The aim of this study was to determine the influence of mechanical fac
tors on free hemoglobin using an F21 hemopump cannula. These factors (
flow, rotary speed, reduction of the Gross section area of the silicon
e tube and foreign material on the screw) were tested in a mock circul
ation with ox blood. Irrelevant increases in free hemoglobin were indu
ced by foreign bodies either in the inlet or on the screw. The hemopum
p flow was easily affected and almost suppressed by small amounts of f
oreign bodies on the screw. Other reasons for elevated concentrations
of plasma free hemoglobin may imply a general disorder of the pump, an
ticoagulation, renal function, red cells or a combination of these fac
tors.