Techniques and Machines for Pediatric Renal Replacement Therapy

Modalities

Peritoneal dialysis (PD) and CRRT are the modalities most frequently used in infants, so far. PD generally is applied in neonates, unless specific contraindications are present (i.e., peritonitis, abdominal masses, or bleeding). PD uses peritoneum as a semipermeable membrane to achieve solute diffusion and plasma water ultrafiltration. Dialysate is infused through an abdominal catheter, and after a period of so-called “dwell time,” waste solution is drained from the abdomen. Typically, to avoid excessive intraabdominal pressure rise during dialysis, especially in high-risk patients (i.e., after cardiac surgery), a “low-flow” prescription of 10 mL/kg dialysate is delivered. This is also useful to prevent hemodynamic instability secondary to reduced venous return by inferior vena cava compression. Dwell times may vary from 10 to 30 minutes according to the needed dose. As a general rule, dialysate tonicity (provided by glucose concentration, 1.36% to 2.5%) is responsible for peritoneal net ultrafiltration. PD is simple and safe, it can be administered without dedicated technology, and a steep learning curve is not needed; it typically is administered by ICU nurses without specific expertise with dialysis. Nonetheless, PD is certainly limited by a lack of efficiency; water removal appears to be particularly difficult in selected patients, which is a major issue in severely overloaded patients. Other important flaws of PD use are the possibility of interstitial fluid accumulation in case of suboptimal dialysate drainage, hyperglycemia, and risk of peritoneal infection. PD obviously is contraindicated in patients with recent abdominal surgery or abdominal bleeding. As a matter of fact, PD frequently is applied to post–cardiac surgery patients, and its main advantage is to be started in a very early phase of oligoanuria or fluid retention. PD recently has been shown to be associated with improved survival in a large cohort of post–cardiac surgery neonates if started in the first 24 postoperative hours when compared with patients who received PD after the second postoperative day.

As an alternative to PD, extracorporeal dialysis in children can be conducted with intermittent hemodialysis and CRRT. These theoretically can be delivered as hemofiltration, hemodialysis, or hemodiafiltration. The choice of dialysis modality may be influenced by several factors, including local expertise and preferences, the required dialytic targets, and the clinical picture of the treated baby. Intermittent dialysis may not be well tolerated in hemodynamically unstable critically ill infants because of its rapid rate of solute clearance and net fluid removal. These children generally are treated by CRRT that reasonably provides effective fluid and solute elimination added to proinflammatory mediators' removal. Circuits with reduced priming volume together with monitors providing an extremely accurate fluid balance are still not commercially available. Current literature, however, focused on the possibility of treating patients' fluid overload at dialysis start. It is more than clear that at the time of dialysis initiation, survivors tend to have less fluid overload than nonsurvivors, especially in the setting of multiple organ dysfunction syndrome (MODS). Differently from the adult patients in whom dialysis dose may play a key role, adequate water content in small children is the main independent predictor of outcome.

With regard to the CRRT modality, the solute clearance in three modes of CRRT at the low blood flow rates typically used in pediatric patients were compared: postdilution continuous venovenous hemofiltration (CVVH) and continuous venovenous hemodialysis (CVVHD) gave nearly equivalent clearances. At the low blood flow rates used in pediatric patients, which raise concerns about high filtration fractions during postdilution CVVH causing excessive hemoconcentration and filter clotting, CVVHD appeared to be the optimal modality for maximizing clearance of small solutes during CRRT. Nevertheless, the advantages of hemofiltration with respect to hemodialysis should be taken into consideration; medium and higher molecular weight solutes are significantly better removed by convective modalities. In light of these thoughts, predilution hemofiltration may be the preferred modality in pediatric patients.