Peritoneal Dialysis Catheter Insertion

Peritoneal Physiology and Transport

During PD, both diffusion and convection are responsible for solute transport. Diffusion results from a difference in solute concentrations across a membrane, which in turn is governed by Fick's first law of diffusion (the rate of transfer of a solute is determined by the diffusive permeability of the membrane to that solute, the surface area available for transport, and the concentration). Convective transport, or solute drag, occurs with water transport during ultrafiltration. Determinants of convective transport include the water flux, the mean solute concentration, and the solute reflection coefficient. The reflection coefficient in semipermeable membranes is related to how such a membrane can prevent solute particles from passing through. When the value is 0, all particles pass through. When the value is 1, no particle can pass through.

The effective surface area of the peritoneal membrane and its intrinsic permeability determine the ability of solutes to be transported. The effective surface area is determined by the number of capillaries perfused and the rate of flow within these capillaries.

There are several barriers to the transport of solutes across the peritoneum. The peritoneal capillary represents the major barrier for peritoneal transport. According to the two-pore theory of capillary transport, small pores of 40 to 50 Å are abundant, and large pores larger than 150 Å are sparse. After the demonstration of aquaporin-1–mediated water transport through red blood cells, these even smaller pores were also described in endothelial cells of peritoneal capillaries and venules. To explain the phenomenon of sieving observed in PD, a three-pore model was proposed. According to this model, about half of the transcapillary ultrafiltration occurs through ultra-small pores 3 to 5 Å in size, and the other half occurs through the small pores. The mesothelium has been shown not to be a significant barrier to small solute transport. However, the interstitium, despite its meshlike architecture, could be such a barrier.

Changes in solute transport are currently assumed to result from the ultrastructural alteration of perfused capillaries. The mass transfer of solutes of low and medium molecular weight is dependent primarily on their size and not on the intrinsic permeability of the peritoneum. The stagnant blood in the capillaries and peritoneal cavity could offer some resistance to solute transport. Macromolecular transport is size selective; therefore, diffusion and convection are limited. Hence, transport is dependent primarily both on the effective surface area and permeability of the membrane and on the molecular size of the solute. In animal models, the negative electric charges at various levels of peritoneum and microvessels appear to restrict clearance of macromolecules, but this is not so in humans. In summary, the transport of solutes of low molecular weight is affected by changes in effective surface area of the peritoneal membrane, and macromolecule transport is determined mainly by the structural alteration of the capillary wall or changes in the interstitium, or both.

Several distributed models and solute transport parameters have been proposed to describe kinetics during a PD exchange. The mass transfer area coefficient (MTAC) is the theoretical instantaneous maximal clearance at time 0 without ultrafiltration. Several simple and other more complicated models have been developed to calculate MTAC. In contrast, in clinical practice, relatively simple measures – such as 24-hour clearance or 4-hour dialysate/plasma (D/P) ratios of low-molecular-weight solutes – are used to assess the efficacy and adequacy of PD. Studies have shown good correlation between D/P ratios and MTAC for all ranges except in the low and high extremes of MTAC. During a dialysis exchange, diffusion accounts for the majority of the mass removal of solutes of lower molecular weight (e.g., urea or creatinine) ; convective transport is a small fraction of total mass removal.

Water transport during PD is driven through an osmotic gradient, generated by agents such as glucose. Under physiologic conditions, the differences among hydrostatic, crystalloid, and colloid osmotic pressures in the peritoneal capillaries and the peritoneal cavity allow for a small amount of transcapillary ultrafiltration into the peritoneum to occur continuously. These pressures are exerted over small pores and through water (aquaporin-1) channels in the endothelium of peritoneal capillaries, which results in transcapillary ultrafiltration. There is a constant backward absorption of water from the peritoneal cavity through transcapillary backwards filtration and fluid uptake through peritoneal lymphatic vessels. Therefore, the net amount of ultrafiltration (water removal from the body) during a PD exchange is a balance between the transcapillary ultrafiltration from the peritoneal capillaries into the peritoneum and the backwards absorption of fluid from the cavity through capillaries and lymphatic vessels. Because peritoneal dialysate is devoid of proteins, it exerts only crystalloid osmotic pressure and hydrostatic pressure in the peritoneal cavity.

