Renal Replacement Therapy

The kidneys work through transport, diffusion, and osmosis, and when this system fails, other means of filtration must be used. The management is primarily achieved through renal replacement therapy. Renal replacement therapy is divided into several modalities including hemodialysis, continuous renal replacement therapy, slow continuous therapy, ultrafiltration, and peritoneal dialysis. This article will give you an overview of the different procedures.

the renal veins.
The artery branches into the interlobar arteries, arcuate arteries, and interlobular arteries that unite to form the afferent arteriole. The high-pressure input system flows against glomerular resistance to elicit the force needed for ultrafiltration, which is the kidney's main role.

Other functions of the kidneys include:
Water and electrolyte balance Excretion of metabolic wastes Acid-base balance Secretion, metabolism, and elimination of hormones Image: "Internal Anatomy of the Kidney" by Philschatz. License: CC BY 4.0 The nephron is the kidney's primary functioning unit. Its purpose is to filter the blood and create urine within the tubular system. The tubular system, though highly efficient and calculated, is also prone to injury. The most common pathologies in the kidneys are due to acute kidney injury and failure, due to a decline in kidney filtration and perfusion. The purpose of renal replacement therapy is to prevent mortality due to uremia and renal failure. Therefore, renal replacement therapy is indicated for patients in the following cases:

Etiology of Acute Kidney Injury
Refractory fluid overload Acidosis (metabolic) with a PH <7.1 Hyperkalemia Uremia

Drug intoxication
Patients with GFR <60, especially <30 with underlying chronic kidney disease It is suggested that renal replacement therapy be initiated before renal failure and the first signs of acute renal injury. These signs are usually complicated uremia ( encephalopathy and metabolic acidosis). The BUN (blood urea nitrogen) is also a factor of when to initiate therapy. Generally, when the BUN reaches 80-100 mg/dl, dialysis is initiated. The rate and mechanism of renal replacement therapy are through diffusion and ultrafiltration.
Image: "Plugged into dialysis" by Freemesm. License: CC BY-SA 3.0 The rate of solute diffusion depends on the solute permeability of the dialysis membrane, blood flow rates, and duration. If rate or duration is increased, a maximum concentration gradient may be achieved for continued diffusion. Solute removal by ultrafiltration is influenced by the transmembrane. Usually, dialysis is performed in the hospital or a dialysis center and now even at home.
The patient is hooked up to a hemodialysis machine in which blood is pumped out of the patient's arm through an external filter dialyzer that contains the dialysis fluid dialysate.
Dialysate contains different concentration levels to allow for diffusion of products from the blood to remove wastes and preserve essential substances that will return to the blood ( blood cells and proteins). Dialysate and its concentration levels are dependent on the patient and nephrologist. Solutes are passed through a concentration gradient as blood leaves the arm. Urea, creatinine, and other wastes are removed from the blood into the dialysate. However, calcium, bicarb, and red blood cells are left unfiltered and move back from the dialysate to the blood returning to the patient's arm. The dialysate flows countercurrent to blood flow through the dialyzer to maximize the concentration gradient between the compartments and, therefore, to maximize the solute removal rate. The result is a reduction in wastes and an elevation in essential proteins and products.

Continuous Renal Replacement therapy
Continuous renal replacement therapy is used when standard hemodialysis is not enough. It is usually given to severely sick patients, the hemodynamically unstable, and those needing greater solute and/or fluid removal. Removing solutes over a daily course of 24 hours allows greater concentrations to be filtered.

Slow continuous ultrafiltration
This process allows for minimal solute removal. It is performed for those with volume overload and allows 7 l of fluid to be removed per day.

Continuous arteriovenous hemofiltration
The fluid is filtered in greater amounts, requiring fluid replacement and preventing volume depletion. The filtration is driven by the arteriovenous pressure difference.
Usually, 1 l of filtrate is replaced per hour. To allow for better solute filtration, replacement fluid is pre-administered to allow for wastes to diffuse from within the red blood cells into the plasma for removal.

Continuous venovenous hemofiltration
This method requires the same concept as continuous arteriovenous hemofiltration.
However, it requires a blood pump for flow rate control.

Continuous arteriovenous hemodialysis
This method allocates dialysis fluid to flow within the filter, separate from the patient's blood in the dialysis machine. Ultrafiltration is enhanced beyond normal to ensure solute clearance. The patient then receives replacement fluid to prevent hypovolemia and restore euvolemia.

Peritoneal dialysis
Image: "Peritoneal dialysis" by National Kidney and Urologic Diseases Information Clearinghouse. License: Public Domain.
This method is less complicated and efficient. Peritoneal dialysis allows the removal of large amounts of fluid. It is slow functioning, so disequilibrium and gradual correction can be achieved. The dialysis is performed in the OR or at the bedside by implanting a catheter into the abdominal wall via a superficial cuff that is placed in the subcutaneous abdominal wall. It is usually medial than midline, below the rectus muscle, to provide better tissue ingrowth around the cuff for better vascularization. Prophylactic antibiotics are given to decrease wound infections. The abdominal cavity is dialyzed by filling it with the dialysate fluid and maintaining it for a period of time. The peritoneum acts as a membrane to allow for solutes to diffuse out of the blood into the solution to be drained.
This process can be repeated up to five times a day. This form of therapy depends on personal preference, the extent of disease, and medical advice. Before one can start hemodialysis, the patient must undergo surgery. Usually, this is the Cimino fistula, an anastomosis between the arteria radialis and the vena cephalica in the distal forearm. This is access created in the patient's distal forearm to allow blood to flow in and out. The main aim of this procedure is to arteriorize the vein, making it stronger to handle a large amount of blood flow and withstand multiple venipunctures without undergoing changes.

Disadvantages of Treatment
Other modalities include an AV graft and a central venous catheter. Maintaining access is vital for preventing infection. Accesses should be cleansed and untouched.
Blood pressure and vaccines, IV's should be avoided in the arm access. Patients should also be instructed to follow low sodium and low potassium diet to reduce the burden on the kidneys.