Augmentation of kallikrein and plasmin inhibition capacity by aprotinin using a new assay to monitor therapy

Aprotinin concentration varies significantly according to cardiopulmonary bypass conditions

Although aprotinin is partially excreted unchanged in the urine, its primary site of metabolism is in the renal lysosomes following proximal tubule resorption. This study tested the hypothesis that plasma aprotinin concentration varies with cardiopulmonary bypass conditions. Thirty-two piglets (weight 13.2 +/- 1.9 kg) received an aprotinin initial dose of 30,000 KIU/kg, a maintenance infusion of 10,000 KIU/kg/h, with a cardiopulmonary bypass (CPB) prime of 30,000 KIU/kg. Aprotinin infusion was terminated at the end of CPB and stopped during hypothermic circulatory arrest (HCA). Piglets were randomized to four groups (n = 8 per group): HCA, 60-minute period at 15 degrees C; low-flow (LF), 10 mL/kg/min low-flow CPB at 25 degrees C; full flow (FF), full flow CPB at 37 degrees C; control at 37 degrees C without CPB. Blood samples were collected at 7 time points: after induction of anesthesia (baseline), after initial dose, 10, 50 and 115 min after start of CPB, just before end of CPB and 30 min after CPB. Plasma aprotinin levels were determined by modified functional assays. Aprotinin levels in the control group were significantly lower at each point after start of CPB than all groups with CPB (p < .001). In particular, during the reperfusion period, aprotinin levels were higher in HCA and LF groups than FF group (p < .05). Throughout CPB, aprotinin levels in the HCA group remained unchanged (p > .40). Bypass conditions affect plasma aprotinin concentration. Recently reported renal and neurological complications with aprotinin use during CPB may reflect excessive dosing and point to the need for real-time monitoring.

Determination of plasma aprotinin levels by functional and immunologic assays

We compared a functional (amidolytic) and an enzyme-linked immunosorbent assay (ELISA) method for determining aprotinin concentration in 82 plasma samples obtained from patients undergoing cardiac surgery with aprotinin therapy. There was good correlation between methods (r = 0.87); however, aprotinin measurements by chromogenic assay were significantly higher than by ELISA [234 +/- 104 kallikrein inhibitory units (KIU)/ml versus 155 +/- 88 KIU/ml; P = 0.0001]. This appeared to be attributable to differences in the potency of the material used to standardize the assays. When results were corrected to allow for potency of the standard, there was no significant difference between chromogenic and ELISA methods (234 +/- 104 KIU/ml versus 240 +/- 137 KIU/ ml), although the ELISA results tended to be higher in some samples. These data suggest that aprotinin concentrations measured by these methods cannot be used interchangeably, and care must be taken when interpreting data from studies measuring aprotinin.

Maintenance of therapeutic plasma aprotinin levels during prolonged cardiopulmonary bypass using a large-dose regimen

Aprotinin concentrations in the range of 127-191 kallikrein inactivator units (KIU)/mL at the end of cardiopulmonary bypass (CPB) (< 2 h duration) reduce transfusion requirements. It has been suggested that prolonged CPB may require higher infusion rates which significantly increase cost. We tested the hypothesis that large-dose aprotinin maintains therapeutic plasma levels during prolonged periods of CPB (< 2 h). Aprotinin was administered as follows: 2 x 10(6) KIU upon skin incision; 0.5 x 10(6) KIU/h x 4-h infusion on initiation of CPB; and 2 x 10(6) KIU added to the CPB prime solution. Aprotinin activity was measured 1) 30 min after initiation of drug administration (Pre-CPB); 2) 30 min after initiation of CPB (CPB + 30); 3) 90 min after initiation of CPB (CPB + 90); and 4) at CPB termination (End CPB). CPB duration (mean +/- SD) was 158 +/- 51 min. Plasma aprotinin concentrations (KIU/mL, mean +/- SD) were: 234 +/- 30 at Pre-CPB; 229 +/- 35 at CPB + 30; 184 +/- 27 at CPB + 90; and 179 +/- 22 at End CPB. In all patients, aprotinin levels at the completion of CPB were in the range previously reported to be effective. The authors conclude that large-dose regimen limited to 6 x 10(6) KIU maintained therapeutic plasma aprotinin concentrations during prolonged CPB.

Blood anesthesia for cardiopulmonary bypass

Cardiopulmonary bypass (CPB) causes bleeding and thrombotic complications, fluid retention and temporary dysfunction of every organ system. This morbidity of CPB is primarily do to activation of blood proteins and cells by contact with nonendothelial cell surfaces of the wound and biomaterials of the extracorporeal perfusion circuit. CPB is not possible without heparin, yet heparin is not an ideal anticoagulant and does not prevent activation of at least five plasma protein systems and five blood cells. Stimulation of these blood elements produces over 25 vasoactive substances that alter vascular tone, capillary permeability, and cardiac myocyte contractility. In addition, CPB produces showers of microemboli that pass filters to obstruct arterioles and precapillaries to produce necrosis of widely dispersed, small groups of cells. Attempts to develop nonthrombogenic synthetic materials have failed; only the endothelial cell is nonthrombogenic and achieves this property by active metabolic processes. Although some biomaterials are less thrombogenic than others, all activate blood elements to initiate clotting and the body's defense reaction. The concept of "blood anesthesia" envisions the use of reversible inhibitors of key blood reactions to temporarily prevent activation of blood elements during CPB. If the initial reactions of blood with nonendothelial surfaces are blocked, production of many vasoactive substances and microemboli by CPB is suppressed. This conserves blood elements that are normally consumed during CPB and makes them available after the inhibitor is reversed. Effective, reversible inhibitors of platelets are entering clinical trials; reversible inhibitors of other key blood relations are being developed and tested at a rapid rate.(ABSTRACT TRUNCATED AT 250 WORDS)