Intra- and inter-Individual variability in nerve block duration: A randomised cross-over trial in the common peroneal nerve of healthy volunteers.

The mean sensory block duration of 380 (95% CI = [342; 418]) minutes on day one was 55 [33; 77] minutes longer than on day two (P < 0.001), but there were no differences in mean duration between days two, three and four. The ratios with 2.5; 97.5 percentiles between inter- and intra-individual variation were 2.4 [0.8; 5.2] for the five-millilitre blockades and 3.0 [0.9; 6.7] for the ten-millilitre blockades. The probabilities of inter- to intra-individual variation-ratios > 1 were 96% and 97%. Conclusion The intra-individual variability is a substantially minor contributor to the overall variability in sensory nerve block duration compared with the inter-individual variability.