Factors affecting transthoracic impedance signals used to measure breathing.

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Abstract
Transthoracic impedance has been measured in normal human subjects with impedance electrodes placed at different levels on the midaxillary lines. The relationship between transthoracic impedance change and inspired volume was linear only when the electrodes were placed high on the midaxillary lines. Below the level of the 4th or 5th intercostal spaces, the x-y plots showed an initial bowing. This initial decrease in sensitivity may have reflected changes in the shape of the lower thorax. Whitney (mercury-in-Silastic) strain gauges were used to measure chest circumference changes at each electrode position. The circumference change was linear with inspired volume only when the gauges were placed at the 4th or 5th intercostal spaces on the midaxillary lines. The sensitivities of both sensors decreased in the supine and prone positions. Both the impedance electrodes and Whitney strain gauges, placed high on the thorax, can be used to measure tidal volume during quiet breathing if the system is properly calibrated.

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