Theory of gas delivery from low flow systems

The prime requirements of any breathing system are that it should remove carbon dioxide from the subject, and supply him or her with the required inhalation mixture. The simplest way to achieve this is with a non-rebreathing system in which all expired gas, carrying CO₂, is discharged through a spill valve and all inspired gas comes directly from the anaesthetic machine via a reservoir bag or demand valve. In a low-flow system, efficient absorption can remove CO ₂, but the inspired gas will be a mixture of expired and fresh gas-assuming that any anaesthetic vaporizer is in the fresh-gas supply line. Gas exchange in the subject makes the expired mixture different from that inspired; therefore the inspired mixture is different from the fresh-gas mixture. Throughout anaesthesia, anaesthetic is being taken up by the patient so the expired concentration is less then the inspired; and therefore, assuming that the vaporizer is in the fresh-gas supply line, the fresh-gas concentration must be greater than the inspired concentration to make up the difference. However, if the vaporizer is inside the breathing system then, soon after turning on the vaporizer, the gas entering it will already contain vapour expired by the patient so that the inspired concentration will be greater than the vaporizer concentration-where that is defined as the concentration delivered when vapour-free gas enters it. In order to control the inspired mixture, it would be possible to monitor that mixture and adjust the fresh-gas mixture-either manually or by a servo mechanism-to attain, and then maintain, the desired inspired mixture. The author discusses the relationship between inspired and fresh-gas concentrations as a guide to manual control, or to enable the servo engineer to devise something more sophisticated than a simple PID controller