Archive for the ‘Ventilators’ Category


A ventilator is an air pump working at pressures necessary to inflate the lungs of a person who is not able, for anyone of many reasons, to breath for themselves.
During mechanical ventilation it is normal for the lungs to be inflated to a positive pressure, this is opposite to normal breathing where the pressure in the lungs is a negative one.
There are many different ventilators from the very simple to the highly complicated, some are powered by air and need no electricity. Some are electrically or electronically driven and need compressed Oxygen and Air, or draw in room air and have low pressure Oxygen added. Whatever the type the basic principle is the same. Two valves, one inspiritory and one expiritory.
Inspiration is when the gas is going into the lungs and expiration is when the gas is going out of the lungs.
At the start of inspiration the inspiratory valve opens and the expiratory valve closes, this allows the gas into the lungs but not out.
When this part of the cycle is completed the expiratory valve opens and the inspiratory valve closes, this allows the gas to leave the lungs, at the same time the machine is preparing for the next breath. So the cycle goes on at a rate set by the doctor.
What tells the machine to change from inspiration to expiration and from expiration to inspiration will vary from machine to machine. There are four systems, see further down the page.

When using any ventilator it is vital that an alarm is used, this will offer protection to the patient should the machine go wrong. Modern machine have alarms built in older ones will have or should have an external alarm system.

Before you work on a machine that you are not familiar with you must make sure that you know how it works.
This may save you a lot of embarrassment if you are asked questions about it in front of a crowd.
Records are very important when maintaining ventilators, all reported faults, repairs and service details should be noted down dated and signed for.
Make out a check sheet outlining all the checks and service operations that are needed, tick each one-off as it is done.
If you have a number of machines they should each be given an identification number.
Machines should be serviced twice a year.
When you get a new machine do make sure that you get the full service book.
Look through the book and decide what spares you will need.
Rubber parts are prone to wear and perishing. Some makers try to make a lot of money on spares so think carefully before you buy. If you are not sure you should follow the makers advice until your experience tells you otherwise.

If you are called to a machine that is reported as faulty, check that it has been set up properly, as much as 75% of reported faults are caused by operator error.
When looking for a potential fault in front of a crowd and under pressure for a quick result do not be panicked, keep your cool.
Start from the beginning, is the electricity turned on? Is the gas on and so on, work through in a planned manner.

This is where it is good to know what your machine should sound like and the rhythm of its operation when all is well with it.
If you have ANY doubt about the machines safety or good working take it back to the workshop, its no good saying you think it is o.k. you must be sure. If there is no spare machine the patient can be ventilated with a resuscitation bag whilst you work on the machine. This can be done for days if necessary.
NEVER EVER RUSH when working on a machine, other people’s lives will depend upon its correct working. Only return it when YOU are happy with it.


During the inspiratory phase as time passes, the volume delivered to the patient builds up and often, the flow to the patient decreases, each of these variables, time, pressure, flow and volume, is sensed in different ventilators and the ventilator is made to change over from inspiration to expiration when the sensed variable has reached a predetermined value.
The mechanism which does the sensing and performs the change over is called the cycling mechanism, there are four basic methods.
1. Time cycled, in which the change over occurs after a pre-set period of time.

2. Volume cycled in which the change over occurs when a pre-set volume has been given.

3. Pressure cycled in which the change over occurs when a pressure reaches a pre-set level, this is closely related to lung pressure.

4.Flow cycled in which the change over occurs when a flow falls to a pre-set level, this is related to inspiratory flow. Some machines may have more than one system of cycling.

1. This will be time dependent and will be governed by the number of breaths per minute set.

There is one more method and that is when the patient makes an inspiratory effort and triggers the machine into an inspiratory phase. In this case a sensor in the machine will detect the negative pressure generated by the patient taking a breath. This control can be made more or less sensitive to prevent false triggers.
This last method will only work on machines that are designed to do so.


the cycle of a ventilator when the gas is being pushed in to the lungs.

the short period when the ventilator holds the gas in the lungs at the pressure reached, this phase is normally very short or absent. see drawing.

the cycle of a ventilator during which the gas is allowed to leave the lungs. see drawing.

the cycle of a ventilator after all or most of the gas has left the lungs, and before the next inspiratory phase has started.

(inspiration/expiration ratio) is the ratio of the inspiratory time over the expiratory time, normally set at 1:2, for example 1 unit of inspiratory time and 2 units of expiratory time, this ratio can be altered on most modern machines, but almost always the inspiration is shorter than the expiration, only in exceptional circumstances is the inspiration time longer than the expiration time.

