Fail-Safe Systems

KEYWORDS

medical gas cylinders

alarm and safety features: anesthesia machine

proportioning devices

pressure fail-safe

A “fail-safe” is a system or device that limits damage or harm. For instance, on locomotives, there is a “dead man’s switch” that the engineer must push every couple of minutes or the train will stop. This is a “fail-safe” in that if the engineer is incapacitated, unconscious, or dead, the train will stop automatically. Another fail-safe is the activation bar on walk-behind lawn mowers. The bar, on the handle of the mower, must be pushed in for the mower blade to spin. If you let go of the bar, the blade stops. Failsafe was also a great movie from the 1960s with Henry Fonda. You should watch it. George Clooney remade it in 2000, and it is pretty good, too.

You will find that virtually all fail-safes, whether in anesthesia or in other disciplines or circumstances, are not 100% foolproof. They can be intentionally or unintentionally circumvented, or they may malfunction in such a way that the event the fail-safe was supposed to prevent occurs anyway.

Let’s use the lawn mower as an example. We said the mower has an activation bar that must be pushed in along the mower handle for the cutting blade to spin. You’re mowing the lawn, but you are so tired of how the blade stops every time your hand shifts or you lose your grip. So you decide to tie the activation bar down to the handle, so when you let go to wipe your brow or adjust your iPod, the blade keeps going. You don’t have to grip the handle as tightly now because your hands were getting tired. You push the mower down a small slope and lose your footing; your foot slips in front of you as you fall backward. Where is your foot going to end up? It’s going to end up under the mower, where it will meet a sharp, spinning, hardened steel blade that unfortunately is going to lop off a good part of your foot. If you hadn’t tied the blade activation bar down, when you lost your footing and lost your grip on the mower handle, the blade would have stopped quickly enough that you would still have all your toes.

When we discuss fail-safes on an anesthesia machine, it is usually in reference to preventing the delivery of a hypoxic mixture of gas to the patient. These devices usually have something to do with preventing giving the patient too much nitrous oxide versus too little oxygen. They do a good job, but we will discuss later that hypoxic mixtures can still be given to a patient despite these built-in fail-safes on anesthesia machines. Fail-safes are found in various places on the anesthesia machine, and we will go over the location and function of each one. Some of them we discuss in other chapters, but will still be discussed here.

CYLINDER COLOR

If your only fail-safe system was this, you would be in trouble. This is a very weak fail-safe and is hardly worth the distinction of being called a fail-safe. It is true, of course, that in the system of coloring in the United States, oxygen is green, nitrous oxide is blue, air is yellow, and so forth. However, remember that this color system varies in different countries. What is more startling is that color coding of medical gas cylinders is not a Food and Drug Administration regulation. Color coding is more or less a suggestion and is a weak standard for the medical gas industry. To be legal, cylinders have to be labeled correctly but not color coded correctly. Multiple reports of medical gas in different color cylinders have been published, and discovery of an air cylinder that was gray, not yellow, has happened to the authors. In our case, the cylinder was properly labeled and contained the proper gas when analyzed but was the incorrect color.

PIN INDEX SAFETY SYSTEM

The Pin Index Safety System is the system that keeps us from putting the incorrect gas cylinder on the incorrect yoke on the back of the anesthesia machine. Each stem of a specific tank has two holes that are in positions that mate with two pins on the appropriate yoke. The holes from an oxygen cylinder do not match up to the pins on the nitrous oxide yoke and vice versa. This is a very good system for ensuring proper placement of gas cylinders on an anesthesia machine. However, misplacement of cylinders is still possible even with the pin index system.

Apart from intentional vandalism, where pins on the yoke were knocked off with a chisel, is it possible to defeat the pin index safety system? The answer is “yes.” When replacing a gas cylinder on the back of the machine, only one washer should be placed between the cylinder stem and the yoke nipple. If old washers are not removed, they can act as spacers, and defeat the pin index safety system by not allowing the pins and holes to mate.

DIAMETER INDEX SAFETY SYSTEM

Most people who know about the Diameter Index Safety System probably think it refers to the ends of the gas supply hoses that attach to the wall outlets. It is true that these connectors are designed to fit specifically into the correct outlet, but this is not the Diameter Index Safety System.

Take a look on the back of your anesthesia machine where the gas hoses enter the machine itself. This is where the diameter index safety system is. Each inlet for each different gas is of a different diameter, so the oxygen hose will not fit on the nitrous oxide inlet and vice versa. This system is more foolproof than the Pin Index Safety System because we rarely if ever disconnect our gas hoses from the machine. Also, to alter the connectors would require deliberate effort.

The main way that this fail-safe system can be defeated is if the gas supply lines are crossed. Let’s say we have the correct connector from the pipeline source to the gas hose, and we have the correct hose attached to the correct inlet, but if the gas coming out of the pipeline is not oxygen when we think it is, we will be delivering nitrous oxide or air to a patient. Crossed pipelines have happened multiple times in the past, and patients have died because they received a hypoxic gas mixture. After construction work or repair to a medical gas pipeline, each outlet needs to be checked to see if it is delivering the correct gas.

OXYGEN–NITROUS OXIDE FAIL-SAFE VALVE

As you can see, none of the just discussed systems (color-coded cylinders, Pin Index Safety System, and Diameter Index Safety System) are foolproof. Someone could either intentionally or unintentionally defeat them. Also, all three of them are outside the anesthesia machine. The oxygen–nitrous oxide fail-safe valve is the first fail-safe we will discuss in this chapter that is inside the machine. Unlike the previously mentioned systems, you will never see this device unless you open up a machine and go looking for it.

The purpose of this device is to cut off or proportionately decrease nitrous oxide in case oxygen pressure is lost. There are different versions of this device as well. We will discuss the two main types. Keep in mind that one type will have an “all-or-nothing” effect, meaning that if the device senses a decrease in oxygen pressure, nitrous oxide delivery will be blocked. The other type proportionately decreases nitrous oxide pressure and volume as oxygen pressure and volume are lost (Figure 11-1).

Figure 11-1 Schematic of a generic oxygen–nitrous oxide fail-safe valve. Depending on the specific type, nitrous oxide delivery will stop completely or proportionately decrease with a decrease in oxygen pressure. (Reproduced with permission from Morgan GE, Mikhail MS, Murray MJ. Clinical Anesthesiology. 2nd ed. New York, NY: McGraw-Hill; 2002. Figure 4-3.)