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Spinal Chloroprocaine Solutions: Density at 37° C and pH Titration

Na, Kimberly B., MD; Kopacz, Dan J., MD

Section Editor(s): White, Paul F. Section Editor

doi: 10.1213/01.ANE.0000093244.01831.D7
Ambulatory Anesthesia: Research Report

The density and pH of a local anesthetic are important characteristics in its use as an intrathecal drug. Preservative- and antioxidant-free formulations of chloroprocaine are available and are being investigated for short-duration spinal anesthesia. In this study, we evaluated the pH and density (to 5 significant digits in g/mL, at 37.0°C) of these new chloroprocaine formulations. In addition to plain 2% and 3% chloroprocaine and 2% lidocaine, mixed solutions of 2% chloroprocaine with epinephrine or with bicarbonate were evaluated. Density was also measured after water dilution and after increasing amounts of added dextrose. Chloroprocaine, 2% or 3%, is hyperbaric relative to cerebrospinal fluid (CSF) before any addition of dextrose (density 1.00123 g/mL and 1.00257 g/mL, respectively). When diluted with water, all the solutions are hypobaric relative to CSF (density <1.00028 g/mL). Plain 2% lidocaine is the only dextrose-free solution measured to be hypobaric (density 1.00004 g/mL). Bisulfite-free 2-chloroprocaine remains very acidic (pH <4.0), but the pH can be increased to more than 7.0 with a small amount of bicarbonate (0.25–0.33 mL/10 mL). The increased density of plain chloroprocaine makes it a useful hyperbaric spinal drug without the addition of dextrose.

IMPLICATIONS: Dextrose-free 2-chloroprocaine is hyperbaric relative to cerebrospinal fluid at 37°C, and therefore can be used for spinal anesthesia without dextrose. Bisulfite-free 2-chloroprocaine remains very acidic (pH <4.0). The pH can be increased to more than 7.0 with a small amount of bicarbonate (0.25–0.33 mL/10 mL).

From the Department of Anesthesiology, Virginia Mason Medical Center, Seattle, Washington

Supported, in part, by the Department of Anesthesiology, Virginia Mason Medical Center, Seattle, Washington.

Presented, in part, at the 41st Annual Western Anesthesia Residents’ Conference, Palo Alto, California, April, 2003.

Disclaimer: Although 2-chloroprocaine has been approved by the FDA, it is not specifically indicated for use in spinal anesthesia. Its use for spinal anesthesia is thus considered “off-label.” All current manufacturers of 2-chloroprocaine distinctly label the product “Not for Spinal Anesthesia.”

Accepted for publication

Address correspondence to Dr. Kopacz, Department of Anesthesiology, Virginia Mason Medical Center, 1100 Ninth Avenue, B2-AN, PO Box 900, Seattle, WA 98111. Address email to

As commercially available solutions of 2-chloropro- caine (2-CP) have not been used for spinal anesthesia, their densities have not been determined. When it was first introduced in 1952, 2-CP was safely used for spinal anesthesia in 214 patients (1). After 1956, 2-CP was only available in solutions containing either the preservative methylparaben, or the antioxidant, bisulfite, both being inappropriate for an intrathecal drug because of the risk of neurotoxicity. In fact, 8 cases of neurotoxicity after unintentional injections of large volumes of 2-CP intended for the epidural space were reported between 1980 and 1982 (2–4). Subsequent laboratory studies have demonstrated the toxicity to be attributable to the bisulfite at a low pH and not to the chloroprocaine drug itself (5,6). Furthermore, in 1980, the United States (US) Food and Drug Administration concluded there was no increased neurotoxicity with chloroprocaine (Nesacaine®-CE) compared to lidocaine, bupivacaine, or mepivacaine (7).

A short-duration intrathecal drug is needed as a replacement for lidocaine in outpatient surgery. Spinal lidocaine has been associated with the potential side effect of transient neurologic symptoms (TNS) involving moderate to severe pain in the lower back radiating through the buttocks to the lower legs (8). In ambulating surgical patients, the incidence of TNS has been reported as frequent as 40%(9). With preservative- and antioxidant-free preparations of chloroprocaine now available (Nesacaine®-MPF, Astra USA, Wilmington, DE; chloroprocaine (generic), Bedford Laboratories, Bedford, OH), the research focus has returned to 2-CP as a potential short-acting intrathecal drug. The purpose of this study was to evaluate the density to 5 significant digits at 37.0°C, and the pH of 2% and 3% metabisulfite-free 2-CP solutions.

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The densities of six independent samples of five different local anesthetic solutions were measured to five significant digits (g/mL) using a DMA 4500/5000 Density/Specific Gravity/Concentration Meter (Anton Paar USA, Ashland, VA). Five primary solutions were studied:

  1. 2% 2-CP(Nesacaine®-MPF, Astra USA)(2 mL).
  2. 3% 2-CP (Nesacaine®-MPF, Astra USA)(2 mL).
  3. 2% lidocaine (Xylocaine®-MPF, AstraZeneca LP, Wilmington, DE)(2 mL) as control.
  4. 2% 2-CP with epinephrine added (0.2 mg epinephrine + 2 mL 2-CP).
  5. 2% 2-CP with 8.4% sodium bicarbonate added (0.2 mL NaHCO3 + 2 mL 2-CP).

