Lipid Micro Emboli in Cardiac Surgery

Abstract: This thesis is based on five studies. It includes the characterization of lipid micro emboli in shed mediastinal blood during cardiac surgery. The reuse of blood directly by cardiotomy suction is a common practice in order to reduce the need of blood transfusions. In shed mediastinal blood we showed that there is an abundance of lipid particles in the size range of 10-60μm. These lipid particles are also recirculated by the cardiotomy suction, through filters and the plastic tubing in the cardiopulmonary bypass (CPB) circuit. The lipid particles are quite probably in an emulsified state which renders them hydrophilic and less prone to stick to the plastic surfaces of tubing and filters. With a series of samples from the mediastinal blood and from the arterial circulation we found that the lipid particles are formed during the operation. Probable sites of origin are the bone marrow after sternotomy and the mediastinal fat tissue manipulated during the surgery in order to expose the heart for cannulation. When the blood was collected in a separate transfusion bag and transfused over a short period of time the increase of lipid micro emboli could be detected in the arterial circulation of the CPB circuit. This was measured by using a Coulter counter to analyze and count emboli in a series of blood samples. The Coulter counter was shown in the first study to be a valuable technique to evaluate embolic loads of lipid micro emboli in blood. The lungs act as physiological filters for the circulation. Embolic material is filtered effectively by the lungs. Using this filtering property of the lungs could therefore be an option in retransfusing the lipid laden mediastinal blood gathered during cardiac surgery. Using a pig model and a shed blood phantom laden with radioactive triolein as embolic material, we registered the hemodynamic results of this during CPB. Furthermore we registered the embolic load in the circulation by measuring beta radioactivity in a series of blood samples. We found that the lungs do act to trap a great deal of these lipid particles but these are then to a certain extent recirculated again. This was especially evident when the cardiac output and pressure were increased by a bolus of volume and vasoconstrictors Lipid emboli find their way into the circulation during cardiac surgery, where they can be detected and counted by the Coulter counter. When simultaneously registering micro embolic signals from a transcranial Doppler and counting lipid micro emboli in blood samples taken from the arterial line, we found a strong correlation between the two phenomena. Lipid material was also tagged by beta radioactivity and found to end up in different organs. By comparing two shed blood phantoms, one with nonemulsified lipids and the other with emulsified lipids we found that the lipid embolic matter increases in the circulation and is more easily deposited in the tissue. The overall purpose was to show that lipid micro emboli are released into the shed mediastinal blood during cardiac surgery. When this blood is retransfused, it is the source of a bombardment of lipid micro emboli in the circulation. These emboli are lodged in various organs. Lipid emboli in the emulsified state are more likely to pass into the circulation during retransfusion of shed mediastinal blood. These emboli can be detected by a transcranial Doppler in the arterial circulation of patients undergoing cardiac surgery.