The role of carbon dioxide in anaesthesia.

DUNCUM in her book "The Development of Inhalahon Anaesthesia" (1) states that carbon dioxide was the first gaseous anaesthetac used surgically and mentions that Hlckman performed surgical operahons on animals under the influence of carbon dioxade as early as 1824. Recently, a large meat-pact6_ng organization introduced the novel procedure of using 30 per cent carbon dioxide as an inhatataonal anaesthetic for hogs, thereby reducmg the noise, confusion, labour, and hme required m slaughtering operattons (2) Be that as it may, carbon dioxide today plays a very important role m modern anaesthesia. I would like to spend a few moments reviewing-with you some of the physiology of carbon dioxide. CO2 is the most important end-product of tissue metabohsm. As such CO2 is constantly being produced by the cells, and ~s carried by the blood to the hmgs for excretion. In the blood, CO2 exists in three forms: (1) as free carbon dioxade, winch is dissolved in the water of the blood, (2) as bicarbonate Of the plasma, and (3) as carbamrao compounds in combmahon with haemoglobin The amount of free dissolved carbon dioxide deterrmnes the tensmn or parhal pressure of the gas. It is the difference in partial pressure of CO2 which is responsible for CO2 transport For exam'~le, the CO2 tension of the body cell is greater than that m the capillary blood, thus CO2 moves into the blood. Simdarly, the partial pressure of CO2 m mixed venous blood, P~CO_~ (3) (normally 46 mm Hg ) is greater than that in the alveoh of the lungs, PACO2 (normally 40 ram. Hg ). Thus CO2 leaves the blood, enters the alveoll, and is subsequently exhaled T h e PACO2 is therefore maintained at a constant level and this value is the same as in the arterial blood The partial pressure or tension of CO2 m the arterial blood is referred to as the PaCO2 It is well known that the pH of the blood is maintained at 7 40 despite the constant produchon of acid metabohtes. Tins is because of the presence of varmus buffers in the blood, of winch the bicarbonate buffer is the most important, both because of its high concen,trataon and because of its ease of alteratton. The body pH is maintained at 7.40, then, because of the constancy of the buffer raho bicarbonate/carbomc acid which is normally 20/1 In the laboratory, one can use the Henderson-Hasselbalch Equahon (4) to calculate PaCO2: pH = 6 1 -~log (H COJp) 0.0801 Pa, CO2 As may be seen, a decrease m this raho will lead to a decrease in pH. This acidosis may be due either to a decrease in the plasma bicarbonate ion concentratmn or, on the other hand, to an increase m the PaCO2.