medical gases Flashcards
how is 02 manufactured?
fractional distilation - industrial O2
oxygen concentrators - field medicine, home O2
describe the process of fractional distilation..
liquidifies air
heats slowly, allowing different components with different boiling points to evapourate at diff temps
collects O2 at temp for 02 boiling point -183 degrees
99% pure
how do oxygen concentrators work
by pressure swing absorption
air is passed over a filter and then through 1 of 2 adsorbers made of zeolite (silica and aluminium)
as pressurised air passes over the zeolite, under pressure the zeolite adsorbs nitrogen and allows O2 to pass through.
this concentrates O2. there is also some argon present
the zeolite will eventually become saturated with nitrogen and to recycle this, it can be depressurised allowing nitrogen to escape.
there is 2 towers allowing one to pressurise while other depressurises allowing constant process of concentrating O2
95% O2 produced, some impurities and argon
what are the pros and cons of O2 concentrators?
pros
* allow portable method of concentrating O2. can be used as a back up on anaesthetic machine, for home O2 or field medicine
* relatively cheap
* reliable
* flows up to 10L/min
cons
* 95% and still has impurities
* needs power
* fire risk
* life span of zeolite is 10yrs
* requires routine maintainance - change of filters
define adsorption
the process of molecules adhering to a surface, without change in chemical structure / properties
define absorption
material bind materials and react with them to change their structure.
how is O2 stored?
cylinders e.g. cylinder manifolds, also as extra portable back up on wards and anaesthetic machine
vacuum insulated evaporator (VIE) - more practical i.e. stored as a liquid so takes up a lot less room
how much can a VIE hold? what are the pressures and temp
1500L of liquid O2
at -150 to -170 degrees C
at 5-10bar pressure
in a large hospital what is the main O2 supply?
VIE
cylinder mannifolds are only a backup
what is the recommended O2 storage capacity for a hospital ?
2 weeks
what are cylinder mannifolds?
group of size J cylinders and piping system that supplys N20, O2 and entanox for the hospital. O2 is mostly as a back up for the VIE.
these cylinders contain gases under pressure 137bar except N20 which is liquid + equilibrium with vapour.
they supply the pipelines at 4 bar
they are connected to a changeover control unit which measures pressure and allows a valve to switch between cyclinders when pressure is low. this will also alarm
each bank carries no less than 2 days supply
what are the pros and cons of cylinder manufolds?
Pros
* provide back up to VIE
* may be primary O2 supply in smaller hospitals
* cheap and simple
* have alarm system for low pressure
* automatic control unit to switch between them
Cons
* less storage capacity than VIE , so less practical as only O2 supply
* fire / explosion risk
how does a VIE work?
concentrated O2 is supplied to a VIE and cooled to -150 to -170 degrees and pressurised to 5-10bar.
this is below O2 critical temp and leads to liquidification of O2.
The liquid O2 is stored in a steel container with a vacuum insulation layer.
The O2 vapourisers and leaves the VIE via pipelines that go through a series of pressure regulating valves to reduce it to a pressure of 4bar. it also passes through a heat exchanger to warm the oxygen.
as the O2 vapourises, it cools the container via latent heat of vapourisation. this helps to maintain a low temp (as well as the vacuum shell)
there are 2 routes for vapourisation incase demand increases
how is the temperature of a VIE maintained?
vacuum insulated
latent heat of vapourisation
white reflective outer shell - reduces absorption of ambient heat.
what happens if no O2 is used in a VIE?
the temp rises (no latent heat) and more gas will vapourise and pressure will rise
if exceeds 15bar it will be released via pressure relief valve.
how is the content of VIE measured?
the VIE is placed on a scale so the mass can be weighed.
using moles = mass / mr
can work out the moles present and hence the volume of gas this could supply
new ones use a pressure transducer which compares pressure at bottom and top to determine how much liquid left.
pros and cons of VIE?
pros - large amount of O2 can be stored efficiently - most practical/ economical. no power
cons - explosion risk, expensive set up, if not used, O2 is wasted, needs thorough testing and checking for pressures/ contamination
how much O2 per L of liquid O2 in VIE?
840L of gaseous O2 supplied per 1L of liquid
what are the safety features of a VIE?
pressure regulators - reduce pressure
heat exchanger - increases temp
pressure relief valve - 15bar - O2 can escape
stored away from hospital incase of explosion
back up cylinder manifolds.
what is the boiling point and critical temp of O2?
