What could explain a measurement of FRC being 50% higher by body box compared to helium dilution in a patient?

A measurement of Functional Residual Capacity (FRC) that is significantly higher when assessed via body plethysmography compared to the helium dilution method can be explained by the presence of increased non-ventilated lung space.

Body plethysmography measures the total volume of air in the lungs, including areas that may not be actively participating in gas exchange, such as areas of the lung that could be collapsed or obstructed but still contribute to total lung volume. If there is significant non-ventilated space in the lungs, such as from conditions causing obstructions or regions that are affected by lung disease, the body box method would capture this additional volume, resulting in a higher FRC measurement.

In contrast, the helium dilution technique only measures the volume of gas present in ventilated areas of the lungs. If there are parts of the lung that are not participating in ventilation, this method would yield a lower FRC measurement because it would not account for the volume of air in non-ventilated spaces. Thus, when there is substantial non-ventilated lung space, the body box will show a higher FRC compared to helium dilution, confirming why the correct answer centers around increased non-ventilated lung space.