Verapamil increases the toxicity of local anesthetics

Vasodilation increases the threshold for bupivacaine-induced convulsions in rats

Bupivacaine affects the vascular resistance by peripheral and central nervous system (CNS) mechanisms. As vasoconstrictors increase the CNS toxicity of IV bupivacaine, vasodilators may decrease its CNS toxicity. We examined the hypothesis that vasodilators decrease the CNS toxicity of bupivacaine in awake, spontaneously breathing rats. Male Sprague-Dawley rats were randomly divided into control (C), nicardipine (N), and phentolamine (P) groups (n = 12 in each group). Racemic bupivacaine was administered IV at 1 mg/kg/min until tonic/clonic convulsions occurred. Saline, nicardipine (0.4 microg/min), and phentolamine (10 microg/min within 5 min, 50 microg/min thereafter) were simultaneously administered with bupivacaine in groups C, N, and P, respectively. Mean arterial blood pressure was significantly increased by infusion of bupivacaine in group C and was maintained at baseline levels before the onset of convulsions in groups N and P. The convulsive dose of bupivacaine in group C was 5.8 +/- 1.5 mg/kg, but was significantly larger in groups N and P (7.6 +/- 1.5 and 8.1 +/- 1.1 mg/kg, P = 0.02 and 0.001, respectively). However, there were no differences in total or protein-unbound plasma concentration of bupivacaine or in concentration of bupivacaine in the brain at the onset of convulsions among the 3 groups. We conclude that nicardipine and phentolamine increase the cumulative dose but do not affect the threshold plasma or brain concentrations required for bupivacaine-induced convulsions.

IMPLICATIONS

Bupivacaine, a long-acting local anesthetic, induces central nervous system toxicity when its plasma concentration is increased. Nicardipine and phentolamine increased the cumulative dose but did not affect the threshold plasma concentrations, required for bupivacaine-induced convulsions, suggesting that both nicardipine and phentolamine inhibited the increase in the plasma concentration of bupivacaine by inducing peripheral vasodilation.

The effects of verapamil and nimodipine on bupivacaine-induced cardiotoxicity in rats: an in vivo and in vitro study

The purpose of this in vivo and in vitro study was to compare the effects of verapamil or nimodipine pretreatment on bupivacaine-induced cardiotoxicity. In the in vivo study, the dose-response curve for the 50% lethal dose (LD50) of bupivacaine was determined for rats. Two separate groups of rats were pretreated with i.v. verapamil 150 microg/kg (n = 35) or i.v. nimodipine 200 microg/kg (n = 35). Each pretreatment group was then subdivided into four groups of at least four rats each. Three minutes after pretreatment, bupivacaine was administered to each of four groups in doses of 2.5, 3.0, 3.25, and 3.5 mg/kg, respectively. Both verapamil and nimodipine pretreatment increased the LD50 and 95% confidence intervals for bupivacaine and increased survival. In the in vitro study, the effects of verapamil or nimodipine perfusion on bupivacaine cardiotoxicity (negative chronotropic, negative inotropic, and arrhythmogenic effects) and coronary perfusion pressure (CPP) were investigated in isolated, perfused rat heart preparations. Depression of heart rate, contractile force, and CPP, and the incidence of arrhythmias caused by bupivacaine alone were similar to those caused by bupivacaine after verapamil pretreatment. In contrast, bupivacaine induced less negative chronotropic effects (P < 0.05, paired t-test) and arrhythmias (P < 0.05, chi2 analysis) after nimodipine pretreatment. The results of this study demonstrate that both verapamil and nimodipine pretreatment decrease bupivacaine-induced cardiotoxicity in vivo, whereas only nimodipine pretreatment decreased bupivacaine-induced cardiotoxicity and arrhythmias in vitro.

IMPLICATIONS

In this experimental study consisting of two stages (in vivo and in vitro), we compared the effects of two calcium channel-blocking drugs (verapamil and nimodipine) on bupivacaine toxicity. Bupivacaine is a local anesthetic frequently used in clinical practice, and cardiotoxicity is one of its severe side effects. Verapamil and nimodipine were both effective in decreasing bupivacaine cardiotoxicity in this rat model.

Effects of calcium channel blockers on bupivacaine-induced toxicity

The purpose of this study was to investigate the influence of calcium channel blockers on bupivacaine-induced acute toxicity. For each of the three tested calcium channel blockers (diltiazem, verapamil and bepridil) 6 groups of mice were treated by two different doses, i.e. 2 and 10 mg/kg/i.p., or an equal volume of saline for the control group (n = 20); 15 minutes later, all the animals were injected with a single 50 mg/kg/i.p. dose of bupivacaine. The convulsant activity, the time of latency to convulse and the mortality rate were assessed in each group. The local anesthetic-induced mortality was significantly increased by the three different calcium channel blockers. The convulsant activity of bupivacaine was not significantly modified but calcium channel blockers decreased the time of latency to obtain bupivacaine-induced convulsions; this effect was less pronounced with bepridil.

Pharmacokinetic interactions with calcium channel antagonists (Part II)

Since calcium channel antagonists are a diverse class of drugs frequently administered in combination with other agents, the potential for clinically significant pharmacokinetic drug interactions exists. These interactions occur most frequently via altered hepatic blood flow and impaired hepatic enzyme activity. Part I of the article, which appeared in the previous issue of the Journal, dealt with interactions between calcium antagonists and marker compounds, theophylline, midazolam, lithium, doxorubicin, oral hypoglycaemics and cardiac drugs. Part II examines interactions with cyclosporin, anaesthetics, carbamazepine and cardiovascular agents.

Lidocaine potentiation of cocaine toxicity

STUDY HYPOTHESIS

The toxic effects of cocaine are enhanced in the presence of lidocaine.

STUDY POPULATION

Male Sprague-Dawley rats weighing 200 to 300 g.

METHODS

Animals received intraperitoneal injections of cocaine (10, 20, 35, or 50 mg/kg), lidocaine (30 or 40 mg/kg), or a combination of all doses of cocaine given simultaneously with 30 or 40 mg/kg lidocaine. The incidence and time to seizure and death were recorded in these groups and compared by chi 2 and analysis of variance analyses, respectively.

RESULTS

At doses of 30 or 40 mg/kg, lidocaine does not induce seizures or death. The effect of simultaneous injection of both cocaine and lidocaine was to dramatically increase the incidence of both seizures and death over that of cocaine alone. The incidence of seizures in animals receiving 35 mg/kg cocaine alone was 10%; this increased to 50% and 80% with the addition of 30 and 40 mg/kg lidocaine, respectively (P less than or equal to .05; P less than or equal to .01). Death did not occur in animals receiving 35 mg/kg cocaine alone; the addition of 30 and 40 mg/kg lidocaine resulted in death in 30% and 60% of animals, respectively (P less than or equal to .01 each group). Similarly, in rats receiving 50 mg/kg cocaine, the incidence of death increased from 0% to 60% and 80% with 30 and 40 mg/kg lidocaine, respectively (P less than or equal to .01).

CONCLUSION

In the rat, overall toxicity of cocaine is significantly increased with simultaneous exposure to lidocaine.