Logic in the safe practice of spinal anaesthesia

An aura of infallibility no longer surrounds the medical profession. This may be a bad thing for patient confidence, but it is certainly good for the practice of medicine. The demand for evidencebased medicine is forcing us to question traditional practices and audit is ferreting out the backsliders ± supposedly. Unless we question our skills and put them to the test, improvement is unlikely. One area of unwarranted complacency is the belief among many experienced anaesthetists that they can identify intervertebral spaces accurately by palpation. Errors are often assumed to be due to lack of skill or experience. Now Broadbent and her colleagues [1] have shown that this may not be the case. A team of four experienced anaesthetists attempted to identify interspinous spaces in the lower back in 100 patients of varying ages and body mass indices. The marked spaces were then identified using MRI scanning. Remarkably, interspaces were correctly identified in only 29% of cases, with the actual space being higher than assumed in 68%. The L324 interspace, one that is popular for lumbar epidural catheter insertion in labour, and certainly the highest that all good anaesthetists should aim for spinal insertion, was identified incorrectly more frequently than any other. In 85 instances, the anaesthetist believed they had identified L324 but they were correct in only 13, the remainder being from 1 to 4 spaces higher than assumed. Among these four anaesthetists, experience did not improve accuracy of identification. As the authors pointed out, in the absence of radiographic feedback there may be little opportunity to become more skilful by learning from one's mistakes. In this respect, pain specialists, who regularly use radiographic control of needle placement, may have the advantage over the average clinical anaesthetist. Other investigators reporting smaller series with fewer observers and using cadaveric dissection [2], plain X-ray of the lumbar spine [3] or injection of radiocontrast after intrathecal catheterisation [4] have also demonstrated how inaccurate can be identification of intervertebral spaces from surface markings. The commonest method of identifying lumbar interspaces is to use Tuffier's line, that joining the two iliac crests, or half of Tuffier's line by dropping a perpendicular from the iliac crest to the lumbar spine. The former is probably more accurate [2], particularly in obese or pregnant patients, in whom the pelvis may be tilted in the lateral position. In either case, however, accuracy depends on the iliac crests being at a constant vertebral level. Other approaches such as counting down from C7 or finding the vertebra that is attached to the twelfth rib are usually less practicable. The former is tedious if treating acute pain, and the latter no help in the obese patient. So we are left with the iliac crest. Herein lies the problem. Tuffier's line is more accurately identified at Xray than by palpation, particularly in the obese. Yet even then it does not bear a constant relationship to an individual vertebra or interspace. Hogan, using data aggregated from cadaveric dissection and radiographic studies [5], showed that Tuffier's line may lie at the modal level of the L425 interspace in 35% of individuals, but in the remainder its distribution extends from L324 to the L5/S1 interspace. This would surely explain why, in the study by Broadbent et al. [1], anaesthetists were more likely to agree with one another than with the MRI scan. But why does this matter? The rise in popularity of a technique often depends on the introduction of a new drug or gadget. The vast increase in the use of epidural blockade in the 1970s followed the advent of bupivacaine, at a time when spinal anaesthesia was in comparative obscurity. Then the re-introduction of atraumatic spinal needles led to a reemergence of the older technique, which is now the commonest mode of anaesthesia for Caesarean section [6]. While epidural blockade was so widely used, anaesthetists became `gung-ho' in their choice of interspace for its insertion. Returning to spinals, many appeared to have forgotten the lesson taught to every medical student, that it is advisable to enter the lumbar subarachnoid space `below the termination of the cord ± usually between vertebrae L4 and L5' [7]. Some anaesthetic textbooks have been less cautious, advising that `the interspaces L223 and L324 are both the safest and the easiest' [8]; `Precise identification of the lumbar spines may be impossible, but this does not matter so long as the first lumbar interspace and those above this level are avoided' [9]; `usually L2 to L3, L3 to L4, or sometimes L4 to L5' [10]; `The use of the L223 space rather than the L324 space helps to ensure that the block will extend high enough' [11]; `Depending on the special anatomic features of the patient, the second, third or fourth lumbar interspace may be found most suitable for needle insertion' [12]. Note that these recommendations all relate to spinal rather than epidural insertion. Such advice may assume that anaesthetists are more accurate than medical students in their identification of levels, and that the adult spinal cord always ends at or above the L122 interspace. Given the above, the first assumption is dubious, but what about the second? As Broadbent and her colleagues point out [1], anatomists have long established that the tip of the conus may lie below the body of L1 in up to 58% of adults [13], and below L122 in about 42% [5, 13]. So, considering the inaccuracy of clinical means of identifying lumbar interspaces, the tendency of anaesthetists to err upwards and the uncertainty of the length of the spinal cord, even in normal individuals, it cannot be logical