The effectiveness of dialysate in inducing ultrafiltration is expressed by the osmotic reflection coefficient. Impermeable solutes such as macromolecular protein exert a reflection coefficient of 1, and freely permeable solutes exert a reflection coefficient of 0. Therefore, during a PD exchange, the reflection coefficient for glucose transport through small pores is very low, at 0.02 to 0.05, and for glucose transport through aquaporin-1, it is 1. The events of water transport that take place during a PD exchange with glucose-based dialysate can be summarized as follows : At the beginning of an exchange (time 0), the glucose concentration of the dialysate is highest, and therefore, crystalloid osmotic pressure and ultrafiltration rate are both also highest. As glucose is absorbed from the dialysate (approximately 61% of the total glucose content of a solution over a 4 hour period), the crystalloid pressure and ultrafiltration diminish. Ultrafiltration volume accumulates progressively, and the ultrafiltration volume peaks before osmotic equilibrium between serum and dialysate is reached; this equilibrium occurs when the net transcapillary ultrafiltration rate progressively diminishes to equal rates of backwards absorption, plus lymphatic absorption. Thereafter, when the back absorption rate exceeds the net transcapillary ultrafiltration rate, the intraperitoneal volume slowly diminishes. With further extension of dwell time, additional fluid would be absorbed. Patients deemed “high transporters” experience rapid transport of small solutes; thus, these patients experience more rapid (and often more extensive) dialysate glucose absorption and less cumulative transcapillary ultrafiltration. Ultrafiltration fails when daily reabsorption equals or exceeds daily transcapillary ultrafiltration.

The Peritoneal Catheter and Access

Key to successful PD therapy is permanent and safe access to the peritoneal cavity. A good catheter provides obstruction-free access to the peritoneum. In addition, it should not be a source of peritoneal infection. Catheter-related problems and infections are responsible for approximately 20% of technique failure. Peritoneal catheters in current use have intraperitoneal and extraperitoneal segments. The extraperitoneal segment passes through a tunnel within the abdominal wall (intramural), exits through the skin, and has an external (outside the exit site) segment. Figure 3-1-1 shows different intraperitoneal and extraperitoneal designs of currently available peritoneal catheters.

Globally, the catheter most widely used is the Tenckhoff catheter, followed by the swan-neck catheter. More than 90% of the catheters used have two cuffs, and the majority incorporate a coiled intraperitoneal segment. Tenckhoff catheters are made of silicone rubber tubing with a 2.6-mm internal diameter and a 5-mm external diameter. The catheter may contain one or two polyester cuffs, 1 cm long. The straight double-cuff catheter is about 40 cm long with an intraperitoneal segment about 15 cm long, an intramural segment about 5 to 7 cm long, and an external segment about 16 cm long. The open-ended intraperitoneal segment has multiple 0.5-mm side openings in the terminal 11-cm segment. The coiled Tenckhoff catheter has a coiled, perforated intraperitoneal end that is 18.5 cm long. Most Tenckhoff catheters have a barium-impregnated radiopaque stripe throughout the catheter length to assist in radiologic visualization.

The swan-neck catheter, a modified Tenckhoff catheter, features a molded bend between cuffs. These catheters can be placed in an arcuate tunnel with both external and internal segments of the tunnel directed downwards. A long tunnel, downward-directed exit, two intramural cuffs, and an optimal sinus length tend to reduce exit and tunnel infection rates. The molded bend between cuffs eliminates the rubber “shape memory” from causing the external cuff extrusion. A downward-directed peritoneal entrance, aided by a slanted polyester disc, a feature similar to one in the Toronto Western catheter (described later), tends to keep the internal segment in the true pelvis, reducing its migration. Insertion of catheters through the rectus muscle decreases pericatheter leaks by promoting fibrous ingrowth onto the polyester cuff. Finally, swan-neck catheters with a coiled intraperitoneal segment minimize infusion and pressure pain. The intraperitoneal segment of the swan-neck catheter is identical to that of the Tenckhoff catheter in that its terminal segment is either straight or coiled.

