Outflow resistance of the Baerveldt glaucoma drainage implant and modifications for early postoperative intraocular pressure control

Purpose: To determine outflow resistance of the Baerveldt glaucoma implant using different tube configurations. Methods: Outflow resistance of 6 tube configurations (C1-C6) of Baerveldt implants was measured under conditions of constant pressure perfusion. Pressures ranged from 2 to 55 mm Hg. Venting slits were created using a 7-0 Vicryl, spatulated suture-needle. Seton tubes were occluded by threading a retrograde suture similar to1.5 cm into the lumen. Results: At pressures between 2 and 55 mm Hg, mean outflow resistance of the normally configured scion (ie, open tube; C1) was 0.41 (+/-0.6) mm Hg/muL/min. Resistance was unchanged (mean 0.41 (+/-0.4) mm Hg/muL/min) by the addition of 4 venting slits (C2) to the seton tube. Occlusion of the open scion tube with a 3-0 Supramid(R) suture (0) significantly increased (P < 0.001) mean outflow resistance to 14.99 (+/-0.6) mm Hg/muL/min. Occlusion of the tube with a 4-0 Supramid(R) suture (C4) significantly increased (P < 0.001) mean outflow resistance to 1.09 (+/-0.5) min Hg/muL/min. In implants where tubes were occluded with a 3-0 Supramid(R) suture, the addition of venting slits (C5) significantly decreased (P = 0.038) mean outflow resistance to 8.98 (+/-0.4) min Hg/muL/min. In tubes occluded with a 4-0 Supramid(R) suture, the addition of venting slits (C6) decreased mean outflow resistance to 0.98 (+/-0.6) min Hg/muL/min. Conclusions: Although these results cannot be directly correlated to the clinical setting, they do show that outflow resistance can be modified at the time of surgery by changing tube configuration of the Baerveldt glaucoma implant. Configuration C5 (3-0 Supramid(R) with venting slits) closely approximates the outflow rate in the normal intraocular pressure range.