(positive end expiratory pressure) the pressure in the lungs normally falls to zero at the end of expiration, but on some machines a peep can beset which leaves the lungs with a slight positive pressure at the end of expiration, this may be as little as 1 or 2 cm H2O or it maybe more, as clinical circumstances dictate.
It is useful in helping to maintain the architecture of the lungs, in clinical practice it has been found useful in situations where the arterial oxygen levels remain low despite high inspired oxygen concentrations.
Some of the harmful effects are, the possibility of lung damage, a reduction in cardiac output, renal function may be impaired, a rise in intra-cranial pressure with the rise in mean intra-thoracic pressure.

(Continuous positive airway pressure. )
This is a form of ventilation that always keeps the lungs at a pressure slightly above atmospheric pressure, it is applied during the expiratory phase when the patient is breathing spontaneously.
It helps to reduce the amount of oxygen that the baby needs to stay the right colour, it makes breathing easier by keeping the alveoli open in situations where they may collapse, and it also helps to remove the CO2 from the expired air.
A typical sort of starting level would be about 5 Cm H2O. It is unwise to go above about 10 Cm H2O.

(Intermittent mandatory ventilation)
I.M.V. is a method of combining spontaneous respiration with regular mechanical respiration at pre-set intervals and tidal volumes(Vt).
The rate at which the ventilations occur is slower than in continuous ventilation.
It is a method of helping people who have been on a ventilator for a long time and whose respiratory muscles have become weak, this helps to strengthen them up so that they may start to breathe for themselves.

(Synchronised intermittent mandatory ventilation. )
S.I.M.V is much the same as I.M.V, except that the mechanical breathing of the machine is synchronised with the natural rhythm of the patients breathing.
It helps to achieve a smoother respiratory pattern.

is the volume of gas taken into the lungs in one breath and is measured in cubic centimetres.

is the volume of gas taken into the lungs in one minute.
The tidal volume multiplied by the number of breaths per minute equals the minute volume.

The volume change produced by each unit pressure increase expressed as litre per cm water pressure.

(intermittent positive pressure breathing) is a term sometimes used to describe the type of breathing when a patient is put on a ventilator in automatic mode.

Most machines are adjustable between 0 and 60 breaths per minute, however some will go to 160 bpm, a few will go to 250 bpm. In high-frequency oscillatory ventilation the respiratory rate maybe adjustable between 300 to 3000 bpm.

a few of the older machines have a system for producing a negative pressure in the lungs at the end of expiration, that is instead of the pressure falling to zero it goes to a minus figure perhaps -2 or sometimes more, like peep there clinical reasons why this is used.

is the speed at which the gas is pushed into the lungs, for example 700 cc pushed in 1 second is faster than 700 cc. in 2 seconds.
The inspiratory flow level is set by reducing the size of the inspiratory tube, so that the gas has to squeeze past to get to the lungs and thus will be slower.

This is the same as the inspiratory flow except that it relates to the gas flowing out of the lungs.

is the valve which when opened allows the gas to pass into the lungs and when closed stops expired gas flowing back into the machine.

is the valve which when opened allows the expired gas to escape from the lungs to the atmosphere and when closed prevents the inspired gas from going anywhere but the lungs.

a control found on some ventilators which sets the machine in automatic mode or opens all the valves to allow the patients lungs to be inflated by hand with a rubber inflation bag often attached to the machine, the bag normally a 2 litres one in capacity.

some ventilators have both oxygen and air piped in, this control mixes the gases to give any percentage of oxygen between 21% and 100%, depending on the patients requirements.

on some ventilators a flow control is provided so that the gas flow to the patient can be adjusted, this is calibrated in litres per minute.

is a valve which prevents the gas pressure in the patients lungs going above a level which could cause damage, these are set around 70 cm H2O.
It is worth remembering that the pressure in the lungs can rise to 100 Cm H2O when a person is coughing and straining.
Cm H2O.is the unit of pressure most often found on ventilator pressure gauges and is short for centimetres of water.

Apparatus working with gases supplied at pipeline pressure(60 p.s.i. or so)must be protected against possible damage should the outlet become blocked.
They may take the form of a non-return valve and pressure relief valve combined and are usually fitted downstream of the vaporisers on the back bar at the end. (on an anaesthetic machine).

is a button on a separate alarm unit or on a machine which has a built in alarm itself, which temporary switches the alarm buzzer off but not the whole unit, this means that the staff can disconnect the patient from the machine, perhaps to suck secretions out the tube, without the danger of reconnecting the patient and forgetting turn on the alarm, this is because the alarm will automatically switch its self back on again after perhaps 45 seconds or so.

triggering is a control which allows the patient to receive an assisted breath from the ventilator when they are able to breathe for themselves.
When the patient tries to take a breath, the machine senses the negative pressure created by the intake of air and gives the patient the air that they require.
The sensitivity control sets how sensitive the machine is to the patients demand for air, when the machine is set to be very sensitive, the patient only has to make a very slight effort to make the machine work, if the machine is set with less sensitivity then the patient has to make a lot of effort to trigger the machine.
As an example, on a baby ventilator the very sensitive position might take -0.25 Cm H2O to trigger the breath and the least sensitive setting might take -3.0 Cm H2O.
This control is found on some of the newer machines, when it is set it will give the patient twice the normal volume of breath every 100 breaths.
For example if the patient is being given a 700 cc. breath, then every 100 breaths a 1400 cc. breath will be given.
This is good for the lungs and makes the patient feel more comfortable, it is supposed to simulate what we do when we are breathing normally, every now and again we take an extra large breath.

