Safety Features in the Anesthetic Machine – Anesthetic Systems

by Brian Warriner, MD, FRCPC

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    00:00 being released into the atmosphere at a lower level. Anesthesiology has become a very safe area of medical care. It's been noted that anesthesiology is one of the few medical specialties where there's been a dramatic improvement in patient safety over the years. The old days where people used to die from anesthetics are gone. Happily! Nowadays, the actual incidents of people dying secondary to anesthesia is extraordinarily rare.

    00:26 Every anesthetic machine, like a modern aircraft, has many, many fail safe devices attached to it. Included in these are the disconnect alarm, which alarms when recognizes a decrease in system pressure, indicating a possibility of a system disconnect, in other words, the high pressure gases that are entering the machine have somehow been cut off. Particularly if oxygen is cut off, this would mean that the major supply of oxygen to the machine was lost, and it's important that the machine lets you know when that happens, so there's a disconnect alarm.

    01:02 Vaporizer failures are almost unheard of nowadays because of the frequent checks that are done, and recalibration that's done. And pressure limiters in the circuit prevent barotrauma or increased pressure damage to the lungs. High pressure hoses, such as the supply hoses from the wall to the machine, are pin-indexed according to the gas they carry, so they can only fit into certain connectors on the anesthetic machine. So that you don't have the risk of connecting a hose or a nitrous oxide hose, to the oxygen system in the machine. So, in addition to the safety features I have already mentioned, all gas supplies that are connected to the anesthetic machine have agent-specific connectors. And vaporizers, which can run out of anesthetic liquid during an anesthetic and need to be refilled, can only be filled by the specific agent that they're designed to manage.

    02:14 The standing bellows, in modern anesthetic machines, minimizes the danger of incorrect gas flows. At one time, we had hanging bellows that could actually function in the absence of oxygen, and that was a major risk. Now they can't.

    02:32 Vaporizers that are positioned improperly, tilted, won't work. And pressure regulators in the machines, are specific for each gas, reduce the gas pressure to an intermediate pressure within the machine, and then to a low pressure before gas gets to the patient.

    02:50 Pipeline and gas tank pressure gauges are prominently displayed on the machine, within the view of the anesthesiologist. And in hospitals, where there are big banks of gas cylinders or liquid oxygen containers, the equipment is checked daily. If there is an oxygen system failure, the machine continues to deliver air, which at least has 1% oxygen in it, to prevent hypoxemia. In other words, it's not really possible to inadvertently deliver 100% nitrous oxide for instance, to a patient. And the oxygen flow-meters are always positioned downstream from the nitrous oxide, or air flow meters, so that a leak in a low pressure system doesn't dilute the oxygen and result in a hypoxic breathing mixture. If there's a leak somewhere in the low pressure system, the last thing that's affected is the oxygen, and the oxygen should be the dominant gas that's given to the patient, even in that situation of a leak. The on/off knob on the oxygen flow meter is fluted and bigger than on the knobs on the nitrous oxide or air flow meter, so it's easy to feel, even if you're not looking at it. Although, believe me, I'm not suggesting you don't look at it when you adjust it. The oxygen flow meter is made to disable the nitrous oxide flow meter if oxygen flows below, if oxygen level is below 21% or there is a failure in the oxygen system. So there's a continuous monitoring of inspired oxygen.

    04:32 And if it ever drops below 21% room air, the system automatically turns off any other gas that might be running nitrous oxide or air. And a stiff button must be pushed on the vaporizer, before vapour can be turned on. So there's either a catch that you compress with your fingers or there's a button on the vaporizer dial that you have to push. And it's a bit of a push before you can turn it and change the vapour dose that you're providing. There's an interlock system which prevents more than one vaporizer from working at one time. So, if you turn on the Desflurane, you can't turn on the Sevoflurane, and vice versa. And scavenging systems are set by international regulations, so we all have the same standards worldwide, when it comes to scavenging systems.

    05:26 And an oxygen analyzer, low pressure analyzer, high pressure analyzer and spirometer, which is a device for measuring breathing patterns, usually within the ventilator, are standard equipment on all anesthetic machines. So failure of anesthetic machines has become a very rare event. However, I'm aware of two events in my own hospital system in the last year that could have been disastrous. In one case, it was my own machine, and the entire breathing circuit and soda-lime canister fell off the machine, it landed on my foot, which was not a pleasant experience, but more importantly, left me with no way to ventilate the patient or deliver vapours to the patient. I was able to quickly change over to a bag and mask system, and start a Propofol infusion, so no damage to the patient occurred, but it was certainly a scary event. And there's been some changes to that machine to try to prevent it again in the future. On another machine, a brand new machine, the amount of Desflurane set by the anesthesiologist was initially supplied properly by the machine.

    06:38 And then for some reason, suddenly it stopped providing the dose that had been set. There was a dramatic drop in the amount of Desflurane, and only the fact that the anesthesiologist was paying attention saved the patient from waking up in the middle of surgery. Those machines are now being evaluated to determine if there's an underlying system problem that needs to be fixed.

    About the Lecture

    The lecture Safety Features in the Anesthetic Machine – Anesthetic Systems by Brian Warriner, MD, FRCPC is from the course Anesthesiology: Introduction.

    Included Quiz Questions

    1. They are activated by many system failures that may occur during anesthesia.
    2. They are limited to just a few machines to avoid confusion when one alarms.
    3. They are turned off when the anesthetic machine is activated.
    4. They activate the operating room cardiac arrest alarm.
    5. They allow the surgeon to know when the patient is not paralyzed.
    1. An alarm is activated upon a decrease in system pressure.
    2. Vaporizer failures are relatively common and, as such, are crucial to detect.
    3. Barotrauma is effectively prevented nowadays due to the presence of accurate flow meters.
    4. Gas supply lines have a universal connection.
    5. The presence of hanging bellows in modern anesthetic machines minimizes the risk of incorrect gas flow.
    1. They have made it highly unlikely that a vaporizer designed for a specific agent would be refilled by mistake with a different one.
    2. The new safety features allow the functioning of even a tilted vaporizer.
    3. Usually, pressure regulations are nonspecific and apply to all gases, which makes the anesthetic machine easier to use.
    4. Pipeline pressures are displayed on a monitor that the anesthesiologist has to check separately, and only after proper indications.
    5. Oxygen flow meters are always placed before the airflow meters.
    1. If there is a failure of the oxygen system, the machine automatically continues to deliver air.
    2. The new safety features allow the dilution of oxygen in the breathing mixture following a lack in the low-pressure system.
    3. The on/off knobs on the new anesthetic machines cannot be easily distinguished.
    4. When oxygen flow drops below 21%, the nitrous oxide flow meter is automatically activated.
    5. An interlock system enables more than one vaporizer to work at the same time.
    1. Oxygen flow meters are always positioned after the nitrous oxide flow meters.
    2. Each hospital has its own regulations regarding the scavenging system standards.
    3. To change the dose of the vapor, the anesthesiologist has to first change some settings on a touch screen.
    4. The risk of connecting a nitrous oxide hose to the oxygen system is not effectively prevented in the current anesthetic machines.
    5. The presence of gas cylinders and liquid oxygen containers in a hospital is beneficial due to their requirement of less frequent equipment checks.

    Author of lecture Safety Features in the Anesthetic Machine – Anesthetic Systems

     Brian Warriner, MD, FRCPC

    Brian Warriner, MD, FRCPC

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