Peritoneal dialysis (PD) is the process of utilizing the peritoneum as a semi-permeable membrane in order to move solutes and water between blood in the peritoneal capillaries and fluid (dialysate) instilled into the peritoneal cavity.1 Peritoneal dialysis is most frequently used in the management of acute renal failure refractory to fluid therapy, but has also been used in management of severe metabolic disturbances, acute poisoning with dialyzable substances (ethylene glycol, ethanol and barbiturates), and severe temperature extremes.2
Biology of the Peritoneal Membrane
The peritoneum is the serosal membrane that lines the abdominal cavity. The visceral peritoneum covers the viscera and other intraabdominal structures, and the parietal peritoneum lines the abdominal cavity. In humans, the surface area of the peritoneum is approximately the same as the body surface area (1 to 2 m2), and the visceral peritoneum accounts for approximately 80% of the total.1 Peritoneal surface area is proportionately larger in comparison to body surface area in infants and children, suggesting that this difference would also be true for dogs and cats.
The foundation of PD is the dynamics of fluid and solute exchange across a semipermeable membrane. Large molecules such as proteins pass slowly or not at all through such a membrane. Smaller molecules (urea and glucose) and ions (sodium and potassium) move easily across the membrane down a concentration gradient until equilibrium is reached on either side of the membrane. Water moves across the membrane from the solution of lower osmolality to that of higher osmolality until equilibrium is reached.3
The rate of diffusion across the peritoneal membrane depends upon the size and charge of the particles, as well as their concentration gradient. Urea (molecular weight 60 Daltons) is smaller and diffuses more rapidly than creatinine (molecular weight 113 Daltons).4,5 Solute movement in PD follows three physical laws: diffusion, convection and osmosis (ultrafiltration). Fluid shifts relate to osmosis created by the addition of appropriate osmotic agents in the PD solutions.
In human medicine PD has been used to treat acute kidney injury (AKI) since 1923. Peritoneal dialysis replaces renal excretory function by utilizing the peritoneum as the semipermeable membrane across which unwanted solutes are eliminated. The most common utilization is for the management of chronic kidney disease, accounting for about 27,000 patients in the US.6 Additional indications for PD in human medicine are the removal of dialyzable toxins and uroabdomen. It is also employed for the treatment of hyperthermia, hypothermia, pancreatitis, and fluid volume overload secondary to congestive heart failure, to correct life-threatening metabolic disturbances (hypercalcemia, hyperkalemia, hepatic encephalopathy) and peritonitis.2,7,8
In veterinary medicine AKI is the prevailing indication for PD. Peritoneal dialysis may be indicated in acute azotemia if blood urea nitrogen (BUN) concentration is greater than 100 mg/dl or serum creatinine concentration is greater than 10 mg/dl and the clinical course is resistant to aggressive medical therapy (rehydration, osmotic or chemical diuresis, and renal vasodilators) for greater than 24 hours.
The technique of PD is also uniquely indicated for acute poisonings in which the offending toxin is dialyzable (ethylene glycol, phenobarbital, ethanol). Severe metabolic disturbances such as metabolic acidosis, hypercalcemia, and hyperkalemia can be rapidly corrected with PD.
There are also a number of intensive care situations in which PD can be applied including uroabdomen, pancreatitis, hyperthermia, and hypothermia. In humans with severe pancreatitis refractory to aggressive supportive care for 48 hours, peritoneal lavage with dialysate has been shown to result in improvement in clinical signs. Hypothermia treated by external rewarming without core rewarming can be extremely dangerous. External rewarming alone causes peripheral vasodilation. Core rewarming by warm PD at 42°C is not difficult and is very effective. It is even more effective than external heat, inhaled warm air, warm water enemas, and blankets. The opposite works as well, cold PD can be used to treat hyperthermia. Most would consider the above uses as peritoneal lavage, which in its simplest sense defines PD.
Peritoneal dialysis is also indicated in the presurgical management of azotemic patients with uroabdomen or urinary tract obstruction. This is one use with the best chance for success because underlying renal function is often normal or the renal pathology is reversible.
Extreme life-threatening fluid overload such as occurs in congestive heart failure can be effectively treated with a dialysis technique called ultrafiltration. In ultrafiltration, hyperosmotic solutions are infused into the peritoneum, causing excess water to be drawn from the circulation into the peritoneal cavity. This ultrafiltrate is then drained from the abdomen.
