Hemodialysis and Peritoneal Dialysis

Chapter 137 Hemodialysis and Peritoneal Dialysis



Julie R. Fischer, DVM, DACVIM



KEY POINTS



• Hemodialysis (HD) and peritoneal dialysis (PD) are chiefly indicated to perform the excretory functions of the kidneys in patients with acute or chronic kidney failure.

• HD and PD can also be used to augment removal of certain toxins or toxicants and to remove excess water from the body in severe fluid overload.

• In HD the patient’s blood is pumped through an extracorporeal circuit. Interface between blood and dialysate (and resulting solute and fluid exchange) occurs outside the body across the many dialyzer membranes.

• PD uses the peritoneal membrane as an interface between the blood and dialysate; dialysate is instilled and removed through a catheter in the peritoneal cavity.

• Indications for dialysis include severe, intractable uremia (acute or chronic), severe hyperkalemia, intractable volume overload, and some acute toxicoses.

• Both HD and PD carry morbidity and mortality risks but, when indicated, potential benefit to the patient usually outweighs the risk of therapy.


GENERAL PHYSICAL PRINCIPLES


Dialysis entails movement of solutes and water across a semi-permeable membrane according to concentration gradients. In clinical medicine, the blood interfaces indirectly with a contrived solution, termed dialysate. The dialysate is formulated to favor movement of waste solutes (e.g., urea, creatinine, phosphorus) out of plasma, to maintain physiologic plasma concentrations of other substances (e.g., sodium, glucose, calcium), and to replenish or load depleted solutes (e.g., bicarbonate). In hemodialysis (HD) the blood-dialysate interface is extracorporeal, across the membranes of a hollow-fiber dialyzer through which the patient’s blood is pumped. In peritoneal dialysis (PD) the vessel-rich peritoneum serves as the dialysis membrane. In both, removal of solutes and excess plasma water occurs across the dialysis membranes by the processes of diffusion, ultrafiltration, and convection.1-3


Most solute removal during a standard HD or PD session occurs by diffusion, relying on random particular motion. Particles arbitrarily encounter dialyzer membranes and may pass through via channels. Odds of channel contact are directly proportional to a given particle type’s concentration and thermodynamic energy. At equal concentrations, smaller molecules diffuse more readily than larger ones because of their higher thermodynamic energy. If concentrations of a solute equalize on both sides of the membrane (filtration equilibrium), diffusion still occurs but net transfer of that solute is zero. Maintenance of the concentration gradient, and therefore diffusion, requires continuous (HD) or periodic (PD) dialysate replacement.1-4


Ultrafiltration can remove excess plasma water. In HD, the outward transmembrane hydraulic pressure generated by the blood pump, plus a vacuum applied to the dialysate side, moves water from the blood across the membrane and into the dialysate.1,2 The amount of fluid to be removed is programmed into the delivery system and is modified as needed. Hyperosmolar dialysate draws fluid into the peritoneal cavity in PD. Dialysate osmolality usually is adjusted by varying dextrose concentration; higher dextrose concentrations effect faster fluid removal.3


Convection, also termed solvent drag, refers to movement of dissolved solutes in conjunction with movement of fluid across the dialysis membrane during ultrafiltration in both HD and PD.1-3 Convection plays a minor role in solute removal during standard dialysis and occurs only with ultrafiltration. Some HD techniques maximize convection by performing simultaneous high-rate ultrafiltration and intravenous fluid replacement. Combined with HD, this is called hemodiafiltration and maximizes removal of middle-molecular-weight solutes (≈12,000 kD).1,2



HEMODIALYSIS


HD requires repeated large-gauge vascular access, usually achieved in animals with jugular catheters. Ideally, a dual-lumen jugular catheter is placed with the catheter tip in the right atrium. For acute dialysis, a temporary catheter is placed percutaneously and usually gives 2 to 3 weeks of access. If the need for HD exceeds the functional life span of this catheter, a permanent HD catheter is placed. Permanent catheters have a Dacron cuff, placed subcutaneously between the venotomy and skin exit site. Fibroblasts migrate into the cuff, stabilizing the catheter and creating a physical barrier to bacteremia.1,2


Before treatment, the patient is assessed and the catheter exit site and ports cleaned meticulously. The HD prescription (dialysate formula, dialyzer and circuit, treatment time, ultrafiltration volume, target blood volume, heparinization) is designed at each treatment, based on physical and biochemical status, as well as prior treatment response. Severely azotemic patients (blood urea nitrogen [BUN] >150 mg/dl) require staged initial azotemia reduction. The urea reduction ratio (URR) is calculated as 1 − (posttreatment BUN ÷ pretreatment BUN). Typically, the first HD treatment targets a 0.3 to 0.6 URR (usually not to exceed a URR of >0.1/hr), the second treatment a 0.5 to 0.8 URR, and the third treatment a 0.9 to 0.95 URR, with treatments usually performed on consecutive days.1,2


