A ventilator is defined as a medical device that results in the movement of air into and out of the lungs.
The expansion of medical ventilation (and in some sense of critical care) arose out of desperation from the polio epidemic of 1952. Prior to this, it was primarily limited to negative pressure ventilation e.g. iron lung
They can be classified in a number of different ways. A key differentiation is between positive and negative pressure ventilators. As the vast majority of clinical use involves positive pressure devices, this will be the focus here.
Positive Pressure Ventilation
This is by far the most common method of ventilation used medically. It has quite a few differences from the normal physiology of breathing (which is negative pressure) and this manifests as certain advantages and disadvantages.
Ventilators may be classified in a number of ways (and many can be found within the same machine and there is some overlap):
The method of cycling
Inspiratory phase gas control
Method of operation
Source of power
Minute volume dividers
In addition, many ventilators have sophisticated modes that can be employed:
As noted, modern ventilators often allow widespread adjustment of their setup.
Method of Cycling
This describes how the ventilator decides the switch between the inspiratory and expiratory phase. This may well be once a certain parameter has been reached:
Length of time - most common
Inspiratory Phase Gas Control
This determines how the inspiratory phase gas is delivered.
Volume - a preset volume of gas is delivered
Pressure - a preset pressure is delivered
This can have a notable impact on ventilation of the lungs, and have different advantages and disadvantages
Method of Operation
This is closely linked to the inspiratory phase gas control, and describes the pattern of gas flow during the inspiratory phase.
Pressure generators create a preset pressure, and this pressure difference compared to the lungs is what allows gas flow. The degree of gas flow will therefore vary depending on the compliance of the patient’s lungs e.g. it may notably decrease with pneumoperitoneum during laparoscopic surgery. An advantage is that it can to some degree compensate for leaks within the circuit.
Flow generators create a predetermined flow, and continue this flow throughout the inspiratory cycle. An advantage is that they will be able to (to some degree) compensate for changes in lung compliance. However, this is at the cost of an increase in circuit pressure towards the end of the cycle, as the lung is being more stretched, whilst the flow into it is continuing - this mode therefore tends to have higher peak airway pressures. Another disadvantage is that there is an inability to compensate well for circuit leaks.
More advanced systems can modulate the above simplistic explanations to try and provide a more ‘natural’ pattern - an initial rapid flow rate that quickly slows.
This refers to how the ventilator actually mechanically delivers gas to the lungs.
A minute volume divider is a more old-fashioned mechanism, the most common being the Manley ventilator. Here the fresh gas flow from the anaesthetic machine is split between 2 bellows. The first is filled with the gas, which then fills the second one which is then delivered to the patient as a tidal breath. The first one is filling up whilst the second one is delivering the tidal breath. As such, the fresh gas flow powers the ventilator, and has to be equal to the patient’s minute ventilation.
A bag in a bottle ventilator is a more common mechanism, replicating the process of hand ventilation. Here the patient’s gas flow is kept in a bag/bellows, surrounded by a bottle. The driving gas is driven into the space around the bag, compressing it and delivering the gas to the patient. Here the fresh gas and driving gas are separate, and so the fresh gas flow requirements are more determined by the circuit.
More modern systems may use electronically controlled bellows or piston systems. Electromagnetic systems control the flow of compressed gas through computer controlled valves. These allow a great deal of control, and are common on ICUs.
There are a number of different modes for delivering ventilation. There is no well defined nomenclature for this, which can be confusing. Many of these modes use some of the different functional aspects described above. Some of the key ones include:
Controlled Mandatory Ventilation CMV is a fairly non description term. It refers to a mode of ventilation where the parameters are tightly controlled by the clinician, and so is really an umbrella term for more detailed modes e.g. VCV, PCV. This approach is often uncomfortable to experience, as it is telling the patient when and how to breath, and hence is more commonly employed during anaesthesia or deep sedation.
