Basic components of a mechanical ventilator

It is a widely recognised fact that ICU doctors are going to spend much of their working life using ventilators, staring at their screens, making fine adjustments to them, talking about them, and generally appreciating them. One might describe the whole cohort of critical care specialists as the “Mechanical Ventilator Fancier’s Society”. It is therefore quite remarkable how little attention is paid to the exploration of these apparatus in the expectations of the formal training process.  Judging by the 2017 CICM Primary Syllabus, college examiners expect absolutely nothing from their primary candidates in this cognitive territory. Then, after an unregulated gap in training, the Fellowship candidates find themselves in a position where this area is assumed knowledge, and are asked questions about pragmatic matters such as troubleshooting the circuit of the inexplicably breathless patient or interpreting abnormal waveforms. 

One might, of course, make the argument that it is quite possible to be a safe and proficient user of a device without expert familiarity with its inner workings, pointing to the personal computer as an example. Moreover, because this topic has zero exam value, and the trainees’ time is finite, one might accuse the author of wasting valuable revision time with this self-indulgent gibberish.  The counterargument is that an in-depth understanding of our instruments informs our use of them, and enriches our practice. To claim mastery of the field of Intensive Care Medicine should probably mean a claim to an understanding which goes somewhat beyond the pragmatic requirements of routine bedside work. 

However, there’s something to be said for satisfying pragmatic requirements.  To define what those might be, one could use the official CICM “Work-Based Competency Assessment: Ventilation”, which mentions that an “acceptable” trainee “describes the principle (sic) components of bellows and turbine ventilators”. In brief, the following components are usually seen in a modern mechanical ventilator:

This list is neither exhaustive (listing every valve and bolt) nor sufficiently broad to cover every possible variation on this theme. Rather, it was designed to cover the main components which one might find inside a normal ICU ventilator, a transport ventilator, or an anaesthesia ventilator. The specific functions of these components are discussed in greater detail in the subsequent chapters of this section. It would certainly be pointless to take this subject back to a time when “an average clinician could …completely disassemble and reassemble a mechanical ventilator as a training exercise or to perform repairs”, but some detail is probably warranted, given our reliance on these devices.

There are not many good peer-reviewed resources for the topic of ventilator design, but wherever one looks one finds an article by Robert L. Chatburn. For instance, much of this chapter is based on the excellent article by Chatburn & Branson (1992) which discusses an all-encompassing taxonomy to classify mechanical ventilator systems. Chatburn seems to have been writing about mechanical ventilation since 1982 and was invited to write the classification chapter for Tobin’s Principles and Practice of Mechanical Ventilation (p.45 - Chapter 2 of the 3rd Ed, 2012). He is also the co-author of Chapter 3 from the same book (“Basic Principles of Ventilator Design”, p. 65-95).

Definition of what a mechanical ventilator actually is

According to the definition offered by Chatburn, a mechanical ventilator is an automated machine in which

“...energy is transmitted or transformed (by the ventilator’s drive mechanism) in a predetermined manner (by the control circuit) to augment or replace the patient’s muscles in performing the work of breathing.”

This definition must be qualified by mentioning that the mechanical ventilator should be automated. The self-inflating bag-valve-mask resuscitator is a ventilator by the above definition, as the user’s muscle energy acts as a drive mechanism and is used to augment or replace the patient’s muscles. However, it would be plainly mad to consider that a mode of mechanical ventilation. Thus, a mechanical ventilator needs to be a device which you can set and walk away from, knowing that it will continue to safely perform its role.

Classification of ventilator system designs

Until surprisingly recently, people have been using variants of a classification system which has undergone little modification since the 1950s. William Mushin’s Automatic Ventilation of the Lungs (1959) was an early textbook of mechanical ventilation which is much referenced, and it was probably quite good in its time (contemporaries gushed that it “deserves to be closely studied by all anaesthetists” and “provides salutary reading for those who feel the urge to design, make or modify an apparatus of this kind”). Of course, it is well out of print, and given its vintage and irrelevance in the modern era, even a veteran software pirate would be entirely unable to track down an illegally scanned copy.

In short, mechanical ventilator classification systems have historically been so pointless and inadequate that Chatburn opened his 1991 article with a quote from Genesis (11:7), “Come, let us go down and there confuse their language, that they may not understand one another’s speech”.  The more mature taxonomy offered by Chatburn is used here to classify ventilators according to the mechanisms and principles of their function. It omits such anachronisms as the inevitable discussion of positive and negative pressure ventilators (of course these days they are all positive pressure devices). The model is extensive, as it covers not only engineering aspects of mechanical ventilator design but also such detail as flow waveform shape and different possible alarm settings. It is reproduced here with minimal modification:

Input Pneumatic Electric AC DC (battery) Power conversion and transmission External compressor Internal compressor Output control valves Control scheme Control circuit Mechanical Pneumatic Fluidic Electric Electronic Control variables Pressure Volume Time Phase variables Trigger Target Cycle Baseline Modes of ventilation Control variable Breath sequence Targeting schemes

Output Pressure waveforms Rectangular Exponential Sinusoidal Oscillating Volume waveforms Ascending ramp Sinusoidal Flow waveforms Rectangular Ascending ramp Descending ramp Sinusoidal Alarms Input power alarms Loss of electric power Loss of pneumatic power Control circuit alarms General systems failure Incompatible ventilator settings Warnings (e.g., inverse inspiratory-to-expiratory timing ratio) Output alarms (high/low conditions) Pressure Volume Flow Time Frequency Inspiratory time Expiratory time Inspired gas Temperature FIO2

Though it is useful later to classify modes of ventilation and make sense of the massive array of totally random-seeming ventilator nomenclature, for the purposes of this engineering-oriented chapter a classification like this is too broad. What the CICM trainee needs is something quick, to memorise and reproduce for the purposes of passing their ventilation WCA.

