Operational instability in extracorporeal filtration of blood

After encountering recurrent flow stasis in continuous arteriovenous hemofiltration (CAVH) clinical systems, a combined experimental and theoretical investigation was undertaken of hydraulic operational behavior of extracorporeal systems operated with blood of high hematocrit. Theoretical analyses were developed for selected modalities of extracorporeal blood purification. These modalities include hemofiltration, volume-controlled hemofiltration, CAVH, and volume-controlled CAVH. These analyses have revealed that steady state hydraulic operation may not always be possible when the venous return blood hematocrit becomes elevated. For all modalities. except volume-controlled hemofiltration. one would expect to encounter a departure from steady state operation at a critical hematocrit specific to that system. One would typically not expect to encounter this operational instability phenomenon, unless venous hematocrit was greater than 50. When operational instability is encountered, progressive hemoconcentration of blood and elevation of hemofilter blood pressure occur despite no further perturbation of the system. For modalities in which blood is pumped, the pressure rises until such time as mechanical integrity of the circuit is breached. For the arteriovenous modalities. the pressure rises, and blood flow and ultrafiltration ultimately cease. Laboratory experiments were performed to validate this theory. Blood was perfused through hemofilters of high hydraulic permeability. The ultrafiltrate pressure was varied stepwise from a positive pressure sufficient to prevent net ultrafiltration (or backfiltration) to approximately -50 mm Hg. Examination of hemofilter pressures and ultrafiltration rates in twenty-four experiments showed a return to steady state operation after most perturbations in ultrafiltrate pressure. In eleven of these experiments (only those in which the venous hematocrit was greater than 47), a subsequent small perturbation in ultrafiltrate pressure was followed by a large upward pressure excursion in the pre- and posthemofilter pressures such that the experiment had to be terminated to avoid disruption of tubing or membrane. No clotting occurred, and the experiments were repeatable with the same or different hemofilters. The experimental results are in reasonably good agreement with the theoretical analysis for pumped ultrafiltration. Hematocrit values at which this operational instability occurs can be predicted based on the shape of the apparent viscosity versus hematocrit curve and system design parameters, in particular hemofilter hydraulic permeability and venous catheter diameter. These hematocrit values may define an upper limit of postfilter hematocrit at which some blood purification systems can operate.