-185 degrees = BP
-118 = critical temp
outline the features of piped O2 supplies in hospitals
supplied from central supplies - either VIE or cylinder manifolds
gases flow through copper alloy pipelines - 4bar
delivered to outlet valves located at different points in the hospital - some connected via shrader valves and some via wall rotameters.
what other piped lines are there?
piped lines for O2, N20, air, suction, scavenging
O2 and N20 and medical air = 4 bar
air for driving tools - 7 bar
suctioning and scavenging - negative pressures
why is copper used in piped lines?
bacteriostatic
prevents degradation of gases
how should cylinders be stored?
size F, G and J - upright to prevent damage of valve
C, D ,E either horizontal or vertical
entanox - horizontal
empty cylinders stored away from full ones.
in fire proof, dry, well ventilated room, normal temp
when are cylinders used?
piped O2 not available - transfer
failure of piped supply
for gases not frequently used - heliox
what sized cylinders do you know?
sizes C to J used in medicine
C = 170L
D = 340L - sometimes transport
E = 680L - anaesthetic machine
F = 1360L - ambulance
G= 3400L - on wards
J =6800L - cylinder manifolds
there are also CD - these are made of aluminium so can withstand higher pressures so store more than conventional C
CD = 460 L , 230bar - most common in transport
what are the medical gas cylinders made of?
traditional/ old ones = molybdenum steel
newer light weight design = aluminium alloys - can withstand higher pressures.
what is this ..
CD oxygen cylinder
modern alluminium alloy cylinder
commonly used in transport
what are the advantages of aluminium vs molybdium steel cylinders?
new CD alluminium ones …
* stronger - higher pressures, store more O2
* MRI safe
* lighter in weight
* have built in pressure gauge and click stop flow meter
* more practical - have a handle
* integral valve
* have a schrader valve connection
the molibdenum steel need a special holder to stand them up as they are heavy with a round bottom. (new ones have a flat bottom)
what is this..
old type molibdenum steel cylinder
looks like a size D
how is a cylidner connected to the anaesthetic machine/ wall?
cylinder yokes - solid block of stainless steal supporting the cylinder
bodok seal - washer to provide airtight seal between valve and yoke
pin index system
how are cylinders marked by manufacuteres?
engraved
* unique serial number
* chemical formula of gas
* tare weight
* test pressure
* dates of tests performed
label on cylinder
* name of gas and chemical structure / proportion if its a mixture
* serial number
* hazards and safety instructions
* size of cylinder, including volume and max pressure
* directions for use including storage and handling
plastic disc around the neck - date of endoscopic exam
what pressures are cylinders stored at?
most 137bar
CD = 230 bar
N20 - 52 bar at 20 degrees (44 bar at 16 degrees
CO2 - 57 bar at 20 degrees
how can O2 contents in a cylidner be measured?
FOr gas cylinders
* boyles law
* gauge pressure
* PV = PV
* we know the volume when full e.g. size E = 680 L if pressure is now 100. can use ratios 100/137 x 680L
for liquid
* weigh cylinder and work out moles
* 1 mole = 22.4 L of gas at 0 degrees and atm pressure.
outline the safety features present in medical gas supplies…
Not mixing up gases:
* colour coding
* schrader
* pin index
* NIST - prevents misconnection
Detecting low/ high pressures:
* pressure releif valves
* gauge pressures
* low pressure alarms detect failure in gas supply - at anaesthetic machine and cylinder manifolds
* back up cylinders on anaesthetic machine
other:
* copper piping
* flexible reinforced hosing - i.e. from wall supply to anaesthetic machine - cant be compressed
* splitting ratios
what are the colours of medical gas cylidners?
shoulders have specific colour
O2 - white (black body)
N20 - french blue (blue body)
entanox - french blue and white checked shoulder
air - black body, white and black shoulders
suction - yellow
scavenging - colourless/ brown
CO2 - grey
helium - brown
heliox - brown and white, brown body
this was old colour
in 2025 will be replaced. all bodies will be white and only shoulders will be different
describe the schrader system…
one way outlet valve
consists of a socket with tapering that fits specific shape of a probe such that the correct pipeline is attached.
