How to catch urea? Considerations on urea removal from hemofiltrate.

Hemofiltration imitates the first step in the natural function of the kidney. After separation from corpuscular and high molecular weight blood components, a filtrate remains which contains urea together with electrolytes and other low molecular weight metabolites. To use a hemofilter in a recirculating closed-loop system, a big quantity of urea must be eliminated. A survey of published attempts to solve this problem is presented. Reasons are given for the difficulty to eliminate urea directly from dilute aqueous solutions. Explanations for ambiguous results of some reactions proposed for urea removal are discussed. The concept of hard and soft acids and bases is used to develop demands to the structure of a reagent which reacts preferentially with urea in aqueous solution. On monomeric model substances-activated aldehydes-this hypothesis is proven in vitro. In spite of the given technical possibility of urea removal, the authors doubt whether solving the problem of urea removal will enable a closed-loop system for alternative simpler or more economic ways of treating renal failure.

Temperature and vascular stability in hemodialysis.

The temperature of the blood returning from an extracorporeal circuit may influence the vascular stability. Mean arterial blood pressure, heart rate and the temperature in the in-and outlet lines for blood and dialysis fluid of the dialyzer (TBa, TBv, TDi and TDo) were continuously measured in 8 patients suffering from vascular instability during standard dialysis. The TBv was adjusted to 37, 36 and 35 degrees C at the start of dialysis by manipulating TDi. The patients were studied two times at each temperature level during a 4-hour dialysis. At the start of dialysis TBa was 35.7 +/- 0.3 degrees C. The patient's mean arterial blood pressure and heart rate decreased and increased, respectively, continuously during TBv 37 and 36 degrees C experiments, but were fairly stable at an TBv of 35 degrees C. The standard TDi of 37 degrees C should be abandoned for a temperature which is similar to TBa (35.7 degrees C) to avoid the vascular effects of heating or cooling the blood in the extracorporeal system.

Urea elimination using a cold activated-carbon artificial tubulus for hemofiltration.

Urea adsorption on active carbon is reversible and temperature-dependent. Urea adsorption isotherms of different carbons were determined at 0 degrees C and 65 degrees C within the equilibrium concentration range of 1.0-3.4 gm/L. At low urea concentrations considerable differences (3.4-13.0 gm/kg carbon at concentrations of 1.0 gm/L) were found between different types of activated carbon. The overall internal surface area was of minor importance compared to the pore size distribution. Adsorbing at low temperature, desorbing at high temperature, and flushing the carbon adsorber with a limited volume of the liquid to be purified yielded an "artificial urine." Compared to the original urea concentration of the filtrate, this "artificial urine" had an increased urea concentration. From a 36-liter volume containing 90 grams urea dissolved in saline, 18 liters were recirculated at a flow rate of 100 ml/min. The influence of adsorption and desorption time intervals was evaluated. After one to one and a half hours the carbon was saturated with urea. After saturation, about 1.4 grams urea were eliminated per cycle. In the "artificial urine" urea concentrations of up to 4.5 gm/L were found when the original solution contained only 2.5 gm/L. In the "patient" volume the urea concentration decreased from 2.5 gm/L to 1.9-2.1 gm/L. Within three hours a total of 22 grams of urea was removed by 3 x 120 grams activated carbon corresponding to removal of 50% of the urea passing the "artificial tubulus." The advantage of this system is that after priming, no additional physiological solution would be necessary. The necessity of excessive safety controls, additional electrolyte adjustment, energy demand in the form of direct current, and great amounts of waste in solid form lead to the conclusion that for intermittent hemofiltration treatment, commercially produced and controlled infusion solution is preferable.

Chronic hemofiltration. A critical evaluation of a new method for the treatment of blood.

The experiences which have been compiled in more than 2400 hemofiltrations confirm that this method represents an alternative way of treating uremic patients. The main advantages of chronic hemofiltration are the comfort of the patient and the ease in handling excess overhydration without extending treatment time, which is less than 3 X 3 hours/week if adequate hemofilters are used. With regard to the improvement of such typical uremic complications as severe hypertension, hypertriglyceridemia or neuropathy, hemofiltration does not seem to be superior to hemodialysis. However, since most hemofiltration patients do not require phosphate binders and, additionally, remarkable amounts of parathyroid hormone are removed during one hemofiltration, it appears possible that hemofiltration might be an important therapeutic alternative for those renal patients who suffer from severe hyperphosphatemia and secondary hyperparathyroidism.