A COMPARISON ON ALKALINIZATION INDUCED EFFECT ON LIDOCAINE ANESTHETIC ACTIVITY

A Sosa¹, M Salazar-Rodríguez² y M Velasco².

¹Department of Pharmacology, National Institute of Health.- ²Department of Pharmacology, Vargas Medical School, Central University of Venezuela, Caracas, Venezuela.E-mail: veloscom@cantv.net

    ABSTRACT: Since some Anesthesiologists have reported side effects with the use of lidocaine in our country, we have made a pharmacological evaluation to define the anesthetic activity of lidocaine. The pharmacological evaluation was made in guinea pigs, by the method of Bülbring and Wajda⁽⁷⁾. For the statistical analysis of the different treatments, Analysis of Variance was used, as well as, the Theorem of Fieller for the potency of lidocaine. The AC50 of commercial 2% lidocaine alkalinizated with 10% sodium hydroxyde was higher than that alkalinizated with 5% sodium bicarbonate and the standard USP (p £ 0,05). We have also evaluated the potency for commercial 2% lidocaine alkalinizated with 10% sodium hydroxyde below 60%. pH of the studied solutions and the adjusted-pH agent were determined, and we concluded that adjusted-pH agent was a determining factor in the anesthetic potency of lidocaine.

Key Words: Local anesthetic activity, Anesthetic potency, Adjusted-pH agent, Alkalinization of lidocaine.

Fecha de Recepción: 16/03/2004 Fecha de Aprobación: 13/04/2004

INTRODUCTION

    Several Anesthesiologists in our country have reported few side effects with the use of lidocaine hydrochloride (lidocaine) i.e. cardiovascular effects that result in cardiovascular collapse, when different pharmacological preparations were used for anesthesia. We have evaluated the raw material used for the use of lidocaine manufacture process.

    Lidocaine is the most widely used local anesthetic. It is an aminoethylamide and is the prototypical member of the amide class of local anesthetics. Lidocaine exhibits its anesthetic activity where a local anesthetic of intermediate duration is needed⁽¹⁾.

    In anesthesia there has been considerable interest in the effects of pH on the potency and the duration of action of tertiary amine local anesthetics, such as lidocaine, which is metabolized in an uncharged free amine base and a cationic protonated form⁽²⁾. As the pH increases, this metabolic process is based on the uncharged species, and lidocaine (pKa 8.2 at 25°C) is thus more permeable through the cell membrane and other lipid diffusion barriers such as the nerve sheath in more alkaline conditions^(3-5). The pH also influences the partitioning coefficient of anesthesic between aqueous solutions and biologic membranes, which results in strong effects on the rate of diffusion of anesthetics to the internal zone of neuronal axon^(3-6).

    As steady state, potency depends, in part, on the local concentration of anesthetic in the region (s) of the membrane where it acts to block sodium channels. Local concentrations of drug depend on the ratio of neutral and charged form, which vary with pH⁽⁶⁾.

    In this research we have studied the anesthetic activity of lidocaine in relation to pH, and the anesthetic activity of commercial lidocaine alkalinizated with 10% sodium hydroxide and 5% sodium bicarbonate.

Methods

    Hartley guinea-pigs of either sex were used, weighing 380 ± 30 g, and they were maintained with water and food ad libitum.

    The study was approved by the author´s Institutional Animal Investigation Committee.

    24 hours after the hair of animals were shaved on the back, the injection was made with a small gauge needle. A volume of 0.1 ml of the lidocaine solutions test was injected. Guinea pigs responded to a mild pinprick of the skin. We applied six pricks, at intervals of 3–5 seconds, to each area as a single test and tested each area at intervals of 5 min, for a period of 30 min (6 tests). Absence of response was also recorded⁽⁷⁾. The normal response observed in the control group, when applying the indicated stimulus, is a contraction of the skin around the injected area. Log dose was plotted as abscissa and number of negative responses (representing degree of anestesia) as ordinate.

    Ten animals were required to test four doses of two compounds. Each animal received an intradermal injection of saline solution and the following five preparations of lidocaine: 1) lidocaine hydrochloride powder (USP standard) was dissolved in saline solution; 2) lidocaine hydrochloride powder (raw material, provided by the manufacturing industry of the commercial lidocaine) was dissolved in saline solution; 3) commercial 2% lidocaine hydrochloride (bach: A and B) alkalinized with 10% sodium hydroxide 4) commercial 2% lidocaine hydrochloride (bach: C) alkalinized with 5% sodium bicarbonate. pH values were measured by a Corning pHmeter 125 (Corning Science Product, New York, USA).

    Statistical analysis was made with one way analysis of variance and a p £ 0,05 was considered statistically significant. The anesthetic potency was determined according to the Theorem of Fieller, a value of p £ 0,05 was considered statistically significant. Anesthetic concentration 50 (AC₅₀) was calculated by the method of Bülbring and Wajda⁽⁷⁾. The linear correlation between the variables was calculated by the method of the best slope, and the equations analyzed by a regression analisys.

RESULTS

    There was not statistically significant differences (p £ 0,05) in parallel to the line of lidocaine. The AC₅₀ of lidocaine solutions were calculated graphically according to Bülbring and Wajda⁽⁷⁾ obtaining values of the significant coefficient of correlation, (Table 1).

