Validity of thoracic gas volume equations in children of varying body mass index classifications

The aim of the study was to evaluate air-displacement plethysmography (ADP) thoracic gas volume (TGV) prediction equations in children representing a wide range of body mass index (BMI).

254 children (5 to 17 years) were recruited from two centres (England and the United States). Subjects were stratified into three BMI categories according to the International Obesity TaskForce (IOTF) criteria: lean (48 male, 42 female), overweight (15 male, 29 female), and obese (52 male, 68 female). As part of the normal ADP procedure, TGV was measured (TGV(Meas)), predicted using child specific equations developed by Fields (TGV(Fields)) and adult derived equations by Crapo (TGV(Crapo)) with percentage body fat (PF) estimates subsequently calculated (PF(Meas), PF(Fields), PF(Crapo)).

Compared with the mean TGV(Meas), the TGV(Fields) estimates were within+/-0.2 L in all groups, except obese males (+0.5 L), while the mean TGV(Crapo) estimates were greater than+/-0.3 L in all groups except lean males (+0.1 L). When converted to PF, the mean PF(Fields) estimates were within+/-1% of the measured value in all groups, except obese males (+1.1%), while the mean PF(Crapo) estimates were greater than+/-1% in all groups, except lean males (+0.5%). Using either prediction equation, Bland-Altman analysis revealed that the greatest PF+/-95% limits of agreement were in the lean and overweight groups and lowest in the obese groups.

The Fields child-specific TGV prediction equations provide accurate mean PF estimates and appear better than using the Crapo equations if a measured TGV cannot be achieved in children of a wide range of BMI. However, individual predictions may result in large PF errors especially in lean children.