It is important to distinguish between these two pressures, mean pressure is pressure in the lungs taking in to account the length of time that the pressure is applied for.
Here are some practical figures that I’ve taken from a paediatric ventilator that shows how the mean pressure alters as the inspiration time increases, the peak pressure remains constant.

This is the maximum pressure that is reached during the breathing cycle, looking at the pressure gauge will not always tell you the peak pressure, this is because the pressure gauge is often not fast enough to register it and sometimes because your eye is not fast enough to notice the quick flick of the needle.
The important pressure is the mean pressure, this should be kept as low as possible.
There are a number of different types of ventilation grouped together according to the speed of breathing. Here they are with a short explanation of each.
The breath rates are only approximate and may well overlap from one machine to another.

There are many different types of ventilator, some are very simple and some very complicated and the method of testing them will vary from machine to machine but here are the main things to check for when testing a ventilator.
1. Leaks in the gas system.

2. That each control does what the makers intended it to do.

3. That any components that are subject to wear and stress, or deterioration are inspected.

4. That the blow off valves are correctly adjusted and working.

5. That the machine cycles in a regular pattern with no unexpected pauses and that the values that you set the machine on at the start of the test remain the same at the end.

6. On machines that use electricity that the electrical parts are in good order and safe.

1. Leaks in the system. In a respirator the gas flows through a series of tubes, bellows and valves to the patient, and then at the right moment out of the expiratory valve, it is thus important that the tubes are in good condition and are tightly connected, and that the valves, once closed, do not leak, and that bellows are not worn.
First inspect the tubes for cracking, check that they are tightly connected and not kinked, check the bellows for holes and splits.
When this has been done, set the controls on the machine to normal sort of settings but with a long inspiratory pause, fit a test lung and start the machine running, the pressure will rise and then hold on the inspiratory pause before falling to zero, if while the pressure is being held on the inspiratory pause you look at the pressure gauge, the needle should remain steady during the pause, this means that the gas system is not leaking, however, if there are any leaks between the expiratory valve and the bellows you will notice that the machine cannot hold the upper pressure and the needle will drop slightly and then when the expiration valve opens will drop fully to zero.
To find the leak you have to work through the gas pathway bit by bit till you find the problem.
One way to look for a gas leak is to get some soapy water and a paint brush and brush it around the joints one by one, then watch to see if any bubbles start forming, if they do then this will show the position of the leak.
If leaks are allowed to remain there is a chance that they will get bigger, and may at an unexpected moment, get so great as to stop the machine.

2. That each control does what the makers intended. With this test you set the machine running, then go through all the controls one by one testing that they are doing what is intended, for example suppose the breathing rate control is intended to go between 0 and 60, use a watch to check that when set on 60 it will give 60 breaths in a minute.
Sometimes controls do not do quite as they are intended to but the service book, or common sense, will tell you how much error is allowable. If the control is a long way off and you have a service book it may tell you how to correct it but quite often you will have to find out for yourself how to correct it.

3. Any components that are subject to wear are inspected. As mentioned before rubber tubes and bellows are subject to perishing splitting and should be inspected.
In a mechanical type of machine such as the East Radcliffe where there are moving parts such as gear boxes, chains and cams, they should be lubricated at each service and pivots checked for wear.

4. That the blow off valves are correctly adjusted and working. This is a most important check as a faulty blow off valve will fail to safeguard the lungs should the machine go wrong. These are often set at about 70 cm H2O, this will vary from machine to machine and maybe set as high as 90 cm H2O, but if in doubt adjust them to 70 cm H2O.
The main problem with these valves is that in normal operation they are not used and this can lead to them being stuck down by moisture or dirt, so that they will not blow off at the correct pressure but at a much higher one, for this reason they should be operated now and again to keep them free.
To test them put your thumb over the end of the patient tube or where the gas comes out of the machine, turn the machine to maximum pressure and watch the pressure gauge, note the reading on the gauge when you hear the valve blow off.

Adjustment is most often done with a screwdriver in a slot on top of the valve.
These valves often work by using a spring to hold down the valve, but on some much lower pressure ones a weight is used.
On some of the more advanced machines such as the Siemens 900 series, the upper pressure alarm is set and should the pressure rise to this level it is automatically cut off and prevented from going any higher, there will also be spring operated blow off valve.

