Hemodialysis

Normal Kidney: Normal kidneys are two bean-shaped organs, each about the size of human fist. They are located in the middle of human back. Human kidneys weigh about 0.5 % of human total body weight. They receive a huge amount (20 %) of the blood pumped by the heart.

Normal Kidneys do the following tasks: 1- Regulate the composition of human blood A- keep the concentrations of various ions and other important substances constant B- keep the volume of water in human body constant C- remove wastes from human body (urea, ammonia, drugs, toxic substances) D- keep the acid/base concentration of human blood constant

2- Help regulate human blood pressure. 3- Stimulate the making of red blood cells. 4- Maintain human body's calcium levels.

Kidney Failure: When the kidneys stop working, the condition is called kidney failure or renal failure. A patient can not get rid of excess fluid/ waste products and they stay in the body. The body may not be able to produce enough of red blood cells. Blood chemistry may become abnormal. This means that the patient no longer has the right balance of solutes such as potassium, calcium and phosphorus in the body

Definition of Hemodialysis: Hemodialysis means cleaning the blood. Blood is circulated through a machine which contains a dialyzer .The dialyzer has two spaces separated by a thin membrane. Blood passes on one side of the membrane and dialysis fluid passes on the other. The wastes and excess water pass from the blood through the membrane into the dialysis fluid which is then discarded. The cleaned blood is returned to human bloodstream.

Hemodialysis machine: It is used for preparing a solution which is called dialysate. Dialysate is composed of substances the same as in blood but in lower concentration. Dialysate is diluted by purified water which is prepared in the water treatment unit. Dialysate tends to absorb from the blood all that should be removed and nothing else.

Wastes and electrolytes move from the blood into the dialysate because of their concentration in the blood is higher.

Historic dialyzers: Rotating Drum Kidney Parallel Plate Dialysers

Modern dialysis with Hollow Fiber: Cylinder filled with up to 11,000 cellulose hollow fibers forming membranes Internal diameter of fibers approx 200-300 micron. Blood flows in fibres, dialysate flows in countercurrent direction outside fibers.

Advantages of modern dialysis with Hollow Fiber: Small priming volume - no requirement for donor blood. Low blood resistance. Gentle on blood. Single use, disposable, inexpensive, pre-sterilized dialysis compartment &emdash; reduces infection risks Good efficiency with reliable & predictable ultrafiltration Strong cellulose membranes give very low frequency of membrane ruptures

Theory of hollow fiber dialyzer: Wastes removed by solute diffusion through membrane pores down specially maintained concentration gradient from blood to dialysate. Dialysate contains blood ionic substances in required concentrations, excess ions diffuse down concentration gradient until equilibrium reached.

Larger proteins & blood cells can’t cross membrane while some do. Pores don’t exactly have the correct size. Membrane can’t selectively remove particles.

Extracorporeal blood circuit:

1- Blood tubing: Made from PVC (polyvinylchloride) which is made moldable & flexible by addition of plasticizers. Tubing molded to achieve smooth inner surface and treated with silicon to minimize blood clotting.

2- Arterial pressure monitoring: Pressure monitoring indicates tubing disconnections or obstructions & aids in controlling fluid removal from blood by pressure gradient . Pressure transducer signal amplified & converted to electrical signal . Alarms register pressure outside limits, blood pump can be shut off.

3- Blood pump: It is a Roller pump - blood tubing placed in curved track of the pump, rotation of roller compresses tubing and forces blood along tube. Pump adaptable to different sized tubing for different sized patients, smaller patients require smaller tubing.

4- Heparin pump: Usually use syringe pump, may use roller pump. Infusion site located downstream of blood pump. If located upstream of pump where subatmospheric blood pressure exists, air embolism risk is increased.

5- Venous pressure monitor: Same method as arterial pressure monitoring, also important in maintaining correct transmembrane pressure for fluid removal. 6- Venous line bubble trap/drip chamber: As blood flows through chamber micro bubbles float to surface & are trapped . Mesh screen acts as mechanical filter to help trap bubbles. It does not protect against macro bubbles.

7- Air leak monitoring. Uses ultrasound transmission. Voltage source vibrates ceramic crystal, which produces ultrasonic waves that pass through bubble trap to ceramic crystal detector . Detector vibrates & electrical output signal produced . Fluids transmit sound more efficiently than air Decrease in intensity of ultrasonic waves when detector indicates presence of air bubbles.

8- Optional automatic clamps: Shuts off blood supply in venous line. Bloodline clamps occlude against maximum pressure in blood circuit using solenoid valve.