Humidity

The humidity of gases is an important topic within anaesthesia and intensive care medicine. An appreciation of the underlying physics helps to ensure that we monitor and deliver optimally humidified gases to our patients, thus minimizing the pathophysiological consequences when normal humidification processes are bypassed. In this article the different techniques and equipment used to measure humidity are illustrated, as well as the methods of humidification used in clinical practice.     

An Apnea Monitor Using a Rapid-Response Hygrometer

Objective. The aim of this study was to detect cyclic changes in the relative humidity (RH) occurring with spontaneous respiration using a rapid-response hygrometer, and to evaluate its potential applicability as an apnea monitor in nonintubated subjects. Methods. Respiratory monitoring using a rapid-response hygrometer was performed in spontaneously breathing, nonintubated subjects. Changes in RH during spontaneous breathing were measured in adult volunteers, breathing room air and nonintubated infants who underwent cardiac catheterization under intravenous anesthesia. The detection of apnea by the hygrometer was assessed in the adult patients during the induction of anesthesia and those undergoing minor gynecologic surgery under epidural anesthesia. The hygrometric sensor was positioned in front of the nostril; a simultaneous recording of CO2 was obtained from nasal cannulas by a sidestream capnograph. Each waveform was collected and acquired by a PC-based computer, and data were analyzed off-line. Results. The hygrometer showed a rapid response to the cyclic changes in RH during spontaneous respiration and could identify respiratory phases of tachypnea as high as 60 breaths per minute in infants. RH rapidly increased to 80% with a plateau on expiration and decreased to 40% on inspiration. These phasic changes, consisting of three distinct phases, expiratory upstroke, plateau and inspiratory downslope, preceded the corresponding capnographic changes by nearly two seconds. Expiratory increase in RH appeared to be influenced by the respiratory flow rate as well as the response time of the equipment. As respiration was depressed due to airway obstruction, the magnitude of RH gradually decreased and then disappeared at the time of apnea. Conclusions. The hygrometer could detect cyclic changes in RH during spontaneous respiration. Apnea was immediately detected by the decreases in the magnitude of RH. The results demonstrate the potential usefulness of a rapid-response hygrometer for monitoring respiratory rate and early detection of apnea.     

Measurement of humidity

This article describes the physical chemistry of evaporation with particular relation to water. The concept of humidity is defined, and factors that influence humidity are discussed in a practical context. The clinical importance of measurement and control of humidity is illustrated. Commonly used methods of measuring humidity are described and their underlying physical principles are explained.     

How to test the safety of homemade antegrade colonic washout fluid?

Normal saline is commonly used for antegrade colonic washout enemas. The saline solution is usually made at home by adding measured quantities of common salt to tap water. The best method for the carers to check the concentration of salt in these homemade solutions has not been reported. Various concentrations of saline solutions were tested for specific gravity (SG) with an aquarium hygrometer and urine analysis sticks by two blinded independent observers. An aquarium hygrometer can read SG with an accuracy of 0.00046 for saline solutions made in tap water and 0.00051 for those made with sterile water. Urine analysis dipsticks sticks failed to achieve this accuracy. An aquarium hygrometer is a simple and reliable way to measure the SG of homemade saline solutions. Its use may avoid the problems associated with over- or under-concentration of salt in the antegrade enema solution. Accepted: 20 December 2000