Thoracic gas volume measured by body plethysmography during anesthesia and muscle paralysis: description and validation of a method.

A method based on body plethysmography for the assessment of thoracic gas volume (TGV) in the anesthetized, paralyzed subject is presented. The compression of thoracic gas following inflation is detected by measuring the difference between the inflation volume and the "box volume" change caused by the expansion of the chest. Model experiments showed good agreement between true and measured volumes with a residual standard deviation of 2 per cent. In studies on human subjects with healthy lungs during halothane anesthesia, the coefficient of variation of repeated measurements was 5 per cent. Comparative measurements with resting lung volume (FRC) determined by multiple breath nitrogen washout disclosed a larger volume by the box technique, a difference which was reduced but not eliminated by deep breathing during the nitrogen washout. This difference, amounting to 0.2 liters on the average, may be explained by abdominal gas and by the detection of trapped gas by the box technique but not by the nitrogen washout.