Acute kidney injury (AKI) is a clinical syndrome, defined as a rapid deterioration in kidney function resulting from injury. While causes of AKI can vary based on the underlying disease process, the goal standard of treatment is dialysis. Peritoneal dialysis is a renal replacement therapy that can be considered an option in the emergency and critical care setting when other dialysis options are financially or geographically unavailable. Providing peritoneal dialysis to AKI patients can be a life-saving measure and is a dialysis option that should be more readily considered.
Kidney Anatomy and Physiology
To review, the renal system (comprised of kidneys, ureters, bladder, and urethra) is responsible for waste-product removal in the body. The kidneys are comprised of a cortex, medulla, renal pelvis, and nephron. The cortex is the outermost layer of the kidney and the medulla is the innermost layer of the kidney. The renal pelvis is an expansion of the proximal ureter and is essentially a large collecting channel for urine. The nephron is the structural and functional unit of the kidney. The kidneys are made up of thousands of nephrons that are responsible for carrying out the kidney’s basic functions. Each nephron works independently to remove body waste, conserve solutes, and produce urine.
The kidneys are vital to life and are responsible for many functions that help maintain overall homeostasis. Their functions include fluid regulation, hormone production, and excretion of metabolic waste products. They maintain the volume and composition of body fluids (water and electrolyte balance), they absorb solutes (proteins, amino acids, glucose), and they remove metabolic waste from the body (urea, uric acid, creatinine). Additionally, the kidneys receive approximately 20–25% of overall cardiac output and help maintain arterial blood pressure. When any of the kidney’s functions become disrupted, there can be systemic consequences.
Acute Kidney Injury
AKI is defined as the abrupt (acute) inability of the kidney to regulate fluid balance, electrolyte balance, and acid-base balance, as well as the ability to excrete body waste products. AKI has been classically categorized based on the types of azotemia that result from the underlying disease process. Azotemia is recognized by abnormally high concentrations of body waste compounds within the blood, primarily blood urea nitrogen (BUN) and creatinine. Azotemia resulting from AKI can further be grouped as prerenal, intrinsic renal, and postrenal, and is reflective of the type of AKI a patient has.
Prerenal refers to “before” the kidney, meaning injury is caused by other physiological or hemodynamic factors. This means that the animal is suffering from another disease process in which renal perfusion is compromised, affecting renal blood flow, and causing ischemic injury. Examples include hypovolemia, dehydration, cardiac compromise, or vasodilatory diseases. Intrinsic renal refers to direct damage to the renal parenchyma. Examples include renal ischemia, exposure to toxic agents, or infectious insult.
Postrenal refers to “after” the kidney, meaning there is an obstruction or impediment in the outflow of urine that prevents urine from being eliminated from the body. Examples include urethral obstruction, prostatic disease, urolithiasis, trauma, and neoplasia.
Principles of Dialysis
Renal replacement therapy (dialysis) is considered the most efficient means of managing the sequelae of AKI, including uremia, acid-base abnormalities, electrolyte derangements, and fluid imbalance. Of the renal replacement therapy options available in veterinary medicine, peritoneal dialysis (PD) is a modality that can be utilized in any emergency/critical care facility that offers advanced care.
Generally speaking, dialysis is the process of transferring water and solute(s) from one compartment to another by means of diffusion across a semipermeable membrane (SPM). The factors that contribute to fluid and solute movement during dialysis are osmosis, diffusion, and convection. Osmosis (ultrafiltration) refers to the movement of water or solutes through a SPM from an area of low concentration to an area of high concentration until equilibrium is achieved. Diffusion refers to the movement of solutes based on the concentration gradient between two compartments, solute charge, and solute molecular weight. Convection refers to the movement of solutes contained in the flow of water (solutes are dragged/trapped with water) and is based on the amount of water movement, SPM pore size, and SPM surface area.
