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Measurement of carbon dioxide production in very low birth weight babies


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.

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) 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.

  • carbon dioxide
  • carbon isotopes
  • calorimetry
  • very low birthweight babies
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