Anesth Analg 2003;97:1646-1651
© 2003 International Anesthesia Research Society
ANESTHETIC PHARMACOLOGY
Physicochemical Compatibility of Propofol-Lidocaine Mixture
Yoko Masaki, PhD,
Makoto Tanaka, MD, and
Toshiaki Nishikawa, MD
Department of Anesthesia, Akita University School of Medicine, Akita-city, Japan
Address correspondence and reprint requests to Makoto Tanaka, MD, Department of Anesthesia, Akita University School of Medicine, Hondo 1–1-1, Akita-city 010–8543, Japan. Address e-mail to mtanaka{at}med.akita-u.ac.jp
 |
Abstract
|
|---|
To examine the physicochemical stability of combinations of propofol-lidocaine mixtures frequently used in clinical practice, we added lidocaine 5, 10, 20, or 40 mg to commercially available 1% propofol 20 mL. To assess chemical stability, propofol concentrations were determined by gas chromatography assay for 24 h after preparation of the mixture. In addition, scanning electron microscopy was used to determine the maximum detectable droplet size in randomly selected fields. Macroscopically, separate, colorless layers were first seen at 3 and 24 h after the addition of 40 and 20 mg of lidocaine to propofol, respectively, whereas the mixture with 5 or 10 mg of lidocaine was macroscopically stable. Propofol concentrations in the mixture with 40 mg of lidocaine decreased linearly and significantly from 4 to 24 h after preparation, whereas those combined with other lidocaine doses were unchanged compared with baseline concentrations. Scanning electron microscopy showed that droplets with diameters 
5 µm first appeared 30 min after the addition of 40 mg of lidocaine to propofol, and the emulsion droplets were enlarged in a time- and dose-dependent fashion. Our results indicate that the addition of lidocaine to propofol results in a coalescence of oil droplets, which finally proceeds to a visible separate layer. Depending on the dose of lidocaine and the duration between its preparation and administration, this combination may pose the risk of pulmonary embolism.
IMPLICATIONS: The addition of lidocaine to propofol results in time- and dose-dependent increases in oil droplet diameters in emulsion. This mixture is physicochemically unstable over time and may cause pulmonary embolism, depending on the dose of lidocaine.
|
Introduction
|
|---|
Propofol (2,6-diisopropylphenol) is widely used for the induction and maintenance of anesthesia as well as for sedation of intensive care patients. Because of its low water solubility, propofol is formulated as an emulsion using soybean oil as the oil phase and egg lecithin as the emulsifying agent in an isotonic aqueous phase with neutral pH (1). The mixing of propofol emulsion with any other drug is not recommended by the manufacturers because emulsions are thermodynamically unstable despite the use of a stabilizing agent (2). However, in clinical practice, lidocaine has been frequently added to propofol emulsion for alleviating pain on injection, which may occur in up to 70% of patients (3).
Previous studies have revealed the physicochemical instability of propofol emulsion with changes in electrostatic charge or propofol concentration (4,5). In addition, appearance of an oily surface layer has been reported after the addition of lidocaine when the propofol-lidocaine mixture is stored at room temperature (6). In an emulsion, coalescence of oil droplets to form larger droplets is one of the separation processes that may lead to a change in macroscopic appearance (7). We have previously described that the physicochemical instability of the propofol-lidocaine mixture is associated with the dissociation of propofol without changes in lidocaine concentration (5). These results suggest that oil droplets in a propofol-lidocaine mixture may coalesce to a size large enough to occlude peripheral capillaries, even before an upper layer of free coalesced oil is visible. Although IV administration of droplets larger than 5 µm in diameter may pose the risk of pulmonary embolism (8), the process of coalescence or analysis of droplet size in propofol-lidocaine mixtures has never been addressed.
Accordingly, the purpose of this study was to evaluate the physicochemical stability of various combinations of propofol-lidocaine mixtures frequently used in clinical practice by measuring propofol concentrations and the size of enlarged droplets for 24 h after mixture. Gas chromatography (GC) was used to detect chemical changes (9,10), and scanning electron microscopy (SEM) was used to analyze changes in droplet diameters as a measure of physical degradations (11).
|
Methods
|
|---|
Propofol (1% DiprivanTM, AstraZeneca, Osaka, Japan) containing no EDTA and lidocaine hydrochloride (0.5%, 1.0%, and 2.0% XylocaineTM, AstraZeneca) was used in this study. After preparation of four different combinations of the propofol-lidocaine mixture (Table 1), 11 samples (0.8 mL) from each mixture were stored in a quiet environment at room temperature (23°C), and no movement was permitted until observation. To detect oil droplets macroscopically on the surface of the mixture, the vials were rotated gently and then centrifuged at 170 g for 1 min. After centrifugation, 100 µL of the mixture was withdrawn for determination of the concentrations of propofol and lidocaine. Macroscopic changes and drug concentrations were measured at 0, 10, 20, and 30 min, then every hour for 6 h, and at 24 h. Propofol (1%), with no addition of lidocaine, was used as a control.
