The use of alarms and built-in safety features in mechanical ventilation seems essential to the non-idiot, as these devices are being relied upon to sustain life in a very immediate sense. All the more disturbing, then, is the data which demonstrate how often these devices fail. Thomas & Galvin’s 2008 review of incident reports to the UK National Patient Safety Agency revealed that they were second only to syringe drivers as the most commonly complained-about source of patient safety incident. In fact, of the incidents associated with “more than temporary patient harm”, a third were due to ventilators which in one way or another stopped working during use.
There is little attention to these matters from the CICM examiners. The WCA document on Ventilation does not mention alarm settings except for where one needs to adjust the limits for different ventilation scenarios. That topic is interesting, but is more specific to each pathophysiological scenario, and is therefore addressed elsewhere. This chapter deals more with the abstract notion of safety in mechanical ventilation, discussing the features which have been incorporated into ventilator design to decrease the likelihood of the patient exploding or asphyxiating in the course of normal operation.
In brief summary:
- Alarms for mechanical ventilators should alert the user to a change in ventilator service delivery and may consist of
- Power alarms (electrical power or gas supply pressure)
- System error alarms (ventilator system failure)
- Output alarms (high or low conditions, eg. pressure, resp rate or volume)
- Inspired gas properties (eg. failure to deliver prescribed FiO2)
- Alarms for transport ventilators should have a visual cue component to accommodate high-noise environments (eg. aircraft)
- Essential automatic safety features should include:
- A pressure release valve that vents gas above a certain safe pressure
- An anti-asphyxia valve which allows the breathing of room air in the event of power failure
- Minimum electrical safety features should consist of:
- Uninterruptable power supplies and redundant batteries
- RCD/LIM and equipotential earthing
- Electromagnetic radiation shielding
The excellent article by Evans et al (2005) is a most comprehensive review of ventilator alarm technology, and would likely satisfy even the most demanding exam candidate. A brief introduction to the area which has an enviable structure to it can be found in Tobins’ textbook (3rd ed), p. 58 (Ventilator Alarm Systems)- it also has the authoritative weight of being written by Robert Chatburn, who is essentially the Pope of mechanical ventilation.
Critical event alarms
One’s dependence on a ventilator extends beyond the crude reliance on it for the supplied fresh gas which one requires to survive. One also expects the ventilator to notify them when there is something happening which threatens to alter or interrupt the aforementioned supply. This is the role of the ventilator’s alarms.
Weirdly, the early ventilators shipped without any sort of alarm systems. ICU physicians in the 1960s routinely had to devise their own alarm systems (eg. this pressure alarm designed by Lamont and Fairley in 1965), and various commercial add-ons trickled onto the market gradually over this period. The earliest alarm monitors were pressure gauges: a sensor (usually an aneroid manometer) would constantly prevent a loud buzzer from sounding while it was pressurised. A depressurised circuit was viewed as the most important possible disaster to be vigilant about – it may represent an accidental circuit disconnection or a self-extubation.
These days, with the variety of sensors available, the ventilator can be trained to freak out over a whole range of minor and major issues. There are various ways to classify these, to put things in some semblance of order:
- By variable: i.e. one might describe the ventilator alarm parameters in terms of the thing they are trying to detect, or the sensor to which they are coupled. Thus one might have power alarms, gas pressure alarms, volume alarms, and so forth.
- By urgency: for example, Chatburn describes Level 1 alarms as notifications of life-threatening situations. These should be relatively free from user interference (i.e. you can’t disable them) and redundant (receiving data from multiple sensors). In contrast, Level 3 alarms might let you know that the lung compliance has changed, or that the patient has not met their minute volume target of spontaneous breaths.
If one had to list a bunch of ventilator alarms for some sort of cruel exam answer, one could do worse than the list offered by Chatburn as a part of his all-embracing classification system for mechanical ventilation. It has been slightly modified and reproduced here:
- Loss of mains electrical power
- Low battery
- Loss of gas supply pressure
- Incompatible or bizarre ventilator settings
- System errors (eg. gas blender unable to produce desired FiO2)
- System failure (eg. turbine fan will not spin, or ventilator circuit cooler has failed)
Output alarms (high or low conditions)
- Flow rate
- Spontaneous respiratory rate
Inspired gas properties
Automated safety features
The critical event alarms mentioned above are typically under the operator’s control and can be fiddled with, silenced, up-adjusted, disabled or just ignored. For the sake of patient safety, a ventilator should also come outfitted with some critical event safety features which are hard-coded and cannot be turned off. At a basic level, these should consist of an over-pressure relief vent and an anti-asphyxia valve.
A pressure-relief valve or vent is a valve which release the circuit gas into the atmosphere whenever a certain peak inspiratory pressure is reached. Usually, people set this at 40 cm H2O. The maximum setting for these is usually 100 cm H2O, which is high enough for most purposes, as it represents an increase of 10% over sea-level atmospheric pressure.
An anti-asphyxia valve which allows an awake-ish patient to breathe room air whenever the ventilator suddenly dies is described as a “desirable” feature by the Tobin’s chapter on transport ventilators (p. 678, 3rd ed). These valves have a certain “cracking pressure”, i.e. they need a certain minimum inspiratory pressure to open, and therefore impose a certain work of breathing on the patient. Representative models were tested by Austin et al (2002), who connected artificial lungs to failed ventilators and recorded the increased work of breathing- turns out, it’s quite substantial. It would almost be better to disconnect the patient from the circuit in the event of prolonged ventilator failure.
Electrical safety for mechanical ventilation
For a high-voltage electrical device which insufflates a patient with flammable gas in the presence of high humidity, it is surprising that the electrical safety aspects of mechanical ventilation are not the subject of much literature. Indeed, all that can be said on the basis of the sparse literature (consisting of local hospital pre-use check guidelines and generic recommendations for medical electrical devices) is that the mechanical ventilator should be attached to an uninterruptible power supply, and should be on an equipotential earthed circuit with a residual current device and line isolation monitor so that microshock can be prevented.
Protection from electromagnetic waves
By this, it is not meant that the ventilator is expected to survive the EMP discharge of a 20—kiloton nuclear blast. It does, however, need to have some sort of basic built-in protection from electromagnetic radiation with which the world is increasingly filled. This ranges from being well insulated from the waves generated by diathermy and defibrillation to being protected from cosmic rays which might become relevant while transporting a patient at high altitude. On a less esoteric note, the greatest risk posed to mechanical ventilators are the ubiquitous mobile phones of ICU staff and family members. This is no joke. Shaw et al (2004) were able to completely destroy a Puritan Bennett 840 using nothing but a Nokia 6120i, which – by ringing within 30cm of the ventilator – caused it to stop ventilating completely. Having given this warning, it must be mentioned that neither the PB840 nor the Nokia are totally representative of modern equipment. At the time of writing in late 2018, it can be safely said that the intensivist poses a greater threat to the ventilated patient than does their phone.