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LETTER TO THE EDITOR

Identification of the Epidural Space

Department of Anaesthesia; Sunnybrook and Women- College Health Sciences Centre; Toronto, Canada; StephenHalpern{at}sw.ca

To the Editor:

We read with interest the article by Evron et al. (1). Unfortunately, there are some ambiguities in the tables that cast doubt on the main conclusions. In particular, it is not possible, in Table 2 of their article, to determine how the P values were derived. Using a two-tailed Fisher's exact test and comparing the "air" to the "lidocaine" groups the P value for the incidence of accidental dural puncture is 0.2, not <0.02. If the two lidocaine groups are combined, the P value becomes 0.035, favoring the combined group. This does not make clinical sense, since the needle in the "air + lidocaine" group is placed using loss of resistance to air. Also, the P value for the incidence of unblocked segments is 0.06 when the "air" group is compared to the "lidocaine" group and 0.03 when air is compared to the " air +lidocaine" group. While the latter value is statistically significant, it may be due to either the increase in volume of 2% lidocaine in the "air + lidocaine" group (3 vs 6 mL), or statistical error (multiple testing).

When reporting P values, it is important to state exactly how they were derived in order to aid in interpretation.

Reference

1. Evron S, Sessler D, Sadan O. Identification of the epidural space: loss of resistance with air, lidocaine, or the combination of air and lidocaine. Anesth Analg 2004;99:245–50.[Abstract/Free Full Text]

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