INTRODUCTION Erythrocytes are known to be affected in patients operated with cardiopulmonary bypass (CPB). Apart from hemolysis, the deformability and filterability of erythrocytes are reduced in patients undergoing open-heart surgery with cardiopulmonary bypass (1, 2). However, there is not much known about the aggregating behavior of erythrocytes in cardiac surgical patients. Normally, erythrocytes in human blood are able to form aggregates called “rouleaux” under conditions of low shear. At high shear, the aggregates are broken almost as rapidly as they are formed. After cessation of the shear the speed at which aggregates are formed are characteristic for the aggregation process. A strong tendency to form aggregates has shown to be related to problems in the microcirculation. In some pathological circumstances such as sepsis, increased erythrocyte aggregation tends to slowdown the microcirculation, resulting in decreased tissue perfusion and organ dysfunction (3). Red cell aggregation in blood samples is determined in vitro from the time-dependent response of the light reflection of blood under shear after a sudden decrease of the shear to zero. This is possible because red blood cell aggregation decreases the amount of scattering in blood (4), which decreases the reflection (5). In general, it can be observed that the reflection rapidly increases after shear cessation, “upstroke”, followed by a decay. The registration of this process is called the “syllectogram”. The technique was developed several decades ago for reflection of light from blood (6-8), and for transmission of light through blood (9). A first commercial aggregometer using light transmission with a cone-plate setup was developed by Schmid-Schönbein (10). Recently, a more accurate whole blood aggregometer, using light reflection on blood and a cylindrical layer of blood, has been introduced to allow measurement of both erythrocyte aggregability and deformability (LORCA, Mechatronics, Hoorn, The Netherlands) (11). The acronym LORCA stands for Laser-Assisted Optical Rotational Cell Analyser. An example of a measurement with this instrument for a blood sample taken during cardiovascular bypass surgery is given in Figure 1. The decay has to be expressed by the LORCA aggregometer software in an aggregation index Artificial Heart and Cardiac Assist Devices The International Journal of Artificial Organs / Vol. 27 / no. 6, 2004 / pp. 488-494 Analysis of red blood cell aggregation in cardio-pulmonary bypass (CPB) surgery R. GRAAFF 1 , Y.J. GU 1,2 , P.W. BOONSTRA 2 , W. van OEVEREN 1 , G. RAKHORST 1 1 University of Groningen, Department of BioMedical Engineering, Groningen - The Netherlands 2 Department of Cardiothoracic Surgery, Academic Hospital Groningen, Groningen - The Netherlands © Wichtig Editore, 2004 0391-3988/488-07 $15.00/0 ABSTRACT: Not much is known about red cell aggregation during cardio-pulmonary bypass surgery (CPB). Blood samples from 19 patients undergoing CPB were anticoagulated with EDTA. Hematocrit was adjusted to 40%. A red blood cell aggregometer (LORCA) measured changes in light reflection from each blood sample after cessation of the rotation, and calculated an aggregation index (AI). Reflection measurements were stored. Because LORCA software failed for 87 of 171 samples, we developed new software, and applied it to the stored reflection measurements. This software failed only in 7 out of 171 cases and showed that all LORCA failures occurred for AI < 40%. The new calculations revealed that the aggregation index significantly decreased from 46.6 ± 10.1 (mean ± standard deviation) baseline to 22.8 ± 8.3 at the end of CPB and recovered to 37.1 ± 13.5 at day 1. It is concluded that the new software can be used to study decreased red cell aggregation during CPB. (Int J Artif Organs 2004; 27: 488-94) KEY WORDS: Red blood cell aggregation, Cardiopulmonary bypass, Algorithms