Congenital heart disease is the most common birth defect and occurs in \~8 per 1000 live births in the United States. Approximately 25% of these infants require surgery in the first year of life to repair or palliate their heart defect. Many cardiac surgeries require the use of cardiopulmonary bypass to maintain systemic blood flow and oxygen delivery during surgery. Cardiopulmonary bypass is not a natural process, and, as a result, contributes to post-operative physiologic derangements including ischemia-reperfusion injury, systemic inflammatory response, and subsequent fluid overload.

Fluid overload, in particular, is a common issue in children undergoing cardiac surgery, particularly in the immediate post-operative period. The rates of fluid overload following cardiothoracic surgery are high, reported between 31% and 100% in different studies depending on the method of assessment and the degree of fluid overload analyzed. Diaz et al demonstrated approximately 55% of children requiring mechanical ventilation or inotropic support in the intensive care unit developed fluid overload. Fluid overload is defined as a positive fluid balance and can occur extra or intravascularly. The buildup of excess extravascular fluid is also known as edema. The etiology of fluid overload and edema is multifactorial and includes fluid retention due neurohormonal pathway activation such as vasopressin and renin-angiotensin system, congestive heart failure, iatrogenic fluid administration, and capillary leak. Intravascular fluid overload can cause elevated central venous pressure, potentially leading to poor renal perfusion and subsequent acute kidney injury (AKI) while extravascular edema compromises abdominal and thoracic compliance and can make ventilation difficult. In the post-operative period, fluid overload has been associated with significant morbidity including AKI, longer mechanical ventilation dependence, prolonged length of stay, and increased mortality.

Unfortunately, management and treatment of fluid overload and edema are not standardized as it is currently difficult to accurately quantify the degree of fluid overload. Methods for monitoring fluid status include trending body weights, monitoring net fluid balance (intake versus output), trending central venous pressure, and physical exam findings. All of these current methods for monitoring fluid status can easily be confounded in the intensive care unit. A paucity of data exists regarding accurate methods of assessing edema in infants. Objective methods of evaluating fluid overload have been attempted, but are limited to measuring only intravascular volume, such as ultrasound of the jugular vein, or are difficult to apply clinically, such as skin bioelectric impedance. Additional research is needed to better understand and directly measure edema in infants.

Ultrasound of the skin is one possible method for quantifying extravascular fluid overload and edema through measurement of the thickness of skin and underlying subcutaneous layers. Ultrasound has previously been utilized in pediatric patients to diagnose skin and soft tissue infections, but there are no dedicated studies performed to solely measure edema. MuscleSound, an ultrasound technology company, has developed an automated software system to measure skin tissue structures, including edema, in adults. This technology has been studied in adults, however, it has not yet been trialed or validated in children. The ability to evaluate edema with a reliable, automated, non-invasive, bedside tool would provide objective measurements into a patient's fluid status. This tool would be of particular importance in infants with congenital heart disease who have many risk factors for fluid overload but whose fluid status can be difficult to appropriately assess.