Power and gas supply requirements for mechanical ventilators

In order to generate a tenuous connection between this resource and the needs of critical care education, one might reach out to the CICM WCA document (“Ventilation”), where a successful trainee “outlines requirement for power and gas supply” at least to a level which is described as “acceptable”. Without defining their anchors any further, the college leaves the assessor to decide which form of underachievement passes for “acceptable”, and what is sufficiently advanced to be described as “good”. This, of course, is worrying to the assessor, particularly where the assessor has not thought about these technical aspects in years, or perhaps decades. The purpose of this chapter is to arm these assessors with the relevant questions and to armour the trainee with sensible answers.

In summary:

Because medical gas supply systems are discussed in greater detail elsewhere, this chapter does not focus so much on what comes out from the wall, but on the requirements of the ventilator, and how these are affected by its design. Probably the best reference for this subject is the article by  Campbell et al (2002); the authors compared several models in terms of their power and gas consumption.

Gas supply for mechanical ventilation

As preambled above, it will suffice to say that most ventilators found in the hospital setting will take their gas from the wall supply. Domiciliary models (eg. CPAP machines designed for home use) will usually ventilate with room air only, and will have a filtered air intake. These may or may not have a little standardised nozzle to hook up to wall oxygen when additional FiO2 is required. In short, gas supply for mechanical ventilation would be some combination of:

• Piped wall (or cylinder) air and oxygen
• Piped cylinder oxygen and room air
• Room air only

Which of these are being used obviously determines the power supply requirements of the device. When the piped wall oxygen supply is used to drive the respiratory circuit flow, there is usually some minimum value for pressure (usually around 250-300 kPa). When cylinder gas is being used to trickle some oxygen into the gas mixture, this is

Power supply requirements for mechanical ventilators

The ventilator usually requires some sort of a power source in order for it to functions. Again, in spite of the fact that there is a rich history of mechanical ventilators having been clockwork-powered or driven by hand, one needs to come to terms with the fact that in this century the majority of devices are powered by some combination of pressurised gas and electricity.

The electricity supplied to the ventilator could be used to generate some, or none, or all of its gas-blowing functions. Broadly, there are several possible permutations of this:

• Wall gas pressure there are a couple of different variants of this (eg. dual-circuit “bellows” ventilators and single-circuit proportional solenoid valve models) but the basic gist is that the pressure of wall gas is used to generate the respiratory circuit pressure. In this case the electrical power supply is used only to power the valves and electronics.
• Wall oxygen only (where wall oxygen supplies the FiO2 but the compressor is used to generate all the respiratory circuit pressure using entrained room air)
• Compressor only (where the ventilator uses a compressor to create all the respiratory circuit pressure and there is no wall gas supplied)

It should therefore make sense that the electrical power supply of a ventilator will work harder in a ventilator which relies on a compressor. It is, by contrast, relatively energy-cheap to just power the solenoid valves and ventilator display.

Thus:

• The advantages of a completely gas-powered model are the longevity of its batteries.
• The advantages of a totally or partially turbine-driven model are its independence from gas supplies.

An ideal power supply for a mechanical ventilator is uninterruptable (i.e. with some built-in redundancy) and portable to permit transport. The availability of electricity even in large hospitals is not to be relied upon, and UPS circuits are usually available in ICU areas (by convention these wall sockets are blue). Internal ventilator batteries offer a third level of redundancy. They weren’t always internal, as demonstrated below – the HELP-115 lead acid battery depicted below comes from an article by Barton et al (1997); it took four people to transport a ventilated patient with this setup.

Push come to shove, most modern ventilators will run on an improvised power source; for example this brochure suggests that with an appropriate DC-to-AC converter one could power their ventilator using their car battery (via the cigarette lighter port). This is good to know. Blakeman et al (2011) found that the batteries of domiciliary models are highly variable in their performance characteristics, with some models lasting only about 100 minutes.

Power requirements of pneumatic ventilators

For example, the completely wall-gas-reliant Maquet Servo-I ventilator requires only 70W to operate even while fast-charging its batteries. At full charge, the energy consumption is about 38W (i.e. approximately the same power requirements as an incandescent light bulb). Most of that goes to the display and user interface: in standby mode, the power requirement drops only to 33W, i.e. while doing nothing clinically useful the Servo-I is wasting its batteries entertaining you with its “Standby” screen.

What batteries does it take, and how long do they last? Most modern ventilators use lithium-ion power sources. In case you were wondering, up to six 3.5Ah battery modules can be connected to the Servo-I simultaneously, offering a total operating time of roughly 3 hours (once discharged completely, the batteries take 12 hours to recharge).

Power and gas requirements of compressor ventilators

A compressor-based ventilator has a turbine which entrains room air and compresses it. This air compressor contributes the pressure for the respiratory circuit. If some oxygen is also being used, this contributes some additional pressure. Most ventilator models which include an oxygen fitting will usually require a high-pressure fitting, allowing for the possibility that the patient will require 100% oxygen at a 200L/min flow rate (i.e where the air compressor turbine could be turned off, as it is not required). Where the turbine is in use, its power requirements will depend somewhat on the requirements of the circuit- higher flow and pressure will obviously require more RPMs from the turbine as compared to low flow or low pressure. Therefore, the power requirements of such a ventilator will dependent on the flow which needs to be generated, and on the percentage of entrained air. To use the Hamilton Model T-1 as an example, these power requirements may range from 50W to 150W.

Factors which influence the power consumption of mechanical ventilators

Apart from its intrinsic design characteristics, the following factors can change the power requirements of a mechanical ventilator:

• Need for continuous mandatory operation
• High levels of pressure required
• High respiratory rate
• High PEEP required

In general, it appears that the power requirements of ventilator models which rely on supply gas pressure are less affected by changes in patient lung characteristics and ventilator settings (Campbell et al, 2002). This makes sense because these models really just need to power the valves and monitor screens. Thus, if one is somehow trapped in a corridor somewhere with a ventilated patient running just on battery, one could conceivably add valuable minutes by turning down the screen brightness.