Inspired humidity in anaesthesia breathing circuits: comparison and examination of effect of Revell circulator

Humidity levels of respired gases in spontaneously breathing patients were measured with a quadrupole mass spectrometer (Medishield Model MS-2) and compared between various anaesthetic systems during clinical anaesthesia. In studies of high flow rebreathing non-absorption systems the mean inspired absolute humidity averaged approximately 16 mg X l-1 (range 14.7 to 16.1 mg X l-1) compared to a semiclosed circle absorption system (BOC MK III) in which values for absolute humidity varied from 6.8 +/- 5.1 mg X l-1 for children through 9.5 +/- 3.0 mg X l-1 for adults), and the closed circle absorption system in which values of 20.5 +/- 2.3 mg X l-1 were only gradually reached. Addition of a functioning Revell circulator to the circle absorption system provided immediately markedly higher levels of inspired humidity during clinical use, either semiclosed (15.4 +/- 5.1 mg X l-1 in children to 18.8 +/- 2.4 mg X l-1 in adults), or closed (23.3 +/- 1.8 mg X l-1 in adults). These markedly higher inspired levels of absolute humidity were attained during forced circulation of respired gas provided by the Revell circulator without the concomitant disadvantages of rebreathing or the potential hazards associated with some humidifying devices.