Estimation of regional lung volume changes by electrical impedance pressures tomography during a pressure-volume maneuver

To assess the degree of linearity between lung volume and impedance change by electrical impedance tomography (EIT) in pigs with acute lung injury and to investigate regional impedance changes during a pressure-volume maneuver.

Experimental animal study in a university research laboratory.

Nine pigs with lung injury induced by lung lavage.

The lungs were insufflated to four different lung volumes. Next the lungs were inflated in steps up to 40 cmH₂O and then in steps deflated.

EIT measurements were performed. Impedance was highly linear with lung volume ( r ²=0.97). From the pressure-volume maneuver regional pressure-impedance (P-I) curves were obtained in the upper half (ventral) and lower half (dorsal) of the thoracic cross-section. Excellent fit was found of the regional P-I curves with a predefined sigmoid equation ( r ²=0.998). The P-I curves after lavage were markedly different than before lavage. The P-I curves recorded after lavage displayed a strong heterogeneity on the inflation limb: Lower corner pressure (traditionally lower inflection point) was significantly higher in the dorsal (28.3±4.1 cmH₂O) than in the ventral region (17.5±4.3 cmH₂O). The deflation limb displayed a more homogeneous pattern. Upper corner pressure and true inflection point, where the curve slope is maximal, in the dorsal region were only slightly higher than in the ventral region (1–2 cmH₂O).

EIT and automated curve fitting provide information on regional lung inflation and deflation which may be of clinical use for optimizing ventilator settings.