In the adult respiratory distress syndrome (ARDS) mechanical ventilation is used to maintain adequate oxygenation, but mechanical ventilation can be hazardous to the lung itself. Different ventilation approaches have been developed to open the lung and to reduce the risk of any trauma to the alveoli during mechanical ventilation [1, 2]. However, in the vertical gradient from anterior to posterior lung density increases, indicating an increase in collapse of alveoli in this gradient due to pulmonary oedema, loss of surfactant and alveolar damage [3]. With increasing pulmonary oedema the differences in the vertical gradient are even more pronounced. The increase in lung weight due to pulmonary edema causes collapse of alveoli at the posterior site of the lungs owing to gravitational forces. Knowing whether and in what region alveoli are collapsed or open might help in the adjustment of mechanical ventilation and/or might give insight in the severity of pulmonary oedema. Therefore, regional information about the division of functional lung volume is needed. CT-scanning of the lung has already been proven useful for the assessment of the regional information [3], but is impractical at the intensive care unit (ICU) since it can not be used at the bedside. With a recent non-invasive bedside method, called Electrical Impedance Tomography (EIT), regional information about the division of functional lung volume can also be obtained. EIT registers changes in impedance due to lung volume changes in a 2-D image plane [4, 5]. In this abstract we will briefly describe the basics of EIT and some experimental results.

The study investigates the effects of non-cardiogenic oedema, especially the accumulation of protein in extracellular fluid, on thoracic impedance and proposes a new method of oedema measurement based on an impedance ratio from a dual-frequency measurement. In vitro measurements in a cell containing an albumin-in-saline solution yield a resistance increase when the albumin concentration increases. Subsequently, 13 patients having acute respiratory failure are measured. The single-frequency Z0 measurements and the proposed impedance ratio are compared with extravascular lung water (EVLW) determined by the double indicator dilution method. The single-frequency measurement correlates poorly with EVLW (r=−0.24, p=0.56). In some patients, a total thoracic impedance increase is found with increasing EVLW. The correlation between the impedance ratio and EVLW is r=−0.79 (p<0.0005). The ratio decreases as EVLW increases. Thus, when oedema is measured using bio-impedance, cardiogenic and noncardiogenic oedema yield different results. It is well recognised that cardiogenic oedema decreases total thoracic impedance. In non-cardiogenic oedema, however, protein accumulation causes an impedance increase. The decrease in the impedance ratio as EVLW increases can be explained by the accumulation of albumin in the extracellular compartment.

The Sheffield electrical impedance tomography; (EIT) system produces images of changes in the distribution of resistivity within tissue. The paper reports on the application of electrical impedance tomography in monitoring volume changes in the limb during venous occlusion. The aim of the study is to assess the feasibility, reproducibility and validity of calf blood flow measurements by EIT. In 14 healthy volunteers calf blood flow is compared, as determined in a calf segment by strain-gauge plethysmography (SGP), with the impedence changes measured by EIT during rest and post-ischaemic hyperaemia. The measurements are repeated to assess reproducibility. The reproducibility for the EIT, assessed from the repeated measurements and expressed as a reproducibility coefficient, is 0.88 during rest and 0.89 during hyperaemia. The reproducibility coefficient for SGP data is 0.83 at rest and 0.67 during hyperaemia. Flow measurements, assessed by means of two methods, correlate well at rest (r=0.89), but only moderately during hyperaemia (r=0.51). The correlation coefficient for the pooled flow measurements is 0.98. It is concluded that EIT is a valid and reliable method for assessing blood flow in the limb. Possible applications of EIT in localising fluid changes are discussed.