Objective: 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.
Design and setting: Experimental animal study in a university research laboratory.
Patients and participants: Nine pigs with lung injury induced by lung lavage.
Interventions: The lungs were insufflated to four different lung volumes. Next the lungs were inflated in steps up to 40 cm H(2)O and then in steps deflated.
Measurements and results: 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 cm H(2)O) than in the ventral region (17.5+/-4.3 cm H(2)O). 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 cm H(2)O).
Conclusions: EIT and automated curve fitting provide information on regional lung inflation and deflation which may be of clinical use for optimizing ventilator settings.