EIT Study With COPD and OHS Patients (EIT Step 2)

Chronic lung disease can sometimes progress to the extent that patients can no longer clear the waste gas from their blood. Treatment can be offered with a mask and machine (ventilator) that helps people breathe and aims to improve their lung condition. It is common for people's lungs to be affected variably, i.e. left more than right or top of lung more than bases of lungs. The way in which the ventilator is set may affect how well the machine deals with these differences. If the lung is better ventilator patients may find the machine more comfortable and it may be more effective.

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.

Optimising ventilator settings in the administration of non-invasive ventilation (NIV) can be improved with the addition of individual physiological data. This approach is limited due to the invasive techniques required to obtain this information, often leading to less ideal NIV settings promoting patient-ventilator asynchrony. It has been recently demonstrated by our group that all patients established on domiciliary NIV have a degree of patient-ventilator asynchrony and that the commonest type of asynchrony are triggering issues. Triggering asynchrony is promoted by mismatch between a patient's intrinsic positive end-expiratory pressure (iPEEP) and applied expiratory positive airway pressure (EPAP) with these ineffective efforts contributing to an increased work of breathing and patient discomfort. Previous strategies used to optimise patient triggering have involved the placement of oesophageal catheters in order to measure neural respiratory drive (NRD) to the diaphragm by electromyography (EMG) but again this process is invasive and often poorly tolerated. Electrical Impedance Tomography (EIT) is a non-invasive, bedside monitoring technique that provides semi-continuous, real-time information about the regional distribution of the changes in electrical resistivity of the lung tissue due to variations in ventilation in relation to a reference state.

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.

In the supine posture obese patients can generate significant levels of iPEEP that contribute to increased levels of neural respiratory drive compared with the upright posture. There has been much debate regarding the optimal ventilator strategy in patients with obesity related respiratory failure, with uncontrolled trial data to support simple continuous positive airway pressure, pressure support (PSV) NIV and volume targeted (VT) NIV. There has been no robust evidence to suggest superiority of a single mode but post hoc data suggests superior control of sleep disordered breathing in patients in pressure controlled mode. It is unclear whether the extended inspiratory time of pressure controlled mode is allowing superior gas exchange by maintaining airway distension and preventing regional collapse.

In COPD, patients' response to treatment can be influenced by disease heterogeneity with some patients showing even distribution of lung damage and others marked differences throughout the lungs. This variation can lead to significant differences in the lung mechanics in different regions with optimal NIV settings for some regions having potentially deleterious effects on neighbouring zones. It has been shown that control of hypoventilation and improved blood gas exchange is essential in order to improve outcomes with NIV in COPD but is less clear if pressure control ventilation as advocated by Windisch and colleagues is required in order to achieve this effect. Inappropriate settings of NIV can also lead to dynamic distension that results in a decrease of tidal volume and an increase in patient discomfort.