Effectiveness of a prolapsed bird's nest filter.

PURPOSE

The authors evaluated the variable deployment of the Bird's Nest filter, including frequency and extent of prolapse in clinical use, and analyzed the effects of this variability on clot-trapping efficiency with an in vitro model.

MATERIALS AND METHODS

In the clinical placement of 20 filters, the average length of cephalic filter wires from the center of the filter was measured from radiographs obtained immediately after placement. To analyze the effects of prolapse, a variable-rate pump was used to mimic the effects of respiration on IVC flow. Four clot sizes (5 x 20, 5 x 40, 10 x 20, and 10 x 40 mm) were evaluated with four filter configurations with wires stretched to different lengths in a cephalic direction from the center of the filter: 2 cm (normal-tight), 5 cm (normal-loose), 8 cm (moderate prolapse), and to a maximum of 12 cm (maximum prolapse). Ten passes for each clot size were performed with each filter configuration in both the horizontal and vertical positions. The smallest clots were also tested with a slower constant-rate pump.

RESULTS

In clinical use, the average length of the filter wire was 5.4 cm (range, 1.2-9.0 cm). With the variable flow pump, the in vitro degree of prolapse did not significantly decrease the trapping efficiency for any but the smallest (5 x 20-mm) clots, for the maximum prolapse configuration in the horizontal position (P = .01). In addition, for these clot sizes, the filter was more efficient with the slower constant rate compared with the faster variable rate; this difference was only statistically significant in the horizontal position for the maximum prolapse configuration (P = .007).

CONCLUSION

The results suggest that in clinical practice, a prolapsed Bird's Nest Filter remains effective for all but small clots.