5. That the machine cycles in a regular pattern. Being a machine, once the controls are set it should continue to cycle with a regular rhythm until you stop it or the gas/electricity runs out or it breaks down.

To test, turn the machine on and set normal values, put a test lung on the patient tubes and the watch and listen to it working.
The test lung can be a 1 ltr rebreathing bag, a blood pressure cuff, an Oxford Bellows may be used by connecting the patient tube to the outlet and with the magnet in place holding the disc valve open.

At a pinch, a bicycle tyre suitably tied on and with the volume reduced could be used.
While watching it is most important to listen to the machine, with practice you soon get to know what all the squeaks, rattles and hissings mean, and an unusual noise or a noise missing can lead you to a fault, watch the moving parts for smooth movement.
After 10 minutes or so check that all the controls are still as you set them.
The main lesson is to really get to know your machine then faults will be noticed before they can do any damage.
When I write of normal values, this means that you set the controls to the average figures that you would expect it to be used on in daily use.
Modern ventilators make very little noise, but still make some.

6. On machines that involve electricity check that electrical parts are in good order and safe. Check that the machine is earthed properly, not only in the plug but also that all exposed metal parts are earthed as well.
Use a proper safety checker to test this (see details in the section on test equipment), If you do not have one use a multi-meter.
Check that the mains cable is not damaged, that the wires in the plug are done up tightly and that the correct fuse is fitted, it is not good enough to put in a 13 amp fuse for everything, check the current drawn by the machine from the information plate on the side, from a service book or by measurement, fit to the machine, if the machine has a fuse holder, the size of fuse suggested by the current drawn, then in the plug fit a fuse to suit the current carrying capacity of the mains cable itself.
Ventilators should be serviced at twice a year on a regular basis.

Some manufacturers suggest that you service their machines every 1000 hours, they will have a clock fitted that tells you how many hours the machine has been working.
So perhaps if you should say you will service them every 1000 hours or 6 months whichever is sooner.
Each machine should be given a number, and a record kept of each time the machine is serviced or if you are called out to look at a possible fault.
Each machine will have its own file and when the service is completed and the sheet filled out it is put into the file, this means that the full service history of the machine can be seen.
This will please the Anaesthetic department and show them that the machines are being looked after properly.
It also protects you should anyone suggest that you are not looking after them properly
If you make out a check sheet of all the things that it recommended that you look at during a service, it will help you to make sure that each service is done properly and that nothing has been missed.

Here is an example of a service sheet that you might design, in this case it is for the Siemens 900 B ventilator that there is in the Intensive Care Unit at the Tribhuvan University teaching hospital, Kathmandu.


1. Fit the parts as supplied in the service kit
2. Check working pressure manometer
3. Check zero of airway pressure meter
4. Check zero of expired minute volume meter
5. Check calibration of pre-amplifier as per book
6. Check for leaks
7. Check patient tubes
8. Check alarm operation
9. Check plug for tightness of wires, and fuse
10. Check high pressure hoses
11. Run and check all functions
12. Note the number of hours on the clock
13. Check the output of the mixer and note the results below.
14. Electrical safety check

30 %. . . . .
40 %. . . . .
50 %. . . . .
60 %. . . . .
70 %. . . . .
80 %. . . . .
90 %. . . . .
100 %. . . .

Comments. . . . . .

Technicians signature. . . . .

The information that you need to put on the sheet can either be obtained from the service book, or you can decide for yourself what to put in or work it between yourself and the senior anaesthetist.
Over a period you may find that you need to add things to the sheet or take things away to improve it.
If you get called to look at a faulty machine note on the record card not only the fault but also any work that you do to the machine, not only to help you in the future should the same fault occur, but to demonstrate that you have indeed corrected the fault.
When a machine has been taken back to the workshop for emergency repair or as part of a regular service, do not be rushed by the doctors into completing the repair or service faster than you are happy with.
Make sure that the service is properly done or that you have cured the fault, then test the machine fully for as long as it takes to satisfy your self that all is well, not till you are absolutely satisfied should you send the machine back.
After a service some times your instinct or experience tells you that a machine is not right, again do not be forced into sending the machine back too soon.
When you do take the machine back get the doctor who reported the fault or the senior doctor in the department to check the machine and tell you that he is happy with it.
Remember that peoples lives will depend upon the machine that you have just worked on.
Here is an example of an out of service record sheet that you should have.



MACHINE TYPE AND MAKE. . . . . . . . . . . . . . . . . . . .
HOSPITAL NUMBER. . . . . . . . . .
SERIAL NUMBER. . . . . . . . . . . .
LOCATION. . . . . . . . . . . . . . . . .
DATE. . . . . . . . . . .


COMMENTS. . . . . .


Read Full Post »