Renal extracorporeal shock wave lithotripsy (ESWL) is a non-invasive therapeutic procedure that may cause pain, though with the arrival of second generation lithotriptors the necessity for general or regional anaesthesia has notably decreased . The anaesthetic management of patients during ESWL in this institution is limited to those patients who did not tolerate a first session without i.v. analgesia. Most of these patients are ambulatory, it is therefore necessary to examine any method that diminishes the requirements for i.v. analgesia.
EMLA cream is a eutectic mixture of lignocaine (2.5%) and prilocaine (2.5%) for topical use. It penetrates to a depth of 5 mm through intact skin after 90 min application . This is because of its high water content and the high concentration of local anaesthetic in base form. It has been successfully employed for venous cannulation , arterial cannulation , lumbar puncture , condylomata acuminata , removal of preputial accretions  and myringotomy in children .
Some authors regard skin to be one of the most important areas of pain perception during ESWL [9,10], while for others  it is not. The aim of this study was to evaluate the effectiveness of EMLA cream in controlling pain during renal ESWL.
The study was approved by the institution Ethics Committee and informed consent obtained from 20 ASA physical status I-II, non-premedicated, ambulatory patients, who had not tolerated the first session of ESWL without i.v. analgesia. One hour before the second session of ESWL, EMLA cream (10 g) and an occlusive plaster were applied to each patient on the skin over the area (64-100 cm2) where the shock waves were to be focussed. The cream and the occlusive plaster were removed immediately before the procedure. Intravenous boluses of fentanyl 0.05 mg were used for rescue medication until an adequate tolerance to the procedure was achieved, when EMLA cream provided insufficient pain control.
A second generation lithotriptor Siemens Lithostar (Siemens AG, Erlangen, Germany), with a voltage range of between 13 and 19 kV, was used for both sessions. The sequence of shocks were the same in the two sessions beginning at 13 kV and employing 3000 shocks to reach an intermediate voltage of 16-17 kV. This voltage range was maintained during 2000 additional shocks. When possible, during the next 2000 shocks the voltage was increased gradually up to 19 kV or up to a lesser voltage when stone fragmentation was achieved. Each patient provided both control and study results.
The results recorded by the same blinded observer at the end of each session were: maximum voltage achieved (kV), successful stone fragmentation (SSF), visual analogue scale (VAS) score (0-10 points) for assessment of pain, and tolerance scale (TS) score (1-4 points). Tolerance scale was defined as follows: (1) absence of facial pain signs; (2) facial pain signs; (3) 2 + verbal reference of pain; (4) 3 + body movements. Criteria for i.v. analgesia administration were: TS score ≥3. Fentanyl (0.05 mg) was given at intervals of 5 min to obtain a TS ≤2.
Statistical analysis consisted of Wilcoxon matched-pairs signed-ranks test for kV, VAS score and TS score; and the exact test for matched pairs based on a binomial distribution for SSF. Signification tests were performed bilaterally, where a P-value of less than 0.05 was considered significant. To compare the characteristics of the patients who needed fentanyl and of those who did not in the second session of ESWL the Mann-Whitney U-test was used.
The patients consisted of 13 females and seven males, the mean values with standard deviation were: height 159.3±15.6 cm, weight 66.9±14.6 kg and age 46.8±17.5 years.
During the second ESWL session, when EMLA was used, a higher kV, a lower VAS score, a lower TS score and a higher SSF rate were obtained. These differences were all significant (Table 1). In nine subjects (45%) i.v. fentanyl was not required. Fentanyl (0.05 mg) was given to two patients, one was given 0.1 mg and another 0.15 mg. Those patients who needed fentanyl and those who did not were studied separately (Table 2). The differences (mean ± SD) in gender (13 female, seven male), age 46.8 ± 17.5 years, weight 66.9 ± 14.6 kg and height 159.3±15.6 cm were not significant. It was noticed that the patients who needed fentanyl obtained a significantly higher TS score and a lower kV during the first ESWL session, without EMLA. Those patients who did not need fentanyl during the first session, the results for kV, VAS, TS and SSF in the session with EMLA were better than in the previous session without EMLA (P<0.05) (Table 3). No complications relating to EMLA cream were detected.
