The Science Journal of the American Association for Respiratory Care

1996 OPEN FORUM Abstracts

BREATH-BY-BREATH HUMIDITY MEASUREMENT IN NORMALS.

P Seakins BE, R Williams PhD, E Meyer Fisher & Paykel Healthcare, Auckland, New Zealand.

Introduction: By observing the relationship between Inspired and expired humidity it is possible to gain some insight into the humidifying load imposed on the airways. The depth into the airways at which inspired gases are conditioned to core temperature and are saturated - termed the isothermic saturation boundary (ISB) - is dynamic. It varies with the temperature and humidity of the inspired gas on a breath by breath basis. Evidence that the ISB is moving can be found in the variation of temperature and humidity of the expired gas. Measurements of breath-by-breath humidity and temperature requires a fast response hygrometer and thermometer. Development of such a device^{1} has allowed us to make these measurements (response time 200mSec). Method: Normal healthy volunteers were asked to breathe each of seven different gas conditions simulating a variety of clinical temperatures and humidities. The order of the gas conditions was randomised and the subjects were blinded to the particulars of each gas. The subjects core temperature, the room temperature, the breath-by-breath inspired and expired temperature and humidity, and a subjective comfort score for each gas were recorded. Each gas was breathed through the mouth for 2 minutes while humidity waveforms were recorded (see figure for typical waveform). A 2 minute rest period was required between gases, during which the instrument calibrations were checked. The gas conditions and typical uses were: 1) Dry room air [23°C, 0mgH_{2}O/L] - non-rebreathing anaesthesia 2) Room air [23°C, 10mgH_{2}O/L] - room air 3) Saturated room air [23°C, 21mgH_{2}O/L] - LFG flow anaesthesia 4) [30/C, 30mgH_{2}O/L] - ISO8185 5) [34°C, 38mgH_{2}O/L] - Humidifier set low 6) [37°C, 44mgH_{2}O/L]- core temperature saturated 7) [40°C, 51mgH_{2}O/L] - above core temperature saturated. The 23°C gases were from a dry air source with cold passover humidifier. All other gases were conditioned by Fisher & Paykel MR730 humidifier with heater wire circuit and MR290 autofeed chamber. Results: Expired absolute humidity (AH) was between inspired AH levels and core temperature saturated in all cases. Gases delivered at less than core temperature, saturated cool the airways during inspiration, resulting in expired gases that are saturated at less than core temperature. The converse applies with inspired gases above core temperature saturated. Gases inspired at core temperature saturated are expired at the same level. Only the dry gases (1&2) were found to be uncomfortable. Conclusions: An air conditioning load is imposed on the airways for all inspired air conditions other than core temperature saturated.

(See original for figure)

1. Wilson AC, Bames Th, Seakins PJ, Rolfe TG, Meyer EJ, 1995, A low cost high speed near infrared hygrometer, Rev. Sci. Instrum., 66(12):5618-5624

Reference: OF-96-183

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