The effects of exogenous epinephrine on a convulsive dose of lidocaine: relationship with cerebral circulation

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.

Concentrations of lidocaine and monoethylglycine xylidide in brain, cerebrospinal fluid, and plasma during lidocaine-induced epileptiform electroencephalogram activity in rabbits: the effects of epinephrine and hypocapnia

When injecting lidocaine into tissues, the mean toxic dose of lidocaine may be increased by adding epinephrine to lidocaine and by decreasing the PaCO(2). In contrast, when lidocaine is introduced directly into an artery or vein, adding epinephrine to lidocaine may decrease the mean toxic dose of lidocaine. Less is known about the effects of decreased PaCO(2) on intravascular lidocaine toxicity. We infused lidocaine in 24 rabbits at 4 mg. kg(-1). min(-1) with/without epinephrine and with/without hypocapnia. We measured the time to onset of lidocaine-induced seizures, total dose of lidocaine at the time of seizures, and concentrations of lidocaine and monoethylglycine xylidide (MEGX), a metabolite of lidocaine, in plasma, brain, and cerebrospinal fluid. Epinephrine decreased onset time by 11% with hypocapnia and by 21% with normocapnia, and it increased plasma MEGX by 1 microg/mL with hypocapnia and 2 microg/mL with normocapnia. Hypocapnia increased onset time by 18% without epinephrine and by 33% with epinephrine, and it increased whole-brain MEGX by 10 microg/mL without epinephrine and by 14 microg/mL with epinephrine. We conclude that, when lidocaine is given intravascularly, hypocapnia increases onset time and lidocaine dose required for seizures. These effects occur with no change in the concentration of lidocaine in plasma or the brain.

IMPLICATIONS

Hypocapnia increases the toxic dose of lidocaine given IV without altering lidocaine concentrations in blood, brain, or cerebrospinal fluid. Whole-brain monoethylglycine xylidide concentration is greater during hypocapnia than during normocapnia, and the addition of epinephrine to lidocaine increases the concentration of monoethylglycine xylidide in plasma.