Simultaneous analysis of propofol and lidocaine concentrations, as previously described (5), was performed by GC with thymol as the internal standard. The GC apparatus (Model GC-6A; Shimadzu, Kyoto, Japan) with a flame ionization detector consisted of a packed column (glass, 2 m; 3.2-mm internal diameter) connected to a SIC chromatocorder 12 (SIC Instruments, Tokyo, Japan). The column was packed with chromosorb WAW DMCS 60/80 mesh coated with OV-17, 2% by weight (GL Science, Tokyo, Japan). The flow rates of nitrogen and hydrogen were 55 and 60 mL/min, respectively. Injector and detector temperature was 250°C. The oven temperature was held at 150°C for 2.5 min and was increased at 20°C/min for 7.5 min. Under these conditions, thymol, propofol, and lidocaine elute at 1.49, 1.95, and 7.07 min, respectively. A 100-µL mixture sample and 100 µL of thymol (10 mg/mL) were made alkaline with 100 µL of 1N NaOH and mixed with 1.0 mL of distilled water and 3 mL of chloroform-ethyl acetate (70:30 vol/vol) on a vortex mixer for 5 min. After centrifugation at 400 g for 5 min at 10°C, 1 mL of the organic layer was transferred to glass vials and dried under a nitrogen stream. After adding 100 µL of acetone, 0.5 µL was injected into the column of the GC system. The propofol and lidocaine calibration standards (9 and 2 mg/mL, respectively) were prepared using the commercial formulations of 1% propofol and 2% lidocaine, as previously described (5), and a response factor of calibration was obtained. The expected propofol concentrations at baseline with 1.0 and 2.0 mL of lidocaine were 0.95 and 0.91 mg/100 µL, respectively. With these methods, the minimum detectable concentration of propofol is 10-6 mg/100 µL. Within-run coefficient of variation is <0.5%, whereas the coefficient of variation for the interday precision is <0.3%. All preparations were performed at room temperature, unless otherwise stated.
An aliquot of a 0.2-mL sample at 0, 10, 20, and 30 min, and 1, 3, 6, and 24 h was diluted with 4.8 mL of cold saline and gently mixed. Whatman 2 qualitative filter papers (5 x 5 mm) were quickly immersed in the diluted samples. Lipid droplets adsorbed on the filter paper were fixed with a combination of equivalent volume of 0.2% glutalaldehyde-1% malachite green mixture (1:1 vol/vol) and 1% osmium tetroxide for 15 min at 4°C, as described previously (11). Filter paper immersed in saline alone was used as a control. These samples were subsequently dehydrated in a graded series of ethyl alcohols and t-butyl alcohol. After 2 h of freeze drying, the samples were coated with gold vapor of approximately 300 Å in thickness. They were observed by SEM (Model JMS-T200, JEOL, Tokyo, Japan) at 5000x and 7500x magnification. In 10 randomly selected SEM fields, the maximum droplet size was measured in comparison with a micron bar in each field.
All data were expressed as mean ± SD. Propofol and lidocaine concentrations over time were analyzed by repeated-measures analysis of variance and, if a significant difference was detected, followed by post hoc paired Student’s t-test to analyze differences between concentrations at baseline and at each time interval. Differences in propofol concentrations with different doses of lidocaine were analyzed using unpaired Student’s t-test with Bonferroni correction to adjust for multiple comparisons. A value of P < 0.05 was considered to be statistically significant.
|
Results
|
|---|
No evidence of macroscopic change was noted in the mixture with 0, 5, and 10 mg of lidocaine for 24 h after preparation. After the addition of 20 and 40 mg of lidocaine, small, colorless, oily droplets were noted on the surface of the mixture at 24 and 3 h and thereafter, respectively.
In each combination of the mixture with 5, 10, and 20 mg of lidocaine, propofol concentrations were unaltered from Time 0 (0.96 ± 0.02, 1.00 ± 0.01, and 0.98 ± 0.03 mg/100 µL, respectively) until 24 h (Fig. 1). Propofol concentrations without lidocaine were unchanged for 24 h compared with Time 0 (0.96 ± 0.02 mg/100 µL). Propofol concentrations in the mixture with lidocaine 40 mg were unchanged until 3 h after preparation but then decreased linearly and significantly compared with the concentration at Time 0 (Fig. 1). However, lidocaine concentrations in each combination of the mixture were unaffected during the entire study period (data not shown).
