Maximum ultrafiltration rate in continuous arteriovenous hemofiltration does not occur at the lowest level of the ultrafiltrate collection chamber.

A common assumption is that increasing transmembrane pressure by lowering the ultrafiltrate receptacle and the accompanying fluid column should always result in increasing ultrafiltration in continuous arteriovenous hemofiltration (CAVH) systems. To test this assumption, CAVH circuits were operated in vitro with use of a recirculating apparatus with an adjustable elevated reservoir. Hydraulic operational characteristics were studied in a variety of CAVH circuits, lowering the height of the ultrafiltrate column stepwise until it was at the lowest height possible. The results for most experiments performed reveal that ultrafiltration rate (UFR) reaches a peak and then declines as the collection receptacle is lowered further. There is also a decline in pre-filter blood flow preceding the peak in UFR. At lower blood flow, UFR decreases for the same transmembrane pressure (TMP). Therefore, as ultrafiltrate pressure is decreased, the effect of increased TMP on UFR is opposed by the effect of decreased blood flow, which decreases UFR. The implication of this in clinical medicine is that one may need to empirically test UFR in a CAVH system in positions other than the very lowest position.

Permeability decay in CAVH hemofilters.

Change in hydraulic permeability over time was measured in four types of continuous arteriovenous hemofiltration (CAVH) hemofilters in order to characterize permeability decay and demonstrate that permeability decay occurs without membrane protein exposure. Polyamide, polysulfone, and polyacrylonitrile membrane hemofilters were placed in a gravity-driven in vitro CAVH apparatus. Distilled and deionized water or saline was used to perfuse the hemofilters. A biphasic pattern of permeability decay was seen in all hemofilter types. A large exponential decline in permeability occurred over the first 1 to 6 hours, with a more gradual decay thereafter. Of all membranes studied, polysulfone hemofilters were the most permeable upon initiation of use but showed the most pronounced early and late permeability decay. Both polyamide and polyacrylonitrile hemofilters showed little permeability decay after a brief exponential decay. These data suggest that early membrane hydraulic permeability decay may not be primarily due to membrane protein coating. Initial high permeability may place some CAVH systems at risk for hemofilter plugging.

Clinical implications of catheter variability on neonatal continuous arteriovenous hemofiltration.

The clinical impact of catheter diameter variability on performance of neonatal continuous arteriovenous hemofiltration (CAVH) systems was shown by demonstration of the effect of catheter inner diameter on catheter and CAVH system blood flow. Diameter of 3.5 Fr and 5 Fr catheters was determined by experimental measurement of catheter flow-pressure drop relationship with known equations for fluid flow. Physical measurements verified the accuracy of the calculated diameters. These diameters were then used in a mathematical simulation to describe blood flow through an entire neonatal CAVH system and predict clinical performance. Catheters of 5 Fr caliber had a 39% variation in diameter among manufacturers, resulting in a 370% variation in blood flows; 3.5 Fr catheters had a 29% variation in internal diameter with a corresponding blood flow variation of 290%. Computer simulation of a neonatal CAVH system revealed a maximum blood flow of 0.8-3.0 ml/min with available 5 Fr catheters. This wide variation is probably responsible for the lack of consistent results in neonatal CAVH systems to date.