With reference to gas supplies in the ICU:
a) Briefly describe the systems for the storage and delivery of oxygen supplied from a wall outlet.
b) List the safety features that are in place to prevent incorrect connection of hoses and regulators to gas outlets (e.g. 02 hose connected to air outlet).
a) Systems for storage and delivery
Large supply of oxygen in a remote storage area and piped to the wall outlets Two main types of supply:
Liquid oxygen tank or vacuum insulated evaporator (VIE)
Oxygen may also be supplied by an oxygen concentrator but this is a relatively new technology
Multiple cylinders are arranged in banks with each bank containing enough cylinders to last for 2 days normal use for that hospital.
Each cylinder is connected to a pipeline which passes to a central control box with High-pressure gauges indicating the contents of the cylinder banks:
High-pressure reducing valves lowering the cylinder pressure from 137 bar to 10 bar
Changeover valve that switches automatically from the in-use bank to the reserve bank of cylinders when the pressure falls to a certain value.
At the outlet of the changeover valve is a second-stage pressure-reducing valve to reduce the pipeline pressure from 10 bar to 4.1 bar, the pressure at the wall outlet.
Main source of supply in large hospitals
Vacuum insulation allows storage of oxygen at or below its critical temperature (-118oC) in liquid form Normally temperature of around -160oC with pressure at about 7 bar (vapour pressure of oxygen at this temperature)
Oxygen is taken form top of storage vessel and passed through superheater coil then pressure regulator to keep pipeline pressure at 4.1 bar
The supply usually has a capacity to last for at least 6 days Reserve manifold of cylinders as back-up
b) Safety features for hoses
Colour coding: (O2 white, air black, suction yellow) Sleeve indexing:
The internal threads are the same for each outlet, but the sleeves are differently configured to prevent placing the wrong hose on the wrong outlet
When attaching regulators to cylinders
Schraeder quick release valves (most commonly where gas line attaches to the high pressure inlet of ventilators)
Additional Examiners‟ Comments:
Most candidates had little or no knowledge about oxygen storage and delivery. Some could describe a VIE, but then gave bizarre pressure levels or storage temperatures, which were possibly just guesses. The safety systems aspect of the question highlighted poor learning – candidates frequently realised that a sleeve index system was used, but couldn't name it.
I sat this paper. Those are my bizarre pressure levels or storage temperatures they are talking about.
At a basic level, the system consists of:
- A central source
- A vapouriser and pressure regulator
- Pressure regulators (decreasing presure along the system)
- Manual and service shut-off valves (to isolate whole sections of the system)
- Pressure release valves upstream of any regulators and shut-off valves, to prevent the whole thing from exploding from pressure excess
- Pressure monitors and alarms to detect the fact that you're out of oxygen
Here's an unhelpfully complicated diagram:
For those of us who wish to quote non-bizarre pressure levels and storage temperatures, here they are:
- The VIE is between -150 and -119° C, and at 1000 kPa
- The pressure in the vacuum chamber is 0.3 kPa
- The blow-off valve goes off at 1500 kPa
- The pressure regulator on the supply side downregulates the delivered gas to 415 kPa
The "sleeve index system" is "a range of male and female components intended to maintain gas-specificity by the allocation of a set of different diameters to the mating connectors for each particular gas" (thank you, ISO/DIS 9170-1(en))
The examiners complained that the candidates knew what this system was, but could not name it. Some might argue that a rose by any other name is still capable of acting as a non-interchangeable gas coupling, but clearly the college are very attached to correct nomenclature.
The information about this can be found in Australian Standard AS 2902-2005, "Medical gas systems—Low pressure flexible hose assemblies". Section 6 ("Connectors and couplers") speaks about the various hoses and outlets in great detail. Unfortunately, the AS is not free. But: we can borrow this excellent resource from cinder.hk, meant for ANZCA Primary candiates. In short, "the screw threads are the same size for each gas. The fittings are made non interchangeable by the presense of the sleeve". After the sleeve is screwed on to the gas outlet, that outlet becomes gas-specific (and thereafter no "wrong" hose can be coupled to it).
Michael Richard Cohen's Medication Errors (2006), in the chapter on "Fatal Gas Line Mix-up", makes the following safety recommendations:
- Non-interchangeable connectors (i.e. the air regulator cannot be connected to the oxygen outlet: it is physically impossible). This is called the Diameter Index Safety System (DISS). In Australia we use the sleeve index system (SIS) described above, which does essentially the same thing.
- Standardised flow meters, regulators and connectors (i.e. they all look the same everywhere in the hospital)
- Easily identifiable gas connectors (obvious colour, as well as outlet shape and texture)
- Observable connections (i.e. not hidden under a shelf or table)
- Regular testing and preventative maintenance of gas supply system (i.e. even if a gas outlet is not in routine us, the engineers need to regularly test it to make sure it still supplies the specified gas)
This excellent lecture from the University of Sydney has a vast amount of obscure information (did you know oxygen tanks are aged at 175°C for 8 hours, and that their walls are only 3mm thick?)
Medical Gas Standard AS 2896-2011 is available online, but you have to pay over $200 to purchase it.
Dorsch and Dorsch have a chapter dedicated to medical gas supply and suction equipment, which can be accessed by Google Books.
Das, Sabyasachi, Subhrajyoti Chattopadhyay, and Payel Bose. "The anaesthesia gas supply system." Indian journal of anaesthesia 57.5 (2013): 489.
Westwood, Mei-Mei, and William Rieley. "Medical gases, their storage and delivery." Anaesthesia & Intensive Care Medicine 13.11 (2012): 533-538.
STANDARD, BRITISH, and BSEN ISO. "Medical gas pipeline systems—." (1998).
UK department of health: Department of Health. Health technical memorandum 02-01. Medical Gas Pipeline Systems, (2006) Part A Design, Installation, Validation and Verification; pp. 41–51