In the past decade, topical anesthesia has been widely used for small-incision phacoemulsification, especially clear corneal cataract surgery. Topical anesthesia is considered safe and effective, greatly reducing the risk of complications and systemic toxicity and eliminating the use of needles.1–4
Lidocaine 4% and bupivacaine 0.5% or 0.75% are among the agents most frequently used for topical anesthesia in small-incision cataract surgery. Both have a rapid onset time.5 Lidocaine is known to be less toxic to the corneal epithelium than bupivacaine.6 Short-term exposure of corneal endothelium to intracameral lidocaine appears to be safe.7,8
Lidocaine is a weak base with a pKa value of 7.8. Hence, at pH 5, only 0.16% of the molecules are nonionized in aqueous solution, calculated from the Henderson-Hasselbalch equation. According to the principle of “nonionic diffusion” or “partition hypothesis,” only the nonionized moiety of a weak electrolyte is expected to penetrate lipid membranes.9,10 Increasing the pH of the anesthetic solution by adding sodium bicarbonate (pH adjustment) should augment the rate of penetration of the local anesthetic agent into the tissue.5,11 In a previous in vivo study, aqueous humor levels of lidocaine after topical application of lidocaine 4% in a pH 7.2 solution were 210% higher than in a pH 5.2 solution.11
In the present study, we examined the pH dependence of the penetration of lidocaine across the human cornea in vitro.
Materials and Methods
Ten pairs of human corneas were obtained from the Eye Bank, Department of Ophthalmology, University of Vienna. The corneas were not considered for corneal transplantation because of 1 or more of the following reasons: enucleation more than 6 hours after death of donor, potential human immunodeficiency virus or hepatitis C virus infection (positive or missing results due to hemolytic sera), advanced donor age, and systemic donor infection or leukemia.12 The use of human corneas in the study was approved by the Ethics Committee of the Medical School, University of Vienna.
The mean donor age of the corneas was 64 years ± 7 (SD). Eight donors were men and 2, women. The mean postmortem enucleation time was 7.9 ± 0.6 hours. Sterile conditions were maintained throughout the enucleation procedure. The mean corneal endothelial cell density, examined by specular microscopy (Labovert® large-angle microscope, Leitz), was 2635 ± 150 cells/mm2. The corneas were stored in a culture medium (Optisol®, Chiron Vision) at 2°C to 4°C. Only transparent corneas with clinically intact epithelium were used. The mean time between enucleation and the experimental procedure was 5.5 ± 2 days (range 2 to 8 days).
A pair of corneas were mounted simultaneously in an in vitro perfusion system (modified Ussing chambers, Figure 1) and short circuited to eliminate electrical driving forces for the charged species of lidocaine using an automatic voltage clamp (VCC 600, Physiologic Instruments).13 Electrode asymmetries and fluid resistance were automatically compensated for by the voltage clamp.
Special care was taken to minimize corneal distortion or edge damage. On the endothelial side of the chamber, a rubber O-ring was inserted. The inner diameter of the Ussing chamber was 8 mm, resulting in an exposed corneal area of 50 mm2. By appropriate addition of Na+-acetate buffer, pH 5, or NaHCO3 solution, the osmolality of the epithelial bathing solutions was set to 290 mOsm/L, measured with a vapor pressure osmometer 5520 (Wescor), resulting in isotonic solutions with pH 5 and pH 7, respectively. The pH values remained unchanged during the entire experiment. The endothelial bathing solution consisted of (mM) 132 Na2+, 132 Cl−, 5.4 K+, 1.2 Mg2+, 1.2 Ca2+, 2.4 HPO42−, 0.6 H2PO4−, pH-adjusted to 7.4 by adding Hepes-Tris buffer. The osmolality of this solution was also 290 mOsm/L. All solutions contained 10 mM glucose. Both bathing solutions were recirculated by a gas-lift system with pure O2 at 37°C.
Transcorneal fluxes of lidocaine were determined by adding 14C-labeled lidocaine to the epithelial bathing solution together with 3H-polyethylene glycol (PEG), MM 4000, as a marker for the extracellular pathway. Fifty microliter aliquotes were sampled from the endothelial bathing solution every 15 minutes for 180 minutes to measure the radioactivity in a liquid scintillation counter (LS 6500, Beckman Instruments Inc.). Transcorneal fluxes of lidocaine were calculated from the differences in the appearance rate of 14C-labeled lidocaine and 3H-PEG in the endothelial solution; hence, lidocaine fluxes were corrected for extracellular movements.
The 14C-lidocaine and 3H-PEG were purchased from New England Nuclear. All other chemicals and reagents were supplied by local dealers.