View larger version (20K):
[in this window]
[in a new window]
|
Figure 1. Propofol concentration (percent of initial concentration) with 5, 10, 20, or 40 mg of lidocaine added to 20 mL of 1% propofol. Values are mean ± SD. *P < 0.05 versus Time 0.  P < 0.05 versus lidocaine 5, 10, and 20 mg.
|
|
In the control (saline-immersed filter paper), no droplets were observed on the fibers (Fig. 2A). Figure 2B shows oil droplets in 1% propofol emulsion alone. Without the addition of lidocaine, droplet diameters larger than 5 µm were not detected, and the mean diameters of the largest droplets in 10 randomly selected fields were unchanged during 24 h (1.1 ± 0.2 and 1.0 ± 0.1 µm at Time 0 and 24 h, respectively). Although the mean maximum droplet diameters did not change significantly compared with Time 0 after the addition of lidocaine 5 and 10 mg, significant increases in droplet sizes were observed at 24 h and between 1 and 24 h after the addition of lidocaine 20 and 40 mg, respectively (Table 2). A droplet with a diameter equal to or larger than 5 µm was never detected after the addition of lidocaine 5 or 10 mg in 10 SEM fields (Fig. 3, A and B) but was first detected at 30 min after the addition of 40 mg of lidocaine (Figs. 2C and 3D). In the mixture with lidocaine 40 mg, consistent increases in the number of fields containing droplets with diameters equal to or larger than 5 or 10 µm were seen from 30 min until the end of the study (Fig. 3D). In this mixture, the maximum droplet diameter detected was 20 µm at 24 h (Fig. 2D). In the mixture with lidocaine 20 mg, droplet diameters larger than 5 µm were noted from 6 h to 24 h after preparation (Fig. 3C).
View larger version (189K):
[in this window]
[in a new window]
|
Figure 2. Scanning electron microscopy (SEM) micrographs of control background (panel A, 7500x), 1% propofol alone (panel B, 7500x), and oil droplets at 30 min (panel C, 7500x) and 24 h (panel D, 5000x) after the addition of lidocaine 40 mg to 20 mL of 1% propofol. The white line in each figure indicates 10 µm.
|
|
View this table:
[in this window]
[in a new window]
|
Table 2. Mean Diameters (µm) of the Maximum Oil Droplets in 10 Randomly Selected Fields of Propofol-Lidocaine Mixtures
|
|
|
Discussion
|
|---|
One of the major findings of our study is the formation of oil droplets larger than 5 µm in diameter after the addition of 20 or 40 mg of lidocaine to 200 mg of propofol. Thus, we recommend that the mixture of 1% propofol 20 mL plus lidocaine more than 20 mg should either be avoided or used within 30 minutes after preparation. Second, from the results of lidocaine 5, 10, and 20 mg added to the same volumes of propofol, increasing the lidocaine concentration in the mixture is more likely to result in faster formation of coalesced oil droplets. Therefore, adding 4% lidocaine 1 mL to propofol 20 mL might result in a more unstable emulsion compared with 1% lidocaine 4 mL because the expected final concentration of lidocaine in the mixture with 4% lidocaine 1 mL is larger (1.90 mg/mL) than 1% lidocaine 4 mL (1.67 mg/mL) despite the same dose of lidocaine.
In a propofol-lidocaine mixture, enlargement of oil droplets dispersed in the aqueous phase has been suggested by previous studies that measured zeta potentials or the appearance of an immiscible oil layer on the surface of the mixture (4). However, time-dependent changes in sizes of oil droplets in the propofol-lidocaine mixture have not been addressed previously because the droplet size in the propofol emulsion is at the lower limit of detection for an optical microscope. Thus, we used SEM in this study to determine droplet diameters and their temporal changes, using one-step fixation for oil droplets on filter papers with a combination of glutaraldehyde-malachite green and osmium tetroxide. These compounds are used to stabilize IV fat emulsions such as 10% Intralipid® (12). In addition to its use as a dye, malachite green stabilizes lipid dissolved in aqueous glutaraldehyde. Accordingly, the specific fixation technique with malachite green permits visualization of flawless oil droplets in the SEM.