The density of each of the above solutions was then tested with equal volume water dilution to produce a hypobaric composition, as well as with the addition of 0.25 mL dextrose (D10; producing a 1.1% dextrose solution) and 2 mL dextrose (D10; producing a 5.0% dextrose solution) to 2 mL of local anesthetic. The density was recorded when the temperature of the solution stabilized at 37°C ± 0.01. Before each sample reading, the density meter was calibrated with deionized, distilled water. Values are expressed as mean ± sd. Baricity is defined using cerebrospinal fluid (CSF) density measurements previously reported (10,11).

The pH of various solutions was tested using a model 8100 pH/temperature/mV meter (VWR Scientific Products, West Chester, PA). Ten mL of preservative-free 2% chloroprocaine, 3% chloroprocaine and 2% lidocaine were measured plain and with increasing concentrations of 8.4% sodium bicarbonate. Six separate samples of each solution were tested. Sodium bicarbonate was added to each 10 mL solution in the following amounts: 0.25, 0.33, 0.5, 0.75, 1.0, 1.5, and 2.0 mL. In addition, mixed solutions of 5 mL 2% chloroprocaine with 5 mL of D10 dextrose (5% dextrose), 10 mL 2% chloroprocaine with 1.25 mL D10 dextrose (1.1% dextrose), and 10 mL 2% chloroprocaine with 1 mL epinephrine (1 mg/mL) were each measured with 4 separate samples. All pH measurements were calibrated at 25.0°C.

The density and pH of the solutions were statistically compared using analysis of variance, with Bonferroni/Dunn for post hoc analysis, and P < 0.05 was accepted as significant.

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2-CP (2% or 3%) is hyperbaric relative to CSF (density, 1.00028–1.00100 g/mL) (10) before the addition of any dextrose (Table 1). Three percent 2-CP has a higher density than the 2% solution (P < 0.0001). Plain 2% lidocaine is the only dextrose-free solution measured to be hypobaric with a density less than 2-CP (P < 0.0001). All solutions become hypobaric relative to CSF when diluted with an equal volume of sterile water (Fig. 1). The expected relationship of increasing density with increasing dextrose concentration was observed. The addition of epinephrine does not alter the density of 2% 2-CP (P > 0.05). However, when dextrose is added, the epinephrine-containing solution becomes denser (P < 0.0001). The combination of NaHCO3 and 2% 2-CP has an increased density over all the other solutions tested (P < 0.0001).

Table 1

Table 1

Figure 1.

Figure 1.

The pH of 2% and 3% 2-CP is significantly less than that of 2% lidocaine (P < 0.0001). Each 2-CP solution is acidic with a mean pH range of 3.26–3.52 (Table 2). Adding only 0.25–0.33 mL sodium bicarbonate per 10 mL of local anesthetic increases the pH over 7.0. Plain 2% lidocaine has a pH of 6.45 ± 0.04, and its pH remains <7.0 until 0.5 mL of bicarbonate is added. There is minimal change in pH in any of the solutions with further added sodium bicarbonate after 0.5 mL (Fig. 2).

Table 2

Table 2

Figure 2.

Figure 2.

The addition of D10 (5%), epinephrine, or sterile water significantly altered the pH of 2% 2-CP (P < 0.0001)(Fig. 3). Except for plain 2-CP and 2-CP in 1.1% dextrose, and because of the very small standard deviation of all solutions, the differences in pH among all other solutions are statistically significant (P < 0.0001). However, compared with the large change in pH after the addition of the first 0.25 mL NaHCO3, these differences are relatively minor. No precipitation was observed for any of the solutions in the immediate mixing time nor was any seen after a prolonged time period (>24 h).

Figure 3.

Figure 3.

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The primary finding of this investigation is that dextrose-free 2-CP solutions are hyperbaric relative to CSF at 37°C. Therefore, plain 2% and 3% 2-CP become quite useful as hyperbaric intrathecal anesthetics and thus eliminated the need to add dextrose to increase baricity. This relationship is not affected by the addition of either epinephrine or 8.4% sodium bicarbonate. The density of dextrose-free lidocaine is consistent with earlier reports by Horlocker and Wedel (12), and Richardson and Wissler (11). Depending on the definition of hypobaricity, lidocaine is categorized as either hypobaric or isobaric. As hyperbaric solutions are clinically used most in the US, commercially prepared spinal anesthetic solutions are packaged in combination with dextrose. Based on the above findings, our recommendation regarding the use of 2-CP in the setting of a spinal anesthetic is to administer it plain, without the addition of any dextrose.

