Experimental and clinical studies about the preemptive analgesia with local anesthetics. Possible reasons of the failure.

Though we fully agree with the truthfulness of the physiopathological sequences proving the phenomena of the hyperalgesia onset and therefore a prolongation of the postoperative pain, we do not believe, as stated by some authors that the studies carried out up to now, both experimental and clinical, are sufficient, the first ones to confirm the preemptive analgesia, while the second ones to be defined as contrasting and totally inadequate to confirm the preemptive analgesia effect. The lack of positive clinical data is why some authors have suggested a "revision" and a "reduction" of the word preemptive analgesia. On the contrary, we believe that differences which seem to originate from the clinical works can also be found in the experimental ones if we examine them with the same methodological principle. In order to classify better such an important treatment like the preemptive analgesia, the aim of this note is to focus the attention on the experimental and clinical data. Our criterion to select the works has based on a backwards research taking into consideration the last reviews and studies about the preemptive analgesia. Since the results obtained with local anesthetics or with opioids seem to counterbalance each other, we have reexamined the experimental and clinical works limiting our attention to the local anesthetics. On a total of 19 assessments carried out in 16 experimental studies, only three of the eight ones comparing pre-vs post-administration prove a real preemptive analgesia effect of the local anesthetic (block of the first and second phase of hyperexcitability) whereas four of the eleven clinical studies examined seem to confirm the validity of the preemptive analgesia effect. These data show how the results of the experimental studies are equal to the results of the clinical ones. However, an important characteristic that seems to come out from these works is connected to two elements strictly linked each other: the intensity and the duration of the nociceptive impulse produced and the level and the type of the induced block. In our opinion, the failure of many of these works can be attributed to the inadequacy of the analgesic levels (intensity of the block) reached and maintained in the pre-and intraoperative period. Therefore we believe that before thinking of a "review" and "reduction" of the word preemptive analgesia is necessary to take a step backward and to face again the methodological problems of the preemptive analgesia.