Modified Ultrafiltration

An important factor affecting morbidity and mortality after cardiac surgery in children is the effect of CPB. During the initiation of CPB, considerable hemodilution occurs. This hemodilution is the result of the priming volume required for the CPB circuit. Under many circumstances this hemodilutional effect is intentional, decreasing blood viscosity and thereby preventing sludging when the patient is cooled to temperatures below 20°C. After CPB, hemodilution is associated with tissue edema and organ dysfunction. Because blood elements are exposed to the nonendothelialized circuitry of CPB, there is also a significant inflammatory response as a result of CPB. Both the effects of hemodilution and inflammation are exaggerated in neonates, infants, and young children because of their disproportionate exposure to the circuit relative to their body size. The inflammatory response leads to an increase in capillary permeability, leading to an overall increase in total body edema postoperatively.

Efforts to reduce the hemodilution and inflammatory effects of CPB have included reducing priming volume, perioperative anti-inflammatory and diuretic therapies,

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MUF is performed after CPB is complete and allows filtration of both the patient's blood and the remaining contents of the CPB circuit, including the venous reservoir.^{[160] [161] [162]} Using the MUF technique, an ultrafilter is interposed in the circuit between the aortic arterial line and the venous cannula. After separation from CPB, the blood is removed from the patient via the aortic cannula and fed through the ultrafilter along with blood from the venous reservoir and oxygen. The outlet of the ultrafilter is fed to the right atrium of the patient. Blood flow through the ultrafilter approximates 200 mL per minute, which is maintained by a roller pump. Suction is applied to the filter port of the ultrafilter, resulting in an ultrafiltration rate of 100 to 150 mL per minute. A constant left atrial or right atrial pressure is maintained, achieving continued hemodynamic stability in the patient. Ultrafiltration is carried out, with the end point being either time (15 to 20 minutes) or the achievement of a hematocrit value of approximately 40. ^[161]

Ultrafiltration offers two major advantages. First, total body water is reduced as a direct result of removing the ultrafiltrate.^[159] This effect counteracts the hemodilution effects associated with the institution of CPB. In addition, the hematocrit is raised after CPB, enhancing oxygen delivery to the tissues. Second, MUF has been shown to remove some of the deleterious vasoactive substances associated with the inflammatory response to CPB.^{[163] [164]} This effect is mediated by reducing circulating cytokines, which are associated with capillary leak syndrome. Examination of the ultrafiltrate shows that it contains low-molecular-weight inflammatory mediators, including C3a, C5a, interleukin-6α, interleukin-8α, tumor necrosis factor, myocardial depressant factor, and various other cytokines. Several studies have shown that, compared with control patients, patients who undergo MUF after CPB have substantially less increase in total body water, demonstrate less complement and interleukin release, require fewer blood transfusions, and have a faster recovery of systolic blood pressure ( Fig. 51-13 ). ^{[159] [161] [165]}

In a clinical study examining the effect of MUF on left ventricular systolic function in children, MUF was associated with an increase in intrinsic left ventricular systolic function and a decrease in end-diastolic pressure, thereby improving left ventricular compliance.^[166] In an experimental study, MUF also demonstrated improvement in CBF and increased cerebral metabolic activity and cerebral oxygen delivery after DHCA ( Fig. 51-14 ).^[167] This last effect acutely improves cerebral metabolism after DHCA and may reduce and reverse the known deleterious effects of DHCA on brain function after CPB. In another clinical study, MUF was demonstrated to reduce postoperative blood use, chest tube drainage, plural effusions, and hospital stay in patients after cavopulmonary operations.^[168]

An alternative to MUF is conventional ultrafiltration during the rewarming period of CPB. In an experimental model examining MUF versus conventional ultrafiltration, only MUF was effective in reducing weight gain and myocardial edema and was associated with improving

Figure 51-13 Systolic blood pressure (BP) after separating from cardiopulmonary bypass and 15 minutes after separating with and without modified ultrafiltration (MUF). Note the significant improvement in systolic BP with the use of MUF. (From Ungerleider RM: Effects of cardiopulmonary bypass and use of modified ultrafiltration. Ann Thorac Surg 65:S35, 1998; reprinted with permission from the Society of Thoracic Surgeons.)

In summary, MUF is a safe adjunct, reversing the deleterious effects of hemodilution and the inflammatory response associated with CPB in children. Perioperative blood loss and blood use is significantly reduced when MUF is used. MUF also improves left ventricular function

Figure 51-14 Cerebral metabolic rate for oxygen measurements (CMRO₂ ) before and after deep hypothermic circulatory arrest. Note the significant increase in CMRO₂ in the MUF animals compared with the control and transfusion groups at stage 3. CTL, control; MUF, modified ultrafiltration; TX, transfusion. (From Skaryak LA, Kirshbom PM, DiBernardo LR, et al: Modified ultrafiltration improves cerebral metabolic recovery after circulatory arrest. J Thorac Cardiovasc Surg 109:744, 1995.)

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