From Ms R. Taylor, FRCA,
and Ms K. Stuart-Smith
SIR,—Day case surgery requires safe, simple and effective analgesia.
Anaesthetists usually request that surgeons infiltrate wound sites with local
anaesthetics, principally bupivacaine, at the end of surgery to achieve patient
comfort without the use of strong narcotic analgesics. This manoeuvre provides
excellent analgesia in the immediate post-operative period, but pain inevitably
recurs in the evening hours when the patient has returned home.1 This simply
displaces patient management from the hospital to the on-call general practitioner,
hardly a satisfactory solution. Ideally, local analgesics should provide patient
comfort for at least 36 hours after surgery, ie, until the inflammatory response
has abated.
In spite of the widespread use of bupivacaine in skin infiltration, studies
examining duration of action, effectiveness and safety are few and contradictory.
Many substances have been added to bupivacaine solutions in an attempt to prolong
their action. We became interested in studies which demonstrated that mixing
the local anaesthetic with dextran solution prolonged the duration of the local
anaesthetic block.2 The method by which dextran is thought to prolong local
anaesthesia is still not fully understood. Dextran forms soluble molecular complexes
with bupivacaine3 and the higher molecular weight dextrans absorb more local
anaesthetic molecules and so presumably improve the duration of action.4 From
the available data, it was not clear how effective the dextran was at prolonging
the action of local anaesthetics. To look into this we performed a prospective,
randomised, double-blind, controlled study on 24 healthy volunteers (all anaesthetists)
to compare the duration of action of two intradermal injections, one containing
0.375 per cent plain bupivacaine and the other 0.375 per cent bupivacaine plus
dextran 70 solution. The bupivacaine solution was a racemic mixture.
A 0.5ml portion of each solution was injected intradermally into the volar aspect
of separate forearms and the areas demarcated. The volunteer was then asked
to test and note sensation to pinprick in each area at regular intervals. The
dextran/bupivacaine mixture varied in its duration of action from 6h to 34h,
with the median time being 20h. For plain bupivacaine the time varied from 6h
to 30h, with a median time of 16h. Overall, there was no difference between
the two solutions. The most alarming finding was the large variation in the
duration of action of bupivacaine. This variation was not related to the person
administering the local anaesthetic, nor the individual being tested, as some
individuals were tested on more than one occasion and provided widely different
results each time. This huge variability may explain the contradictory results
of the past literature: the duration of bupivacaine-induced analgesia appears
to be a matter of luck, whether dextran is present or not.
How then, is the action of bupivacaine to be prolonged in a consistent manner?
This letter is a plea to those working in the pharmaceutical world to suggest
a simple an effective way to make our patients’ lives more comfortable.
Ruth Taylor
Specialist Registrar
Karen Stuart-Smith
Lecturer
Department of Anaesthesia,
Bristol Royal Infirmary
References