Hemofiltration

Membrane Design

Hemodialysis and hemofiltration share a common ex-tracorporeal circuit design: blood is continuously extracted from the patient through a central venous catheter or through a specially constructed vascular access. Occlusive roller pumps applied to the blood tubing pull blood from the patient and push it through a hollow-fiber dialyzer, whence it then returns to the patient via the same vascular access device. Attempts to use the patients own blood pressure (continuous arteriovenous hemofil-tration) as the driving force for blood purification have not been widely successful, as the pressure gradients required to extract blood through the catheter, push blood through the dialyzer at high enough pressure to generate ultrafiltrate in clinically useful volumes, and return the blood to the patient far exceed systemic arterial pressure. The hollow-fiber dialyzers themselves are large and impractical to wear or implant, and have limited service lifetimes (<100 h). A dialyzer or hemofiltration membrane with high hydraulic permeability and extended lifetime would be a fundamental improvement in a wearable or even implantable device.

Automated ECF Sensing

The kidney accomplishes at least six vital homeostatic roles on time scales spanning minutes to days [8]. The most critical short-term homeostatic role is clearly con trol of extracellular fluid (ECF) volume; should ECF regulation go awry, death from pulmonary edema or cardiovascular collapse may shortly ensue. In dialytic practice today, extracellular volume is estimated by patient weight. A patient's true 'dry weight' is quantitated by removing fluid volume during the dialysis session until the patient experiences symptomatic hypovolemia. Total body weight is in fact only a very crude surrogate for effective intravascular volume, a conceptual quantity encompassing blood volume, cardiac output, and systemic vascular resistance. A significant fraction of the human monitoring in hemodialysis is directed towards ECF volume control. Nurses or technicians weigh the patient, adjust the dialysis machine, and remain vigilant for signs and symptoms of volume overload or cardiovascular collapse. Automated ECF volume sensing is a key technology in automated dialysis.

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