Staged azotemia reduction permits cerebral acclimation to the osmolality change accompanying resolution of azotemia. This lessens the risk of dialysis disequilibrium syndrome, a clinical manifestation of cerebral edema that varies from ataxia, altered mentation, and pupillary abnormalities, to seizure, coma, and death from brain stem herniation. Mannitol, given during initial treatments (particularly in cats), acts as prophylaxis against dialysis disequilibrium syndrome. Once renal values are lowered into the normal range, thrice-weekly HD can maintain a nonuremic state and good quality of life until renal function improves.1,2


Highly and specifically trained personnel are critical to safe and effective HD. Filtering and reverse osmosis provide purified water for the dialysate to minimize patient exposure to harmful agents. The dialysis delivery system monitors and regulates dialysate composition and temperature, rate of blood flow, anticoagulant delivery, blood circuit pressures, and ultrafiltration. Before treatment, dialysate and bicarbonate concentrates are connected to intake hoses, and the extracorporeal circuit and dialyzer are primed with saline or dextrans. Following systemic heparinization, the patient’s catheter ports are connected to extracorporeal blood lines, and a pump draws blood into the circuit, through the dialyzer, and back to the patient. If a treatment parameter is breached, the system alarms and suspends dialysis pending correction.


During treatment, physical status and treatment parameters (e.g., flow rate, chamber pressures, ultrafiltration response) are monitored closely. Blood pressure, heart rate, and clotting times are measured every 15 to 30 minutes or as needed, and ideally venous oxygen saturation and hematocrit are monitored continuously with an in-line probe. General appearance and mentation are monitored continuously.


At the end of treatment, a rinse-back procedure returns the circuit blood to the patient. The catheter is capped in a sterile manner and the catheter lumens filled with a lock solution (usually 100 to 5000 U/ml heparin). A neck wrap protects the catheter until the next treatment. Dialysis catheters are guarded jealously and handled only by dialysis personnel. With stringent care, some dialysis catheters have been maintained in dogs for a year or longer.1,2



PERITONEAL DIALYSIS


PD involves little specialized equipment and is not technically difficult, but it is extremely labor intensive and demands meticulous sterile technique. PD requires insertion of an indwelling catheter into the abdominal cavity, and the degree and durability of catheter function chiefly determine success or failure of treatments. Most catheters are Silastic tubing with multiple fenestrations and one or two Dacron cuffs. The fenestrations are positioned in the peritoneal cavity; the cuffs are placed in the rectus sheath or the subcutaneous tunnel created between the point of rectus penetration and skin exit. Tissue growth into the cuffs anchors the catheter and provides a physical barrier to dialysate leakage and ascending infection. Catheter placement is performed percutaneously, laparoscopically, or via laparotomy; laparotomy gives the option of simultaneous partial or complete omentectomy, which may substantially increase and prolong catheter function. If the need for PD may exceed 24 hours, omentectomy is recommended.3,4


PD is accomplished by infusion of dialysate into the abdomen through the catheter, allowing the dialysate to remain for a prescribed dwell time, draining the fluid into a waste bag, and repeating the process. Each drain-infuse-dwell series is called a cycle or exchange. Exchanges can be performed with a straight transfer set, but ideally the dialysate bag, catheter, and drainage bag are connected by a closed Y system that permits drainage followed by dialysate infusion without catheter disconnection. Before drainage, a small amount of clean dialysate should be flushed through the line into the drainage bag. The abdomen is then drained and subsequently filled with fresh dialysate for the next dwell. This drain-then-infuse sequence flushes any contaminants in the system into the drainage bag instead of into the abdomen, and in humans markedly reduces occurrence of secondary septic peritonitis.3,4


PD solutions contain sodium, chloride, a buffer (usually lactate), varying concentrations of calcium and glucose or dextrose, and varying other additives (amino acids in some newer solutions). A simple PD solution may be made by adding dextrose (30 ml of 50% dextrose in 1 L = 1.5% dextrose solution) and heparin (250 to 2000 u/L) to lactated Ringer’s solution. Adding heparin for the first days after catheter placement decreases the risk of catheter occlusion from fibrin deposition. Initial exchanges for marked azotemia or overhydration are performed every 1 to 2 hours with 30-minute to 40-minute dwell times. This high exchange frequency continues for 24 to 48 hours, or until clinical stabilization with BUN of 70 to 90 mg/dl and creatinine of 4 to 6 mg/dl. Then a less intensive schedule (e.g., exchanges 3 to 4 times per day with 4 to 6 hour dwells) is adopted.3


Maintenance of sterility is critically important because septic peritonitis is a common, often terminal, complication of veterinary PD. Sterile gloves should be worn during connection, disconnection, and bag changes (bag spike contamination is the most common source of peritonitis in human PD) and hands washed frequently. Cover line connections with chlorhexidine-soaked dressings during infusions and drains. Clean injection ports with chlorhexidine or alcohol before use, and use new, single-dose vials for dialysate additives.3


Careful monitoring of the patient and PD procedures is critical to effective and safe treatments and allows complications to be addressed at the earliest point. Critical monitoring data for PD patients should be collected and maintained in an organized fashion for analysis and future treatment planning.

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  2. Hyperthermia and Fever
  3. Ventilator-Associated Lung Injury
  4. Allergic Airway Disease in Dogs and Cats and Feline Bronchopulmonary Disease

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Hemodialysis and Peritoneal Dialysis

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