The tissue content of lidocaine in the cornea was obtained from the time span until the unidirectional transcorneal 14C-lidocaine fluxes reached their steady state.14
The results are given as means ± standard deviations. The statistical significance of the mean differences was analyzed using paired t tests. Probabilities less than 0.05 were considered significant.
Unidirectional transcorneal lidocaine fluxes reached a steady state after about 90 minutes. At all sampling times, transcorneal fluxes of lidocaine were significantly higher at pH 7 than at pH 5 (P < .05). The mean steady-state flux at pH 5 was 59 ± 34 nmol/min·cornea and at pH 7, 101 ± 37 nmol/min·cornea. This difference was significant (P < .002). Permeation of lidocaine via PEG-accessible shunts was always below 15% of the total flux.
The time-dependent accumulation of lidocaine on the endothelial side of isolated human corneas at pH 5 and pH 7 is shown in Figure 2. The total amount of lidocaine transferred through the cornea to the endothelial side within 180 minutes was significantly higher at pH 7 than at pH 5 (16.1 ± 5.5 μmol/cornea versus 9.1 ± 4.7 μmol/cornea; P < .001).
The steady-state tissue content of lidocaine in the cornea at pH 7 was 2.8 ± 0.9 μmol/cornea and at pH 5, 1.7 ± 1.2 μmol/cornea (not significant). The increase in corneal lidocaine content at pH 7 may be attributed to the increased solubility of the mainly undissociated lidocaine in membranes.
Corneal penetration enhancers such as the disinfectants chlorhexidine and benzalkonium chloride increase corneal drug penetration.15,16 However, their use is often limited by problems of corneal toxicity.17 Another way to enhance ocular drug penetration is to modify the physicochemical properties of drugs.18 Alkalinization of local anesthetic agents is a well-known method to improve the anesthetic effect and its duration.19–21 Neutral solutions of lidocaine induce significantly less pain during infiltration than acidic solutions22–25 and have a more rapid onset.26 In a study in rabbit eyes,5 various topical anesthetic agents at different concentrations and acidity were investigated. Compared with unbuffered solutions, buffered preparations of lidocaine and bupivacaine had a significantly longer anesthetic effect.
In the present study, penetration of lidocaine across human corneas in vitro was about 70% faster at pH 7 than at pH 5. This agrees with the results in a previous clinical study in which the accumulation of lidocaine in the aqueous humor of the anterior eye chamber was 3-fold higher when topical lidocaine was administered in a solution of pH 7.2 than in a solution of pH 5.2.11 The fact that in the present in vitro study transcorneal lidocaine penetration was stimulated less by alkalinization may be related to differences in the penetration areas (central part of cornea in the present in vitro study versus cornea and conjunctiva), to the effect of solution pH on lacrimation and blinking in vivo,27 to the buffering action of tears,28 or to structural changes in the isolated corneas related to the postmortem time and storage. It is not reasonable to study lidocaine solutions with pH values higher than 7.2 to 7.4, since in alkaline solutions, lidocaine tends to precipitate during autoclavation.11
In conclusion, the present in vitro study showed that lidocaine penetration across the human cornea and the corneal content of lidocaine was significantly higher at pH 7 than at pH 5. According to the Henderson-Hasselbalch equation, at pH 7, 16.00% of lidocaine is in its undissociated state compared with 0.16% at pH 5. These findings agree with the notion of nonionic diffusion or the pH partition hypothesis of diffusion, according to which a weak electrolyte penetrates lipid biomembranes primarily in its undissociated state.9,10 In the cornea, lipid barriers are primarily formed by the cell membranes of the epithelium and endothelium, whereas the stroma of the cornea is predominantly hydrophilic.29 Alkalinization of the lidocaine solution by adding NaHCO3 is easy to perform and inexpensive.
