Comparisons of the Anesthetic Potency and Intracellular Concentrations of S(−) and R() Bupivacaine and Ropivacaine in Crayfish Giant Axon in Vitro

L-bupivacaine Inhibition of Nociceptive Transmission in Rat Peripheral and Dorsal Horn Neurons

BACKGROUND

Although the widely used single L-enantiomers of local anesthetics have less toxic effects on the cardiovascular and central nervous systems, the mechanisms mediating their antinociceptive actions are not well understood. The authors hypothesized that significant differences in the ion channel blocking abilities of the enantiomers of bupivacaine would be identified.

METHODS

The authors performed electrophysiologic analysis on rat dorsal root ganglion neurons in vitro and on spinal transmissions in vivo.

RESULTS

In the dorsal root ganglion, these anesthetics decreased the amplitudes of action potentials. The half-maximum inhibitory concentrations of D-enantiomer D-bupivacaine were almost equal for Aβ (29.5 μM), Aδ (29.7μM), and C (29.8 μM) neurons. However, the half-maximum inhibitory concentrations of L-bupivacaine was lower for Aδ (19.35 μM) and C (19.5 μM) neurons than for A β (79.4 μM) neurons. Moreover, D-bupivacaine almost equally inhibited tetrodotoxin-resistant (mean ± SD: 15.8 ± 10.9% of the control, n = 14, P < 0.001) and tetrodotoxin-sensitive (15.4 ± 15.6% of the control, n = 11, P = 0.004) sodium currents. In contrast, L-bupivacaine suppressed tetrodotoxin-resistant sodium currents (26.1 ± 19.5% of the control, n = 18, P < 0.001) but not tetrodotoxin-sensitive sodium currents (74.5 ± 18.2% of the control, n = 11, P = 0.477). In the spinal dorsal horn, L-bupivacaine decreased the area of pinch-evoked excitatory postsynaptic currents (39.4 ± 11.3% of the control, n = 7, P < 0.001) but not touch-evoked responses (84.2 ± 14.5% of the control, n = 6, P = 0.826). In contrast, D-bupivacaine equally decreased pinch- and touch-evoked responses (38.8 ± 9.5% of the control, n = 6, P = 0.001, 42.9 ± 11.8% of the control, n = 6, P = 0.013, respectively).

CONCLUSIONS

These results suggest that the L-enantiomer of bupivacaine (L-bupivacaine) effectively inhibits noxious transmission to the spinal dorsal horn by blocking action potential conduction through C and Aδ afferent fibers.

EDITOR’S PERSPECTIVE

Chiral Aspects of Local Anesthetics

Thanks to the progress made in chemical technology (particularly in the methodologies of stereoselective syntheses and analyses) along with regulatory measures, the number of new chiral drugs registered in the form of pure enantiomers has increased over the past decade. In addition, the pharmacological and pharmacokinetic properties of the individual enantiomers of already-introduced racemic drugs are being re-examined. The use of the pure enantiomer of a drug that has been used to date in the form of a racemate is called a "chiral switch". A re-examination of the properties of the pure enantiomers of racemates has taken place for local anesthetics, which represent a group of drugs which have long been used. Differences in (R) and (S)-enantiomers were found in terms of pharmacodynamic and pharmacokinetic activity as well as in toxicity. Levobupivacaine and robivacaine were introduced into practice as pure (S)-(-)-enantiomers, exhibiting more favorable properties than their (R)-(+)-stereoisomers or racemates. This overview focuses on the influence of chirality on the pharmacological and toxicological activity of local anesthetics as well as on individual HPLC and capillary electrophoresis (CE) methods used for enantioseparation and the pharmacokinetic study of individual local anesthetics with a chiral center.