Although the likelihood of circulating particulates filtered and retained in the lung for prolonged periods may depend on their shape as well their diameter, the upper limit of acceptable size at which pulmonary embolism occurs is reported to be 5 µm in rabbits (13) and 7 µm in dogs (14). Because of the ethical limitation for studies involving humans, the exact acceptable size of spherical particulates has been widely debated, but 5 µm is generally regarded as the upper limit in humans (15). Therefore, we set the upper limit at 5 µm as the acceptable droplet diameter in our study.
The present study also indicates that even if no macroscopic evidence of emulsion instability was apparent, i.e., no visible, separate, oily layer in the mixture, there is an immediate possibility of oil droplet formation large enough to cause pulmonary embolism when the amount of lidocaine added to propofol exceeds a certain limit. The increase in droplet diameter is time-dependent and also depends on the amount of lidocaine added to propofol. In the mixture of lidocaine 40 mg and propofol 200 mg, droplet diameters of 5 µm were first observed at 30 minutes, macroscopic evidence was apparent at 3 hours, and the oil droplet had increased to a maximum diameter of 20 µm at 24 hours after preparation. The time delay from the first appearance of a droplet size 5 µm until the macroscopic detection of the immiscible oil layer is in accordance with Stokes’ law (7), which states that the time taken for enlarged oil droplets in an oil/water emulsion to separate from the aqueous phase depends on the size of the droplet, i.e., 3 minutes for 100 µm, 5 hours for 10 µm, and 20 days for a 1-µm droplet diameter. However, because the addition of lidocaine 5 and 10 mg to propofol 200 mg neither affected the droplet size nor propofol concentrations, they may be physicochemically stable for at least 24 h after preparation.
For the first time in the literature, the SEM measurements were made for the actual measurement of the oil droplet diameters of the propofol alone and the propofol-lidocaine mixture, thus enabling us to demonstrate the process of coalescence to form larger oil droplets after the addition of lidocaine to propofol. In addition, we have previously demonstrated that the upper layer of free coalesced oil contained propofol (5). These results indicate that dispersed, fine oil droplets, in which most of the propofol is dissolved, coalesce in the aqueous phase to form larger droplets and finally aggregate into a separate surface layer visible to the human eye. Also, actual measurements of droplet diameters by the SEM in our study identified a wide distribution of droplet sizes in commercially available 1% propofol (Fig. 2B). Although the mean maximum droplet diameter was approximately 1 µm in our study, a previous study using photon correlation spectroscopy analysis, which only allows determination of the mean value, demonstrated that the average diameter of 1% propofol emulsion was 0.21 µm (12). This discrepancy may be explained by the fact that a far larger proportion of the droplet relative to the total population in a field belongs to much smaller droplets (<1 µm in diameter) (Fig. 2B).
The rationale for adding lidocaine to propofol is to reduce pain associated with IV injection (3). Among various doses of lidocaine having been tested, the minimum effective dose of lidocaine is reported to be 30 mg added to 160 mg of propofol (16), the concentration of which is between 20 and 40 mg of lidocaine used in our study. From the antimicrobial point of view, these clinically relevant concentrations of lidocaine seem not to be effective in the prevention of bacterial growth (17,18). In addition to lidocaine, propofol-opioid mixtures have also been used to reduce pain on IV injection and to supplement analgesia to propofol-based anesthesia (19,20). Because the appearance of larger coalesced droplets cannot be predicted from macroscopic examination or chemical stability, actual measurement of the droplet diameters by SEM technique is indispensable before these mixtures can be recommended for clinical use.
Although we used propofol without preservatives in this study, various formulations of propofol containing different additives (sodium EDTA or sodium metabisulphite) or lengths of medium-chain triglycerides in the carrier emulsion are commercially available (15,21). As expected, these formulations have different physical stability characteristics. Propofol formulation containing metabisulphite exhibits less physical stability associated with a decreased pH value and zeta potential (pH value of 4.5–6.4; zeta potential, -40 mV) than the formulation containing EDTA (pH value of 7–8.5; zeta potential, -50 mV) (15). Although the exact mechanism for the reduced stability in the propofol-lidocaine mixture is not clear from our results, the addition of lidocaine (acidic solution) would be expected to further reduce the pH value and zeta potential of the emulsion, suggesting that the emulsion stability of the metabisulphite-containing formulation may be inferior to that containing EDTA. Whether the propofol formulation containing EDTA exhibits similar physicochemical stability compared with ours remains to be determined.
