[Microscopic characteristics of epidural filter pores]

The increased use of the epidural route for administering opioids to treat chronic pain and the need to reduce complications as much as possible, has led some authors to recommend using micro filters to reduce catheter contamination. This study was motivated by the lack of technical information documenting epidural filters used routinely, as well as by the scarcity of literature describing their characteristics. Our aim was to investigate the true nature of the membrane pores, their characteristics and dimensions.

Samples from 30 epidural filters labelled "Porosity: 0.2 microns" from three different manufacturers were studied. Filters from Vygon, Braun and Abbot were labelled A, B and C, respectively. The samples were placed in six groups of five filters each, and 15 random studies were made of each sample. Three of the six groups were used to study prefiltration surfaces and the others to study postfiltration surfaces. Each sample was metalized with gold and its center was then studied by scanning electron microscope. Given that the pores were anfractuous, they were measured by taking the diameter of the largest circle fitting inside that could predict the size of the smallest spherical non elastic body that might be retained. The samples for measuring thickness were cryofractured for determining the number of filtration planes in the 15 filters.

Prefiltration surface: Pore diameters were 0.70 (0.66 to 0.74), 0.45 (0.41 to 0.49), and 2.077 (2.01 to 2.15) microns on the filtration surfaces of manufacturers A, B and C, respectively. The differences were significant (p < 0.01) and the pore shapes were also different. Postfiltration surface: The function pores of filters from manufacturers A and B measured 0.26 (0.25 to 0.28) and 0.26 (0.24 to 0.28) microns, and the differences were not significant. The pores of filters from company C were significantly larger (p < 0.01), measuring 0.46 (0.43 to 0.49) microns. There were significant differences (p < 0.001) in pore size on the pre- and postfiltration surfaces from all three manufacturers. Gauge: The five A, B and C filters averaged 130, 118 and 165 microns thick, respectively, with an average number of 140, 220 and 210 filtration planes, respectively.

The pores of filters for epidural use labelled "0.2 microns" actually had much larger pores on their prefiltration surfaces and throughout the membrane thickness. On the postfiltration surface, however, the diameters of pores on filters manufactured by Vygon and Braun approached 0.2 microns. Pores on filters manufactured by Abbot, however, were approximately 0.46 microns. We believe that in the future manufacturers should include more information in the documentation accompanying their filters.