Peritoneal dialysis has had a low success rate in veterinary patients because of physiologic differences when compared to humans and the manner in which it is used. In veterinary medicine PD is often considered as a "last resort" and only used in patients with a grave prognosis. This may result in an inaccurate impression of the success of PD, that it is rarely effective. This impression arises from the treatment population selected and the fact that personnel are often inexperienced in its use. Peritoneal dialysis catheter failure is more of a problem because of the quadruped stance and omental characteristics of veterinary patients. However, peritoneal dialysis is definitely a viable and effective technique in veterinary medicine not only in acute cases but in those animals with chronic disease requiring chronic ambulatory peritoneal dialysis (CAPD). Peritoneal dialysis is utilized in approximately 15% of the dialysis patients in the developed world.9 It has been reported that CAPD therapy has effects that help to maintain renal perfusion including: smaller variation in body weight, more constant blood pressure, continued mild overhydration with higher mean pulmonary arterial pressure, persistent high blood osmolality, and continued removal of proteins from the blood (B2 microglobulins, albumin, plasminogen-activator inhibitor type 1, and immunoglobulins).10
In a recent review of 22 cats that received PD the most common indication was acute on chronic kidney failure (7/22,32%). Urolithiasis accounted for 23% (5/22) of cases, 18% (4/22) of cats were acute renal kidney injury due to toxicity. Acute kidney injury attributed to spay complications occurred in 14% (3/22) of the cats.11
Peritoneal dialysis is contraindicated in patients with intraabdominal conditions that prevent safe dialysate exchange such as abdominal wall trauma and peritoneal infections or adhesions that lead to the loss of more than 50% of the peritoneal surface. Peritoneal dialysis is contraindicated in severe catabolic states or when marked hypoalbuminemia is present, because large amounts of protein can be lost through the peritoneum during dialysis. Severe ascites, obesity, recent abdominal surgery, bowel distention, or abdominal masses may interfere with catheter placement or adequate volume exchange and are relative contraindications for PD.7
Protocol for Peritoneal Dialysis/Catheter Types and Placement
The key to successful PD is the catheter and its placement. An ideal catheter allows efficient inflow and outflow, is biocompatible, resists infection of both the peritoneum and subcutaneous tunnel, and retards leakage at the peritoneal exit site.12 One of the most common causes of catheter failure in small animals is catheter obstruction by omentum, resulting in failure to drain the dialysate from the abdomen. There are many catheter designs available and most are modifications of a fenestrated silicone tube with Dacron® cuffs applied to promote fibrous attachments at the peritoneal and cutaneous exit sites. Simple tube catheters with stylets can be placed percutaneously in conscious animal using local anesthetics in an emergency situation.2,13,14 A percutaneous cystotomy tube catheter (Stamey Percutaneous Suprapubic catheter set, Cook, Spencer, Indiana) has also been used successfully for acute short-term PD. The Tenckhoff catheter, developed in 1968, is a straight soft Silastic tube fenestrated at the distal end and furnished with 2 Dacron® velour cuffs.15 Due to the high rate of omental entrapment, surgical omentectomies are advocated when using this catheter in animals in which more than 3 days of dialysis is anticipated.2,13 Peritoneal dialysis for acute kidney injury should be performed for a minimum of 48-72 hours, but the animal's condition often necessitates a longer period of treatment.
An alternative catheter style called the fluted-T (Ash Advantage Peritoneal Dialysis Catheter, Medigroup) has shown good results in dogs. The flutes are designed to offer minimal resistance to influx and efflux of fluid while preventing omental adhesion. There is diminished hydraulic resistance during inflow and outflow because most of the dialysate flow pathways are around the catheter rather than within it. There is more complete drainage of the peritoneum through the limbs of the catheter which are directed both cranially and caudally in the abdomen. This silicone catheter has two Dacron® cuffs which when implanted in the rectus muscle and subcutaneous layers become anchored by ingrowth of fibroblasts.16,17 The catheter is flexible and is designed to temporarily fold at the crosspiece to facilitate insertion. The fluted aspect of the catheter is 30 cm in length, but it can be cut to a shorter length for small patients. A subcutaneous tunnel is directed laterally for placement of the superficial cuff and the exit site of the catheter.18 A similar approach is used in both dogs and cats. Alternatives to the fluted-T catheter are the 15 French Blake surgical drain19 (Johnson and Johnson, Arlington, Tx), the Swan Neck straight or curled Missouri catheter (Kendall Healthcare, Mansfield , MA) and the 10 cm length peritoneal dialysis catheter, coaxial design (Global Veterinary Products, New Buffalo, MI). These catheters are placed surgically. Although not specifically designed for PD, the Blake drain functions in a manner similar to the fluted-T catheter and has been utilized for PD in human infants.2
Depending on the catheter selected there are 3 placement methods: laparoscopically, percutaneously (guide wire or trocar), and surgical.20,21 In veterinary medicine, in the emergency situation, a simple catheter that is placed percutaneously for short term use is often chosen. Using aseptic technique, the catheter (over the trocar) is inserted through a stab incision 3-5 cm lateral to the umbilicus oriented toward the pelvis.2,14 The trocar is tunneled subcutaneously for several centimeters before being inserted through the abdominal muscles into the abdomen. The catheter is then threaded over the trocar until fully in the abdomen.2
For peritoneal dialysis that is expected to be performed for longer than 24 hours, a surgically placed catheter should be utilized. Although some catheters such as the Fluted-T or the Quinton Swan Neck Curl Catheter are designed to be placed either via laparoscope or blind trocarization in human medicine, it is preferential to place these catheters surgically in dogs and cats. Omentectomy is necessary to provide adequate exchanges for long durations. The curled tip catheters or the tip of the Missouri catheters should be positioned in the inguinal area. The subcutaneous tunnel should be such that there is a gentle bend in the catheter that does not kink, and that exits caudally and off midline by 3-5 cm.
