Pre-emptive analgesia and the paravertebral space—an ignis fatuus
Editor—Lönnqvist has once again cast the spotlight on pre-emptive analgesia.1 The idea to induce a process of change, akin to immunization, against pain at some point in advance of a predictable insult, has mesmerized as it was floated in an editorial on the strength of retrospective observational studies.2 This seminal publication used the term ‘prevention’ and was a leap into the intellectual dark by a basic scientist. Clinicians took a leap of faith and followed. Since, it has been called ‘pre-emptive’, become doctrinaire, and is interpreted by reference to animal models as something different from the common-sense prophylaxis of taking an analgesic before the dentist extracts your teeth or using an analgesic as premedication.Early studies of the paravertebral space led me to advocate a standard regimen of two percutaneous paravertebral local anaesthetic injections bracketing the T5 or T6 intercostal spaces by injecting at T3 and T7 before surgery.3 4 The reasoning was that a consistently effective dose would then be at the space through which surgery is conducted, which is most disrupted and from which the bulk of nociception is generated. Injection ablates the pressor response to this stimulating process (primary objective).5 Spread of local anaesthetic up and down from the point of injection results, in the majority of cases, in antinociception for the dermatomes affected by incision and muscle splitting of lateral thoracotomy (secondary objective). Only if the surgery is rapid is there any lingering analgesia to tide the patient over into recovery—and that only covers intercostally mediated nociception (tertiary objective).6 7 Achievement of the latter is a bonus. Nowadays, it is achieved more consistently by placing a catheter at the termination of surgery.
I teach the same routine of bracketing the T6 space for thoracoscopic cases. The pressor responses to port insertion and pleural abrasion (below T2) are modified. As in the case of thoracotomy, the only pre-emption achieved is in pressor responses to surgery.
Other studies, such as those quoted by Lönnqvist, have demonstrated that, on occasion, a single injection appears to function outside the therapeutic profile, sometimes beyond 24 h. Here is the falsehood and danger of the pre-emptive doctrine! The conclusion drawn should not be that this is the unique fingerprint of pre-emptive analgesia but that more likely are factors, such as sensitivity, neuropraxia and even neurolysis (perhaps surgical). It must be a given that these situations are abnormal, not be taken for granted, nor seen as evidence of a successful therapeutic process. Harm, not good, more likely has been done. Alarm bells should ring. Signs of an abnormally prolonged action of an agent should, in this field, alert to the potential of a chronic post-thoracotomy pain syndrome developing. And, sadly for aficionados of the pre-emptive doctrine, even a complex crossover or a quantum defining methodology will always be subject to the same problem of a differential diagnosis of drug sensitivity and temporary or permanent nerve damage.8–10
Despite the diverse nature of algesia of thoracic surgery and the nature of the treatment, it is a consistent observation that there are no significant differences in pain analogue measures at rest 4 h after the end of the main algesic insult.7 It is irrespective of whether the analgesia is given as a premedication [opioid or local anaesthetic (epidural or paravertebral)], perioperatively or after operation rather than any advantage programmed by pre-insult administration of analgesic.
Of the four objectives for pain relief techniques for thoracic surgery, only two are met by paravertebral injection—the humanitarian, and some reversal of the effects of surgery on respiratory and metabolic function. Epidural techniques meet a third, more ill-defined and less easily measured objective, that of active promotion of healing (unnecessary for most thoracic surgery but obligatory for such operations as oesphagectomy). There is no evidence, but wishful, that a fourth objective—preventing transition to a chronic pain syndrome—is achieved by any analgesic technique or way of administration.6 7 Attempting to achieve pre-emptive analgesia by injecting local anaesthetic into the paravertebral space is a good example of what J.S. Haldane (1860–1936) called an ignis fatuus.11 My intuition is that the same is true for the whole doctrine.
I. D. Conacher
Newcastle upon Tyne, UK
E-mail: i.d.conacher{at}btinternet.com
Editor—I read with great interest and pleasure the letter from Dr Conacher who has extensive experience in the field of thoracic anaesthesia in general, and paravertebral blockade in particular. It is also always nice to read communications that show a great knowledge not only in medicine but also regarding history and literature and I am pleased to be spared the ‘Occam's razor’ principle this time around.12
As alluded to in the editorial, some scientists and practitioners frown upon the potential concept of pre-emptive analgesia and it is evident that Conacher agrees with this line of reasoning, based on arguments clearly laid forward in his letter. I cannot but agree that most attempts outside animal and volunteer studies have failed to take this concept into clinical medicine. However, for a number of reasons put forward in the editorial, I propose the idea that paravertebral nerve blockade may, and I repeat may, make this block different from other regional techniques. Furthermore, my conclusion in the editorial, as in this reply, is that paravertebral blockade deserves more widespread use, whether producing pre-emptive analgesia or not, as it has been shown to provide better postoperative analgesia than alternative methods. I will leave for the reader to judge whether this conclusion is valid and as near the truth as we can get today, or if I just represent one of those ‘few enthusiasts’.
Stockholm, Sweden
E-mail: per-arne.lonnqvist{at}karolinska.se
References
1 Lönnqvist PA. Pre-emptive analgesia with thoracic paravertebral blockade? Br J Anaesth 2005; 95:727–8
2 Wall PD. The prevention of postoperative pain. Pain 1988; 33:289–90[CrossRef][Web of Science][Medline]
3 Conacher ID and Kokri M. Postoperative paravertebral blocks for thoracic surgery: a radiological appraisal. Br J Anaesth 1987; 59:151–61
4 Conacher ID. Resin injections of thoracic paravertebral spaces. Br J Anaesth 1988; 61:657–61
5 The Thoracic Paravertebral Space and the Relief of Pain of Thoracic Surgical Origin Conacher ID. Rate pressure product and paravertebral block. 1989;36–50 MD Thesis, University of Dundee
6 Conacher ID. Post-thoracotomy analgesia. Anesthesiol Clin North America 2001; 19:611–25[Medline]
7 Conacher ID and Slinger PD. Pain Management. In Kaplan JA and Slinger PD (Eds.). Thoracic Anesthesia 2003. 3rd Edn. Philadelphia Churchill Livingstone pp. 436–62
8 Conacher ID. Anaesthesia for thoracoscopic surgery. Best Pract Res Clin Anaesthesiol 2002; 16:53–62[Medline]
9 Soni AK, Sudarshan G, Conacher ID. Evaluation of a clinical method to detect pre-emptive analgesia. International Monitor of Regional Anesthesia Abstract Issue. 11th Annual ESRA Congress 1993; 1:Suppl, 47
10 Conacher ID, Sudarshan G, Soni A. Pain disaggregation theory. Br J Anaesth 2003; 61:657–61
11 Haldane JS. Effects of low atmospheric pressures. Respiration 1922.New Haven Yale University Press pp. 366
12 Conacher ID. 2000—Time to apply Occam's razor to failure of hypoxic pulmonary vasoconstriction during one lung ventilation. Br J Anaesth 2000; 84:434–6
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