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Measurement of carbon dioxide production in very low birth weight babies
C C Kingdon1, F Mitchell, O A Bodamer
1Department of Child Health, St George's Hospital Medical School, Cranmer Terrace, London SW17 0RE, UK. markeane@aol.com
Background:
CO2 production is most commonly measured by using indirect calorimetry to quantify elimination of CO(2) in breath (VCO2). An alternative is to measure the rate at which CO2 appears in the body pool (RaCO2) by infusing a (13)C labelled bicarbonate tracer. VCO2 and RaCO2 generally differ but are related by c, 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 for c but the only neonatal study to determine c 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.
Method:
Eleven spontaneously breathing, continuously fed, very low birthweight infants (median birth weight 1060 g, median gestational age 29 weeks) were studied.
Results:
Mean (SD) VCO2 was 9.0 (2.0) ml/min (standard temperature and pressure dry, STPD) and mean (SD) RaCO2 was 9.6 (2.1) ml/min (STPD). The mean (SD) value of c 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 measuring c for a given study population rather than assuming a value based on adult studies. The close approximation of RaCO2 and VCO2 in this group of babies implies that the labelled bicarbonate infusion technique could be used to measure simply CO2 production during mechanical ventilation.