Measurement of the relative contributions of rib cage and abdomen/diaphragm to tidal breathing in man.

A simple mathematical model of the chest wall was constructed so that during tidal breathing the relative volume contributions of the rib cage and abdomen/diaphragm could be measured in man, using four mercury-in-rubber strain gauges around the trunk. From the dimensions of the trunk and the change in circumference determined by the four gauges, the separate contributions of rib cage and abdomen/diaphragm could be determined using a purpose-built analog computer. The system was evaluated in 13 laboratory personnel, and in 13 other subjects before and after anaesthesia. There was a linear relationship between tidal volumes computed and measured at the mouth, over the residual volume to (FRC + 1 litre) range, with an error of +/- 8%. The relative contribution of rib cage to tidal breathing showed a large scatter from 5 to 42% with a non-significant tendency to decrease with age.