Peritoneal Dialysis
In human medicine, PD has been used for the treatment of acute and chronic renal failure since 1923. In veterinary medicine, PD is primarily indicated for use in the management of AKI that is nonresponsive to traditional medical therapies. Specific AKI indications of PD include oliguria (urine output <0.5 ml/kg/h), anuria (lack of urine output), BUN concentration 100 mg/dL (35 mmol/L), creatinine concentration >10 mg/dL (884 µmol/L). It is not uncommon for AKI patients to have coexisting electrolyte derangements (i.e., hyperkalemia), acid-base disturbances (metabolic acidosis), plus or minus volume overload.
PD is a form of dialysis that is based on fluid and solute exchange across a SPM, in which the large surface area of the peritoneum is utilized for removal of uremic toxins, with the peritoneum acting as a natural SPM between the peritoneal capillaries and a dialysis solution (dialysate). The process of PD relies on the properties of diffusion, convection, and osmosis (ultrafiltration) for removal of fluid and solutes.
Fluid exchange occurs through the process of osmosis. An osmotic gradient is created through the presence of osmotic agents (i.e., dextrose) in the dialysate that draws fluids across the peritoneal SPM (moves fluid from a low to a high concentration). Solute exchange occurs through the processes of diffusion and convection. Waste products that are more highly concentrated in the blood diffuse across the peritoneal SPM into the dialysate, which means they are removed during each PD fluid exchange. The peritoneal membrane has three different pore sizes that allows for the movement of small and larger molecular weight solutes. For clearance of small molecular solutes (diffusive property), frequent dialysate exchange is needed to maintain the significant concentration gradients. For clearance of larger molecular solutes (convective property), increased dialysate dwell time is needed to allow solute equilibrium. Waste products can also be moved across the peritoneal SPM when they are trapped within the flow of fluid (convection), which allows for additional solute to be transferred.
Peritoneal Dialysis Therapy
Implementing PD is meant to be a temporary measure to replace the functions of the kidneys so they can heal from disease.
PD relies on first obtaining peritoneal access using a multi-fenestrated catheter via surgical insertion. There are specific, commercially available PD catheters available from human medical supply companies that are specific for PD use (i.e., include Dacron cuffs placed in the rectus abdominus muscle and subcutaneous layers to promote body wall adhesion and prevent leakage). While there are commercially available permanent peritoneal catheters, alternative temporary catheters can be placed. A temporary catheter is often a single-lumen, long-term style placed percutaneously using Seldinger technique and a local anesthetic. In veterinary medicine, examples of alternative peritoneal catheters include a Jackson Pratt suction drain or the Blake silicon drain. Once placed, the PD catheter should be aseptically connected to the dialysate solution and closed collection system using a 3-way stopcock.
A dialysate solution is a mixture that passes through the peritoneal SPM to maximize elimination of uremic toxins, prevent depletion of normal blood solutes, replenish depleted solutes, and minimize physiologic and metabolic disturbances during and after PD sessions. Conventional PD dialysate solutions are buffered balanced electrolyte solutions that contain differing concentrations of glucose, lactate, sodium, potassium, and calcium. While there are commercially available dialysates, veterinary hospitals can make their own. If a commercially available dialysate solution is not available, the preparation of the in-house dialysate must follow strict aseptic techniques. This includes wearing a mask and sterile gloves and drawing up/injecting dialysate components (i.e., dextrose, heparin) sterilely (i.e., using alcohol wipes for ports). The crystalloid Lactated Ringer’s solution (LRS) is an ideal dialysate base because of its balanced isotonicity in which dextrose can be added for osmolality; LRS is also the most similar composition to commercial dialysates. In addition to dextrose, add unfractionated heparin to the in-house dialysate (LRS + dextrose) or commercial dialysate to decrease clot formation and improve dialysate outflow. The most current recommendations state to add 500 U/L of heparin because heparin is minimally absorbed at this dose. When preparing dialysate bags, the dialysate solutions should be prepared only as needed (not in advance) to limit potential contamination/infection risk. Prior to and during infusion, the dialysate should be warmed to body temperature (99.5°F–102.5°F; 37.5°C–39°C) for improved patient tolerance.
