Although it has been reported that a total dose of a local anaesthetic is an important factor for the development of epidural anaesthesia [1,2], the effects of changes in the concentration and volume of a local anaesthetic are unclear [3-6]. Sakura and colleagues  demonstrated that there was no difference in the extent of the sensory block produced by epidural lidocaine 200 mg in a 1% solution compared with a 2% solution. However, there has been no study on the effect of changes in the concentration of a local anaesthetic on the spread of epidural anaesthesia in pregnant females. The present study compared the qualities of epidural anaesthesia using two different concentrations of lidocaine without changing the total dose during Caesarean section.
Forty ASA I patients who were scheduled for elective Caesarean section with epidural anaesthesia were enrolled in the study. The Ethics Committee of our institution approved the study, and informed consent for participation in the study was obtained from all patients. None of the patients had cardiopulmonary or neurological disorders.
No sedative premedication was given before surgery. Patients received at least 500 mL acetated Ringer's solution before anaesthesia, followed by 8 mL kg−1h−1 in the operating room. With each patient in the lateral position, the epidural space was identified using the midline approach with an 18-G Tuohy needle and loss of resistance at the L1-L2 interspace. An epidural catheter was then inserted 5 cm into the epidural space. The catheter was aspirated to exclude intrathecal or intravenous placement. Each patient was placed in the supine position throughout the study. Oxygen (3 L min−1) was administered via a facemask. By means of sealed envelopes, patients were randomly assigned to receive either lidocaine 1% 30 mL (Group 1) or lidocaine 2% 15 mL (Group 2), both solutions with 5 μg mL−1 epinephrine. Each epidural injection was performed over 1 min.
Electrocardiograms (lead II) and haemoglobin oxygen saturation were monitored continuously throughout the procedure. Heart rate (HR) and mean arterial pressure (MAP) were measured by an automatic oscillographic method every 1 min until delivery and thereafter every 5 min. A decrease in systolic arterial pressure of >25% from the preanaesthetic value was treated with intravenous ephedrine in 5 mg increments.
The quality of block was assessed by a blinded investigator at 5, 10, 15, 20 and 30 min after injection and at the end of surgery. The sensory block was evaluated by pinprick. The onset of analgesia was defined as complete loss of pinprick in the L1 dermatome. The degree of motor block was assessed using the modified Bromage scale for both legs (0: no paralysis; 1: unable to raise the extended leg; 2: unable to flex knee; 3: complete motor block) were assessed. Surgery was started when the sensory block was established at or above T6. Intravenous fentanyl at 50 μg increments was given to treat pain. Apgar scores were assessed at 1 and 5 min after birth.
Results were expressed as mean ± SD or median (range). Data were compared between groups with an analysis of variance, followed by Fisher's protected least significant difference test or U-test. P < 0.05 was considered significant.
The patient characteristics in the two groups were comparable (Table 1). Onset of analgesia was rapid with both concentrations (Table 2). The development of analgesia and of motor block after the epidural injection are shown in Fig. 1. No significant differences in the spread of analgesia were observed at any time between the groups. There were no differences in maximal cephalad and caudal spread with the two concentrations. The progression of motor blockade was faster in Group 1, but the difference was not significant.
Five and four patients felt pain during the operation in Groups 1 and 2, respectively, and seven patients were treated with fentanyl. There was no difference in the decrease in systolic blood pressure between the two groups. Ephedrine was given to six and eight patients in Groups 1 and 2, respectively, resulting in prompt recovery of blood pressure.
The effects of various concentrations and volumes of local anaesthetic solutions on the extent of sensory block are controversial [3-6]. Liu and colleagues  compared the effects of a threefold difference in concentration and volume of 2-chloroprocaine and found that there was a greater extent of sensory block with a high volume of a low concentration. On the other hand, Sakura and colleagues  reported that there were no differences in the upper level of sensory block to cold, pinprick and touch induced by lidocaine 1% 20 mL or lidocaine 2% 10 mL. The results of the present study extended these observations to pregnant patients.
Local anaesthetics in the epidural space cross the dura and exert their effects on spinal roots and spinal cord tracts. Thus, it is possible that dilution and distribution of an epidurally administered local anaesthetic in the cerebrospinal fluid lead to similar intensities of motor block regardless of the concentration .
The amount of epinephrine injected with the lidocaine in Group 1 (150 μg) was twice the amount in Group 2 (75 μg). Since epinephrine may exert an antinociceptive effect through spinal descending inhibitory pathways , this difference in the epinephrine doses might have influenced the results of our study. However, Brose and Cohen  reported that epidural lidocaine 2% 20 mL plus epinephrine 5 μg mL−1 induced similar analgesic effects (onset of analgesia, number of patients achieving a T4 blockade after 20 min, number of patients requiring supplemental analgesics) compared with lidocaine 2% plus epinephrine 2.5 μg mL−1. Moreover, Sakura and colleagues  demonstrated that epidural lidocaine 1% 10 mL with or without epinephrine 5 μg mL−1 produced similar time-courses of dermatome levels tested by pinprick. Therefore, we may assume that the difference in the doses of epinephrine did not have a clinically relevant impact on the results of our study.
Using a sensory testing device for quantitative measurement of cutaneous current perception threshold, Sakura and colleagues  demonstrated that the concentration of lidocaine affected the intensity of sensory block during epidural anaesthesia. Liu and colleagues  assessed motor block by isometric force dynamometry and demonstrated similar intensities of motor block by epidural block with different concentrations of 2-chloroprocaine. Although only qualitative measures of analgesia and motor block were applied in the present study, we believe that they were clinically relevant to estimate the effects of the epidural block.
In summary, the study demonstrated that lidocaine 300 mg given epidurally as a 1 or 2% solution produced comparable anaesthetic effects in pregnant females, as evaluated by the spread of analgesia and the Bromage scale. The finding suggests that the effects of epidural anaesthesia depend primarily on the total dose of the local anaesthetic.
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