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There is a renewed interest in understanding the precise role of lymphatics in the ultrafiltration kinetics during peritoneal dialysis. In the normal state, lymphatics draining the peritoneal cavity are the principal means of removal of intraperitoneal isosmotic fluid and macromolecules. During a hypertonic peritoneal dialysis exchange, after peak intraperitoneal volume is achieved, fluid removal proceeds at an almost linear rate, causing intraperitoneal fluid volume to reduce. The isosmotic fluid removal from the peritoneal cavity could occur through the microcirculatory capillaries or through the lymphatic capillaries draining the peritoneal cavity. Animal and human studies suggest that this fluid loss occurs primarily through lymphatics. The two indirect methods of lymph flow measurements, plasma appearance and peritoneal disappearance of tracer colloid, show conflicting results. Although direct measurement of lymph flow rates through cannulation of mediastinal lymph vessels in animals suggests a significant flow through the lymph channels in response to intraperitoneal fluid instillation, lymph flow modification at the lymph node level may prevent use of this technique to assess the precise role played by lymphatics in fluid kinetics during peritoneal dialysis. By analogy with ascites and by extrapolation from previous studies of drain volumes after infusion of isotonic and hypertonic solutions, the average daily lymph absorption rate during CAPD may be predicted to be at least 1 liter per day.

Intraperitoneal fluid is absorbed continuously by convective flow into the peritoneal cavity lymphatics. We evaluated the role of lymphatic absorption in the kinetics of peritoneal dialysis during standardized four hour exchanges in six children using 40 ml/kg of 2.5% dextrose dialysis solution. Cumulative lymphatic absorption averaged 10.4 +/- 1.6 ml/kg and reduced the total net transcapillary ultrafiltration during the dwell time by 73 +/- 10%. Due to the considerable lymphatic absorption rate, maximum intraperitoneal volume was observed before osmolar equilibrium. Extrapolated to four study exchanges per day, lymphatic absorption decreased the potential daily drain volumes in the children by 27 +/- 5% and daily peritoneal urea and creatinine clearances by 24 +/- 4% and 22 +/- 5%, respectively. Compared with four hour exchanges using two liters of 2.5% dextrose dialysis solution in 10 adult CAPD patients with average peritoneal transport, the children had more rapid equilibration of urea, greater absorption of dialysate glucose, higher lymphatic absorption and lower net ultrafiltration (P less than 0.01 to P less than 0.05). Lymphatic absorption therefore causes a relatively greater reduction in net ultrafiltration and solute clearances in children than in adults.

Net ultrafiltration was measured directly during hypertonic peritoneal dialysis exchanges in rats. Simultaneously, lymphatic absorption was measured by monitoring the disappearance of albumin in the instilled dialysis solution from the peritoneal cavity. The albumin method for measuring lymphatic absorption was also tested in rats absorbing Lactated Ringer's solution from the peritoneal cavity where absorption rate could also be measured directly. The findings suggest the following: 1.) lymphatic absorption rate is similar with both hypertonic dialysis solutions and Lactated Ringer's solution; 2.) lymphatic absorption is substantial and net ultrafiltration is well below true transcapillary ultrafiltration; and 3.) in our model, lymphatic absorption occurs at a relatively constant rate over six hours of dwell time.

The contribution of peritoneal cavity lymphatic absorption to ultrafiltration kinetics and solute clearances in continuous ambulatory peritoneal dialysis was evaluated in patients with normal (group 1) and high (group 2) peritoneal permeability X area during 4-h exchanges using 2 liters 2.5% dextrose dialysis solution with 30 g added albumin. Cumulative lymphatic drainage in all continuous ambulatory peritoneal dialysis (CAPD) patients averaged 358 +/- 47 ml per 4-h exchange and reduced cumulative net transcapillary ultrafiltration at the end of the exchange by 58 +/- 7.2%. The peak ultrafiltration volume was observed before osmotic equilibrium between serum and dialysate was reached and occurred when the net transcapillary ultrafiltration rate had decreased to equal the lymphatic absorption rate. Thereafter the lymphatic absorption rate exceeded the net transcapillary ultrafiltration rate, and intraperitoneal volume decreased. Extrapolated to 4 X 2 liters, 2.5% dextrose, 6-h exchanges per d, lymphatic drainage reduced potential daily net ultrafiltration by 83.2 +/- 10.2%, daily urea clearance by 16.9 +/- 1.9%, and daily creatinine clearance by 16.5 +/- 1.9%. Although lymphatic absorption did not differ between the two groups, lymphatic drainage caused a proportionately greater reduction in net ultrafiltration in group 2 (P less than 0.025), because these patients had more rapid dialysate glucose absorption (P less than 0.05) and less cumulative transcapillary ultrafiltration (P less than 0.01). These findings indicate that cumulative lymphatic drainage significantly reduces net ultrafiltration and solute clearances in CAPD and that ultrafiltration failure in CAPD occurs when daily lymphatic absorption equals or exceeds daily transcapillary ultrafiltration. Reduction of lymphatic absorption may provide a means for future improvement in the efficiency of CAPD.