Presternal catheters were designed to allow for an exit site above the abdomen. The chest is a rather rigid structure with minimal wall motion; the catheter exit located on the chest wall is subjected to minimal trauma; therefore, chances of contamination are decreased. Also, in patients with abdominal ostomies and in children with diapers, a chest exit location reduces the chances of cross contamination. Implantation directly over the sternum should be avoided, so as to prevent catheter damage during any cardiac surgery that necessitates sternotomy. A long catheter tunnel, combined with three cuffs, may reduce pericatheter bacterial contamination of the peritoneal cavity and hence reduce the incidence of peritonitis. The presternal catheter is composed of two silicone rubber tubes, cut to an appropriate length and connected end to end at the time of implantation. The lower tube, including the internal segment, is identical to that of the swan-neck abdominal catheter. A titanium connector is used to connect the two components at the time of implantation. The Moncrief-Popovich catheter is a modified swan-neck coiled catheter with a longer subcutaneous cuff (2.5 cm instead of 1 cm). This catheter is inserted with the Moncrief–Popovich implantation technique. The other catheters in use are the T-fluted catheter; the self-locating catheter; the Cruz catheter; the Toronto Western Hospital catheter; the Ash (Life) catheter; the column disc catheter; and the Gore-Tex peritoneal catheter.

Rigid catheters for acute dialysis, rarely used in developed nations, are still used in some countries. Complications of rigid catheter insertion include minor bleeding; leakage of dialysis solution; extravasation of fluid into the abdominal wall, particularly in patients who have had a previous abdominal operation or multiple catheter insertions; and inadequate drainage as a result of omental wrapping, loculation, or misplaced catheter in the upper abdomen. Loss of a part or the entire rigid catheter after manipulation of a poorly functioning rigid catheter has been reported. The incidence of peritonitis varies widely with rigid catheters; the rate may be dependent on the duration of dialysis and catheter manipulation, among other factors.

For long-term use, PD catheters such as the Tenckhoff or swan-neck can be inserted surgically at the patient's bedside by an experienced nephrologist or by a surgeon or through a laparoscopic insertion, a procedure that has gained favor.

Catheter-Related Complications

The most common complications of PD catheters include exit and tunnel infection, external cuff extrusion, poor function, dialysate leaks, peritonitis, and infusion or pressure pain. Several factors adversely influence the normal healing process and lead to early infections: foreign body-induced tissue reaction, poor tissue perfusion, mechanical factors, sinus bacterial colonization, delayed epithelialization, local cleansing agents, exit direction, and several other systemic problems. After the exit site is well healed, a factor that predisposes to infection is bacterial colonization of the sinus tract in association with local trauma.

The catheter tip, as it rests against the pelvic wall or intra-abdominal organs, may cause localized pain from irritation. The jet effect of rapidly flowing dialysis solution may also cause abdominal pain. In some rare instances, compartmentalization from adhesion formation around the catheter may cause severe abdominal pain. Coiled catheters are less likely to induce abdominal pain than are straight catheters.

The extrusion of the external synthetic cuff can be prevented by creating the tunnel in a shape similar to the shape of the catheter and placing this cuff approximately 2 to 3 cm beneath the skin. In the absence of catheter infection, shaving off the extruded external cuff may help prolong the life of the catheter.

Entrapment, or “capture,” of the catheter by the active omentum may cause outflow obstruction in the postimplantation period. Omental “capture” as a late event is rare. From time to time in some patients, drainage slows as a result of catheter translocation, obstruction by omentum, or fibrin clot formation. Laxatives or addition of heparin, 500 U per liter of dialysis solution, or both may be successful in restoring good dialysate flow. In some patients, catheters have migrated out of the true pelvis. If the catheter continues to function appropriately, it is not necessary to reposition it. If the catheter fails to function after simple maneuvers are implemented, more aggressive measures (e.g., laxatives, forced flushing) may be tried. When these measures fail, laparoscopic repositioning of the catheter tip back to the true pelvis and anchoring may be necessary. The Toronto Western catheter has two silicone discs in the intraperitoneal segment of the catheter that hinder the free movement of catheter tip out of the pelvis after placement.

Insertion of the deep cuff into the center of the rectus muscle, as opposed to midline placement, has significantly reduced the incidence of early leakage of pericatheter dialysis solution. Pericatheter leaks are rare with catheters that have a bead and polyester flange at the deep cuff (Toronto Western Hospital catheter, swan neck Missouri catheter, swan neck presternal peritoneal catheter). In contrast to the early leaks, which are usually external, the late leaks infiltrate the abdominal wall through prior healed incisions. PD catheters may cause hemoperitoneum by causing minor tears of small vessels. On occasion, a peritoneal catheter erodes into the mesenteric vessels, leading to hemoperitoneum. In rare cases, a peritoneal catheter damages the internal organs, which leads to intra-abdominal bleeding. Transvaginal leakage of peritoneal fluid is rare, but the possibility should be considered in an appropriate clinical setting.