Rib Cage Movement during Halothane Anaesthesia in Man

Abstract

Chest wall movement, partitioned into rib cage and abdomen/diaphragm contributions, was measured using four mercury-in-rubber strain gauges and an analog computer. The relative volume contribution of the rib cage and the abdomen/diaphragm to tidal volume was measured in 13 subjects before and during anaesthesia with thiopentone and halothane. In awake subjects, movement of the abdomen/diaphragm contributed more than 70% of the tidal volume, the smaller rib cage contribution ranged from 5 to 30%. Manual ventilation during induction of anaesthesia showed that the rib cage was less compliant than the abdomen/diaphragm, but when suxameth-onium was given there was a disproportionate increase in rib cage compliance. In nine out of 12 subjects, halothane anaesthesia resulted in a large decrease in the fractional contribution of the rib cage. In two of these subjects there was paradoxical breathing, the rib cage and the abdomen/ diaphragm movement being 180° out of phase. This effect was produced easily in three other subjects by inserting a resistance (1 kPa litre−1 s) into the anaesthetic circuit. The technique also produced information about changes in volume of the trunk induced by anaesthesia. Eleven subjects showed an increase in end-expiratory abdominal volume (mean increase 120 ml) during halothane anaesthesia while there was a mean reduction in end-expiratory rib cage volume of 29 ml. We concluded that halothane depressed both phasic and tonic postural reflex activity, which affected predominantly the rib cage musculature. This reduced the amplitude of phasic rib cage movement, impaired stability of the rib cage and predisposed to paradoxical ventilation. The results also suggested that the reduction in lung volume during anaesthesia may result from a loss of postural control of the chest wall and a central shift of blood volume.