Ultrafiltration in Cardiac Surgery

____________ _ Cardiopulmonary bypass (CPB) may result in hemodilution and volume overload. Restoration of a normal hematocrit is important for post-operative myocardial performance. Diuretics, conventionally used in the post-operative period may be ineffective due to renal impairment or lead to electrolyte imbalance. Hollow-fiber hemofilters, made of an anisotropic polysulfone membrane (AMICON Diafilter) have been used for the ultrafiltration of blood in cardiac surgery during CPB. The hemofilter is inserted in the arterial line of the CPB. With this method ultrafiltration of the priming volume and restoration of the pre-operative hematocrit can be achieved and edema can be reversed. In our experiments, performed in dogs (n = 5) we removed 1 L of an ultrafiltrate of plasma water in 15 minutes without hemodynamic changes or electrolyte imbalance. In the same time hematocrit increased from 19 to 29% (p < 0.05) and protein concentration from 2.7 to 5.4 g/dl (p < 0.05). Simultaneous studies with Crlabelled RBC's demonstrated that blood volume was not significantly altered from pre CPB values during ultrafiltration, suggesting that the origin of the removed ultrafiltrate was mainly from the extravascular space. Dr. B. Walpoth is supported by the following Swiss grants: -Kommission Zur Foerderung des Akademischen Nachwuchses des Kanton Zurichs -Schweizerische Stiftung Fur Kardiologie Part of the work has been performed at the Department of Surgery (Chairman Prof. A. SENNING) of the Medical School of the University of Zurich, Switzerland. Direct communications to: B. Walpoth, M.D., Dept. of Cardiovascular Surgery, Stanford University Medical Center, Stanford, CA 94305. 68 The Journal of Extra-Corporeal Technology In conclusion, the insertion of a hemofilter in the CPB either during or at the end of bypass allows the removal of the priming volume and prevents postoperative hemodilution and fluid overload without compromising hemodynamic stability. The use of hemofilters in cardiac surgery is safe and indicated for patients with CHF, fluid overload, renal impairment and long bypass time. In addition, the hemoconcentration achieved with the hemofilters may reduce transfusion requirements and improve myocardial performance. Introduction, ____________ _ The extracorporeal circuit in cardiopulmonary bypass (CPB) requires that the circulating blood volume of the patient is expanded by up to 2 liters or more. While in earlier years the extracorporeal circuit (ECC) was primed with whole blood or blood products it has become practice in recent years to use crystalloid solutions for this purpose. As a consequence the patient's blood is diluted by the priming volume causing a fall of hematocrit and protein concentration of up to 50%. Due to the decreased oncotic pressure part of this volume is redistributed during the intra-operative period to the extravascular space resulting in fluid overload and edema. · Conventionally at the end of surgery the entire or part of the volume of the ECC is returned to the patient, who is frequently left with a low hematocrit and the fluid overload. Both the lowered hematocrit and the fluid overload represent a burden for the post-operative myocardial performance. 3 To prevent this complication either the priming volume including the red blood cells (RBC's) is discarded or the RBC's from the leftover blood (in the oxygenator) are separated by centrifugation or filtration and then returned to the Volume 16, Number 2, Summer 1984 patient. 4 • Both these approaches involve only the oxygenator blood of the CPB while the patient remains with a low hematocrit and fluid overload. A simple method to reverse the detrimental effects of hemodilution is intra-operative ultrafiltration. In recent years hemofilters with high ultrafiltration characteristics have become available and are used as an alternative to hemodialysis in the end stage renal disease. 6 A hollow fiber filter made of an anisotropic polysulfone membrane (Amicon-Diafilter-30)a was inserted in the ECC. This filter is made of 6000 fibers of 25 em length with a surface area of 0.6 m . With an internal diameter of 0. 2 mm, a pore size of only 1000 A and a molecular cut-off rate of maximally 50,000 Daltons, the filter prevents the loss of proteins. The insertion of such a hollow fiber filter in the ECC allows removal of an ultrafiltrate of plasma water close to the CPB priming volume without hemodynamic changes ,1 and the effects of hemodilution can be reversed with preservation of RBC' s. 8 Method ____________ _ Five anesthetized mongrel dogs were subjected to one hour of normothermic CPB after thoracotomy and full heparinization using two venous and one arterial cannulae and an adult size bubble oxygenator (Bel leo )b. The animals' average weight was 23 kg. Two liters of Ringer's lactate solution served as priming volume, reducing the hematocrit to below 20%. After one hour of total CPB, partial bypass was continued and an Amicon-30-hollow fiber-filter was incorporated in a parallel postpump circuit to the arterial line (Fig. 1). One liter of ultrafiltrate was removed in each animal and discarded. Two-hundred-fifty ml of oxygenator blood were not given back to the animals and discarded with the system. The transmembrane pressure was kept below 500 mmHg by adjusting the blood flow which varied according to the hematocrit between 533 and 300 ml/min (QB), yielding a maximal filtration flow (QF) of 75 ml/min. The following parameters were studied at the beginning and at the end of filtration: filtration rate and volume, blood flow (Biotronics)c and pressure, hematocrit, electrolytes, cell count of blood and filtrate. In addition, blood volume measurements were performed by the technique of chromium labelled Erythrocytes (Cr* RBC) . Electrocardiogram and hemodynamic parameters were continuously reVolume 16, Number 2, Summer 1984 METHOO !J' lf:M!J'ILTRATION FIGURE 1. Experimental Protocol 0 = Oxygenator P = Roller pump M = Manometer H = Hemofilter Flow = Blood flow corded on a (Brush Gould System 2000)ct eightchannel recorder. This included central venous pressure, pulmonary artery pressure (Swan-Ganz 7F catheter)e, aortic pressure and left ventricular pressure (dip tip Milar 5F catheter)r and its first derivative dP/dt, a parameter of isovolumic contractility. To assess myocardial edema, transmural left ventricular biopsies were taken before and after filtration. The water content was measured by the wet and dry weight method. 10 Data are expressed as mean ± standard deviation (SO), statistical significance was obtained by the Student's T-test. Results, _____________ _ Before the end of CPB 1 liter of ultrafiltrate was removed from each dog in a mean time of 16 minutes. The mean values of pressures and flow rates during filtration are summarized in Figure 2. However, blood flow (QB) and filtration rate (QF) varied with the hematocrit: at the beginning of the aAmicon Corporation, Danvers, MA 01923 bBELLCO (Oxibel D600), Dideco, s.p.a. Mirandola 1-41037 Italy cBiotronics 3/S Extracorporeal, Silver Spring, MD dBrush-Gould, Inc. Cleveland, OH 44114 eEdwards Laboratories, Inc. Santa Ana, CA 92711 fMilar Instruments, Inc. Houston, TX 77223 The Journal of Extra-Corporeal Technology 69 procedure with a hematocrit of 19%, blood flow (QB) was 533 ml!min and filtration rate (QF) 75 ml!min. With a hematocrit of 29% at the end of filtration QB was 300 ml!min and QF 54 ml!min.