Influences of lung parenchyma density and thoracic fluid on ventilatory EIT measurements

Abstract

Ventilatory impedance changes can be measured by electrical impedance tomography (EIT). Several studies have pointed out that the ventilatory-induced impedance change measured over the lungs shows a linear relationship with tidal volume. However, EIT measures the ventilatory impedance changes relative to a reference. Therefore, changes in the reference due to lung parenchyma destruction (increase of thoracic impedance) or lung water (decrease of thoracic impedance) might influence ventilatory EIT measurements. A study was designed to evaluate the influence of the density of lung parenchyma and the thoracic fluid content on ventilatory EIT measurements. Eleven emphysema patients with a variable degree of lung parenchyma destruction, nine haemodialysis patients with general fluid overload and ten healthy subjects were measured. The impedance changes were measured with the subject in the supine position breathing a constant tidal volume of 1 litre starting at the maximum end-expiratory level. In the emphysema group a significantly lower impedance change between ins- and expiration was found in comparison with the healthy subjects (11.6 +/- 6.4 AU l-1 versus 18.6 +/- 4.2 AU l-1, p < 0.05), whereas the haemodialysis group showed a significantly larger impedance change between ins- and expiration before haemodialysis (30.5 +/- 13.1 AU l-1, p < 0.05). A significant decrease in ventilation-induced impedance change during dialysis was found (30.5 +/- 13.1 AU l-1 versus 21.4 +/- 8.6 AU l-1, p < 0.01). Furthermore, a significant correlation between lung function parameters, which indicate the severity of lung parenchyma destruction, and the measured impedance change was found in emphysema patients. From these results it can be concluded that the density of lung parenchyma and the thoracic fluid content have a serious impact on the ventilation-induced impedance change.

Evaluation of Electrical Impedance Tomography in the Measurement of PEEP-Induced Changes in Lung Volume

“…The lower VIC in the nondependent part of the lung is influenced by the decrease in lung volume due to an increase in the severity of ARF. 7,10 In our study, a significantly higher LIS was found in the group at high PEEP levels, indicating the increase in severity of ARF. The significant increase in IR at higher PEEP levels suggests an increase of Vt in the dependent part of the lung, and thus alveolar recruitment at higher levels of PEEP.…”

“…However, VIC is not solely determined by the Vt, but also by the amount of lung parenchyma and lung edema. 7 Therefore, in the second part of the study, intraindividual PEEPinduced changes were measured by EIT.…”

“…4 However, between different subjects that have all been ventilated with the same Vt, the ventilation-induced impedance changes may differ due to the influence of the amount of lung parenchyma and lung edema. 7 Therefore, the main contribution of EIT would appear to be the ability to determine independently the nondependent and dependent lung contribution to Vt in an ARF patient.…”

Regional ventilation distribution determined by electrical impedance tomography: Reproducibility and effects of posture and chest plane

“…As shown in previous studies, effects of acute lung injury, bronchial carcinoma, lung oedema, emphysema, etc. can be identified 6,25,39–41 . Moreover, the effects of artificial ventilation can be assessed and the functional response of lung tissue to various ventilator settings determined 7,9,15,23,25,27,30 .…”

Electrical impedance tomography in acute lung injury

“…For example, on measuring changes in ventilation, we may be overlapping intrathoracic fluid changes (pleural effusion, pulmonary extravascular water, pulmonary blood volume), which likewise can cause variations in the impedance results. 62 It is therefore essential to associate the changes visualized by the system with the clinical changes of the patient (a circumstance not exclusive of EIT), in order to ensure their correct interpretation.…”