Quantitative measurement of ventilation at the mouth and nose with heat sensors

Airflow at the nose and mouth is routinely measured during polysomnography for the evaluation of respiratory disturbances during sleep. The usual monitoring device is a small thermocouple placed in front of the nose and mouth that qualitatively detects airflows in or out. A more accurate pneumotachometer on a tight-fitting mask is less frequently used because of the discomfort that is introduced and its potential interference with sleep. The authors tested ten heat-sensing devices modified to increase accuracy without increasing discomfort. The designs of these ventilation monitors differed in choices of specific heat detector, techniques of mounting on the face, and types of heat-trapping covers employed. The goal was to find a comfortable oronasal monitor and to define a procedure that could identify those units that generate signals approaching quantitative ventilatory flow. The signal from each device was compared with a "standard" signal derived from respiratory belts during contrived respiratory maneuvers that assessed specific potential sensing problems. The devices behaved differently, and design perturbations could be controlled to construct flow detectors to closely match actual flow. The more accurate devices could be as comfortable as commonly used monitors that do not have the enhanced accuracy. The principles and techniques described for improving the measurement of oronasal ventilation by heat sensor can be invoked to develop monitors that can closely measure actual quantitative ventilatory flow.