BACKGROUND CO₂production is most commonly measured by using indirect calorimetry to quantify elimination of CO₂ in breath (Vco ₂). An alternative is to measure the rate at which CO₂ appears in the body pool (Raco ₂) by infusing a ¹³C labelled bicarbonate tracer. Vco ₂ and Raco ₂ generally differ but are related byc, a factor that adjusts for the incomplete recovery of infused tracer in the breath. The literature relating to human studies cites a wide range of values forc but the only neonatal study to determinec empirically estimated a mean value of 0.77.

AIM To estimate fractional recovery rate, c, in very low birthweight babies, and assess the feasibility of using the isotopic technique to measure CO₂ production during mechanical ventilation.

RESULTS Mean (SD) Vco ₂ was 9.0 (2.0) ml/min (standard temperature and pressure dry, STPD) and mean (SD) Raco ₂ was 9.6 (2.1) ml/min (STPD). The mean (SD) value ofc was estimated as 0.95 (0.13). The 95% confidence intervals of the mean were 0.87–1.03.

CONCLUSIONS The results emphasise the importance of measuringc for a given study population rather than assuming a value based on adult studies. The close approximation of Raco ₂ and Vco ₂ in this group of babies implies that the labelled bicarbonate infusion technique could be used to measure simply CO₂ production during mechanical ventilation.