Weight-correction of carbon dioxide diffusion coefficient (DCO 2 ) reduces its inter-individual variability and improves its correlation with blood carbon dioxide levels in neonates receiving high-frequency oscillatory ventilation.

BACKGROUND: Carbon-dioxide elimination during high-frequency oscillatory ventilation (HFOV) is thought to be proportional to the carbon dioxide diffusion coefficient (DCO2 ) which is calculated as frequency x (tidal volume)2 . DCO2 can be used to as an indicator of CO2 elimination but values obtained in different patients cannot be directly compared.

OBJECTIVES: To analyze the relationship between DCO2 , the weight-corrected DCO2 (DCO2 corr) and blood gas PCO2 values obtained from infants receiving HFOV.

METHODS: DCO2 data were obtained from 14 infants at 1/s sampling rate and the mean DCO2 was determined over 10 min periods preceding the time of the blood gas. DCO2 corr was calculated by dividing the DCO2 by the square of the body weight in kg.

RESULTS: Weight-correction significantly reduced the inter-individual variability of DCO2 . When data from all the babies were combined, standard DCO2 showed no correlation with PCO2 but DCO2 corr showed a weak but statistically significant inverse correlation. The correlation was better when the endotracheal leak was <10%. There was significant inverse but weaker correlation between the HFOV tidal volume (VThf) and the PCO2 . In any baby, DCO2 corr >50 mL2 /sec/kg2 or VThf > 2.5 mL/kg was rarely needed to avoid hypercapnia.

CONCLUSIONS: Weight-correction of DCO2 values improved its comparability between patients. Weight-corrected DCO2 correlated better with PCO2 than uncorrected DCO2 but the correlation was weak.