of fresh
gas. This new gas is inspirated by the patient via inward
tubing and consequently expired via outward tubing.
Modern machines use complex electronic feedback and control
systems in order to regulate and maintain proper use of ventilation.
The process is complicated by the interference of resistance
and alveolar compliance but usage of sensors can measure and
feedback in order to correct the error and maintain a constant
output – be it pressure, volume or flow – by valve
adjustment.
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REFERENCES
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Aitkenhead, R., Smith, G., (1996),
Textbook of Anaesthesia 3rd Edition, Churchill Livingstone
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Al-Shaikh, B., Stacey, S. (2001) Essentials
of Anaesthetic Equipment 2nd Edition, Harcourt Publishing
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Branson, R.D., Hess, D.R., Chatburn,
R.L., (1995) Respiratory Care Equipment, JB Lippincott
Company Publishing
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Draeger (2003), Series on Advanced
Ventilation, [online] at URL: http://www.draeger.com/us/MT/Library/or/case_studies/advanced_vent.pdf,
last viewed 8/5/03
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Goerig, M., Filos, K., Ayisi, K.W.,
(1987), George Edward Fell and the Development of Respiratory
Machines, In: Atkinson, R.S., Boulton, T.B., (Eds.), (1987),
The History of Anaesthsia, Royal Society of Medicine
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Ingelstedt, S., Jonson, B., Nordstrom,
L., Olsson, S.G., (1972) A Servo-Controlled Ventilator
Measuring Expired Minute Volume, Airway Flow and Pressure,
Acta Anaesthesiol Scand
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Kestner, J. (1981) The Mechanical Ventilator,
In: Rattenborg, C.C.,(Ed.), (1981) Clinical Use of Mechanical
Ventilation, YearBook Medical Publishers
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Mapleson, W.W., (1962) The Effect of
Lung Characteristics on the Functioning of Artificial
Ventilators, Anaesthesia
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Pilbeam, S.P., (1986), Mechanical Ventilation:
Physiological and Clinical Applications, MultiMedia Publishing
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Rendell-Baker, L., Pettis, J.L., (1987),
The Development of Positive Pressure Ventilators, In:
Atkinson, R.S., Boulton, T.B., (Eds.), (1987), The History
of Anaesthesia, Royal Society of Medicine
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Rowland, M. (1992), Biology, Nelson
Publishing
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Young, J.D., Sykes, M.K., (1994) Artificial
Ventilation: history, equipment and techniques, In: Moxham,
J., Goldstone, J., (Eds.), (1994) Assisted Ventilation
2nd Edition, BMJ Publishing Group
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