Extracorporeal Organ Support Therapy

KEY POINTS

Extracorporeal organ support therapy comprising peritoneal dialysis, intermittent hemodialysis (HD), various continuous renal replacement therapies (CRRTs), and therapeutic plasma exchange is a standard tool in the PICU.

Common indications for renal replacement therapy (RRT) include acute or chronic renal failure, fluid overload, severe electrolyte disorders, and hyperammonemia.

The standard components of the CRRT circuit consist of blood inflow tubing, a hemofilter, blood outflow tubing, and the ultrafiltrate drainage tubing from the hemofilter to the drainage bag. Roller head pumps control blood flow and ultrafiltrate flow. The hemofilter may also have infusion ports for replacement fluid and/or dialysis fluid depending on the CRRT technique.

Pediatric CRRT requires either systemic anticoagulation with heparin or regional anticoagulation with citrate infused pre-filter, the effect of which is then reversed with a calcium infusion post-filter to prevent filter clotting.

Fluid removal during CRRT is caused by the hydrostatic pressure gradient (the transmembrane pressure) between the hemofilter inflow and the ultrafiltrate collection bag.

Solute removal during RRTs occurs by osmosis (peritoneal dialysis), diffusion (all HD techniques), or convection (all continuous hemofiltration techniques).

Careful monitoring for intake, output, serum chemistries, complete blood count, and coagulation profile is required during RRTs.

Therapeutic plasma exchange is considered first-line therapy in the treatment of Guillain-Barrè syndrome (GBS), thrombotic thrombocytopenic purpura, antibody-mediated transplantation rejection, myasthenia gravis, and fulminant Wilson disease.

Extracorporeal organ support therapy (ECOST) is a standard and integral part of modern intensive care. It is distinguished from extracorporeal life support, which can maintain cardiac, cardiopulmonary, or pulmonary support for prolonged periods in a variety of diseases (see Chapter 40) (1).

Various renal replacement therapies (RRTs) (2,3,4), liver

support therapies (e.g., molecular adsorbent recirculating system [MARS]) (5,6,7), and total plasma exchange (TPE) (8) represent the most common forms of ECOST. Table 41.1 shows the general indications for the use of these therapies at the Royal Children’s Hospital, Melbourne, Australia, over the past 25 years. Use of this technology is less common in children than in adults; in the four centers in which the authors work, the frequency is one to three patients per month. The many different modes of ECOST have reasonable alternatives that allow widespread international variation about the preferred mode for a given situation; this variation is reflected in Table 41.2 from centers in the United States and Australia.

All extracorporeal therapies have similar complications related to vascular access, infection, mechanical/equipment failure, thromboembolism, and bleeding. The principles that apply to all devices and circuit configurations in relation to blood flow and solute clearance are similar and depend on the nature and volume of the solute or protein that is to be removed. This chapter reviews the technical aspects of commonly performed adjunctive extracorporeal therapies such as dialysis, hemofiltration, plasma exchange, MARS, coupled plasma filtration and adsorption (CPFA) and their use in children with critical illness.

GENERAL CONCEPTS

Developmental Renal Physiology

Renal Blood Flow and Glomerular Filtration Rate

A newborn infant has a renal blood flow (RBF) of ˜20% that of an adult. Consequently, the glomerular filtration rate (GFR) is also markedly reduced, as are tubular excretion and reabsorption. The normal age-related changes in GFR and RBF are given in Table 41.3.

Clearance

Renal clearance is a quantitative measure of the rate of removal of a substance from the blood by the kidneys. It is expressed in terms of the volume of blood that could be completely cleared of a substance in 1 minute (mL/min). Clearance is often normalized for body surface area (expressed as mL/min/m²).

The ECOST Circuit Components

The basic ECOST filter circuit is shown in Figure 41.1. This consists of a double-lumen catheter, the hemofilter, as well as roller pumps for blood filtration, ultrafiltration, and replacement fluids. It is important to understand the technical functioning of these components for safe use in critical care (9,10).

TABLE 41.1 GENERAL INDICATIONS FOR ECOST AT ROYAL CHILDREN’S HOSPITAL (1987-2012)

▪ INDICATION	▪ PATIENTS (%)	▪ FILTERS PER PATIENT (AVERAGE)
Fluid/electrolyte	39	2.15
Sepsis	27	2.1
Antibody removal	16	5.7
Renal failure	7	3
Elevated ammonia	6	2
Drug overdose	1	19
Other	4	8