Wall suction outlet regulator

The wall suction outlet regulator is the valve-like mechanical interface between the suction catheter and the howling interstellar abyss of the hospital's centralised vacuum system. Question 22.2 from the first paper of 2010 asks which critical features of this device prevent you from stupidly connecting it to the patient's trachea. 

According to the college, those critical features are:

1)  Colour coding (oxygen is white, suction is yellow)
2)  A unique sleeve index arrangement for each wall gas

Other features can be thought of. In general, this device and its family of vacuum equipment are well-covered in the third edition of "The Principles of Vacuum And Clinical Application in the Hospital Environment", from the Ohio Medical Corporation. The time-poor exam candidate will have little time to spend on reading this 32 page document, and so what follows is a brief summary of medical suction equipment.

• In most hospitals, in the basement there is a large industrial-scale vacuum pump.
• The pump empties a large central reservoir, which could be a few hundred litres in capacity.
• All the wall suction outlets connect to this reservoir.
• The pressure in the reservoir is kept around half to one-third of the atmospheric pressure; whenever the wall suction is accessed the tank fills with air, and if the reservoir pressure increases to a certain level, the attached vacuum pump will empty it again.
• Given that the system needs only to suck fluids, it is unnecessary to generate a "hard" vacuum. Half an atmosphere is more than enough. To equate that pressure to commonly used ventilator values, it is equivalent to about 500 cmH₂O.  In Australia, wall suction is typically even higher - around 600mmHg, or 80kPa.
• This pressure is far greater than that required by any sane person for routine clinical applications. For this reason, a pressure regulator is required.
• The vacuum pressure regulator limits the maximum amount of suction pressure which the patient might be exposed to.
• These may be simple in construction (eg. a needle valve or stopcock controller) or more complex, incorporating a mechanical bellows. The latter may be able to automatically adjust the negative pressure to maintain a desired level of vacuum in the face of changing vacuum supply or suction demand.
• An excellent review of suctioning pressure safety practices by David Arbon (2011) quotes an older verison of the NSW ICCMU guidelines (2014)  which recommend that the maximum occluded suction pressure should be limited to - 80 to 150mmHg (20kPa). 
• The recommendation to use a lower level of negative pressure for tracheal suctioning is hardly new. It has been an accepted fact since at least 1978, when some dog tracheas were sacrificed to the demonstration of negative pressure damage. The author was forced to conclude that "the extent of tracheobronchial trauma was directly related to the magnitude of negative pressure applied", and that "aspiration efficiency proved to be the same regardless of the negative pressure used".