Insuring Safer Anesthesia Care for MH-Susceptibles (or How Everything Old Is New Again)

By MHAUS President Henry Rosenberg MD, CPE

Safer Anesthesia Care: The Anesthesia Machine

When an MH susceptible patient requires a general anesthetic one concern is whether he/she will be inadvertently exposed to potent gas anesthetics such as sevoflurane or isoflurane that may trigger an MH episode. This may happen because anesthetic gases may remain in the circuit of the anesthesia machine for a period of time after the anesthetic administration is ended. In order to prevent this from happening the anesthesia provider prepares the anesthesia machine by flowing Oxygen or air through the tubing of the machine for a period of time to insure that the concentration of anesthetic gases drops to levels that are thought not to trigger an MH episode. In addition, many recommended that the vaporizers, the devices that convert the liquid anesthetic to a gas were disconnected from the machine. For machines that were made up till roughly the mid 2000s, that preparation took about 20 minutes. So, unless the surgery was the first case of the day, time had to be taken to prepare the anesthesia machine. However in recent years there have been significant changes to the design of the anesthesia machine. The machine is basically a sophisticated set of tubes and valves and canisters that convert the liquid anesthetic to a gas and then mix it with a delivery gas such as Oxygen, nitrous oxide or air in precise proportions. The gas is delivered through a built in ventilator to move the gases in and out of the patient's lungs. The more modern anesthesia machines incorporate computer controlled valves and allow the anesthesia provider to ventilate the patient's lungs in different ways in order to maximize the movement of Oxygen and removal of carbon dioxide. All the machines have a calcium based chemical compound in a canister to remove exhaled carbon dioxide from the gases as well, but they do not retain the anesthetic gas.

One of the consequences of the increased sophistication was the addition of internal tubing and valves. Therefore rather than take 20 minutes to reduce the anesthetic concentration coming out of the machine after it was used to deliver an anesthetic, it now takes up to 60 minutes to achieve a very low concentration of anesthetic gas (Reference 1). This additional 40 minutes is a real inconvenience in a busy operating room. Sometime, the schedule does not permit the MH susceptible to be the first case of the day, or sometime the MH patient comes to the OR as an emergency so there is only a few minutes to prepare the anesthesia machine. Some facilities therefore have kept an anesthesia machine in reserve for the MH susceptible patient, but that is not a particularly good use of resources.

The anesthetic gases are hydrocarbons (compounds of carbon/hydrogen/Oxygen) with chlorine, fluoride or similar compounds attached to the basic molecule. One potential way to facilitate the purging of the machine of these compounds is to flow the gas through a canister or device that contains activated charcoal. . Activated charcoal is granular carbon that is made to be highly porous and therefore has an enlarged surface area. The charcoal granules adsorb a wide variety of chemical compounds and binds them within the pores of the granules. It is familiar to emergency medicine physicians as well as toxicologists because it is used to absorb certain toxic or dangerous chemicals that a person has ingested. It is usually placed in the stomach by means of a tube. The result is that the toxic chemical or drug does not enter the blood stream but stays in the intestinal track. In my search of the literature I found reference to the use of activated charcoal to bind and adsorb anesthetic gases as early as the 1920s. It was also suggested to be used in gas masks to bind chlorine in gas warfare.

In the 1970s a professor of Anesthesiology at the University of Alabama, Ed Ernst, suggested that a canister of activated charcoal placed in the anesthetic machine's circuit could reduce the concentration of anesthetic gases to facilitate rapid awakening and also might be useful in the management of Malignant Hyperthermia!

In the 1970s the interest in these charcoal filters was further stimulated because of concern that anesthetic gases vented in the operating room could be a danger to the health of nurses and physicians. So the filters were designed to attach to the anesthesia machine where the exhaled gases were vented and purge the gas of the anesthetic vapors. Eventually the problem of anesthesia gas pollution was solved by venting the waste gas to a vacuum system and venting the gas to the atmosphere outside the ORs where it was massively diluted by the atmosphere. Therefore the technology languished in semi oblivion.

Then a few years ago a group of investigators in Utah reexamined the use of activated charcoal to rapidly reduce the concentration of anesthetic gases in the anesthesia machine to facilitate awakening from anesthesia and developed an FDA approved device to do that. When Dianne Daugherty and I met these investigators at the American Society of Anesthesiologists meeting and spoke extensively to Dr. Joe Orr, we urged them to continue to develop the technology to be used to prepare an anesthesia machine for an MH susceptible.

A few years later they accomplished this feat. A simple to use, disposable set of small canisters to fit on the anesthesia machine that would rapidly decrease the concentration of the anesthetics that the patient would be exposed to. The device is now marketed by a company named Dynasthetics. (In the interests of disclosure, the company advertises its products on the MHAUS web site).

A study examining the effectiveness of the device was published in the journal Anesthesia and Analgesia this past year (reference 2). At the MHAUS hotline consultants/professional advisory council meeting in May of 2011, we examined the evidence for the use of these devices in preparation of the anesthesia machine and several of the consultants then trialed the devices clinically. We were all impressed that the device (called 'Vapor Clean') did what the investigators said it would: rapidly reduce the concentration of anesthetic gases entering into the patient from the anesthesia machine to practically zero!

As a result, MHAUS has developed a consensus statement on preparing an anesthesia machine for an MH susceptible patient:

'MHAUS suggests following the manufacturer's recommendations, Vapor-Clean Filters may be used as an alternative to, or in addition to, the present MHAUS recommendation for preparation of anesthesia machines for MH-susceptibles.'

Furthermore, it is hypothesized that applying the filter during an MH crisis might facilitate resolution of the crisis by removing exposure to the trigger anesthetic.

As a side note, it is really interesting to me to note that the recognition that activated charcoal could be of value in the management of MH was made so many years ago but languished because no one pursued the design of a practical, inexpensive device that passed rigorous testing to prove efficacy .

Finally, I should also point out that no rigorous study has been conducted in humans or even animals to determine the lower limit of anesthetic gas concentration that can trigger an MH episode. We do feel confident though that background anesthetic concentrations found in an operating room environment will not trigger MH or MH-like problems in humans. The simple, inexpensive device activated charcoal filter does reduce anesthetic concentrations below that level.

References:

1.Kim TW, Nemergut ME. Preparation of modern anesthesia workstations for malignant hyperthermia'susceptible patients: a review of past and present practice. Anesthesiology 2011;114:205'12

2.Birgenheier N, Orr J, Westenskow D. Activated charcoal effectively removes inhaled anesthetics from modern anesthesia machines. Anesth Analg 2011;112:1363'70

Additional references to the comments made in the text are available on request.

Please note that I have no personal commercial interests in Dynasthetics or the activated charcoal filters.