Technical Aspects of Pediatric Continuous Renal Replacement Therapy

Prescription

The CRRT prescription in the pediatric patient follows the same principles as those in an adult patient, but with dosing based on weight or body surface area. The flow rate of dialysate (Q _d ) or filter replacement fluid (Q _f ) can be referred to as the dialysis dose . The optimal dialysis dose is not known, but Goldstein et al. provided preliminary and limited data consistent with the dialysis dose discussed by Ronco et al. of 2000 mL/hr/1.73 m ² (35 mL/kg/hr). Higher-dose CRRT (8000 mL/hr/1.73 m ² ) may play a role in treatment of the child with hyperammonemia (values >400 umol/L). The maximum dialysis dose may be limited by the total ultrafiltration rate allowed by the filter. A higher dialysis dose also affects removal of medications, nutrition, and electrolytes, such as phosphorus. In most children, a comparable dialysis dose rate of 2 to 4 L/hr/1.73 m ² is readily achievable.

Regarding the modality of treatment, continuous venovenous hemofiltration (CVVH) and continuous venovenous hemodialysis (CVVHD) have been described with comparable solute clearances. Although CVVHD is primarily a diffusion-based therapy, some degree of convection will be present because of the prescribed net fluid balance and ongoing removal of intravenous fluids from the patient. Goldstein et al. described a mean contribution by convective clearance of 17% to the total dose of dialysis in patients on CVVHD. Net patient fluid removal usually is between 0.5 and 2.0 mL/kg/hr, depending on patient volume status and hemodynamics. This rate is a direct extrapolation from work on hemodialysis by Donckerwolcke and Bunchman, who demonstrated this to be a safe and effective ultrafiltration rate in pediatric hemodialysis. In the severely edematous and hemodynamically marginal patient, increasing pressor support to optimize mean arterial blood pressure may allow for more aggressive ultrafiltration in the initial 24 to 48 hours of CRRT, resulting in improved cardiac and pulmonary function as volume overload is reduced. The advancement in technology has allowed for improvement in the accuracy of fluid removal. A prospective study assessing the accuracy of a volumetric-based fluid measurement in CRRT using pediatric patients treated for an accumulated duration of 318 hours. They demonstrated that measured ultrafiltration (UF), using an independent scale, was different than the reported UF by the software/CRRT platform per patient over 48 to 112 hours per patient was −8 +/−1.7 mL/hr to 10 +/−1.8 mL/hr. A key consideration is to consider the rapidity of fluid removal. Too rapid a fluid removal in the face of fluid overload (which may have taken days to achieve) may be a risk factor for extending real injury. It should be titrated to minimize the possibility of further extending renal injury by reducing effective circulating volume. Many leading centers are developing strategies and protocols to effect this very important component of the prescription.

Blood Priming

As in infant hemodialysis, the relationship of the patient's blood volume to the extracorporeal volume of the CRRT circuit must be taken into consideration. Patients weighing less than 10 kg have blood volumes of approximately 80 mL/kg, whereas larger children have blood volumes closer to 70 mL/kg. If the circuit volume is in excess of 10% of the patient's total blood volume, blood priming often becomes necessary.

Blood priming has its own set of potential problems. Banked blood has an inherently low pH and low ionized calcium concentration related to the presence of anticoagulant. In addition, high potassium content develops during prolonged storage. Another concern is that this low pH potentiates the bradykinin release when an AN-69 membrane is used. This unphysiologic blood prime potentially may cause further hemodynamic instability in the child during initiation of CRRT. A couple of different approaches can be used to prevent a serious decrease in the patient's hematocrit while avoiding hemodynamic instability.

One approach involves direct transfusion of blood into the patient at the time of dialysis initiation. The packed red cells obtained from most blood banks possess a high hematocrit of approximately 50% to 60% and should be reconstituted to a hematocrit of 30% to 35% with 0.9% saline or 5% albumin, to minimize risk of clotting the circuit. The blood can be infused directly through the venous port of the dialysis catheter. This infusion should occur simultaneously with initiation of CRRT, the circuitry for which has been primed with 0.9% normal saline. As the patient's blood infuses into the circuit from the arterial port, the primed saline is drained into a collection bag that has been connected temporarily to the venous end of the filter system. Once blood has reached the venous collection bag, then the CRRT machine is paused temporarily. The collection bag then is removed, and the venous end of the filter system is connected to the venous port of the dialysis catheter. Sodium bicarbonate boluses can be used to help minimize cardiac instability secondary to a “membrane reaction” during the initiation of CRRT.

Another approach involves the zero balance ultrafiltration (“Z-BUF”) technique. The CRRT machine is primed according to the manufacturer's recommendations. Banked blood, diluted to an approximate hematocrit of 30% to 35% with 5% albumin, then is used to prime the circuit. Before the circuit is connected to the patient, the arterial and venous ends of the circuit are connected to each other, and the circuit is hemofiltered or dialyzed on itself using a physiologic solution of electrolytes such as Normocarb. Calcium chloride is added to the solution to improve the ionized calcium of the prime. The blood flow rate is set to 100 mL/min, and the circuit is then ultrafiltered at a rate of 2 L/hr for 15 to 20 minutes while a neutral circuit volume (zero balance) is maintained. Hemofiltration or hemodialysis of the circuit seems to be equally effective at normalizing pH and electrolyte content of the prime. This technique also may help blunt the initial bradykinin-type reaction seen with initiation of CRRT. Careful monitoring of the pressures in the circuit is essential. Once the blood has circulated for the allotted time, the machine is placed in pause, and the arterial and venous lines are connected to the patient. The circuit flow then is resumed.

A third option, which lessens the membrane hypersensitivity reaction, uses a different membrane, such as a polyarylethersulfone membrane. The larger HF 1000 can be used in small infants but requires careful attention to relative volume of circuit/filter compared with patient blood volume. The bradykinin-release phenomenon can be avoided with this filter, but potential complications of acidotic, hypocalcemic, and hyperkalemic blood products of the blood prime still must be addressed at the time of startup. The use of an HF20 filter, which requires less, if any, blood prime at the time of startup of CRRT, minimizes these blood volume priming issues.