The effect of differential doses of bicarbonate on lidocaine pH and ionization
Abstract Number: 90
Abstract Type: Original Research
Epidural top-up using lidocaine 2% with or without epinephrine is the most commonly utilized technique to obtain surgical anesthesia in a laboring patient undergoing unanticipated Cesarean delivery (1). Bicarbonate is often added to the solution to raise the pH and improve the onset of blockade (2), however, the optimal dose required with commercially available solutions has not been well delineated. The aim of this study was to measure the effect of different doses of bicarbonate on both pH and percent unionized form of various 2% lidocaine solutions.
The pH of mixtures of lidocaine 2% with or without epinephrine and bicarbonate were measured using a Corning pH meter 320 (Corning, NY). We recorded the pH of 9 different solutions outlined in Table 1 at time 0 and 24 hours later. Five samples of each solution were determined to account for variability. Samples were prepared in plastic syringes from newly opened vials and kept covered from light to minimize degradation changes. Using a derivative of the Henderson-Hasselbalch equation, percent unionization was also determined for each sample, and the average for each group was calculated. Paired t-test and ANOVA were used as appropriate with p<0.05 considered statistically significant.
Findings are outlined in Table 1. Lidocaine containing premixed compared to freshly added epinephrine was significantly more acidotic (pH 4.34 vs. 6.30) and less unionized (0% versus 3%). Acidity and ionization differences were largely corrected by adding 1 ml per 10 ml of bicarbonate. Adding bicarbonate in a ratio of 2:10 resulted in the highest amounts of unionized lidocaine. After 24 hours, all samples containing bicarbonate increased in both pH and percent unionization (p< 0.05).
Commercially available lidocaine with epinephrine solution is significantly more acidotic and ionized than lidocaine with freshly added epinephrine. These results suggest that it may be optimal to use plain lidocaine and add epinephrine to the solution rather than using the commercially available lidocaine and epinephrine solutions. Future studies should assess whether these differences in percent unionized lidocaine results in faster block onset or improved block density.
1) SOAP Annual Meeting 2009 Abstract (A36). A Survey of Obstetric Anesthetic Practice for Cesarean Section.
2) Anaesthesia. 2001;56(8):790-4.