The effect of pH and PCO2 on epidural analgesia with 2% 2-chloroprocaine.

Increasing the pH of local anesthetics with sodium bicarbonate has been reported to hasten their onset of action. The purpose of this study was to compare the onset and duration of epidural analgesia with the use of sodium bicarbonate and tromethamine to increase the pH of 2% chloroprocaine (2CP). Five groups of patients were studied: Group I received 2CP; Group II received 2CP buffered to a pH of 7.1 with tromethamine; Group III received 2CP buffered to a pH of 7.1 with sodium bicarbonate; Group IV received 2CP buffered to a pH of 7.7 with tromethamine; and Group V received 2CP buffered to a pH of 7.7 with sodium bicarbonate. The final pH and PCO2 of each solution were measured. Time to onset of analgesia was significantly delayed with either of the tromethamine buffered groups (II [5.6 +/- 1.0 minutes] and IV [5.4 +/- 0.4 minutes]) when compared with data from the unbuffered control (I [4.4 +/- 0.1 minutes]) and the sodium bicarbonate buffered (III [4.5 +/- 0.8 minutes] groups and Group V [2.7 +/- 0.9 minutes]). Only when sodium bicarbonate buffer adjusted to pH 7.7 (Group IV) was onset significantly more rapid than the unbuffered 2CP (I) and tromethamine buffered 2CP (II and IV). Multiple regression analysis revealed that onset times were significantly related to both pH and PCO2. The coefficient of determination for this model was 0.5156.(ABSTRACT TRUNCATED AT 250 WORDS)