Equipment, Instrumentation and Technology is 20 percent of the NCE, the same as basic sciences, and most students give it a fraction of the time. The upside is that the content is finite and the questions are concrete. Gas supply is the best place to start, because almost every machine question traces back to it.
Pipeline versus cylinder
The machine has two sources for each gas.
- Pipeline: the hospital supply, delivered at about 50 to 55 psig. This is the primary source.
- Cylinders (E cylinders on the back of the machine): the backup. Their high pressure is reduced by a regulator to about 45 psig.
Because the pipeline pressure is higher than the regulated cylinder pressure, the machine draws from the pipeline whenever it is connected and working, even if the cylinder valve is open. That design is also the trap: if the cylinder is left open and the pipeline fails, the cylinder empties silently in the background.
Keep cylinder valves closed when the pipeline is in use. Open one to check it, then close it.
E cylinder facts
| Gas | Color (US) | Full pressure | Full volume | State in the cylinder |
|---|---|---|---|---|
| Oxygen | Green | about 2,000 psig (1,900 to 2,200) | about 660 L | Gas |
| Nitrous oxide | Blue | about 745 psig | about 1,590 L | Liquid and gas |
| Air | Yellow | about 1,900 psig | about 625 L | Gas |
The international color for oxygen is white. The NCE uses US standards, but it is worth knowing why a picture might show a white cylinder.
Pressure tells you oxygen, weight tells you nitrous
This is the most tested idea in the section.
Oxygen is a compressed gas, so pressure falls in direct proportion to the contents. Half the pressure means half the oxygen.
Nitrous oxide is stored as a liquid with vapor above it. As long as there is liquid left, the pressure stays at about 745 psig. The gauge only starts to fall once the liquid is gone, when roughly a quarter of the contents (about 400 L) remain. The only way to know how much nitrous is in a cylinder is to weigh it and compare with the tare weight stamped on the cylinder.
The time remaining calculation
Expect this as a calculation item. For an oxygen E cylinder:
Liters remaining = (gauge pressure / 2,000) x 660
Minutes remaining = liters remaining / flow in L/min
Example: the gauge reads 1,000 psig and you are running 6 L/min.
- 1,000 / 2,000 x 660 = 330 L
- 330 / 6 = 55 minutes
Some texts use a factor of 0.3 (660 / 2,000 = 0.33, or 0.3 for a conservative estimate). Read the stem for which numbers it gives you.
The Pin Index Safety System
Each E cylinder yoke has pins that only fit holes in the matching cylinder valve, so you cannot hang the wrong gas.
- Oxygen: 2 and 5
- Nitrous oxide: 3 and 5
- Air: 1 and 5
Pipelines use the Diameter Index Safety System (DISS) at the machine inlet, and quick connects at the wall. A missing or doubled washer can defeat the pin index, which is why it is a known failure point.
The fail-safe and the proportioning system
Two devices are designed to prevent a hypoxic mixture, and the exam tests what each one cannot do.
The fail-safe valve (oxygen failure protection device) cuts or reduces the flow of the other gases when oxygen supply pressure falls. It responds to pressure, not flow and not concentration. If someone hooks nitrous oxide to the oxygen pipeline (a crossed pipeline), the fail-safe sees normal pressure and does nothing.
The proportioning system (for example a mechanical link between the nitrous and oxygen flow controls) keeps the fresh gas at at least about 25 percent oxygen when only oxygen and nitrous are used. It does not protect against a third gas or a crossed pipeline.
The only monitor that catches a hypoxic mixture from a crossed pipeline is the oxygen analyzer in the breathing circuit. That is why it must be calibrated and alarming.
Flowmeter order
The oxygen flowmeter sits last in the bank, downstream, closest to the common manifold. If a flowmeter tube upstream cracks, oxygen is the gas least likely to leak out. Placing oxygen anywhere else allows a hypoxic mixture with a leak.
Crossed pipeline, what you actually do
The oxygen analyzer alarms and the patient desaturates on what should be 100 percent oxygen.
- Open the oxygen cylinder.
- Disconnect the oxygen pipeline from the machine. Just opening the cylinder is not enough, because the higher pipeline pressure will keep feeding the contaminated gas.
- Ventilate, and consider an Ambu bag on room air if there is any doubt about the machine.
Quick hits for test day
- Pipeline 50 to 55 psig, cylinder regulated to 45 psig.
- O2 E cylinder: about 2,000 psig and 660 L. Pressure tracks contents.
- N2O: 745 psig until about 400 L remain. Weigh it.
- Pin index: O2 2-5, N2O 3-5, air 1-5.
- Fail-safe responds to pressure only.
- Oxygen flowmeter is last in line.
- Crossed pipeline: open the cylinder and disconnect the pipeline.
The equipment lessons on CRNA Dojo are built for exactly this domain, and the question bank has hundreds of machine items, including hotspots on the machine itself.
Sources
- Dorsch and Dorsch, Understanding Anesthesia Equipment
- Ehrenwerth and Eisenkraft, Anesthesia Equipment: Principles and Applications
- Barash, Clinical Anesthesia
