Available tools to calculate perioperative blood loss include visual observation of the surgical field, gravimetric before-and-after weighing of sponges, direct weighing, and colorimetric technology. Visual estimation of blood loss alone, the historical standard, is inaccurate. Readily available physical findings during clinically significant blood loss include, but are not limited to, capillary refill time, skin mottling, body temperature gradients, pulse amplitude, and heart sounds. A determination of clinically significant blood loss requires utilization of all conventional vital signs and laboratory values, indices of oxygenation and cardiac output (SaO2 and ETCO2), and, when possible, arterial oxygen content (CaO2), cardiac output (CO), oxygen content difference between the arterial and venous sides of the circulation (a-v DO2), and mixed venous oxygen (SvO2). The gold standard to measure cardiac output (CO) in critically ill patients has been the pulmonary artery catheter (PAC), an invasive monitor that requires expert management. To rapidly assess a patient’s CO response to fluid resuscitation at the bedside, surrogate measures of CO have been adopted. At this point in time, there is not a consistent definition of fluid responsiveness in children, and pulse wave form analysis in isolation has not been shown to predict which patients will benefit from a blood transfusion. Emerging tools to guide oxygenation management include hemoglobin co-oximetry (SpHb) and near-infrared spectroscopy (NIRS). These noninvasive monitors provide continuous observation of Hgb levels and regional oxygen saturation (rSO2), respectively. NIRS is increasingly used as a management tool in an attempt to prevent hypoxemia and subsequent end-organ injury in hospitalized children and adults. However, the ideal NIRS rSO2 value(s) for currently used clinical applications are unknown. Systematic reviews have failed to conclude that NIRS is appropriate for routine patient oxygenation assessment. The Hgb co-oximeter is a logical solution to noninvasive, continuous Hgb monitoring (SpHb) of critically ill patients who are at risk of serial phlebotomy.