The density of an anesthetic solution is an important determinant of its spread in the CSF. The baricity of an intrathecal drug is largely responsible for the spread of a sensory block. Baricity is defined as the ratio between the density of the local anesthetic solution and the density of CSF at 37°C. Hyperbaric is a ratio more than 1.0, and hypobaric is a ratio less than 1.0. We use the CSF density range of 1.00028 to 1.00100 g/mL (10), defining the limits in most populations of hypo- and hyperbaricity, respectively. Because there is variation in CSF density among patients, three standard deviations below the mean has been historically used as the upper limit of hypobaricity. Yet the lower limit of hyperbaricity has not been defined. By applying the use of three standard deviations above the mean CSF density, we defined the lower limit of hyperbaricity as 1.00100 g/mL.

Richardson and Wissler (10) found that postmenopausal women (8 patients) have more variability in their CSF density (1.00070 ± 0.00018 g/mL). The upper limit of the CSF density may be as much as 1.00124 g/mL in a few patients (mean + 3 sd). Therefore, it is possible that the baricity of 2% 2-CP may be isobaric in a few patients, namely postmenopausal women, whose CSF density occurs in the outlier range.

Typically, dextrose is the easiest method to increase the density of an anesthetic solution and is necessary to achieve a hyperbaric solution, as most local anesthetics are hypobaric relative to CSF in plain concentrations (11,12). However, adding too much dextrose can decrease the reliability of a block. It has been shown that 0.8% glucose with bupivacaine (density, 1.0045 at 23°C) produced a more consistent block height than 8% glucose added to bupivacaine (density, 1.0203 at 23°C) (13). With similar relative density results in our study, it can be expected that 1.1% dextrose is more than adequate to increase the density compared with 5% dextrose and might produce a more reliable block height when used clinically. Although hyperbaric solutions are used for the majority of spinal anesthetics in the US, isobaric and hypobaric solutions remain useful for selected clinical situations. An easy 1:1 dilution of water with 2-CP produces a hypobaric solution as demonstrated in this study.

In recording the density of local anesthetics, temperature and precision of measurements are essential factors to consider. The density of a solution is inversely related to its temperature (14). Therefore, readings need to be evaluated at physiologic temperature (37.0°C) to make a fair assessment of the density in comparison with CSF. The density of bupivacaine decreases by 0.00007 g/mL with each increase in temperature of 0.20°C (11). In earlier work, Stienstra et al. (15) showed the necessity for precise measurements to 5 significant digits, reporting that differences as small as 0.0006 g/mL can influence the spread of local anesthetics in the spinal canal.

In addition to outpatient surgery, a short duration anesthetic such as 2-CP may also be of value in the obstetrical patient having combined spinal-epidural anesthesia for labor analgesia or a single injection spinal anesthetic for postpartum tubal ligations. In the study by Richardson and Wissler (10), the CSF density in pregnant women was determined to be 1.00030 ± 0.00004 g/mL, which is significantly less than that seen in men and nonpregnant women. Therefore, the use of plain 2-CP in the intrathecal space in the parturient will be even more hyperbaric considering the decreased CSF density.

Both the 2% and 3% concentrations of the current formulation of 2-CP are acidic, with a mean pH of 3.34 ± 0.01 and 3.31 ± 0.02, respectively, comparable to earlier preparations (16–18). However, these prior studies were done with earlier formulations of 2-CP containing either EDTA or bisulfite. Two separate reports were published on the amount of bicarbonate necessary to alkalinize 3% 2-CP to physiologic range (more than 7.0) (17,18). Although bisulfite-containing 3% 2-CP (Nesacaine®-CE) had a slightly higher pH (3.84 – 3.89) than current formulations, alkalinization of the newer product is achieved with less bicarbonate (0.25–0.33 mEq NaHCO3 per 10 mL of anesthetic versus 1.0–1.5 mEq per 30 mL of bisulfite-containing 2-CP). In 1987, bisulfite was replaced with EDTA (Nesacaine®-MPF). The pH of this formulation (2% 2-CP, 3.40; 3% 2-CP, 3.38) was closer to that of the current formulation, and alkalinization to physiologic pH could be accomplished with 0.5 mL per 20 mL (16).

Stevens et al. (19) have shown that the onset time for epidural anesthesia is decreased when the pH of 2-CP is increased closer to physiologic pH by the addition of sodium bicarbonate. The relevance of onset time and pH is unknown with regards to spinal anesthesia. The equilibration of pH between a local anesthetic and CSF is much more rapid with intrathecal injection, most likely negating any benefit of first increasing the pH with sodium bicarbonate. Nonetheless, if desired, the addition of only 0.25–0.33 mL of sodium bicarbonate per 10 mL of local anesthetic is enough to increase the pH of 2% and 3% chloroprocaine to the mean pH of 7.22 ± 0.05 and 7.07 ± 0.02, respectively.

In conclusion, the commercially available preparation of 2-CP (2% or 3%) is a hyperbaric solution. It is feasible to use this solution as a hyperbaric spinal drug as it currently is packaged. The preservative-free solution continues to have an acidic pH, which can be readily alkalinized if necessary. However, the impact of alkalinization in an intrathecal drug is likely to be minimal.

The authors would like to thank Suzanne Noe, Amgen Inc., Bothell, WA, and Sharon Kochik, Benoroya Research Institute, Seattle, WA for their technical assistance.

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