Continuous postoperative pain control using a multiple-hole catheter after iliac bone grafting: comparison between ropivacaine and levobupivacaine

The aim of this study was to compare the analgesic effects of ropivacaine and levobupivacaine in continuous infiltration anaesthesia delivered via a multiple-hole catheter for the purpose of postoperative analgesia after iliac bone grafting. Thirty-four patients scheduled for iliac bone grafting in the maxillofacial region participated in this study. The patients were randomized to a ropivacaine group (Ropi group) and a levobupivacaine group (Levo group). After harvesting the iliac bone for grafting, a multiple-hole catheter was placed on the periosteum of the iliac bone. When surgery was completed, continuous administration was started at 4 ml/h of 0.2% ropivacaine (Ropi group) or 0.25% levobupivacaine (Levo group). Pain was evaluated in the recovery room and at 4h after surgery, as well as at 9:00 and 18:00 on postoperative days 1, 2, and 3, using a visual analogue scale. Side effects were also recorded. No significant difference in the visual analogue scale scores at rest or in motion was observed between the two groups. In addition, there were no side effects in the two groups. Both 0.2% ropivacaine and 0.25% levobupivacaine provided comparable analgesic effects in continuous infiltration anaesthesia delivered via a multiple-hole catheter after iliac bone grafting.

Bilateral superficial cervical plexus block combined with general anesthesia administered in thyroid operations

Background

We investigated the analgesic efficacy of bilateral superficial cervical plexus block in patients undergoing thyroidectomy and to determine whether it reduces the adverse effects of general anesthesia.

Methods

We prospectively recruited 162 patients who underwent elective thyroid operations from March 2006 to October 2007. They were randomly assigned to receive a bilateral superficial cervical block (12 ml per side) with isotonic saline (group A; n = 56), bupivacaine 0.5% (group B; n = 52), or levobupivacaine 0.5% (group C; n = 54) after induction of general anesthesia. The analgesic efficacy of the block was assessed with: intraoperative anesthetics (desflurane), numbers of patients needing postoperative analgesics, the time to the first analgesics required, and pain intensity by visual analog scale (VAS). Postoperative nausea and vomiting (PONV) for 24 h were also assessed by the “PONV grade.” We also compared hospital stay, operative time, and discomfort in swallowing.

Results

There were no significant differences in patient characteristics. Each average end-tidal desflurane concentration was 5.8, 3.9, and 3.8% in groups A, B, and C, respectively (p < 0.001). Fewer patients in groups B and C required analgesics (A: B: C = 33:8:7; p < 0.001), and it took longer before the first analgesic dose was needed postoperatively (group A: B: C = 82.1:360.8:410.1 min; p < 0.001). Postoperative pain VAS were lower in groups B and C for the first 24 h postoperatively (p < 0.001). Incidences of overall and severe PONV were lower, however, there were not sufficient numbers of patients to detect differences in PONV among the three groups. Hospital stay was shorter in group B and group C (p = 0.011). There was no significant difference in operative time and postoperative swallowing pain among the three groups.

Conclusions

Bilateral superficial cervical plexus block reduces general anesthetics required during thyroidectomy. It also significantly lowers the severity of postoperative pain during the first 24 h and shortens the hospital stay.

The toxicity of local anesthetics: the place of ropivacaine and levobupivacaine

PURPOSE OF REVIEW

Ropivacaine and levobupivacaine were developed after evidence of bupivacaine-related severe toxicity. Despite a comparable analgesic profile, quantitative differences become evident with regard to their specific rate of systemic toxicity. The present article provides a concise review of the toxic potencies of levobupivacaine and ropivacaine.

RECENT FINDINGS

As lipophilicity is known to be a major determinant in local anesthetic toxicity, the clinical safety profile of ropivacaine seems to be more favorable than that of levobupivacaine. Experimental studies and case reports confirm this hypothesis, showing that ropivacaine is characterized by fewer (cardio) toxic effects and, most probably, a greater margin of safety. Both agents also may dose dependently damage neurons and skeletal muscle tissue at the injection site. Although their specific rate of neurotoxicity appears to be rather low, levobupivacaine is characterized by an outstanding myotoxic potential.

SUMMARY

Compared with bupivacaine, both agents may be considered as 'more well tolerated' but not as 'totally well tolerated', as they are still capable of inducing systemic and local toxicity. However, ropivacaine seems to have the greatest margin of safety of all long-acting local anesthetics at present.

