Impedance-measuring ventilation monitor for infants

Abstract

A two-electrode transthoracic impedance measuring system has been used to measure tidal volume and ventilation in infants. When the electrodes are positioned bilaterally immediately below the axillae, the impedance change is linear with lung volume change. It can be used to measure tidal volume within 1.5 ml (SD) when the system is calibrated by matching its output with that from a small spirometer or a reverse body plethysmograph for a few breaths. Unsatisfactory records are obtained if the electrodes are placed low on the chest (below the nipple line). Total ventilation can be measured which is automatically calibrated in ml/min when the tidal volume signal is accurately adjusted. A built-in system can be used to correct for the artifact related to cardiac activity, if it exists, thereby increasing the accuracy of the monitor for recording ventilation. Body movement, particularly of the arms, interferes with the impedance signal, but the lack of discomfort for the subject minimizes such movement artifacts. Satisfactory quantitative measurements of tidal volume and ventilation have been recorded for up to 6 hr without requiring recalibration. The instrument permits long-term monitoring of tidal volume and ventilation without restricting the airway or interfering with the medical management of the infant.