In conclusion, the mixture of lidocaine 5 or 10 mg and propofol 200 mg is compatible and chemically stable for 24 hours after preparation. The addition of lidocaine 20 or 40 mg to propofol 200 mg results in coalescence of oil droplets, which finally proceeds to a visible separate layer, indicating physicochemical incompatibility. Although no complication including pulmonary embolism after IV administrations of the propofol-lidocaine mixture has been reported, the mixture of 1% propofol 20 mL plus lidocaine more than 20 mg should best be avoided for clinical use or administered immediately after preparation.
|
Acknowledgments
|
|---|
The authors thank Satoshi Saito, Central Research Laboratory, Akita University School of Medicine, Akita-city, Japan, for his technical assistance in SEM preparations.
|
References
|
|---|
- Tan CH, Onsiong MK. Pain on injection of propofol. Anaesthesia 1998; 53: 468–76.[ISI][Medline]
- Macpherson RD. Pharmaceutics for the anaesthetist. Anaesthesia 2001; 56: 965–79.[ISI][Medline]
- Doenicke AW, Roizen MF, Rau J, et al. Reducing pain during propofol injection: the role of the solvent. Anesth Analg 1996; 82: 472–4.[Abstract]
- Lilley EMM, Isert PR, Carasso ML, Kennedy RA. The effect of the addition of lignocaine on propofol emulsion stability. Anaesthesia 1996; 51: 815–8.[ISI][Medline]
- Masaki Y, Tanaka M, Nishikawa T. Changes in propofol concentration in a propofol-lidocaine 9:1 volume mixture. Anesth Analg 2000; 90: 989–92.[Free Full Text]
- Eriksson M, Englesson S, Niklasson F, Hartvig P. Effects of lignocaine and pH on propofol-induced pain. Br J Anaesth 1997; 78: 502–6.[Abstract/Free Full Text]
- Washington C. The stability of intravenous fat emulsions in total parenteral nutrition mixtures. Int J Pharm 1990; 66: 1–21.
- Driscoll DF, Bhargava HN, Li L, et al. Physicochemical stability of total nutrient admixtures. Am J Health Syst Pharm 1995; 52: 623–34.
- Yu HY, Liau JK. Quantitation of propofol in plasma by capillary gas chromatography. J Chromatogr 1993; 615: 77–81.[ISI][Medline]
- Fan SZ, Yu HY, Chen YL, et al. Propofol concentration monitoring in plasma or whole blood by gas chromatography and high-performance liquid chromatography. Anesth Analg 1995; 81: 175–8.[Abstract]
- Tanimura H, Miki K, Hikasa Y. Scanning electron microscopic study of intravenous fat emulsions. Nippon Geka Hokan 1979; 48: 439–49.[Medline]
- Babl J, Doenicke A, Mönch V. New formulation of propofol in an LCT/MCT emulsion: approach to reduce pain on injection. Eur Hosp Pharm 1995; 1: 15–21.
- Illum L, Davis SS, Wilson CG, et al. Blood clearance and organ deposition of intravenously administered colloidal particles: the effects of particle size, nature and shape. Int J Pharm 1982; 12: 135–46.
- Kanke M, Simmons GH, Weiss DL, et al. Clearance of 141Ce labeled microspheres from blood and distribution in specific organs following intravenous and intraarterial administration in beagle dogs. J Pharm Sci 1980; 69: 755–62.[ISI][Medline]
- Han J, Davis SS, Washington C. Physical properties and stability of two emulsion formations of propofol. Int J Pharm 2001; 215: 207–20.[ISI][Medline]
- Gajraj NM, Nathanson MH. Preventing pain during injection of propofol: the optimal dose of lidocaine. J Clin Anesth 1996; 8: 575–7.[ISI][Medline]
- Vidovich MI, Peterson LR, Wong HY. The effect of lidocaine on bacterial growth in propofol. Anesth Analg 1999; 88: 936–8.[Abstract/Free Full Text]
- Wachowski I, Jolly DT, Hrazdil J, et al. The growth of microorganisms in propofol and mixtures of propofol and lidocaine. Anesth Analg 1999; 88: 209–12.[Abstract/Free Full Text]
- Sherry E. Admixture of propofol and alfentanil: use for intravenous sedation and analgesia during transvaginal oocyte retrieval. Anaesthesia 1992; 47: 477–9.[ISI][Medline]
- Wilson RJT, Ridley SA. The use of propofol and alfentanil by infusion in military anaesthesia. Anaesthesia 1992; 47: 231–3.[ISI][Medline]
- Rau J, Roizen MF, Doenicke AW, et al. Propofol in an emulsion of long- and medium-chain triglycerides: the effect on pain. Anesth Analg 2001; 93: 382–4.[Abstract/Free Full Text]
Accepted for publication July 7, 2003.
Top
Abstract
Introduction
