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Continuous renal replacement therapy (CRRT) is a form of dialysis using a hemodialysis machine, which takes place over 24 hours at a very slow rate. When the kidneys are not functioning properly, CRRT can be used to adjust the blood chemistry. The slow, constant process of CRRT makes it a better form of dialysis for critically ill or injured people, providing gradual and more tolerable regulation of the blood chemistry for these sensitive patients.
Regular, out-patient dialysis occurs every few days and takes a couple of hours. CRRT occurs much slower over a 24 hour time period. The slower rate allows for more precise and quick-acting control over the chemistry of the blood. This fine control is necessary for critically ill or injured patients, especially patients who are hemodynamically unstable, meaning they are unable to regulate their blood pressure, and those who are highly sensitive to fluid changes, infection, or shock.
Cleaning of the waste products from the blood and altering blood chemistry during CRRT involves the use of diffusion, a principle where particles at a higher concentration move to a space where the particle concentration is lower. In CRRT, the blood is removed from the patient, passes into the hemodialysis machine and then though tiny tubes composed of a semi-permeable membrane or filters with tiny holes. The tiny tubes are surrounded by a dialysate solution which contains a lower concentration of such waste products as urea, forcing the waste out of the blood, through diffusion, and carrying it away in the dialysate solution. This solution also contains either a high or a low concentration of bicarbonate and potassium than the blood to preferentially add or remove these components, depending on the patient’s needs. Larger-sized, beneficial components in the blood, such as red and white blood cells and platelets, cannot fit through the tube's membrane and remain in the blood.
Water balance must also be achieved in the blood with CRRT. During hemodialysis, water will either be added to or removed from the blood using the principle of filtration or the forced movement of fluid across a semi-permeable membrane using pressure. The hemodialysis machine uses a pump to place pressure on the blood, forcing it through the filtering tubes. When water needs to be removed from the blood, strong pressure is placed on the blood in the hemodialysis tubes, forcing water out of the blood and into the dialysate solution. Blood with a low concentration of water will be exposed to a lower pressure, and less water will be forced out of the blood.