Accurate Measurement of Systemic Oxygen Consumption in Ventilated Children with Congenital Heart Disease

Measuring systemic oxygen consumption (VO2) is a fundamental part of hemodynamic and oxygen transport assessment when using the Fick principle. This measure is pivotal for children with congenital heart disease, at cardiac catheterization and in the Intensive Care Unit (ICU) after cardiopulmonary bypass surgery (CPB). According to the direct Fick principle (Fick, 1870), VO2 may be combined with the difference between arterial and venous oxygen content, and the pressure gradient, to allow the calculation of each parameter of systemic hemodynamics and oxygen transport. Parameters that may be calculated include systemic and pulmonary blood flows (Qs and Qp) and resistances (SVR and PVR), systemic oxygen delivery (DO2), and oxygen extraction ratio (ERO2). Importantly, these parameters can be derived in a variety of simple or complex circulations in congenital heart defects before and after surgical repair or palliation, including 1) biventricular circulation with or without left to right or right to left shunt (Li, Hoschtitzky et al. 2004; Li, Schulze-Neick et al. 2000; Schulze-Neick, Li et al. 2001; Schulze-Neick, Li et al. 2002), 2) functionally single ventricular circulation such as hypoplastic left heart syndrome before and after the Norwood procedure (Li, Zhang et al. 2006; Li, Zhang et al. 2006; Li, Zhang et al. 2007; Li, Zhang et al. 2007; Li, Zhang et al. 2008), and 3) one-and-a-half ventricular circulation such as after the bidirectional cavopulmonary shunt operation (Hoskote, Li et al. 2004; Li, Hoskote et al. 2005). If VO2 needs to be measured, then accuracy of the measurement cannot be overemphasized (Kendrick, West et al. 1988; Laitinen and Rasanen 1998; Shanahan, Wilson et al. 2003). Any error in VO2 measurement will translate directly into an equivalent magnitude of underestimation or over-estimation of hemodynamics and oxygen transport parameters, which may misdirect surgical and clinical treatment strategies. Prognostic cardiac catheterization is often used for evaluation of systemic and pulmonary blood flows and vascular resistances, particularly pulmonary vascular resistance, in patients with primary or secondary pulmonary hypertension, and in patients with functionally single ventricular abnormalities undergoing staged surgical palliations. In this latter group, elevated pulmonary vascular resistance is a risk factor for poor outcome (Gentles, Gauvreau et al. 1997; Gentles, Mayer et al. 1997; Mair, Hagler et al. 1990), emphasizing the need for accurate hemodynamic assessment before staged palliations.