Once the peritoneal catheter is placed and the dialysate solution is prepared, PD therapy can begin. The three arms of the 3-way stopcock circuit are 1) PD catheter from patient, 2) line from dialysate solution, 3) line to closed collection bag. To minimize the risk of contamination/infection, each line connection should be covered with chlorhexidine-soaked dressings (i.e., gauze) and access to ports (i.e., dialysate bag, drug vials) should be scrubbed chlorhexidine or alcohol prior to penetration.
At initiation of PD, dialysate exchanges typically occur hourly for the first 24 hours and are adjusted thereafter based on the patient’s response. The PD exchange process first involves infusion of the dialysate into the peritoneum over 5–10 minutes. Recommended dialysate dosage volumes range 10–40 ml/kg. Following infusion, the dialysate is then maintained within the peritoneum in what is called a dwell time, which is 40–45 minutes, in order to reach equilibrium and allow for maximum removal of water and solutes (i.e., diffusion, ultrafiltration). Lastly, the peritoneum is emptied via gravity over 10–15 minutes. During the dwelling/draining period, fluid and solutes (primarily waste products—urea and creatinine) are drawn across the membrane through the diffusion and convection processes. For each PD exchange cycle, the patient must be monitored for signs of abdominal discomfort, nausea, or respiratory compromise, any of which would warrant smaller dialysate infusion volumes. The rate of fluid exchange is greatest at the beginning of each PD cycle and becomes less effective over time as the osmotic gradient dissipates from glucose absorption. The rate of solute exchange is most determined by the volume of dialysate solution instilled into the peritoneum.
The PD cycles continue hourly until the patient’s status is stabilized/improved and/or target values are met. The adequacy of dialysis is evaluated by interpreting all relevant clinical data, including both the patient status (i.e., parameters, hydration status, appetite, energy level) and laboratory status (i.e., renal parameters, electrolyte, and acid-base status). Once the patient improves, PD can be extended to every 4–6 hours. It is critical that during every PD interaction, strict asepsis is followed. PD should continue until the patient has appropriate urine output, can maintain normal fluid balance, has adequate renal values, and is clinically improving.
Contraindications for PD therapy include recent thoracic or abdominal surgery, hernias (i.e., diaphragmatic, abdominal, inguinal), peritonitis, severe coagulopathy, or hypoalbuminemia.
Common complications of PD include catheter occlusion or migration, dialysate leakage, septic peritonitis, hypoalbuminemia, dyspnea (from increased abdominal pressure), hyperglycemia, and dialysis disequilibrium syndrome (DSS). DSS is a rare complication characterized by dementia, seizures, or death. However, since PD results in a gradual decline of uremic toxins, patients are less likely to develop DSS when compared to extracorporeal renal replacement (i.e., intermittent hemodialysis, continuous renal replacement therapy).
Patient Care
While the procedural technique of performing PD is relatively simple, patient care and monitoring require dedicated and advanced nursing skills.
Veterinary technicians and nurses are generally entrusted with the PD procedure, and it is important to ensure that the dwell is appropriately timed to maximize effectiveness. Using a cage-side timer is ideal to keep track of time for each aspect of the PD procedure (infuse, dwell, drain). For consistency, care of the PD patient should be limited to one veterinary technician/nurse (typically one-on-one) per shift to ensure consistency of the procedure and patient care.
During each PD procedure, maintaining accurate records by recording the volume infused in, dwell time, volume drained out, and calculation of the net difference in fluid volumes should be standard practice. In PD, these precise measurements of dialysate input and output during each exchange are essential. In addition, the color, turbidity, and/or odor of the peritoneal fluid should be assessed with every PD procedure. The attending DVM should be notified of any changes regarding amounts of volumes or the fluid. The PD catheter should also be inspected daily for evidence of occlusion/obstruction and integrity of the insertion site.