these pipelines are secured via non-interchangable screw threads (NIST) on the anaesthetic machine
what is the pin index system?
a system for attaching the correct gas cylinder to the anaesthetic machine - for safery
Air - 1 5
O2 - 2 5
N20 3 5
CO2 1 6
entanox 7
how are cylinders tested?
internal endoscopic exam - every 5 years - looks for cracks and defects
tested against 220 bar pressure
1 in 100 cylinders are tested for effects of impact, pressure, tensile strength
how are the hoses maintained and tested?
tug test - correct connection at schrader
replace every 2-5 years
hoses are reinforced and non-compressible
what is the filling ratio?
the weight of cylidner when full of N20 / weight when filled completely with water
in UK this is 0.75
gives room for vapour to exist and prevents extremely high pressures and explosion if ambient temp is to rise
in hotter climates 0.67
can you describe a cylinder valve block?
larger cylinder dont have a pin index and instead have a bull nose valve
how is nitrous manufactured?
ammonium nitrate is heated to 250 degrees
NH4NO3 –> N20 + 2H20
thermal decomposition
impurities - ammonium, nitric acid, N2, NO2
these are removed by chelating and washing with acidic/ alkaline washes
important to remove these due to pulmonary oedema and fibrosis.
how is nitrous oxide stored…
french blue cylinders
as a liquid in equilibrium with vapour phase
filled to 75%
why may a nitrous oxide cylinder be cold ?
latent heat of vapourisation
how much L of N20 do size E cylinders provide?
1800L
draw a graph to show the emptying of O2 and N20 cylinders..
O2 - linear, boyles
N20 initially constant P as liquid vapourises, then boyles when all liquid has gone.
- This is actually a simplification; in reality the initial pressure is not perfectly constant because as the n2o vaporises from the liquid phase it cools slightly (latent heat of vaporisation), and therefore gauge pressure does fall slightly.
what is entanox?
50:50 mix of O2 and N20
how is entanox sotred?
as a compressed gas into cylinders with white and blue checked shouldered at 137 bar
should be kept above 5 degrees and horizontally to reduce risk of liquidifcation.
how is entanox produced?
liquid nitrous is carried into O2 gas to create a 50:50 mixture by the poynting effect
the mix has its own properties - i.e. critical temp -7 degrees = pseudocritical temp
therefore above this temp, the mix is gas, below this there is risk of liquidification.
what is meant by the poynting effect..
when 2 gases are mixed, the critical temp and pressure will be between that of the 2 individual gases alone. This is now called the pseudocritical temperature.
Hence liquid nitrous oxide becomes gaseous when mixed with oxygen gas.
- Critical temp of O2 alone = -118
- Critical temp N20 alone = 36.5 degrees
- When mixed = pseudocritical temperature = -7 degree
what are the risks of liquidification of entanox? how is this risk minimised?
if the mixture separates it will consist of O2 gas above liquid N20
initially O2 rich mixture delivered
then hypoxic mixture
horizontal storage, above 5 degrees C
a dip tube that reaches bottom of cylinder to siphon nitrous oxide first
why is risk of liquidification not important in pipelines?
the pressure is only 4 bar so even below pseudocritical temp not enough pressure to liquidify
how is CO2 manufactured?
heating Ca/ Mg carbonate (limestone) to 850 degrees
CaCO3 –> CaO + CO2
what is the boiling point and critical temp of CO2?
-78degrees boiling point
-30degrees critical temp
how is CO2 stored?
grey cylinders
as liquid with in equilibtium with vapour phase.
57 bar at 20 degrees
what are the uses of CO2?
fire extinguishes
inflation of the abdomen
cryotherapy
what are the effects of CO2 on the body?
CVS - vasodilation, increase HR, arrhythmias
resp - hyperventilation
neuro - narcosis above 10kpa
tell me about the medical uses of helium
- Helium (He) is an inert gas with a molecular weight of 4
- Available as heliox = 79% helium, 21% O2
- Lower density than nitrogen
- Therefore, when turbulent flow is present, a helium-oxygen mixture will increase flow compared to a nitrogen-oxygen mixture.
- This has significant clinical application and is used to treat upper airway obstruction where flow is mainly turbulent.
- Helium is also used in the measurement of lung volumes because it has very low solubility and so does not transfer.