Table 1

Anesthesic Concentration 50 (AC₅₀)and pH of the 2% lidocaine solutions

    Bach A, B and C: commercial 2% lidocaine prepared by the pharmaceutical company. USP standard: lidocaine hydrochloride reference standard prepared in saline solution. AC₅₀: concentration that induces 50% of the local anesthetic effect of lidocaine hydrochloride in guinea pigs.

(*) p < 0.05 vs USP standard.

    The anesthetic activity was statistically significant different (p £ 0,05) between commercial 2% lidocaine (bach A and B) alkalinized with 10% sodium hydroxide in comparison with the material raw and lidocaine hydrochloride (USP standard), (Figure 1).

    In order to investigate the cause of the low anesthetic potency of commercial 2% lidocaine (bach A and B) alkalinized with 10% sodium hydroxide, the Pharmaceutical Industry came to manufacture a lot pilot of commercial 2% lidocaine (bach C) alkalinized with 5% sodium bicarbonate. One was not statistically significant different between the anesthetic activity obtained with commercial 2% lidocaine (bach C) alkalinized with 5% sodium bicarbonate, material raw 2% lidocaine hydrochloride, and USP standard, (Figure 1).

    From the comparison of pH of the samples that the exhibited anesthetic potency between 50-60%, with the one of the samples with potency near 100% were that the values of pH were within the rank of optimal anesthetic answer (pH: 5 - 7), independent of the obtained anesthetic potency, (Table 1).

DISCUSSION

    In this study, we have demonstrated that although the studied solutions of lidocaine had less between 50-60% of anesthetic activity, parallel to the line of lidocaine was not significant among them (p £ 0,05), reason why the active principle of the commercial can be inferred that lidocaine were acting by means of a mechanism of similar action and/or that the drugs were of the same chemical nature.

    The anesthetic potency of commercial 2% lidocaine (bach: A and B) alkalinized with 10% sodium hydroxide and commercial 2% lidocaine (bach: C) alkalinized with 5% sodium bicarbonate, with pH very near (6.23 – 6.49), was different. Whereas the anesthetic potency of commercial 2% lidocaine was preserved with 5% sodium bicarbonate (99.46%), and with 10% sodium hydroxide, smaller anesthetic response to observed so much with raw material as with the USP standard.

    Alkalinization has been shown to increase onset, potency and duration of motor nerve blockade^(8-10). Increasing the pH of the local anesthetics solution towards the physiologic range has been reported to improve the quality of neural blockade in vitro⁽¹¹⁾ and results in an improved nerve penetration and more rapid onset time of nerve blockade^(11,12). Nevertheless, these results bring about the difference in the potency of lidocaine solutions studied are not related the values of pH, but, possibly to the influence of the adjusted-pH agent used in the process of manufacture of the product.

    Our finding that sodium bicarbonate increases potency of the lidocaine compared with alkalinized with sodium hydroxide, and the appearance of side effects in the patients reported with commercial 2% lidocaine hydrochloride (bach: A and B) alkalinized with 10% sodium hydroxide, we feel that the sodium hydroxide is not a safe agent close fitting jacket of pH in the solutions of lidocaine. Also, Sinnott et al.⁽¹³⁾ emphasized that sodium hydroxide is not approved for clinical use as an alkalinizing agent for local anesthetics and do not advocate its use for this purpose until rigorous clinical testing of its safety and efficacy is performed.

    On the other hand, Galindo⁽¹⁴⁾ demonstrated rapid times of onset of neural blockade in patients receiving local anesthetics to which increments of sodium bicarbonate were added. The addition of sodium bicarbonate to local anesthetic solutions resulted in a physiological pH (7.0 – 7.4). This addition was associated with a marked shortening of the analgesia onset time and prolongation of analgesic action⁽¹⁴⁾. Also, Galindo⁽¹⁴⁾ demonstrated that addition of sodium bicarbonate to bupivacaine, mepivacaine and lidocaine, to raise the pH of the solutions to 7.0 and 7.4, consistently resulted in improved quality and longer duration of epidural anaesthesia.

    The addition of bicarbonate to local anesthesia in order to adjust its pH produces not only a change in pH but also an increase in PCO₂⁽¹⁵⁾. Carbon dioxide also diffuses into the axon, acting as a membrane-penetrating acid and facilitating the formation of the local anesthesia cationic form within the axon⁽⁵⁾.

    When extracelular pH is increased by addition of sodium bicarbonate, decreased intracelular pH through diffusion of carbon dioxide (produced from the reaction of H⁺ and HCO₃^- in extracelular fluid) may also play a role in enhancing local anaesthetic blockade through protonation of intracellular free-base local anaesthetic (‘ion trapping’) and increasing the concentration gradient for the free-base local anaesthetic across the plasma membrane^(10,16,17). Carbon dioxide may enhance the availability of anesthetic to sites of action within the membrane, e.g., by increasing the concentration of the protonated form within the membrane through bicarbonate salt formation⁽¹⁸⁾.

    In our study, addition of sodium bicarbonate to lidocaine significantly increased the potency of lidocaine and solution pH, as determined by measurement of anesthetic activity. The choice of the adjusted-pH agent is essential for sensible measures of the actions of lidocaine, and was determinant in anesthestic potency of lidocaine.

ACKNOWLEDGEMENTS

    This research work was undertaken with a Grant provided by the Institute of Health Siences.

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