Use of Predicted vs Measured Body Weight for Targeting Tidal Volumes in Children

When targeting tidal volume for children receiving mechanical ventilation, it may be strategically more lung protective to use the lower of either measured body weight (MBW) or predicted body weight (PBW), according to a study published in Chest.

Researchers performed a retrospective analysis of spirometry data from outpatient pulmonary functions tests from 2 separate datasets of children to determine the difference in forced vital capacity (FVC; mL/kg) using measured vs predicted body weight in children. Dataset 1 included children from the Canadian Health Measures Survey (n=5394) ages 6 to19 years, and the second dataset included pulmonary functions tests of children from the Children’s Hospital Los Angeles (n=8472) who were 6 to 20 years of age. Children with obstructive or restrictive lung physiology were excluded from the study.

The results demonstrated that children in the first dataset with a body mass index (BMI) Z-score >1.5 had FVC in the normal range (65-76 mL/kg) when PBW was used and below the normal range when using MBW, whereas children with a BMI Z-score <–0.5 had a median FVC at or below the normal range when using PBW and at or above the higher end of the normal range when using MBW. FVC obtained from PBW increased slightly with increasing BMI Z-score groups.

Children in the second dataset were found to have a statistically significant decrease in FVC from MBW with an increase in BMI Z-score from –0.5 to 0.49 (P <.001). There was also a statistically significant mild increase in FVC with PBW as BMI Z-score groups increased from 0.5 to 1.5, followed by a slight decline afterward (P <.001). Therefore, FVC fell in the normal range of 65 to 76 mL/kg when MBW was used in children with BMI Z-score <–0.5 but was below the normal range with PBW. In obese children (BMI Z-score >1.5), FVC fell into the normal range of 65 to 76 mL/kg using PBW, but was below the normal range when MBW was used.

Researchers concluded that as BMI Z-scores increase, there is a decrease in FVC when obtained using MBW but an increase when using PBW. FVCs are maintained for BMI Z-score >0.5 when using PBW and MBW for those with a BMI Z-score <–0.5, which suggests that lung volumes in children who are overweight are best estimated using their height (PBW), while volumes in children who are underweight (ie, those with failure to thrive) are best estimated from their weight. Researchers found the results indicated that “as children become obese, their lungs do not properly continue to grow in size.” Clinicians should consider setting the tidal volume for mechanical ventilation at the lowest measured or PBW, as researchers found it may be the most lung protective strategy for this specific patient population.

Reference

Kim GJ, Newth CJL, Khemani RG, Wong SL, Coates AL, Ross, PA. Does size matter when calculating the “correct” tidal volume for pediatric mechanical ventilation? A hypothesis based on forced vital capacity [published online April 20, 2018]. Chest. doi:10.1016/j.chest.2018.04.015