Hemodynamics in the Extracorporeal Aortic Cannula : Review of Factors Affecting Choice of the Appropriate Size

____________ _ To minimize hemolysis rate and damage to blood due to characteristics of aortic cannulae, critical blood velocity, blood cannula interactions and shear stress ranges for extracorporeal circulation (ECC) must be determined. At blood flow rates and viscosity levels typically experienced during ECC mean blood velocity, critical blood velocity, shear stress, and kinetic energy are determined in several aortic cannulae, 3-8 mm. Twenty-five percent glycerol is the priming solution. In ten Fr and twenty-four Fr aortic cannulae at 25°C, the velocity above which flow becomes turbulent occurs at flow rates of 975 ccjmin and 2750 ccjmin respectively. Turbulence within these cannulae occurs at flow rates above 650 ccjmin and 2000 ccjmin at 37°C. The recommended flow rate for the cannula sizes tested, determined by the average flow rate at each limit studied, resulted in maximum recommended flows of 883, 1283,2134,2899,3840, and 4954 ccjmin for cannulae sizes 3-8 mm respectively. Blood velocity and shear stress at typical ECC flow rates caused significant turbulence and hemolysis, which must be minimized by the choice of cannulae of the correct size. Address correspondence to Lynn M. Pfaender, 69 Butler Street, S.E., Atlanta, Georgia, 30303. Presented at the 19th International AmSECTConference, 3-5 March, 1981, San Francisco, California. Introductio"'--------------Since the development of extra corporeal circulation one of the most difficult problems encountered has been to reduce red blood cell damage. Hemolysis due to blood flow is related to exceeding certain previoqsly determined extremes of shear stress, turbulence, velocity, and energy loss. During laminar flow a shear stress factor of 132 dynesjcm2 produces 113 mg% of free plasma hemoglobin at the end of I 00 minutes of pumping4 In vitro blood turbulence occurs at a Reynolds number greater than 10002•3•4; if flow is turbulent, hemolysis increases.4 An exponential rise in hemolysis occurs above a critical blood velocity of 120 emf sec 1 to 200 emf sec. 5 The mechanical destruction of blood in a jet injection apparatus occurs at a kinetic energy of 10,000 to 20,000 gm·cm2·sec-2_6.7 If these in vitro findings could be translated to clinical extracorporeal circulation, 8 then shear stress in excess of 132 dynesjcm2, turbulence measured by a Reynolds number in excess of 1000, flow velocity in e-xcess of 120 emf sec to 200 emf sec, and kinetic energy loss in excess of 10,000 to 20,000 gm·cm2-sec-2 would cause a significant amount of hemolysis. These extremes should be considered in the design and the choice of extracorporeal circuit components. Today the ascending aorta is a standard cannulation site for arterial reentry of. extra corporeal blood flow during open heart surgery.9•10 The cannula diameter chosen should be determined according to the blood 224 The Journal of Extra-Corporeal Technology Volume 13, Number 4, 1981 flow rate desired. The choice of unnecessarily small bore arterial cannulae for extracorporeal circulation adds to the damage of red blood cells. The purpose of this paper is to aid in the selection of cannulae of acceptable inner diameters for chosen blood flows using reported ranges of shear stress, turbulence, mean and critical blood velocity, and kinetic energy loss. Backgrounu_ ________________________ __ Shearing stress in flowing blood can directly activate or damage blood elements; such shear-induced blood damage as a result of passage through the extracorporeal circuit has been reported.l 1·12•13 Shear stress results from applied forces causing two fluid layers or a fluid layer and the tube wall to slide over one another in a direction parallel to the plane of contact (Figure 1). Shear stress is equal to the force causing the flow divided by the area over which the force is applied14 and behaves in a linear manner with a Newtonian fluid. 15 For the purpose of research, investigators have assumed that blood flowing in a tube many times the diameter of a red blood cell behaves approximately as a Newtonian fluid. 15 Blood· flow produced by a roller pump is considered steady flow because the pressure and flow waveforms are considered independent of frequency.16 Consequently, blood flow in this study has been assumed to behave in a Newtonian manner. The shear stress measured at the cannula wall serves as an index of expected mechanical damage to the red blood cells.1·15 Within circular tubes the average shear stress at the wall ( TW) during expected turbulent flow