Background: Although Indirect Calorimetry (IC) is increasingly used to measure Energy Expenditure (EE) in preterm and term infants there is little information as to the optimum time period over which measured EE represents actual EE. Subjects: 11 healthy preterm infants<32wks gestation and <1500g birthweight. Methods: EE was measured by open canopy IC using the Datex Deltatrac II during three 48hr metabolic balances, two prior to discharge and the third at Term+8wks corrected age. Cumulative measurements of EE took place within 24hr blocks. Measurements were interrupted to allow normal parental and nursing care to be undertaken. EE was recorded at minute intervals and averaged for the total measurement time and for shorter measurement times within the 24hrs. The first 15mins of each measurement period and after each disturbance of the subject were discarded to allow time for equilibration (data previously obtained). Comparisons were made between the mean EE at each measurement time and that for the total within that 24hrs so that the technical error[=√(·Sd2/2N)] for each time could be calculated.Results: 51 periods of measurement were undertaken with a mean measurement time of 653mins (range 225 - 1214mins). The mean total EE during the first two balances was 93kcal/24hrs whilst during the third was 278kcal/24hrs. Table Conclusion: There is a rapid reduction in technical error associated with increasing length of measurement of EE. We suggest a minimum measurement period of 6hrs.

Table 1
Full size table