The main clinical advantages of adjusting the pH of anesthetic solutions to more physiological values include less local irritation and lacrimation as well as augmentation and prolongation of the anesthetic action.11
1. Fine IH, Fichman RA, Grabow HB, eds, Clear-Corneal Cataract Surgery and Topical Anesthesia. Thorofare, NJ, Slack Inc, 1993
2. Kershner RM. Topical anesthesia for small incision self-sealing cataract surgery; a prospective evaluation of the first 100 patients. J Cataract Refract Surg 1993; 19:290-292
3. Zehetmayer M, Radax U, Skorpik C, et al. Topical versus peribulbar anesthesia in clear corneal cataract surgery. J Cataract Refract Surg 1996; 22:480-484
4. Johnston RL, Whitefield LA, Giralt J, et al. Topical versus peribulbar anesthesia, without sedation, for clear corneal phacoemulsification. J Cataract Refract Surg 1998; 24:407-410
5. Sun R, Hamilton RC, Gimbel HV. Comparison of 4 topical anesthetic agents for effect and corneal toxicity in rabbits. J Cataract Refract Surg 1999; 25:1232-1236
6. Marr WG, Wood R, Senterfit L, Sigelman S. Effect of topical anesthetics on regeneration of corneal epithelium. Am J Ophthalmol 1957; 43:606-610
7. Werner LP, Legeais J-M, Obsler C, et al. Toxicity of Xylocaine to rabbit corneal endothelium. J Cataract Refract Surg 1998; 24:1371-1376
8. Kim T, Holley GP, Lee JH, et al. The effects of intraocular lidocaine on the corneal endothelium. Ophthalmology 1998; 105:125-130
9. Rowland M, Tozer TN. Clinical Pharmacokinetics; Concepts and Applications, 3rd ed. Baltimore, MD, Williams and Wilkins, 1995; 109-118
10. Levin RJ. The Living Barrier; a Primer on Transfer Across Biological Membranes. London, Heineman Medical Books, 1969; 65-85
11. Zehetmayer M, Rainer G, Turnheim K, et al. Topical anesthesia with pH-adjusted versus standard lidocaine 4% for clear corneal cataract surgery. J Cataract Refract Surg 1997; 23:1390-1393
12. Bourne WM. Corneal preservation. In: Kaufman HE, Barron BA, McDonald MB, Waltman SR, eds, The Cornea. New York, NY, Churchill Livingstone, 1988; 713-724
13. Ussing HH, Zerhan K. Active transport of sodium as the source of electrical current in the short-circuited frog skin. Acta Physiol Scand 1951; 23:110
14. Turnheim K, Plass H. Determination of the sodium transport pool in epithelia from tracer fluxes: a simplified approach. Am J Physiol 1985; 248(2 pt 2):F308-F313
15. Ramselaar JAM, Boot JP, van Haeringen NJ, et al. Corneal epithelial permeability after instillation of ophthalmic solutions containing local anaesthetics and preservatives. Curr Eye Res 1988; 7:947-950
16. Tang-Liu DD, Richman JB, Weinkam RJ, Takruri H. Effects of four penetration enhancers on corneal permeability of drugs in vitro. J Pharm Sci 1994; 83:85-90
17. Burstein NL. Corneal cytotoxicity of topically applied drugs, vehicles and preservatives. Surv Ophthalmol 1980; 25:15-30
18. Sasaki H, Yamamura K, Mukai T, et al. Enhancement of ocular drug penetration. Crit Rev Ther Drug Carrier Syst 1999; 16:85-146
19. Lewis P, Hamilton RC, Brant R, et al. Comparison of pain with pH-adjusted bupivacaine with hyaluronidase for peribulbar block. Can J Anaesth 1992; 39:555-558
20. Liu JC, Steinemann TL, McDonald MB, et al. Topical bupivacaine and proparacaine: a comparison of toxicity, onset of action, and duration of action. Cornea 1993; 12:228-232
21. Sarvela PJ. Comparison of regional ophthalmic anesthesia produced by pH-adjusted 0.75% and 0.5% bupivacaine and 1% and 1.5% etidocaine, all with hyaluronidase. Anesth Analg 1993; 77:131-134
22. Hinshaw KD, Fiscella R, Sugar J. Preparation of pH-adjusted local anesthetics. Ophthalmic Surg 1995; 26:194-199
23. Fitton AR, Ragbir M, Milling MA. The use of pH adjusted lignocaine in controlling operative pain in the day surgery unit: a prospective, randomised trial. Br J Plast Surg 1996; 49:404-408
24. Masters JE. Randomised control trial of pH buffered lignocaine with adrenaline in outpatient operations. Br J Plast Surg 1998; 51:385-387
25. Palmon SC, Lloyd AT, Kirsch JR. The effect of needle gauge and lidocaine pH on pain during intradermal injection. Anesth Analg 1998; 86:379-381
26. Gormley WP, Hill DA, Murray JM, Fee JP. The effect of alkalinisation of lignocaine on axillary brachial plexus anesthesia. Anaesthesia 1996; 51:185-188
27. Sieg JW, Robinson JR. Vehicle effects on ocular drug bioavailability. II: evaluation of pilocarpine. J Pharm Sci 1977; 66:1222-1228
28. Yamada M, Kawai M, Mochizuki H, et al. Fluorophotometric measurement of the buffering action of human tears in vivo. Curr Eye Res 1998; 17:1005-1009
29. Pepose JS, Ubels JL. The cornea. In: Hart WM Jr, ed, Adler's Physiology of the Eye; Clinical Application, 9th ed. St Louis, MO, Mosby Year Book, 1992; 29-70