DEW-POINT HYGROMETRY SYSTEM FOR MEASUREMENT OF EVAPORATIVE WATER-LOSSIN INFANTS

Evaporation of mater from the skin is an important mechanism in therma l homeostasis. Resistance hygrometry, in which the water vapor pressur e gradient above the skin surface is calculated, has been the measurem ent method of choice in the majority of pediatric investigations. Howe ver, resistance hygrometry is influenced by changes in ambient conditi ons such as relative humidity, surface temperature, and convection cur rents. We have developed a ventilated capsule method that minimized th ese potential sources of measurement error and that allowed second-by- second, long-term, continuous measurements of evaporative water loss i n sleeping infants. Air with a controlled reference humidity (dew-poin t temperature = 0 degrees C) is delivered to a small, lightweight skin capsule and mixed with the vapor on the surface of the skin. The dew point of the resulting mixture is measured by using a chilled mirror d ew-point hygrometer. The system indicates leaks, is mobile, and is acc urate within 2%, as determined by gravimetric calibration. Examples fr om a recording of a 13-wk-old fullterm infant obtained by using the sy stem give evaporative water loss rates of similar to 0.02 mgH(2)O . cm (-2). min(-1) for normothermic baseline conditions and values up to 0. 4 mgH(2)O . cm(-2). min(-1) when the subject was being warmed. The sys tem is effective for clinical investigations that require dynamic meas urements of water loss.