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 modualities including hemodialysis, continuous renal replacement therapy, slow continuous therapy, ultrafiltration and peritoneal dialysis. This article will give you an overview of the different procedures.
Arm plugged into dialysis

Image: “Plugged into dialysis” by Dan. License: CC BY 2.0

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Function of the Kidneys

The urinary system is our body’s most vital function in terms of maintaining health and balance. Its organ of control is contributed by the renal system, specifically the kidneys. The kidneys are retroperitoneal organs found on the posterior left side and lower right ( due to the liver). Their vascular system is supplied by renal arteries and drained through the renal veins.

Internal Anatomy of the Kidney

Image: “Internal Anatomy of the Kidney” by Philschatz. License: CC BY 4.0

The nephron is the primary functioning unit of the kidney. 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. Most common pathologies in the kidneys are due to acute kidney injury and failure, due to a decline in kidney filtration and perfusion.

Etiology of Acute Kidney Injury

Causes of acute kidney injury

  • Medication overuse such as NSAIDs, angiotensin receptor blockers, chemotherapy medications
  • Hypercalcemia, hypernatremia
  • Diuretic abuse
  • Burn injuries
  • Sepsis
  • Glomerulonephritis/nephrosis
  • Bacteremia
  • Fungal infections
  • Lupus
  • Multiple myeloma
  • Tumor lysis syndrome
  • Scleroderma
  • Renal hypertension
  • Nephrolithiasis

Symptoms of Acute Kidney Injury

Manifestations of acute kidney injury

Signs and manifestations of acute kidney injury and failure include but are not limited to the ones listed in the following table:

Organ System Manifestation
Integumentary Purpura, allergic nephritis rash, vasculitis, ischemia
Ophthalmology Keratitis, jaundice, uveitis
Cardio Endocarditis, a fib from emboli
Musculoskeletal Rhabdomyolysis
Renal Hematuria, granular casts( red, white, muddy brown)<0.5 ml per kg per hour of urine output for >12 hours.
Neuro Encephalopathy

Necessity for Renal Replacement Therapy

Management of acute kidney and chronic injuries is performed for patients with severe and morbidity inducing kidney injuries. The different methods of therapy for these patients are subdivided into:

  • hemodialysis (the most common)
  • continuous renal replacement therapy
  • renal transplant (if all fails)

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:

  • 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 is through diffusion and ultrafiltration.

schema of a semipermeable membrane

Image: “Plugged into dialysis” by Freemesm. License: CC BY-SA 3.0

The rate of diffusion of solutes is determined by solute permeability of the dialysis membrane, blood flow rates and the 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.

Quick-Hits for determining need for renal replacement therapy

  • Uremia/plasma urea > 30 mmol/L
  • Creatinine > 600 mmol/L
  • Chronic hyperkalemia
  • Hypervolemia (refractory)
  • Acute renal failure (ARF)
  • Chronic uncontrolled renal failure with ARF

Different Methods Used for Renal Replacement Therapy


Hemodialysis allows a mean of replacement for those with declining or lost renal function. It removes toxins, wastes and substances hemodynamically in patients with acute kidney injury or stage 5 chronic kidney disease (end stage renal disease). Ultrafiltration and diffusion through a semipermeable membrane are the common methods implemented through this treatment to replace fluids.

Arm plugged into dialysis

Image: “Plugged into dialysis” by Dan. License: CC BY 2.0

Hemodialysis dosing and intervals are case dependent. Standard treatment is performing dialysis every other day for 3 – 4 hours for those who are hemodynamically stable.  The dialysis is performed usually in the hospital or 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 which contains the dialysis fluid dialysate.

Dialysate contains different concentration levels to allow for diffusion of products from 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 rate of solute removal. The result is a reduction in wastes and an elevation in essential proteins and products.

Simplified hemodialysis circuit

Image: “Simplified hemodialysis circuit.” by YassineMrabet. License: CC BY 3.0

Continuous Renal Replacement therapy

Continuous renal replacement therapy is used when standard hemodialysis is not enough.  It is usually given to severely sick patients, 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

Allows for minimal solute removal. Performed for those with volume overload and allows 7 l of fluid to be removed per day.

Continuous arteriovenous hemofiltration

Fluid is filtered in greater amounts, requiring fluid replacement, 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

Requires the same concept as continuous arteriovenous hemofiltration, however, requires a blood pump for flow rate control.

Continuous arteriovenous hemodialysis

This method allocated 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. replacement fluid is then given to prevent hypovolemia and restore euvolemia.

Peritoneal dialysis

Schematic diagram of peritoneal dialysis

Image: “Peritoneal dialysis” by National Kidney and Urologic Diseases Information Clearinghous. License: Public Domain.

A less complicated and efficient method. 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 by 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 maintained 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 5 times a day.

Disadvantages of Treatment

Side effects of renal replacement therapy

Several factors may contribute to the disadvantages and side effects patients feel post dialysis. Most common is hypotension. The sudden filtration, volume loss and restoration may be accompanied with syncope, dyspnea, cramping, nausea and emesis.

Socially and economically, dialysis can be expensive and be a barrier for those without traveling means. It forced patients to travel or stay idle for long periods, rendering them unable to work or carry out daily activities.

Peritoneal dialysis poses a risk for abdominal hernias,  weight gain, constipation as well as sepsis from infection.  Dialysis dementia is also a side effect from chronic dialysis due to aluminum toxicity. Aluminum is found in dialysate and may participate in the patient. Finally anemia due to renal failure may occur. For better absorption, IV iron is given during hemodialysis and monitored for iron overload.

Preparing for Renal Replacement Therapy

Cimino fistula

Image: “Radiocephalic fistula” by kbk. License: CC BY-SA 3.0

Form of therapy depends on personal preference, extent of disease and medical advice. Before one can start hemodialysis, an operation must be performed on the patient. Most preferred is the Cimino fistula, an anastomosis between the arteria radialis and the vena cephalica in the distal forearm. This is an access created in the distal forearm of a patient to allow blood to flow in and out.

Other modalities include AV graft and central venous catheter. Maintaining care to the 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’s. Patients should also be instructed to take a low sodium and low potassium diet to lessen the burden on the kidneys.

Popular Exam Questions on Renal Replacement Therapy

The answers are below the references.

1. A 75-year-old female with a past medical history of chronic glomerulonephritis presents to the clinic complaining of worsening bone pain. Patient states to being on hemodialysis three times a week. Which of the following findings on lab results would you most likely expect to see?

  1. Increased PTH, decreased calcium, decreased phosphate, increased calcitriol
  2. Decreased PTH, increased calcium, increased phosphate, normal calcitriol
  3. Increased PTH, decreased calcium, increased phosphate, decreased calcitrol
  4. Decreased PTH, increased calcium, decreased phosphate, decreased calcitriol

2. A 50-year-old male comes into the ER with shortness of breath and fatigue. He is currently on diuretics for CHF which has not been helping recently. Lab results show a GFR of 15,  increasing uremia and hyperkalemia. Which is an indication for emergent dialysis in this patient?

  1. Fluid overload due to pulmonary edema in a patient with ESRD
  2. Hyperkalemia
  3. Chronic CHF
  4. GFR of 15

3. An 80-year-old female was found unconscious on the bedroom floor and rushed to the ER. Pt was found to have dysathria and myoclonus. She recently came into the ER for a progressive dementia and CHF exacerbation in which she received renal replacement therapy. Which of the following would be the most likely precipitating factor leading to this syncopal episode?

  1. Medication intoxication
  2. Dialysis dementia
  3. Febrile seizure
  4. Alzheimer’s dementia

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