Transmembrane pressures generated by filtrate line suction maneuvers and predilution fluid replacement during in vitro continuous arteriovenous hemofiltration

Abstract

A recirculating in vitro CAVH system was designed which generated pulsatile blood and filtrate flows. Monitors recorded hydrostatic pressures simultaneously in the arterial, venous and filtrate lines during varying plasma or blood flow rates and predilution (vs postdilution) replacement fluid flow rates. Similar hydrostatic pressure monitoring was carried out during multiple maneuvers to generate suction on the filtrate side of the hemofilter (Amicon^® D-20's and Renaflo^®‘s).

With a plasma flow (Qp) of 100 cc/min and predilution replacement fluid infusion rate of 500 cc/hr, the arterial pressure was 5% greater than during postdilution (p < 0.05). With a blood flow (Qb) of 50 cc/min, predilution fluid replacement rates of 500 and 1000 cc/hr, and vacuum suction applied to the filtrate compartment, the arterial pressure was 33% lower than during postdilution fluid replacement (p < 0.03). Nonetheless, the ultrafiltration rate (UFR) was 10 to 30% higher (p < 0.03). At many other combinations of Qp, Qb and replacement rates and modes, there were no significant changes in arterial pressure.

Despite these arterial pressure changes, > 70% of the transmembrane hydrostatic pressure (TMP) was due to the negative pressure induced by filtrate suction (gravity, Gomco^®, wall suction, IMED^®). The actual pressure in the filtrate compartment measured during Gomco^® or wall suction was 3/4 of that stated by their gauges, presumably due to leakage. Maximum wall suction never generated TMP's > 150 mmHg. Using an IMED^® 960 as the suction device (bypassing the air alarm), for a Qp of 50, IMED^® settings of 300, 600 and 900 cc/hr generated TMP's of 67 ± 3, 77 ± 3, and 97 ± 3 mmHg, respectively. When Qp was 20 cc/min, an IMED^® setting of 999 cc/hr generated TMP's consistently < 136 mmHg but UFR was only 668 ± 246 cc/hr. With the IMED^® set at 999 cc/hr and Qp of 30 cc/min, the TMP was < 132 mmHg and the UFR was 944 ± 10 cc/hr. No filters ruptured.

These suction maneuvers do not generate enough pressure to rupture filters. When Qp is low the IMED^® may not generate the UFR one expects. Predilution will probably alleviate this concern.