What causes the lowered FRC during anaesthesia?

Abstract

Functional residual capacity (FRC) by means of body plethysmography chest-abdomen dimensions by whole body computerized tomography, central blood volume (CBV) by double-indicator dilution technique and extremity (peripheral) blood volume (PBV) by segmental thigh and upper arm plethysmography, were assessed in lung-healthy patients who were to undergo general anaesthesia and elective surgery. Anaesthesia was induced by thiopentone and was maintained either by a continuous drip of thiopentone or by inhalation of halothane. Muscle relaxation was obtained by pancuronium bromide. Anaesthesia caused a reduction of the total thoracic volume by an average of 0.75 1. This followed from a cranial shift of the diaphragm and to less extent by a reduced transversal chest area. FRC was concomitantly reduced by an average of 0.5 1 and the CBV, mainly thoracic blood, was reduced by 0.25 - 0.3 1. PBV was slightly reduced by 0.1 1, and it is concluded that these fractions of CBV and TBV must have been pooled in the abdomen. This pooling resulted in more or less maintained transversal abdomen area despite the cranial shift of the diaphragm.