A technique using anesthesia-containing nanoparticles—drawn to the targeted area of the body by magnets—could one day provide a useful alternative to nerve block for local anesthesia in patients, suggests an experimental study in the April issue of Anesthesia & Analgesia, official journal of the International Anesthesia Research Society (IARS).
"We have established proof of principle that it is possible to produce ankle block in the rat by intravenous injection of magnetic nanoparticles associated with ropivacaine and magnet application at the ankle," write Dr. Venkat R.R. Mantha and colleagues of University of Pittsburgh School of Medicine. With further study, the nano-anesthesia technique might allow more potent doses of local anesthetics to be delivered safely during local anesthesia in humans.
Magnets Used to Attract Anesthesia-Containing Nanoparticles
The experimental pilot study evaluated the use of magnet-directed nanoparticles containing the local anesthetic drug ropivacaine (MNP/Ropiv) to produce anesthesia of the limbs. The researchers engineered nanoparticle complexes containing small amounts of ropivacaine and the iron oxide mineral magnetite. The MNP/Ropiv complexes were injected into the veins (intravenously, or IV) of anesthetized rats.
The researchers then placed magnets around the ankle of the right paw for 15, 30, or 60 minutes. The goal was to use the magnets to draw the nanoparticles to ankle. Once there, the particles would release the anesthetic, numbing the nerves around the ankle.
Sensation in the right paw was assessed by comparing the right paw to the left paw, which was not affected. Other groups of rats received standard nerve block, with ropivacaine injected directly into the ankle; or IV injection of ropivacaine alone, not incorporated into nanoparticles.
Injection of MNP/Ropiv complexes followed by magnet application produced significant nerve block in the right ankle, similar to a standard nerve block. The left ankle was unaffected.
Nano-Anesthesia Could Permit Safe Use of Higher Anesthetic Drug Doses
The ankle block produced by MNP/Ropiv injection was greatest when the magnet was applied for 30 minutes—likely reflecting the time of maximum ropivacaine release. High ropivacaine concentrations were found in right ankles of the MNP/Ropiv group, suggesting "sequestration of the drug locally by the magnet."
In rats receiving MNP/Ropiv, the nanoparticles contained a total of 14 milligrams of ropivacaine—a dose high enough to cause potentially fatal toxic effects. Yet none of the animals in the MNP/Ropiv group had apparent adverse effects of ropivacaine. This was similar to the findings in rats receiving 1 milligram of plain ropivacaine. Thus the safe dose of ropivacaine combined with nanoparticles could be at least 14 times higher, compared to IV ropivacaine alone.
Magnet-directed nanoparticles have previously been used for targeted delivery of chemotherapy drugs. The new study suggests that a similar technique could be used to attract local anesthetic-containing nanoparticles to specific areas, as an alternative to local anesthetic block—like that used for foot and ankle surgery, for example.
Additional animal experiments would be needed before the MNP/Ropiv technique can be tested in humans. But if it proved safe, the magnet-directed approach could provide a useful new alternative for regional anesthesia—delivering high concentrations of local anesthetics directly to the desired area, without increasing toxic effects.
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