A simple method of carbon dioxide estimation in expired air.

A SIMPLE method for the estimation of carbon dioxide in air or gas mixtures containing anaesthetic gases finds many applications in the investigation of anaesthetic problems and in the control of artificial respiration by mechanical respirators. This adaptation of already established techniques utilizes apparatus which may be found in most hospitals. It is rapid, and can be performed at the bedside or in the operating-room. The principal has been employed for some time (Higgins and Marriott, 1917; Wilson, Orcutt and Peterson, 1932; Wilson, 1933), and has recently been adapted by Brinkman in an instrument known as the " Carbovisor". The gas mixture to be examined is brought into equilibrium with an indicator solution lightly buffered with sodium bicarbonate. The tension of carbon dioxide in the gas mixture determines the pH achieved in the buffered solution and the pH, in turn, determines the colour of the indicator. The colour of the indicator is estimated in a photo-electric colorimeter, using a convenient filter. The indicator solution consists of bromthymol blue 0.005 per cent and sodium bicarbonate 0.05 per cent and it has been found to be stable in storage provided that it is kept in bottles the interior of which have been carefully lined with paraffin wax.. The sample of gas to be analysed is drawn into a syringe with a long needle attached; 10 ml are required. The tip of the needle is then placed underneath the surface of a small quantity of the indicator solution and 2 ml of gas expelled from the syringe; 2 ml of the indicator solution are then drawn in. The syringe is then closed, either with a blanked-off needle hub or by thrusting the point of the needle into a rubber bung. The syringe and