Ann Thorac Surg 2006;81:648-649
© 2006 The Society of Thoracic Surgeons
Original article: Cardiovascular
Invited commentary
Dixon M. Moody, MD, FACR
Department of Radiology/Neuroradiology, Wake Forest University School of Medicine, Medical Center Blvd, Winston-Salem, NC 27157-1088
(Email: dmmoody{at}wfubmc.edu).
Brondén and colleagues [1] present data from a bio-distribution study of tritiated triolein introduced through a cardiotomy reservoir, passing through a 40-micron arterial line filter and dispersed into the left-sided circulation of pigs during cardiopulmonary bypass. The tracer was injected at the midpoint of a 40-minute cardiopulmonary bypass, and the animal was euthanized shortly after being weaned from bypass. The triolein had slightly more than 20 minutes to distribute and re-distribute throughout the circulation. The percent relative distribution in the organs was kidney 39%, spleen 28%, liver 6%, brain cortex 5%, and less for each of the other organs. The fact that no tracer was found in venous blood at euthanasia rules out the possibility that the distribution of radioactivity was simply a reflection of blood volume. It is reasonable to assume that the radioactive triolein detected represents fat emboli lodged in the microvascular bed of the various organs. The radioactivity of zero in the venous blood also suggests that washout of the fat emboli from the microvascular bed with subsequent re-distribution to the reticuloendothelial system was not robust at the time of euthanasia, but it may have been active during the first few circulation passes after injection.
The relatively small amount of tracer detected in the brain is puzzling. The authors point out that other investigators studying cardiopulmonary bypass in pigs have measured higher blood flow (using microspheres) to the brain than the kidney, and it may be noted that Ketamine, which dramatically increases cerebral blood flow, was used as an anesthetic agent by this team of investigators. Contrary to their speculation that "the 40 micron arterial filter will probably break down larger emboli to a size less than 40 microns" my colleague, David Stump, found that an arterial line filter in a saline-filled clear plastic tubing circuit, into which tiny triolein droplets were injected, will promote the formation of huge agglomerations of lipid emboli (unpublished data). Studies have shown that solid particles greater than 150 microns will not pass through the porcine rete mirabile proximal to the major pial vessels. Therefore triolein destined for the brain may have been trapped by the rete as discussed in the article. Accordingly it would be interesting to see this experiment repeated in a dog model.
Intuition suggests that embolic microvascular blockage of a significant portion of the cerebral microcirculation, even if transient, is detrimental for later higher integrated function. However, it has proven difficult for my group to find widespread conspicuous ischemic lesions after cardiopulmonary bypass in human autopsy material or after research cardiopulmonary bypass in dogs, where cold triolem has been injected into the bypass circuit. It may be that the injury caused by small fat emboli is not sufficiently severe to cause infarct, but it may initiate a cascade of events resulting in apoptosis of particularly vulnerable cells (oligodendrocytes) whose accelerated drop-out eventuates in later cognitive impairment (through loss of myelin). In any case, strategies to reduce the number of lipid emboli to brain are probably worthwhile.
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- Brondén B, Dencker M, Allers M, Plaza I, Jönsson H. Differential distribution of lipid microemboli after cardiac surgery Ann Thorac Surg 2006;81:643-649.[Abstract/Free Full Text]
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