EXECUTIVE SUMMARY
A MH crisis begins with the skeletal muscles contracting and not relaxing. This causes them to deplete their energy stores, which leads to the membranes becoming unstable and cells dying.
- For the first few minutes of the crisis, the process is asymptomatic.
- As cells release potassium, acid, or lactic acid and heat, the patient may experience an increased heart rate, variable blood pressure, and higher carbon dioxide levels.
- When the OR team sees that a patient’s carbon dioxide has increased, it is a signal that something is wrong.
Patients who experience malignant hyperthermia (MH) in the OR are genetically predisposed to the condition. However, they may be unaware they are susceptible. It is a pharmacogenetic disease that usually becomes a crisis after a pharmaceutical trigger.
“The common drugs that trigger malignant hyperthermia are the volatile anesthetics and succinylcholine,” says Joseph Tobin, MD, board member with the Malignant Hyperthermia Association of the United States. “The crisis begins with the skeletal muscle cells, beginning to contract and not relaxing, which causes them to deplete their energy stores,” Tobin explains. “Normally, once you are paralyzed, your muscles should be fully relaxed. But in these cases, the muscles contract anyway, and there’s a problem from inside the cell.”
As the muscles contract, they deplete their energy supply, a process that is happening without the OR team’s knowledge, at least at first. “It can be asymptomatic for the first few minutes,” Tobin notes. “Then, as the cells deplete their energy, the membranes become unstable and the cells die.” This process results in the cells releasing potassium, acid, or lactic acid and heat. “These problems cause an increased heart rate, a variable blood pressure, and increased carbon dioxide, which is reflected in the expired carbon dioxide, measured by the anesthesia team,” Tobin says.
When the OR team sees that a patient’s carbon dioxide has increased, it is a sign that something might be amiss. “They don’t know that the patient is significantly ill yet,” Tobin notes. “It’s just the beginning of what’s happening. If it it’s unrecognized, the process continues.”
If the process proceeds, the patient’s potassium level can rise, leading to cardiac dysrhythmias. Still higher, the patient could suffer cardiovascular collapse and death.
When surgery teams are trained to identify signs of MH, they might suspect MH when they see the rising carbon dioxide level, and monitor the patient’s heart rate trend while under anesthesia, watching for unexpected changes, Tobin says. “If lab testing is available, they could measure serum electrolytes and the blood gas,” he adds. “But, if it’s not available, the anesthesia team must consider alternative explanations for the elevated heart rate and carbon dioxide levels.”
There are several potential alternative causes of rising carbon dioxide levels:
• Caused by laparoscopic procedure. “One thing we often do in outpatient surgery is laparoscopic procedures, including gallbladder removal,” Tobin says. “Tiny punctures push carbon dioxide into the body, which absorbs it, and it’s exhaled as CO2.”
• Sepsis. This is a difficult diagnosis when a patient is under anesthesia, and there might not be a fever with either sepsis or MH, Tobin notes.
Sometimes, OR patients develop sepsis as the result of kidney stone procedures in which an ultrasound machine is used to break up the stones, he explains. “We provide anesthesia for these patients because it’s painful,” Tobin says. “Sometimes, behind the stone there is an infection. When the stone breaks open, the infection is released from the area where it is located and it gets into the blood stream.”
An estimated 5% to 10% of the hotline calls received by the Malignant Hyperthermia Association are associated with lithotripsy, outpatient procedures to break up kidney stones, Tobin says. “When in doubt, we treat for both sepsis and malignant hyperthermia by using antibiotics and the rescue drug dantrolene.”
• A malpositioned endotracheal tube. If the patient’s breathing tube is not placed well or ajar, then the patient might not be ventilating appropriately, which could lead to increased carbon dioxide. One way to determine that the endotracheal tube is malpositioned is to listen to the patient’s breath on both sides of the body, Tobin says. “It takes a couple of minutes to assess,” he says. “If it’s put in too far, you can diagnose it by markings on the endotracheal tube that tells you it is in too far.”
• An obstructed airway. “An obstructed airway could cause it,” Tobin says. “If you have a big patient, the person might relax and go to sleep, obstructing their own airway in the neck because they’re heavy and the muscles are relaxed.”
This also causes snoring and CO2 levels to build up. The OR team can determine if an obstructed airway is causing the CO2 rise by listening to the patient’s breathing sounds, such as wheezing or strider, Tobin suggests. “Also, measure the exhaled tidal volumes,” he adds. “Too little volume means the airway is obstructed.”
Physicians and surgery centers could prevent a malignant hyperthermia crisis by identifying patients who might be susceptible to the condition. For instance, all patients could be screened for a family history of anesthesia intolerance. Also, patients who have been hospitalized for exercise-induced rhabdomyolysis and severe muscle cramping could be at risk of MH, Tobin says.
The good news is that if a patient has a history of MH or a history that suggests susceptibility, the preventive action is to use clean technique, an anesthesia approach that does not include volatile anesthetics or succinylcholine. “If a surgeon knows of a patient’s potential for malignant hyperthermia, then tell anesthesia ahead of time to use clean technique,” Tobin says. “The induction and maintenance of anesthesia [in clean technique] is most commonly performed with propofol and an opioid. Propofol will keep you asleep, and the opioid will take care of pain. If we need to paralyze someone, we can use a paralyzing drug in another class.”
The alternative drug could be a nondepolarizing neuromuscular blocker. If a patient’s family history suggests MH or if the patient has visited the ED because of severe muscle cramping, Tobin says he would use a clean technique for that person’s surgery. “Not everyone would agree with that,” he says.
But there is some science suggesting that ED patients with exercise-induced rhabdomyolysis who were biopsied for MH were susceptible to the condition at a ratio of 45 out of 100 vs. a general population susceptibility of one in hundreds to 1,000 people, Tobin adds.
There is a genetic screening test that can be sent to a genetic reference lab, but it is only 40% sensitive, Tobin cautions. “A positive test is helpful, but a negative test is of no information at all.”
A biopsy test is the gold standard for diagnosing MH, but it is costly, challenging, and available at only five centers in North America, including the MH Biopsy Testing Center in Winston-Salem, according to Tobin. “It’s a physiologic test done on living tissue, taken from the operating room and immediately delivered to the testing laboratory,” he explains. “The biopsy is taken from the outer thigh, three to four inches long, and it’s teased into small fascicles. Then, the muscles are made to contract by exposure to caffeine and halothane.”
The five centers perform about 150 to 200 biopsies for MH per year, Tobin says. Since the screening biopsy is so difficult for patients to procure, it is unreasonable to ask possibly susceptible patients to obtain the test before surgery.
A better precaution is to ask at-risk patients to undergo their procedures in surgery centers that are prepared for a MH crisis, including the ability to use the clean technique, Tobin suggests.
Also, the FDA recently approved an activated charcoal filter that could absorb volatile anesthetics that patients exhale, reducing the volume of anesthetic agents that patients might rebreathe. The filter costs about $60 per patient, making it most useful for patients at high risk who are in a MH crisis, Tobin says.
“Another technology advance is that the FDA approved a new formulation of dantrolene, which can deliver 250 mg in 5 milliliters vs. older formulations, which deliver 20 mg in 60 milliliters,” he says. “It’s faster. If you’re in an outpatient surgery center, and MH begins to happen, it will take 30 seconds for the new formulation to work vs. minutes with the older formulation.”
A closer examination of what happens during malignant hyperthermia.
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