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Arm plugged into dialysis

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

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 (liver).

Blood supply and functions

The kidneys are made up of up to 1.3 million nephrons each that receive 25% of cardiac output in order to carry out the various functions of the kidney.  The supply is via the renal arteries and drained through the renal veins.

The artery branches into interlobar arteries, arcuate arteries and interlobular arteries that unite to give rise to 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.
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

Causes of acute kidney injury are classified as:

  • Prerenal causes(30%of the cases)
  • Burn injuries
  • Sepsis
  • Hemorrhage
  • Dehydration
  • Congestive heart failure
  • Renal artery occlusion
  • Hepatorenal syndrome
  • Intrarenal causes(65% of the cases)
    • Acute tubular necrosis following medication overuse such as NSAIDs, angiotensin receptor blockers, chemotherapy medications.
    • Acute glomerulonephritis/nephrosis following infections, ischemia or toxic chemical ingestion.
    • Acute interstitial nephritis following inflammatory and infiltrative conditions such as Lupus, Multiple myeloma, Scleroderma, and lymphoma.
    • Other conditions such as Nephrolithiasis
  • Postrenal causes (5% of the cases)
    • Ureteral obstructions by stones, peritoneal fibrosis or blood clots
    • Bladder neck obstruction.
    • Urethral obstructions following strictures.
    • Benign prostate hypertrophy and other tumors.

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

Hemodialysis comes with acute, intermediate and long term adverse effects as follows:

  • Acute complications include
    • Hypotension due to excessive fluid loss. The sudden filtration, volume loss, and restoration may be accompanied by syncope, dyspnea, cramping, nausea, and emesis.
    • Air emboli
    • Cardiac arrhythmias
    • Muscle cramps due to redistribution of blood.
  • Intermediate complications include
    • Dialysis disequilibrium syndrome.
    • Hemorrhage from the access due to anticoagulant use.
  • Chronic complications
    • Amyloidosis due to β2 microglobulin accumulation that isn’t removed by dialysis.
    • Dialysis dementiais also seen due to aluminum toxicity. Aluminum is found in dialysate and may penetrate into the patient’s system.

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 an infection.

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, the 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 access created in the distal forearm of a patient to allow blood to flow in and out. The main aims of this procedure are to arteriorize the vein and make it stronger to handle a large amount of blood flow as well as withstand multiple venipunctures without undergoing changes.

Other modalities include AV graft and central venous catheter. Maintaining care to 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 low sodium and low potassium diet to lessen the burden on the kidneys.

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