Evaluation of Electrical Impedance Tomography for the Diagnosis of Chronic Rejection in Lung Transplants Recipients

Lung transplantation is the only treatment option available for patients affected by end-stage lung diseases such as chronic obstructive pulmonary disease (COPD), cystic fibrosis or idiopathic pulmonary fibrosis, not responding to maximal medical therapy. Despite the advance in the surgical techniques, immunosuppression treatment and prevention of acute cellular rejection episodes as well as opportunistic infections, the long terms outcomes following lung transplant remain unsatisfactory. Chronic lung allograft dysfunction (CLAD) is the first cause of long term mortality in lung transplant recipients responsible of 5 years mortality post-surgery of 50%. CLAD can lead to chronic respiratory failure and can presents in two different phenotypes: Restrictive forms of CLAD were all lung volumes are decreased (commonly called RAS, Restrictive Allograft Syndrome) that has been recently associated with antibodies mediated rejection. A more common form of obstructive CLAD has been identified as the well-known and defined Bronchiolitis Obliterans Syndrome (BOS). In patients with BOS, the progressive obliteration and a narrowing of the distal airways result in breathlessness, airflow obstruction and air trapping.

Currently, Forced Expiratory Volume at 1 second (FEV1) measurement is the gold standard to assess BOS severity. This test is non invasive and easily performed at bedside. Thus, it only gives a broad idea of the regional consequences of BOS as may be influenced by large airway obstructions. Chest computed tomography (CT) gives more detailed imaging of the regional consequences of BOS but is time-consuming and expose patients to radiations. Others imaging techniques such as ventilation/perfusion scintigraphy have been studied but cannot be performed at bedside.

As to date there is no curative treatment for BOS, preventive treatments such as long term azithromycin, bronchodilators, inhaled steroids or plasmapheresis have to be started at early stage of the disease to improve outcomes for the recipients Electrical impedance tomography (EIT) is a new technology that involves wearing a belt of sensors around the chest that provides information on how well the lungs are being filled with air by the ventilator. It allows the assessment of these differences, which previously required the use of invasive equipment to obtain. Information is gained by repeatedly injecting small alternating electric currents (usually 5 mA) at high frequency of 50 - 80 kHz through a system of skin electrodes (usually 16) applied circumferentially around the thorax in a single plane between the 4th and 6th intercostal space. While an adjacent pair of electrodes 'injects' the current ('adjacent drive configuration'), all the remaining adjacent passive electrode pairs measure the differences in electric potential. A resistivity (impedance) image is reconstructed from this data by a mathematical algorithm using a two dimensional model and a simplified shape to represent the thoracic cross-section.

The resulting image possesses a high temporal and functional resolution making it possible to monitor dynamic physiological phenomena (e.g. delay in regional inflation or recruitment) on a breath by breath basis. It is important to realize that the EIT images are based on image reconstruction techniques that require at least one measurement on a well-defined reference state. All quantitative data are related to this reference and can only indirectly quantify (relative) changes in local lung impedance (but not absolute).

EIT can be used in mechanically ventilated patients to assess recruitment and to optimise ventilator settings to reduce risk of iatrogenic ventilator associated lung injury. To date, EIT has never been used in lung transplant recipients.