Maintaining sterility is of critical importance because of the high risk of introduction of bacteria directly into the peritoneal cavity (potential to cause peritonitis and/or sepsis). Nursing care includes keeping all port connections sterile, wearing sterile gloves when addressing/accessing port connections or PD catheter insertion site, wearing exam gloves when handling the patient and ports (i.e., PD 3-way stopcock, IV catheter, urinary catheter, and collection set), and using alcohol wipes when administering medications or collecting a urine sample from the collection set.
Careful monitoring of patient parameters in addition to PD parameters is also important. This includes routine assessment of mentation, heart rate (+/- ECG), respiratory rate, blood pressure, temperature, mucous membrane color, and capillary refill time. The patient should also have serial body weights to assess hydration status—weight should be recorded consistently on the same scale and without dialysate in the abdomen.
It is important to monitor fluid balance, as part of PD care. When monitoring fluid balance, careful recording of the amount of fluids in and amount of fluids out must be done. Fluids in include IV crystalloids, IV medications (if they comprise a significant volume to the patient), and oral intake (nutrition, water). Fluids out is essentially urine output (UOP).
Close monitoring and recording of UOP is critical in the AKI and PD patient. Quantifying UOP should be done every 1–4 hours and is depending on the frequency of PD exchanges. When recording a patient’s UOP, it is helpful to record the volume three ways: 1) total UOP, 2) ml/h UOP, 3) ml/kg/h UOP. For example, a 10-kg dog has 200 ml of urine over the past 4 hours; this should be recorded as 200 ml urine, 50 ml/h urine, and 5 ml/kg/h urine. In addition to the volume of UOP being recorded, the color of urine should be documented. Performing urinary catheter care should be done every 6–8 hours to maintain cleanliness and minimize the risk of introducing bacteria that could cause a urinary tract infection.
Serial lab work should be performed to evaluate the specific parameters related to renal disease. These include packed cell volume and total plasma protein for hydration status, blood gases, electrolytes, serum BUN and creatinine. These values should be monitored at a minimum every 24 hours, more frequently depending on the patient’s response and disease process.
Analgesia is essential for patient comfort and well-being and should be administered as necessary. In general, opioids are the preferred analgesic choice because of their potency with minimal cardiovascular effects. In addition to providing analgesic relief, monitoring of a patient’s pain should also be included as part of patient assessment. Pain can be monitored using pain scoring systems, such as the Colorado State University Canine/Feline Pain Scale.
Other aspects of patient care for the PD patient include nutritional support, antimicrobial usage, and treatment for the primary/underlying disease process. The use of nasoesophageal or nasogastric feeding tubes is recommended for short-term nutritional supplementation. Enteral nutrition can be provided as either bolus feedings or a constant rate infusion (CRI) of the patient’s energy requirements. In human AKI patients, early nutritional support has been shown to improve nitrogen balance and to decrease morbidity. Antimicrobials may be indicated depending on the disease or prophylactically. When considering antimicrobial usage, good stewardship should be followed, which includes timely selection, appropriate usage based on susceptibility, and an antibiotic’s action and spectrum. There should be specific consideration in the renal patient because they may have decreased ability for appropriate drug elimination/excretion. The PD process can result in subtherapeutic concentration in the blood and the antimicrobial dosage and frequency may need to be adjusted.
Conclusion
Acute kidney injury is a common condition encountered in the emergency and critical care setting. Peritoneal dialysis is a renal replacement therapy that can be utilized as a temporary measure to replace the functions of the kidneys so they can heal from disease. PD therapy plays an important role in the management of AKI and can be implemented when other more advanced types of renal replacement therapies are not available. Understanding the pathophysiology of AKI, PD processes, and the patient care involved are vital to promote a positive patient outcome.
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