Basic ventilator components

If one were to behold a ventilator with a critical eye, one would find that it is really composed only of four main parts:

• Power source
• Controls
• Monitors
• Safety features

The power source consists of something to supply the gas which will be delivered to the patient, as well as the energy required to run the ventilator components. Thus, this category encompasses the gas supply system, the batteries and power source for the mechanical ventilator.

The controls are some means of regulating the timing and characteristics of the delivered gas. These components consist of an entire array of parts, each of which probably merits an entire chapter of their own:

• A gas blender is required to control the mixture of air, oxygen, anaesthetic gas or whatever else you might be using the ventilate your patient. One may not need any such gas blender if one is discussing some sort of stripped-down domiciliary model which runs on room air alone, and which does not accept an exogenous oxygen source.
• A gas accumulator might be a component of a ventilator which requires a precise control of gas mixtures and which cannot rely on proportioning valves to produce this level of precision, eg. where the gas flows are very low. An example of this is the accordionlike “bellows” of an anaesthetic machine; it is used to maintain a reservoir of a stable gas mixture.
• Inspiratory flow regulator – basically, any device which ensures that the respiratory circuit receives the prescribed gas flow. This is usually a solenoid valve. This thing sits in front of the gas supply (either from the wall or from the compressor turbine) and ensures that the patient is only exposed to carefully measured amounts of that gas. Given that the wall gas in ICU piping outlets is supplied at a standard pressure of 400kPa (approximately 4 atmospheres), it is obviously an essential component. 
• Humidification equipment is a requirement in most settings. This can take the shape of an active humidifier (i.e. a device which heats and evaporates water into the supplied gas mixture) or a passive humidifier like a heat/moisture exchanger. Generally, domiciliary CPAP machines which supply room air via some sort of face mask can rely on the patient’s own upper airway for humidification.
• The circuit, that wobbly mess of corrugated tubing, is often forgotten in discussions of ventilator equipment, but it plays an important role (try to ventilate the patient without one). Its characteristics, for example its compliance and resistance to air flow, are important features.
• Expiratory pressure regulator (i.e PEEP valve) is a means of maintaining and controlling positive airway pressure. These are basically carefully controlled expiratory flow obstructions, usually in the form of a solenoid valve (though crude mechanical models also exist for old-school ventilators).

The monitors are means of sensing and presenting the characteristics of gas delivery so that one might be able to assess the ventilator’s performance (and probably also the patient’s condition).

• Gas concentration is usually measured by either voltaic cells or spectrophotometers. For example, the oxygen supply sensor is usually an oxygen cell, which produces an output voltage proportional to the partial pressure of oxygen in the inspiratory gas pipe.  
• Flow is pretty much the main thing the ventilator supplies, so it makes sense to want to monitor it in some way. All commercially available mechanical ventilators have some method of monitoring flow. These methods include:
  • Hot wire anemometry, where the effect of gas flow on cooling a heated platinum wire is detected as a change in the wires' resistance
  • Variable orifice flowmeters, where a pressure drop across a narrow pipe is used to calculate flow
  • Screen pneumotachography, where a pressure drop across a mesh screen is used to calculate flow
  • Ultrasonic flowmeters, where two transducers are used to analyse changes in ultrasound wave transit time caused by the velocity of the intervening medium.
• Pressure in the circuit had historically been accomplished by means of aneroid manometers, i.e. pressure sensors that measure air pressure by the action of the air in deforming the elastic lid of an evacuated box. In modern ventilators, these have been superceded by integrated silicon wafer pressure transducers, at a fraction of the cost and with greatly improved accuracy.
• Volume is not measured directly in modern ventilators it is calculated from flow measurements. In older ventilator designs (eg. the bellows and the piston models) a directly measure volume was the main variable over which the intensivist had any control.

The safety features are some devices and measures which ensure that the patient does not come to any additional harm from being ventilated (beyond the already brutal effects which are integral to the process). These consist of filters and alarms.

• Inspiratory filters of the ventilator promote purity of inspired gas (eg. by removing airborne particles and bacteria from the inspired gas mixture).
• Expiratory filters protect the ICU staff. Expired gas is filtered to prevent the ventilator from constantly belching out great clouds of aerosolised pathogens generated in the horrific toilet-like bog water of the patient’s airways.
• Expiratory filters are also usually needed to protect the ventilator components from the necessarily hot and humid expired gases, which would degrade the quality of sensor measurements and decrease the lifespan of the device
• Alarms are usually integrated into the software as safeguards against unintentional changes to the ventilator settings and weird misapplications of ventilation. Broadly, these are systems to let you know that the patient condition or ventilator performance has trespassed the parameters which (you decided) are safe. Nonsoftware alarm-like features are also integrated into ventilators, for example mechanical blow-off valves to release excess pressure when the patient coughs.