Change in chest wall dimensions on induction of anaesthesia: a reappraisal

We used a linearized magnetometer to measure the changes in rib cage and abdominal dimensions which occurred during induction of anaesthesia with i.v. methohexitone when spinal movement was prevented. By plotting the changes on an adaptation of the Konno and Mead plot, we inferred changes in chest wall volume, which decreased in 11, did not change in two and increased in four patients (n = 17). The mean change was a decrease (P < 0.05). These results differ from previous studies using magnetometry but are consistent with those of three-dimensional imaging. A decrease in chest wall volume could explain the decrease in functional residual capacity associated with general anaesthesia.