Measurement of tidal volume by using transthoracic impedance variations inrats

The application of impedance pneumography for monitoring respiration in sma ll animals has been limited by problems with calibration. With improved ins trumentation, we describe the calibration of tidal volume in anesthetized r ats. The detection of changes in voltage, reflecting the electrical impedan ce variations associated with respiration, was optimized by using disposabl e adhesive silver-silver chloride electrodes, advanced circuitry, and analo g-to-digital recording instrumentation. We found a linear relationship betw een change in impedance and tidal volume in individual rats (R-2 greater th an or equal to 98%), which was strongly influenced by rat weight. Consequen tly, a calibration equation incorporating change in impedance and rat weigh t was derived to predict tidal volume. Comparison of the predicted and true tidal volumes revealed a mean R-2 greater than or equal to 98%, slopes of similar to 1, intercepts of similar to 0, and bias of similar to 0.07 mi. T he predicted volumes were not significantly affected by either frequency of respiration or pulmonary edema. We conclude that impedance pneumography pr ovides a valuable tool for the noninvasive measurement of tidal volume in a nesthetized rats.