Changes in diaphragmatic position in association with the induction of anaesthesia

Images of a saggital section of the right hemidiaphragm were obtained using an ultrasound sector scanner in 20 patients in the supine position immediately before, and after, the induction of anaesthesia (with thiopentone). In the awake patient, the mean excursion of the part of the diaphragm that showed the greatest tidal movement was 1.56 +/- 0.52 (SD) cm. The end-expiratory position of this part of the diaphragm was noted before and after induction. Craniad movement of this position was seen in 10 patients. In a further eight, the end-expiratory position did not change, and in two patients it moved caudally. The mean movement was 0.36 +/- 0.52 cm in a cranial direction, which was statistically significant (P less than 0.01) but was only 23% of the movement associated with quiet breathing. The extent or direction of movement was not related to the weight of the patient (expressed as a proportion of the expected weight). The findings do not support the hypothesis that the reduction of lung volume on induction of anaesthesia is caused solely by movement of the diaphragm.