Effect of infant position on breath amplitude measured by transthoracic impedance and strain gauges.

Continuous monitoring of respiration by transthoracic electrical impedance gives a signal that has certain not well understood irregularities. Among them is a change in the amplitude of the signal when there is no apparent change in the infant's tidal respiration. One factor that could hypothetically account for alterations of the impedance signal is a change in current path through the thorax secondary to a change in body position. To test this hypothesis we have studied the relationships between breath amplitude measured by transthoracic impedance, one strain gauge on the chest and one on the abdomen, and tidal volume by integrated flow in four body positions. Median breath amplitude was found to vary significantly with body position according to the measuring device. The median impedance breath amplitude increased by 27% in the supine position compared with the prone position, with no associated change in tidal volume. Differences in the strain gauge signal amplitude for these positions were not statistically significant. Correlation between breath amplitude measured by impedance changes and tidal volume was minimal (r = 0.114). These results indicate that infant position affects impedance breath amplitude independently of changes in tidal volume.