PATHWAYS FOR FLUID LOSS FROM THE PERITONEAL-CAVITY

During peritoneal dialysis, fluid is transported out of the peritoneal cavity by lymphatic and nonlymphatic pathways, thereby decreasing net ultrafiltration by 40-50% and reducing small solute clearance by 15-2 0%. The direct lymphatic pathway consists of the diaphragmatic lymphat ics, which directly connect the peritoneal cavity to the bloodstream. The interstitial lymphatic and direct blood entry pathways convey flui d that has been driven into the interstitial space of the tissue surro unding the peritoneal cavity by the increased intraperitoneal pressure , and return it to the bloodstream. Since flow through lymphatic pathw ays is only a portion of the flow through all pathways, total fluid lo ss is greater than lymph flow. The best technique for estimating lymph flow is direct measurement by cannulation of lymphatic vessels, a tec hnique that is not clinically feasible. The tracer disappearance techn ique, which measures the rate at which macromolecules leave the perito neal cavity, is an indirect measure of fluid loss. The tracer appearan ce technique, which measures the rate at which macromolecules reach th e blood from the peritoneal cavity, slightly overestimates lymph flow because some tracer may enter the bloodstream directly from the tissue s. Much of the previous controversy over the contribution of the lymph atic pathways to total fluid loss can be resolved by understanding the differences in what these techniques measure.