The pain produced during an ESWL session may be caused by many factors: stone movement, impingement of the shock wave on the peritoneum or renal pelvic tissues or, in some patients, by the shock wave accidentally hitting the 12th rib. Pain originating from skin nerve endings can also play an important role [9,10].
It may be assumed that, if EMLA cream had not been used, all the patients would have been given fentanyl during the second session, none of them having been able to tolerate the first session without i.v. analgesia. The conditions for the study were kept reasonably constant, as each patient was his/her own control and the area of impact for the shock waves was the same in the two sessions except in one patient. Forty-five percent of the patients did not require i.v. analgesia during the second ESWL session. In these patients EMLA cream alone increased kV, decreased VAS and TS scores and resulted in a higher number of SSF. For the remaining patients (55%) the fentanyl requirements were low. Bierkens et al., using EMLA cream (30 g) over 100 cm2 area and the same lithotriptor as the one employed in the present study, found a decrease in the fentanyl requirements in a group of patients using EMLA compared with a group using a placebo but the results did not achieve significance. MacDonald and Berry  found no differences using EMLA cream (60 g) over a 400 cm2 area and a second generation lithotriptor Stortz Modulith SL20. Both groups employed higher doses of EMLA cream than in the present study. The results reported here suggest a beneficial effect of EMLA perhaps because a more gradual sequence of increments in kV was used. Moreover, in none of these studies were the criteria for fentanyl administration described. Monk et al., using EMLA cream (30 g) over a 300 cm2 area and an immersion Dornier HM3 (Dornier Medical Systems, Inc., Marietta, GA), found a significantly lower VAS score with EMLA only at high voltage levels but did not observe a decrease in supplemental analgesic requirements. Conversely, Tiselius , using 30 g of EMLA cream over an area of 225 cm2 and a Dornier HM3 lithotriptor at lower voltage than Monk et al., found a significant decrease in the analgesic requirements in premedicated patients receiving EMLA cream compared with placebo. We think that this was because the voltage he employed was initially lower and increased gradually.
A positive factor in the present study is that each patient was also the control, a very useful approach to small population samples. On the other hand, the placebo effect was not considered. The role of patient anxiety in the second session is difficult to interpretate. Anxiety may have been lower because the procedure was already known, or perhaps it might have been higher because of the painful and unsuccessful experience of the first session. One of the characteristics of our protocol was that no patient received i.v. analgesia during control sessions. Thus, it can be assumed that EMLA cream alone was effective in controlling pain in 45% patients who had not tolerated a previous session of ESWL without i.v. analgesia. In the remaining 55% of the patients the fentanyl requirements were low. Moreover, it was possible to compare the patients who needed fentanyl and the patients who did not, in respect of VAS score, TS score and kV during the first session. The former had presented significantly worse TS score and kV, the VAS score had also been higher but this difference was not significant.
Although kV seems to be more objective than VAS or TS scores, it may be that kV is unreliable in the context of this study because the therapeutic results (SSF) using a similar voltage for the first and second sessions may be different as a result of the patient's better tolerance and comfort during the second session. This would allow shock waves to be focussed more properly on the selected area. A SSF or a possible subclinical fragmentation of the stone after the first ESWL session could lead to easier fragmentation in the second session at a lower voltage. In this setting, we believe that VAS and TS scores can be used to evaluate EMLA cream's effectiveness more accurately.
In conclusion the topical application of EMLA cream (10 g) 60 min before renal ESWL, prevented the need for i.v. analgesia in 45% of the subjects, improving the tolerance and comfort of those patients who could not tolerate the previous session. We attribute these favourable results to the gradual sequence of increments to maximum voltage (compared with the results of others [1,9,11]), and to the cutaneous anaesthesia produced by EMLA cream (compared with the control session).
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