["Alternative" effects of local anesthetic agents]

Membrane-Mediated Activity of Local Anesthetics

The activity of local anesthetics (LAs) has been attributed to the inhibition of ion channels, causing anesthesia. However, there is a growing body of research showing that LAs act on a wide range of receptors and channel proteins far beyond simple analgesia. The current concept of ligand recognition may no longer explain the multitude of protein targets influenced by LAs. We hypothesize that LAs can cause anesthesia without directly binding to the receptor proteins just by changing the physical properties of the lipid bilayer surrounding these proteins and ion channels based on LAs' amphiphilicity. It is possible that LAs act in one of the following ways: They 1) dissolve raft-like membrane microdomains, 2) impede nerve impulse propagation by lowering the lipid phase transition temperature, or 3) modulate the lateral pressure profile of the lipid bilayer. This could also explain the numerous additional effects of LAs besides anesthesia. Furthermore, the concepts of membrane-mediated activity and binding to ion channels do not have to exclude each other. If we were to consider LA as the middle part of a continuum between unspecific membrane-mediated activity on one end and highly specific ligand binding on the other end, we could describe LA as the link between the unspecific action of general anesthetics and toxins with their highly specific receptor binding. This comprehensive membrane-mediated model offers a fresh perspective to clinical and pharmaceutical research and therapeutic applications of local anesthetics. SIGNIFICANCE STATEMENT: Local anesthetics, according to the World Health Organization, belong to the most important drugs available to mankind. Their rediscovery as therapeutics and not only anesthetics marks a milestone in global pain therapy. The membrane-mediated mechanism of action proposed in this review can explain their puzzling variety of target proteins and their thus far inexplicable therapeutic effects. The new concept presented here places LAs on a continuum of structures and molecular mechanisms in between small general anesthetics and the more complex molecular toxins.

[Update on the pharmacology and effects of local anesthetics]

Local anesthetics (LA) are broadly used in all disciplines and it could be considered that relatively little is reflected on the mechanisms of action of this old substance group. However, several molecular mechanisms of LAs mediating wanted and unwanted effects remain to be explored. Furthermore, the number of indications for application of LAs seems to be expanding. The local anesthetic effect of LAs is primarily mediated by a potent inhibition of voltage-gated sodium channels. However, this effect is due to much more than the interaction of LAs with one single molecule. Most recent studies indicated that the development of selective local anesthetics might be possible and LAs also interact with several other membrane molecules. Although the relevance of these effects is still unclear, they might play a role in systemic analgesia, tissue protection and anti-inflammatory effects of LA. The therapeutic index of systemically applied LA is very narrow. Systemic application is formally not permitted because the impending systemic toxicity is still a life-threatening complication. Although the cardiac and central nervous toxicity at least partly result from an unselective block of neuronal and cardiac sodium channels, preclinical studies suggest the involvement of several mechanisms. A local LA toxicity is less clinically impressive; however, all LAs induce a significant tissue toxicity for which the underlying mechanisms have been partly identified. This review reports on recent findings on mechanisms and on the clinical relevance of some LA-induced effects which are of relevance for anesthesiological activities.

[Midfemoral nerve block for foot surgery: is there an anatomical-clinical correlation between motor response and latency?]

BACKGROUND AND OBJECTIVE

The latency times of midfemoral sciatic nerve blocks vary greatly. This study investigated the correlation between the type of motor response to nerve stimulation on the one hand and latency and block efficacy on the other.

PATIENTS AND METHODS

We enrolled 215 consecutive patients (184 women) undergoing orthopedic foot surgery. A tourniquet was applied above the malleolus. The puncture location was found by palpating to locate the groove between the vastus lateralis and biceps femoris muscles, at the mid-point of the line between the posterior edge of the greater trochanter muscle and the insertion of the biceps femoris muscle in the popliteal fossa. A solution of equal proportions (1:1) of 1.5% mepivacaine (with bicarbonate 1:10) and 0.75% levobupivacaine was injected at a dose of 0.45 mL x kg(-1) (maximum 40 mL) using a 10-cm needle. Nerve stimulation was applied at 100-300 ms, 02-0.4 mA, and 2 Hz. Latency was classified as response in less than 15 minutes, in 15 to 30 minutes, or later than 30 minutes.

RESULTS

The evoked motor response was inversion in 30 patients, flexion or extension in 38, plantar flexion in 101, dorsiflexion in 37, and eversion in 9. Shorter latencies (15 minutes) were observed in all patients with inversion or flexion/extension and in 84 (83%) of the 101 patients with plantar flexion. Mid-range latencies were observed in 13% of those with a plantar flexion response and in 29.7% of those with dorsiflexion. All 9 patients with eversion and 17 (45.9%) of the 37 patients with dorsiflexion had the longest latencies. The surgical block was complete for all patients.

CONCLUSIONS

This approach provides an effective block with minimum latency in patients who have a flexion or extension motor response in the foot and/or fingers, inversion, or plantar flexion, which assumes that the injection has reached the common trunk of the sciatic or tibial nerve. However, a longer latency is associated with a peroneal motor response, particularly eversion.