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Calcium Metabolism Derailment
Malignant hyperthermia is one of the most dreaded complications in general anesthesia. It is triggered by certain anesthetics and – if not treated – it ends lethally in 70 – 80 % of the cases. In most cases, it develops during narcosis. However, it might also occur in post-surgical cases.
The cause of malignant hyperthermia is a genetic defect in the calcium metabolism of the skeletal muscle cells. It is assumed that there is a mutation in the gen, which codes for the intracellular ryanodine receptor. This receptor makes for the release of calcium out of the sarcoplasmatic reticulum of the muscle cell, which eventually leads to muscle contraction.
In the case of malignant hyperthermia, a ryanodine receptor defect leads to continuous muscle contraction. The re-absorption of calcium into the sarcoplasmatic reticulum and the relaxation phase associated with it fail to occur.
The consequence is an increased cellular metabolism (hypermetabolism) with a subsequent massive increase in CO2, tissue hypoxia, increased warmth production, and lactate acidosis. If not treated, cellular decay with an increase in creatine kinase (CK) and myoglobinuria.
These Anesthetics Require Careful Usage
Trigger substances for malignant hyperthermia are inhalation anesthetics (halothane, enflurane, isoflurane etc.) and the muscle relaxant succinylcholine. Stress can also be a trigger. Furthermore, ketamine and local anesthetics of the amide type are discussed as possible trigger substances.
You can fall back on certain ‘safe anesthetics’ for patients with previously diagnosed malignant hyperthermia. They include:
- Laughing gas
- Non-depolarizing muscle relaxants
Recognizing Malignant Hyperthermia
The most frequent early signs of malignant hyperthermia are tachycardia or tachycardic cardiac dysrhythmia in almost 90 % of the cases. Unfortunately, this is very unspecific and is often misinterpreted as too low depth of anesthesia.
A by far more specific early symptom is hypercapnia. Within a few minutes, the CO2-partial pressure can rise to the three- or fourfold. Indirectly, this becomes apparent through the warming of the CO2-absorber. A direct indicator is capnometry, which is part of standard monitoring in surgery and in the intensive care unit. Also, muscular rigor, acidosis, and hyperkalemia are early signs.
Masseter spasms and rigor of the remaining muscles are specific for a succinylcholine induced malignant hyperthermia.
Masseter spasms at the beginnging of the narcosis are always suspicious and should lead to the termination of the narcosis and the initiation of further diagnostics and therapy. As a late sign, the creatine kinase concentration and body temperature rapidly rise (up to 1° Celsius in 5 minutes). Initially, the blood pressure is often elevated and, eventually, it is decreased as a sign of circulatory failure.
On suspicion of malignant hyperthermia, you should take immediate action. This includes removing the trigger substances, terminating surgery as soon as possible, and – if necessary – to change narcosis to safer anesthetics (see above).
Ventilation should be changed to hyperventilation with 100 % oxygen and three- to fourfold increased respiratory minute volume. Dantrolen is given intravenously to terminate the continuous muscular contraction. It is a muscular relaxant, which inhibits the release of calcium of the sacroplasmatic reticulum.
Symptomatic measures include the balance of the acidosis with sodium bicarbonate, cooling of the patient, volume loading, and electrolyte balance. At myoglobinuria or increased serum-myoglobin, forced diuresis should be performed to avoid the complication of a crush kidney. In the event of absent contraindications, tachycardia can be treated with the administration of beta-blockers.
Diagnostics of Malignant Hyperthermia
Malignant hyperthermia can be detected with a muscle biopsy and subsequent in-vitro-contracture test with caffeine and halothane. If a predisposition for malignant hyperthermia is present, an increased contraction behavior under the influence of caffeine and halothane can be observed.
The test should be performed after an incident during narcosis or post-surgically if relatives are affected by malignant hyperthermia (family history in premedication discussion).
If malignant hyperthermia is detected, the patient should be informed about the existing risk of narcosis and an anesthetic pass with a respective note should be made out.