**2.1 The inaccuracies of predictive equations**

294 Congenital Heart Disease – Selected Aspects

In ICU patients, the importance of the accurate measurement of VO2 has been increasingly realized in the past decade or two. Significant alterations in systemic oxygen transport and the contribution of VO2 in the impaired balance of oxygen transport during the early postoperative period after CPB are now better understood (Chiara, Giomarelli et al. 1987; Li, Hoschtitzky et al. 2004; Li, Schulze-Neick et al. 2000; Li, Zhang et al. 2007; Li, Zhang et al. 2007; Oudemans-van Straaten, Jansen et al. 1996). VO2 has its own meaning in the balance of oxygen transport, which has been largely ignored. A hypermetabolic response with increased VO2 occurs in patients after CPB, due mainly to *1)* a systemic inflammatory response (Li, Hoschtitzky et al. 2004; Oudemans-van Straaten, Jansen et al. 1996), *2)* rewarming from hypothermic CPB and fever (Li, Hoschtitzky et al. 2004; Li, Schulze-Neick et al. 2000), and *3)* the use of inotropes (Li, Zhang et al. 2006). The increase in VO2 is an important contributor to the imbalance of oxygen transport in the early postoperative period, when cardiac function and oxygen delivery are depressed due to myocardial injury by surgery and ischemia-reperfusion (Li, Zhang et al. 2006; Li, Zhang et al. 2007; Wernovsky, Wypij et al. 1995). VO2 varies greatly between patients and within individual patients over time. Variation in VO2 results from varied circulatory, metabolic, and hormonal responses to CPB (Li, Hoschtitzky et al. 2004; Oudemans-van Straaten, Jansen et al. 1996), from patient body temperature (Li, Hoschtitzky et al. 2004; Li, Schulze-Neick et al. 2000;), and from pharmacological (Li, Zhang et al. 2006) and ventilator manipulations (Li, Hoskote et al. 2005; Li, Zhang et al. 2008) (please see section 4 for details). In this dynamic milieu, continuous or repeated monitoring of VO2 is necessary to reflect changes

Accurate measurement of VO2 allows precise assessment of systemic hemodynamics and oxygen transport parameters in varied circulations after complete repair or palliations (Li, Hoschtitzky et al. 2004; Li, Hoskote et al. 2005; Li, Schulze-Neick et al. 2000; 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; Schulze-Neick, Li et al. 2001; Schulze-Neick, Li et al. 2002). Actual measurements are superior to the indirect indicators, such as blood pressure and arterial and venous oxygen saturations that are most commonly used in postoperative management. Superiority of actual measurements is seen particularly clearly after the Norwood procedure, when profound hemodynamic instability and oxygen transport imbalance occurs. Furthermore, actual measurements of hemodynamics and oxygen transport parameters are fundamental to bedside physiological research on factors affecting the imbalance of oxygen transport, research aimed at improving the management of critically ill children. With direct and continuous measurement of VO2 using state-of-the-art technique respiratory mass spectrometry, we have conducted extensive studies in neonates 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; Li, Zhang et al. 2008; Li, Zhang et al. 2008; Li, Zhang et al. 2008). We use the Norwood circulation and physiology in this Chapter as a model to understand

oxygen transport and the factors affecting oxygen kinetics in children after CPB.

The objectives of this chapter are two-fold. 1) To review the currently available techniques of VO2 measurement, including published predictive equations and indirect Fick principle using themodilution, their advantages and disadvantages, with special emphasis on respiratory mass spectrometry to assess VO2 in children undergoing cardiac cauterization and after CPB in the ICU. 2) Using the Norwood physiology as the model to introduce the concept of oxygen transport and further emphasize the importance of direct and continuous

over time.

Although techniques for metabolic monitoring using indirect calorimetry or respiratory mass spectrometry are available for the direct measurement of VO2, it is still common practice to estimate VO2 values from tables or published predictive equations (LaFarge and Miettinen 1970; Lindahl 1989; Lundell, Casas et al. 1996; Wessel, Rorem et al. 1969). Despite attempts to improve the accuracy of estimated VO2 values, large discrepancies are still observed between measured and estimated values (Laitinen and Rasanen 1998; Shanahan, Wilson et al. 2003; Wolf, Pollman et al. 1998). Such discrepancies present challenges in the clinical application of predictive equations e.g., in the catheterization laboratory setting, because subsequent hemodynamic calculations will be impaired. In ICU patients during the early postoperative period after CPB, estimation of VO2 is even further exposed to inaccuracies due to significant variability of VO2 between and within patients over time (Li, Zhang et al. 2006; Li, Zhang et al. 2006; Li, Zhang et al. 2007; Li, Zhang et al. 2008). Furthermore, estimating VO2 by predictive equations gives a single value for a given patient and makes no provision for the dynamic patient milieu that is inevitable in the early postoperative period (Li, Zhang et al. 2006; Li, Zhang et al. 2006; Li, Zhang et al. 2007; Li, Zhang et al. 2008).

We have compared results from four commonly used equations for estimating VO2 (LaFarge and Miettinen 1970; Lindahl 1989; Lundell, Casas et al. 1996; Wessel, Rorem et al. 1969) against VO2 measured directly by respiratory mass spectrometry. Both the equations and the direct measurements were applied to children with congenital heart defects, during cardiac catherization and in the ICU after CPB. We found poor agreement between the direct measurements and all estimated values, especially in children younger than 3 years of age and in the ICU patients (Li, Bush et al. 2003; Rutledge, Bush et al. 2010).