A Comparison of Intracellular Lidocaine and Bupivacaine Concentrations Producing Nerve Conduction Block in the Giant Axon of Crayfish In Vitro

Clinically, lidocaine requires a larger concentration than bupivacaine to block nerves. Bupivacaine has a higher lipid solubility, tissue permeability, and affinity for sodium channels than lidocaine, resulting in greater anesthetic potency. Local anesthetics require access to the sodium channel from the intracellular milieu. In this study, we sought to determine the intracellular concentration of lidocaine and bupivacaine when a nerve was blocked in the giant axon of a crayfish. A solution of lidocaine or bupivacaine was perfused, and a nerve block was determined as the absence of an evoked action potential after tonic or phasic electrical stimulation. The intracellular lidocaine or bupivacaine concentration was measured using a lidocaine- or bupivacaine-sensitive glass micro-electrode. A phasic block was more effectively and rapidly achieved with a smaller concentration of bupivacaine than with lidocaine. The intracellular concentration and intra- to extracellular ratios were significantly larger with lidocaine than with bupivacaine when nerve conduction was blocked. These findings suggest that bupivacaine has a higher potency than lidocaine, at least in the giant axon of a crayfish in vitro. The implications of the present results are that bupivacaine is a more potent nerve block and produces a use-dependent (phasic) block at smaller concentrations than lidocaine.

Bupivacaine, levobupivacaine and ropivacaine: are they clinically different?

Two new, long-acting local anaesthetics have been developed after the evidence of bupivacaine-related severe toxicity: levobupivacaine and ropivacaine. Both these agents are pure left-isomers and, based on their three-dimensional structure, they have less toxic potential both on the central nervous system and on the heart. Several clinical studies have evaluated their toxicology and clinical profiles: theoretically and experimentally, some differences can be seen, but the reflections of these characteristics into clinical practice have not been evident. Evaluating randomised, controlled trials that have compared these three local anaesthetics, this chapter supports the evidence that both levobupivacaine and ropivacaine have a clinical profile similar to that of racemic bupivacaine, and that the minimal differences observed between the three agents are mainly related to the slightly different anaesthetic potency, with racemic bupivacaine>levobupivacaine>ropivacaine. However, the reduced toxic potential of the two pure left-isomers supports their use in those clinical situations in which the risk of systemic toxicity related to either overdosing or unwanted intravascular injection is high, such as during epidural or peripheral nerve blocks.

Direct cardiac effects of intracoronary bupivacaine, levobupivacaine and ropivacaine in the sheep

1. The racemic local anaesthetic agent bupivacaine is widely used clinically for its long duration of action. Levobupivacaine and ropivacaine are bupivacaine enantiopure congeners, developed to improve upon the clinical safety of bupivacaine, especially the risk of fatal arrhythmogenesis. 2. In previous preclinical studies of the safety of these drugs with intravenous administration in conscious ewes over a wide dose range, we found that central nervous system (CNS) excito-toxicity reversed the cardiac depressant effects when doses approached the convulsant threshold and thus precluded accurate comparison of their cardiovascular system (CVS) effects. 3. To study CVS effects over a wide range of doses with minimal CNS and other influences, brief (3 min) infusions of bupivacaine, levobupivacaine or ropivacaine were administered into the left main coronary arteries of previously instrumented conscious ewes (approximately 50 Kg body weight). After dose-ranging studies, the drugs were compared in a randomized, blinded, parallel group design. Equimolar doses were increased from 8 micromol (approximately 2.5 mg) in 8 micromol increments, to either a fatal outcome or a 40 micromol (approximately 12.5 mg) maximum. 4. All three drugs produced tachycardia, decreased myocardial contractility and stroke volume and widening of electrocardiographic QRS complexes. Thirteen of 19 animals died of ventricular fibrillation: four of six with bupivacaine (mean+/-s.e.mean actual fatal dose: 21.8+/-6.4 micromol), five of seven with levobupivacaine (22.9+/-3.5 micromol), four of six with ropivacaine (22.9+/-5.9 micromol). No significant differences in survival or in fatal doses between these drugs were found. 5. The findings suggest that ropivacaine, levobupivacaine and bupivacaine have similar intrinsic ability to cause direct fatal cardiac toxicity when administered by left intracoronary arterial infusion in conscious sheep and do not explain the differences between the drugs found with intravenous dosage.