Studies on respiratory humidity. II. Humidity in anesthetic circuits and water loss via anesthesia systems

Through the use of an automatic photo tube dew-point hygrometer, the author succeeded in measuring dew point of gas flows continuously in anesthetic circuits. Simultaneous thermometries were done on the nasal or oral mucosa, on the respiratory gas flows in the anesthetic mask or the endotracheal tube, and on the gas in the inhaling conduit. Experiments were performed on ten adults patients undergoing various types of surgery under general inhalation anesthesia. Anesthetic technics were varied intentionally during the measurements. Thus, both absolute and relative humidities of exhaled and inhaled gases, and respiratory water and heat losses were calculated under various anesthetic conditions, and physiological and clinical considerations were discussed. The conclusions obtained from this research are as follows: (1) When a non-rebreathing system is applied, moisture content of exhaled gas is minimal, and respiratory losses of both water and heat are maximum. With a semi-closed circle method, according to decreasing fresh gas flows, the humidity of the inspiratory and expiratory gases becomes higher, and both heat and water losses through respiration are lessened. When a closed circle method, with carbon dioxide absorption, is employed, temperature and humidity of gas in the inhaling conduit are highest, and the expired gas offers the maximum temperature and moisture contenL Both water and heat losses from anesthesia become minimal when administered in a closed system. (2) While the water and heat that a patient loses through respiration increase with increasing breathing capacity, they are still small parts of the total water and heat losses of the patient. Water and heat losses via anesthesia systems are not so predominant in maintaining water balance and heat regulation of patients during anesthesia and surgery.