Physiological factors underlying transthoracic impedance variations in respiration.

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Abstract
The relationship between the changes in transthoracic impedance ([DELTA] Z), respired volume ([DELTA] V), and thoracic circumference ([DELTA] C) during both spontaneous and forced positive-pressure respiration eas studied in anesthesized dogs. By sectioning the phrenic nerves and stimulating them distally to tetanize the diaphragm it was possible to separate the contributions to [DELTA] Z produced by the diaphragm alone and by the remaining respiratory muscles. The following conclusions were drawn: 1) The ratio [DELTA] Z/ V had essentially the same value for both spontaneous and forced breathing. The magnitude of [DELTA] Z/[DELTA] V was maximum at the level of the 8th ribs and decreased rapidly with distance (along the midaxillary lines) above and below the 8th ribs, 2) Tetanus of the diaphragm was associated with a sustained increase in [DELTA] Z which varied in magnitude with rib level in the same manner as during spontaneous or forced respiration. 3) Both [DELTA]C and downward displacement of the diaphragm produced independent components of [DELTA] Z which appear to combine linearly to produce the [DELTA]Z obtained with normal tidal volumes.

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