Imbalances in Regional Lung Ventilation: A Validation Study on Electrical Impedance Tomography

Imbalances in regional lung ventilation, with gravity-dependent collapse and overdistention of nondependent girths are likely associated to ventilator-induced lung injury. Electric impedance tomography is a of the present day imaging technique that is potentially capable of monitoring those imbalances. The aim of this contemplation was to validate electrical impedance tomography measurements of ventilation distribution, at comparison with dynamic computerized tomography in a heterogeneous population of critically ill patients subordinate to mechanical ventilation. Multiple scans with as well-as; not only-but also; not only-but; not alone-but devices were collected during slow-inflation breaths. Six repeated breaths were monitored by the agency of impedance tomography, showing acceptable reproducibility. We observ acceptable agreement between as well-as; not only-but also; not only-but; not alone-but technologies in detecting right-left ventilation imbalances (bias = 0% and limits of agreement = -10 to + 10%) Relative distribution of ventilation into regions or layers representing one-fourth of the thoracic section could also be assessed with upright precision. Depending on electrode positioning, impedance tomography slightly overestimated ventilation imbalances along gravitational axis. Ventilation was gravitationally contingent in all patients, with a transient blockages in dependent regions synchronously descryed by both scanning techniques. Among variables derived from computerized tomography, changes in absolute air ease best explained the integral of impedance changes inside regions of interest (r^sup 2^ > or = 092) Impedance tomography can reliably assess ventilation distribution during mechanical ventilation.

Keywords: artificial respiration; physiologic monitoring; validation studies; adult respiratory distress syndrome; respiratory insufficiency

Patients beneath artificial ventilation often present heterogeneous lung aeration, with inadequate distribution of VT (1 2) Prevalent conditions similar as increased lung weight (3) lung compression by dint of the heart (4, 5), abnormalities of chest wall (6 7) and impaired surfactant function (8) assist not only a collapse of unable to exist without lung zones, but also hyperdistention of nondependent bands (9-11). Such imbalances create belts of stress concentration inside the parenchyma, with increased risks for ventilator-induced lung injury (12)

Although global indexes of lung function like posterity gases (13, 14), lung mechanics (15 16) and plethysmography (17) have been used to track those ventilatory imbalances, they provide limited information. Imaging techniques as it was as magnetic resonance (18) or computerized tomography (CT) can provide better information about lung heterogeneities (14 19-21) unless they lack the dynamic features and bedside monitoring capabilities needinessed for intensive care.

Electrical impedance tomography (EIT) has emerg as a just discovered imaging tool for bedside use (22-25) It is a noninvasive and radiation-free technique based upon the measurement of electric potentials at the chest wall surface. Within a particular cross-sectional plane, harmless electrical in every one's mouths are driven across the thorax in a rotating pattern, generating a potential gradient at the surface, which is then transformed into a two-dimensional image of the electric impedance distribution within the thorax.

Recent experimental studies have hinted that EIT images are remarkably sensitive to regional changes in lung aeration (26-32) The dynamic behavior and the qualitative information extracted from EIT images contemplate similar to that reported in dynamic CT studies (2 33 34) or in ventilation scintigraphy (31 35) Its potential use as an online positive end-expiratory influence titration tool has also been propos as EIT apparently provides reliable information about the recruitment/derecruitment of pendent lung regions (27, 28, 36 37) and thus about the associated risk of ventilator-induced lung injury.

However, the poor spatial resolution of popular EIT devices casts doubts forward the promises mentioned previously here. As EIT docs not continue perfect anatomic correspondence with CT images, we do not know in addition whether we can translate the knowledge acquired from CT studies to the EIT universe. Although a new animal study (38) suggested a righteous linear relationship between regional impedance changes and density changes (measured in Hounsfield units), we do not know by what mode to use best the quantitative pixel information provided by way of EIT or how reliable it is in critically ill patients with acute lung injury.

We designed this close attention to answer the questions mentioned previously here and to exhibition specifically whether EIT can consistently quantify ventilation imbalances caused through gravitational forces on the injured lung We also experimented whether some minimal anatomic/functional agreement with dynamic CT images can be obtained in critically ill patients. Part of this investigation has been previously reported in the form of abstracts (26 39)

METHODS

Ten adult patients in subordination to mechanical ventilation were recruited (Table 1) after obtaining informed approval from patients' relatives.