Volume Controlled Ventilation This has been touched upon above. In this mode, a preset volume is delivered. This is achieved by delivering gas at a fixed flow rate. When looking at the relative waveforms, this results in a fixed gas flow rate until the tidal volume is delivered, at which point the ventilator stops. There may be a short inspiratory pause here, prior to exhalation. The airway pressure will rise steadily to a peak pressure, but may drop to a slightly lower plateau pressure during the inspiratory pause. This can be modified to slow the fixed flow rate, and therefore maximise the spread of the gas flow across the whole inspiratory cycle. This can therefore reduce peak pressures, reducing airway injury and optimise the filling of the alveoli with different time constants (speeds of filling) The ventilator will then switch to the expiratory phase, which is passive.
Pressure limited volume controlled This is one of the ways that the VCV and PCV advantages have been brought together. This mode delivers a fixed flow rate of gas, aiming for a target TV, much as with VCV. However, once a maximum preset pressure is reached, the gas delivery changes from fixed flow to simply maintaining this airway pressure. This allows some ongoing flow to occur to alveoli with a lower time constant, whilst not exceeding the desired airway pressure. The flow waveform is then that of a decelerating pattern.
Pressure Support/Assisted Spontaneous Breathing (ASB) In this mode, the patient's efforts to take a breath are detected by the ventilator. In response to this effort, the machine raises the airway pressure to a preset value, thereby giving positive pressure to assist with the breath. The breath detection is often when an inspiratory flow rate is detected by the machine. The speed of the pressurisation can also be adjusted, termed the ramp time. The inspiratory period will end when the patient stops taking a breath, detected by a fall in the inspiratory flow rates (i.e. is flow cycled) compared to the maximum flows (usually set to 25%). This will not ventilate the patient if they are not spontaneously breathing, although most machines have back up ventilation that will initiate a controlled mode of ventilation (e.g. VCV at 500ml x 12bpm) after a specified apnoea time e.g. 30s.
Synchronised Intermittent Mandatory Ventilation (SIMV) In this mode there are essentially 2 different types of ventilation ongoing. Time is divided up into cycles. In the mandatory phase, the machine wants to give a mandatory breath (either VCV or PCV in nature). If the patient triggers a breath in this window, then the mandatory breath will be given at this time. Outside this phase, the patient can breathe normally, and will often be given pressure support to their breaths (but not a mandatory breath).
Assist Control Ventilation (ACV) Here there is a TV set, as with VCV (or it can be a PCV set up). This will be given as a mandatory mode in the normal way, at a set RR. However, if the patient initiates a breath, the ventilator will given the mandatory TV breath at this time. The respiratory effort is detected by either a drop in pressure or by a change in flow.
Biphasic Positive Airway Pressure (BIPAP) This is essentially spontaneous ventilation occuring on top of mandatory ventilation. The mandatory component cycles between two different airway pressures, similar to PCV. However, as the flows to the circuit are kept high, the patient is able to breath over these two levels. It is essentially like having two different levels of CPAP when the patient is breathing, or is just PCV if they are not. Note: the term BIPAP is also sometimes used for a mode of NIV.
Airway Pressure Release Ventilation (APRV) is a variant of BIPAP where there is a long inspiratory (high pressure) phase compared to expiratory (low pressure). This gives the optimum conditions for alveolar recruitment, so may be used in cases of stiff alveoli to aid oxygenation. The low pressure level is generally set fairly low to aid expiration and reduce the CO2 retention that may occur.
Pressure regulated volume control (PRVC) There are a few other names for this mode, but it essentially blends the advantages of VCV and PCV by delivering the desired volume but with a PCV waveform. It does this by initially delivering a VCV and measuring the plateau pressure that result. This is used as the pressure value for the next breath as a VCV mode. Measurements of the ongoing resulting TVs allow regular adjustment of the inspiratory pressure (if needed) to continue to achieve the desired volume.
Links & References
Al-Shaikh, B. Stacey, S. Essentials of anaesthetic equipment (3rd ed). Churchill Livingstone Elsevier. 2007
Dornan, R. Principles of IPPV. e-LFH. 2016.
Gould, T. Ventilation - Basic modes 1. e-LFH. 2016.
Gould, T. Ventilation - Basic modes 2. e-LFH. 2016.