Authors: Manal Mirreh, MD, Lindsey Weidmann, MD, and Lea Matthews, MD – Children’s Hospital of Philadelphia, Philadelphia, PA
A 14-year-old female with a history of ANCA-associated vasculitis, end-stage renal disease requiring dialysis, and severe left ventricular dilation is undergoing left ventricular assist device (LVAD) implantation. Cardiopulmonary bypass (CPB) is initiated uneventfully, and the patient is maintained normothermic with no use of aortic cross-clamp or cardioplegia. During CPB, her serum potassium level is noted to rise to 6.5 mEq/L. Given the risk of worsening hyperkalemia following transfusion of red blood cells, which of the following intraoperative ultrafiltration strategies is most appropriate to manage her electrolytes prior to separation from CPB?
EXPLANATION
Hyperkalemia is defined as a serum potassium higher than the upper limit of normal, commonly considered to be 5.5 mEq/L. Homeostatic mechanisms regulate potassium balance to maintain high intracellular levels required for cellular functions (eg, metabolism and growth) and low extracellular concentration to preserve the steep concentration gradient across the cell membrane needed for nerve excitation and muscle contraction.1
Preoperative chronic kidney disease and the administration of potassium-rich cardioplegic solutions are common causes of hyperkalemia during CPB. Additional sources of exogenous potassium include the transfusion of older units of packed red blood cells or irradiated blood products, due to red cell membrane injury, which increases extracellular potassium.2
Cardiopulmonary bypass during pediatric cardiac surgery is linked to a robust inflammatory response, fluid overload, and end-organ dysfunction, all of which contribute to postoperative morbidity and mortality. To address these complications, a range of intraoperative ultrafiltration strategies—including conventional ultrafiltration, modified ultrafiltration (MUF), zero-balance ultrafiltration (ZBUF), and hybrid approaches such as ZBUF-MUF—have been employed over the years to reduce these adverse effects and enhance postoperative recovery. However, no clear consensus exists on which ultrafiltration technique provides the greatest benefit for infants and children undergoing open-heart surgery.
Ultrafiltration is the process of running blood through a device with a semipermeable membrane to remove “free water” (water, electrolytes and substances with a molecular size smaller than the membrane pore size). It can be performed before, during, and/or after CPB. The goals of ultrafiltration during cardiopulmonary bypass include the removal of excess crystalloid volume, hemoconcentration to increase hematocrit, and the clearance of electrolytes and inflammatory mediators.2
Ultrafiltration performed during cardiopulmonary bypass, known as conventional ultrafiltration (CUF), is primarily used to remove excess fluid that accumulates from various sources, including pre-bypass fluid administration, cardioplegia solutions, valve testing saline, and crystalloid added to the venous reservoir during periods of reduced venous return.3
Zero-balance ultrafiltration (ZBUF), also referred to as dilutional ultrafiltration (DUF), was initially introduced during the rewarming phase in pediatric patients. With ZBUF, ultrafiltrate is continuously removed and simultaneously replaced with an equivalent volume of crystalloid to maintain a net-zero fluid balance. The primary goal is to continuously clear electrolytes such as potassium and lactate, as well as inflammatory mediators, thereby producing a circulating volume with a composition closer to that of the replacement solution. Several crystalloids are utilized for ZBUF, including lactated ringers, Plasma-Lyte, and normal saline. In patients with chronic kidney disease, normal saline is preferred due to lack of potassium in the solution. However, use of normal saline is not ideal due to its high chloride content and may also exacerbate hyperkalemia due to potential for hyperchloremic acidosis. Case reports have described the use of Dialysate solution, in an attempt to avoid this occurrence, as it contains a lower potassium content than lactated ringers or Plasma-Lyte.4
Modified ultrafiltration (MUF) is performed after cardiopulmonary bypass to remove free water and concentrate the blood, resulting in increased hematocrit. It also allows recovery of whole blood from the bypass circuit, unlike cell salvage systems that return only red blood cells. While proposed benefits such as reduced edema, lower inotropic needs, and improved pulmonary function remain debated, MUF is widely accepted for its efficiency in raising hematocrit and reducing transfusion requirements.5 In conclusion, among the available ultrafiltration techniques, zero-balance ultrafiltration (ZBUF) is the most appropriate choice in this setting, as it effectively reduces serum potassium levels and is particularly beneficial for managing hyperkalemia during cardiopulmonary bypass.
REFERENCES
1. Kremen, J, Matoo TK, Somers, M. Hyperkalemia in children: Causes, clinical manifestations, diagnosis, and evaluation. In: UpToDate, Post TW (Ed). UpToDate. [Accessed May 10th, 2025]. Available from: https://www.uptodate.com
2. Ozdemir D, Chan R. The Challenges of Hyperkalemia on Cardiopulmonary Bypass. The Academy Newsletter. Summer 2012;(Summer):6‑9
3. Andersen ND, Meza JM, Turek JW, Mavroudis C, Backer CL. Management of pediatric cardiopulmonary bypass. In: Mavroudis C, Backer CL, eds. Pediatric Cardiac Surgery. 5th ed. John Wiley & Sons Ltd; 2023:161-189. doi:10.1002/9781119282327.ch
4. Heath M, Raghunathan K, Welsby I, Maxwell C. Using Zero Balance Ultrafiltration with Dialysate as a Replacement Fluid for Hyperkalemia during Cardiopulmonary Bypass. J Extra Corpor Technol. 2014;46(3):262-266
5. Matte GS. The Bypass Plan. In: Perfusion for Congenital Heart Surgery: Notes on Cardiopulmonary Bypass for a Complex Patient Population. Wiley Blackwell; 2015:55-59