Careful records of the dialysate volume infused and recovered during each exchange period should be maintained. Less fluid may be recovered from the abdomen than was delivered for the first few exchanges. As dialysis continues outflow should approximate or exceed inflow if the patient is adequately hydrated. The following monitoring protocol is recommended for AKI patients.
1. Weigh the animal twice daily before dialysate infusion.
2. Monitor central venous pressure every 4-6 hours.
3. Record systemic blood pressure every 6-8 hours.
4. Check body temperature every 6-8 hours.
5. Record heart rate and respiratory rate every 2 hours; note if there is an increased respiratory effort especially if dialysate is infused.
6. Perform adequate peritoneal catheter exit-site care and check for any sign of infection daily.
7. Evaluate BUN, creatinine, electrolytes, albumin, packed cell volume, and blood gas analysis once to twice daily.
8. Record or weigh the amount of dialysate infused and recovered with each exchange.
In cases of AKI, the goals of PD are to reduce azotemia and resolve the clinical signs of uremia. Additionally, correction of fluids, electrolytes and acid-base imbalances until the animal's renal function can maintain these parameters alone. Conversion of the anuric or oliguric state to a polyuric state and stabilization or improvement of azotemia are the primary indications for discontinuation of PD.
Complications with peritoneal dialysis are common but manageable if recognized early. The most common complications include:
1. Catheter flow problems
2. Exit site leaks
5. Pleural effusion
6. Dyspnea caused by increased abdominal pressure
7. Changes in hydration status
8. Electrolyte abnormalities
Catheter flow obstruction by fibrin or omentum leading to dialysate retention is the most common problem. In one study, 30% of dogs undergoing peritoneal dialysis developed such obstruction.22 Careful catheter placement and management are important preventative steps. The occurrence of decreasing volumes of dialysate during egress or abdominal pain on dialysate infusion are evidence of omental entrapment. If omental entrapment occurs, catheters can be repositioned or replaced to correct this problem.
Peritoneal dialysis is a realistic treatment option for veterinary patients with acute nonresponsive renal failure, dialyzable toxin exposure, or chronic renal failure. The protocol requires careful intraperitoneal catheter placement and care, aggressive exchange prescriptions, and careful monitoring for complications. Veterinarians should recognize that peritoneal dialysis is an extremely effective tool in human medicine and therefore should consider peritoneal dialysis as a treatment modality for the appropriate patient.
Example of Catheter Costs
Kendall Healthcare Products Co www.kendallhq.com/catalog/healthcarecatalog.asp
8888413100 Quinton Swan Neck Infant Peritoneal Dialysis Catheter $120.60
Curl Cath Catheter with 2 felt cuffs 15.25 in. (38.9 cm)
8888413101 Quinton Swan Neck Pediatric Peritoneal Dialysis Catheter $245.00
Curl Cath Catheter with 2 felt cuffs 16.7 in. (43 cm)
8888412643 Quinton Swan Neck Pediatric Missouri Peritoneal Dialysis $235.62
Catheter with 2 felt cuffs, Left 14.8 in. (38 cm)
8888412601 Quinton Swan Neck Missouri Peritoneal Dialysis Catheter $200.93
With 2 felt cuffs, left 17.4 in. (44.5 cm)
8888413401 Quinton Swan Neck Missouri Peritoneal Dialysis Catheter $245.00
Curl Cath Catheter with 2 felt cuffs 24.5 in. (62.2 cm)
Ash Advantage Catheter $528.00
Cook Stamey Percutaneous Suprapubic Catheter $42.50
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18. Ash Advantage Peritoneal Catheter: Implantation Instructions; Aurora, IL, Medigroup Inc., 1996.
19. Alexander JW, Aerni SE. Comparison of Fluted Silicone and Polyvinyl Chloride Drains for Closed Suction Drainage Following Cholecystectomy. University of Cincinnati and Johnson and Johnson Products Inc., 1984.
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