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J Thorac Cardiovasc Surg 2002;123:381-383
© 2002 The American Association for Thoracic Surgery
Brief Communications |
From the Department of Surgical and Perioperative Science, Cardiothoracic Surgery, Umeå University Hospital, Umeå, Sweden.
This work was supported by Swedish Society for Medical Research and funds of the Medical Faculty, University of Umeå, the Sahlberg Fund, Swedish Medical Research Council (12X-11204), Swedish Diabetes Association, Swedish Heart and Lung Foundation, and Northern Swedish Heart Foundation.
Received for publication April 26, 2001. Accepted for publication June 18, 2001. Address for reprints: Gunnar Engström, MD, PhD, Cardiothoracic Surgery, Department of Surgical and Perioperative Science, Umeå University Hospital, S-901 85 Umeå, Sweden (E-mail: gunnar.engstrom.us{at}vll.se).
The possible link with cognitive changes caused by arterial microembolization during cardiopulmonary bypass (CPB) drives cardiac surgery toward off-pump procedures. One of these CPB effects is blood trauma. CPB indeed causes hemolysis, but in terms of erythrocyte flow function, some conflicting results exist.
1,2 With regard to CPB, the focus of science has been on leukocytes and systemic inflammatory response, 3,4 whereas the capillary flow function of blood remains an unexplored field. Of great interest are new observations about fat deposits in brain capillaries versus the reinfusion of pericardial suction blood. 5 The possible role of activated leukocytes and platelets has also been discussed. 6 We aimed at investigating the embolic potential of the different blood subcomponents. This was accomplished by using an in vitro narrow-pore flow model with diameter approximating the size of human capillaries.
Materials and methods
Ten elective CPB cases were selected at random: 7 coronary artery bypass grafting, 1 aortic valve replacement, and 2 combined procedures. Blood was sampled before CPB (before anesthetic induction), 5 minutes after CPB onset, and at the end of CPB. The CPB was of standard design: crystalloid antegrade cardioplegia, moderate hypothermia, no arterial filters, and recirculation of unprocessed pericardial suction blood. Of a 15-mL blood sample, 250 µL was used to determine differential blood cell counts, and the remaining was centrifuged (5 minutes at 1500g) for plasma removal. Plasma was saved for flow analysis (see below) and for plasma-free hemoglobin. The blood cells were then prepared to express its subcomponents. With the buffy coat remaining, the rest of the plasma was removed by means of centrifugation and washing in 2 cycles (Krebs-Ringer-HEPES buffer [KRH]). One third of this blood was collected as sample A, with leukocytes remaining. The remaining blood was recentrifuged, with removal of the buffy coat, and resuspended as sample B. Sample B is the traditional method of leukocyte reduction and was used in historic CPB investigations. Half of sample B was filtered through a leukocyte-eliminating filter 7 and referred to as sample C, with only erythrocytes remaining. The narrow-pore flow characteristics of these samples and the plasma were measured with the KGE flowmeter (ESTRAB, Umeå, Sweden). 8 This computer-assisted system with pneumatic valve control is used to measure the flow rate at high temporal resolution by means of communication with a digital balance. Polycarbonate membranes with 5-µm capillaries were used. The flow curves were divided by means of blank filtration to express relative flow and with linear regression to find the initial filtration rate (IFR) and the capillary clogging slope (CS). Narrow-pore filtrating of blood is a widely accepted method to simulate capillary flow with many clinical implications. 9 Paired and unpaired Student t tests were used, and changes were verified by means of analysis of variance testing.
Results
Minor hemolysis occurred, corresponding to 0.6% of the circulating erythrocytes at the end of CPB. The erythrocyte particle concentration dropped at CPB onset as a result of priming and provided correction for hemodilution(Table 1). Mainly the neutrophilic granulocytes increased during CPB. InFigure 1 the flow curves of the different blood subcomponents are displayed (ie, plasma, washed and resuspended whole blood cells, blood cells after traditional buffy-coat removal, and erythrocytes only). The plasma flow suggested that the CPB priming reduced the capillary relative viscosity (increased IFR, P < .001). However, plasma flow then decreased (in terms of both IFR and CS, P < .05) with further CPB. This response indicated particle contamination because the hemodilution remained constant. The pattern of whole blood cells suggested deteriorated capillary flow properties at the end of CPB (sample A;Figure 1, B; IFR, P < .01). This pattern remained when the traditional leukocyte buffy-coat removal was used (sample B, P < .05). However, when all leukocytes were eliminated, the difference disappeared (sample C). The IFR and CS values are indicated inTable 2.
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1 In our preparation the buffy-coat aspiration only removed 55% of the leukocytes, a problem known from the literature. 7 When all leukocytes were eliminated with an absorbing filter, 7 there was no detected difference with CPB. The decrease in capillary flow hides 2 mechanisms: an increase in leukocyte count and possible leukocyte activation. Another concern was the plasma-flow pattern at the end of CPB, with changes in both IFR (indicating relative capillary viscosity) and slope (clogging particles). These particles are not blood cells but instead represent low-density contamination, such as fat cells, platelet aggregates, or cell debris. The model was very sensitive, and the 10 random CPB cases had significant outcomes. The experimental model is not sensitive to platelets, erythrocyte aggregation does not occur because the sample was free from plasma, and lysed cells were removed by washing.
Blauth and coworkers 10 demonstrated vessel occlusion in the retina during CPB by using fluorescein retinal angiography. It is possible that these embolic particles would become trapped in our model. The model may provide useful information for in vitro monitoring of capillary blood function during CPB. Of particular interest is arterial-line filters and processing of pericardial suction blood.
References
This article has been cited by other articles:
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  K. G. Engstrom and M. Appelblad Fat reduction in pericardial suction blood by spontaneous density separation: an experimental model on human liquid fat versus soya oil Perfusion, January 1, 2003; 18(1): 39 - 45. [Abstract] [PDF]
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K. G. Engstrom The embolic potential of liquid fat in pericardial suction blood, and its elimination Perfusion, January 1, 2003; 18(1_suppl): 69 - 74. [Abstract] [PDF]
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 M. Appelblad and G. Engstrom Fat contamination of pericardial suction blood and its influence on in vitro capillary-pore flow properties in patients undergoing routine coronary artery bypass grafting J. Thorac. Cardiovasc. Surg., August 1, 2002; 124(2): 377 - 386. [Abstract] [Full Text] [PDF]
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