Validation of a [13C]bicarbonate tracer technique to measure neonatal energy expenditure.

The use of a stable isotope-labeled [13C]bicarbonate infusion to measure energy expenditure is advantageous, as a complete collection of expired air is not required. This technique allows for facile measurements of energy expenditure in intubated neonates. The aim of the present study was to determine the accuracy of energy expenditure estimates in postsurgical neonates by using the [13C]bicarbonate method compared with the current standard, indirect calorimetry. Eight neonates who were receiving total parenteral nutrition [98 +/- 21 (SD) kcal x kg(-1) x d(-1); 3.1 +/- 0.7 (SD) protein g x kg(-1) x d(-1)] were studied on postoperative d 15.5 +/- 11.9. A primed continuous 3-h intravenous infusion of NaH13CO3 and indirect calorimetry were performed simultaneously. Energy expenditure was calculated separately from the Weir equation and from the dilution of 13CO2 in the breath in combination with the individual energy equivalents of CO2 from the diet. The rate of CO2 appearance and energy expenditure calculated from the bicarbonate method (0.725 +/- 0.021 mol x kg(-1) x d(-1); 89.5 +/- 2.5 kcal x kg(-1) x d(-1)) highly correlated (r = 0.94 and 0.98, respectively) with the CO2 excretion and energy expenditure determined by indirect calorimetry (0.489 +/- 0.016 mol x kg(-1) x d(-1); 60.2 +/- 2.0 kcal x kg(-1) x d(-1)) when analyzed nonproportionately to weight. Bland-Altman analysis demonstrated the 95% confidence interval to be +/- 8.2 kcal x kg(-1) x d(-1). Linear regression analysis revealed a highly statistically significant equation relating the two energy expenditures: Indircal (kcal/d) = -9.341 + [0.705 x Bicarb (dcal/d)]; p < 0.001, r2 = 96.4%. We conclude that energy expenditure in neonates can be accurately determined using the [13C]bicarbonate method and a regression equation. Therefore, the bicarbonate method may be useful for determining energy expenditure in neonates not readily accessible to indirect calorimetry, such as those being mechanically ventilated or on extracorporeal life support.