Changes in Regional Lung Ventilation Following Mechanical Insufflation-Exsufflation

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).

Patients with respiratory muscle weakness have reduced cough strength and this can cause difficulties with clearing respiratory secretions. This impaired secretion clearance can lead to respiratory infections and acutely can causing small airway occlusion with subsequent ventilation-perfusion mismatch adversely effecting pulmonary mechanics. The use of cough assist devices, such as mechanical insufflation-exsufflation (MIE) can improve secretion clearance. Although there is evidence that these devices can improve secretion clearance the optimal pressures or technique required to provide effective secretion clearance is not known. Furthermore the use of high pressure swings that are now frequently used in clinical practice could lead to lung derecruitment. The use of EIT would allow a novel and effective method of assessing the ability to clear secretions and optimise ventilated lung in neuromuscular patients with poor cough and assess post procedure derecruitment.

MIE: A pilot physiological study will be performed in patients with neuromuscular disease with acute (10 patients) and chronic (10 patients) secretion management issues that require clinical use of MIE. These patients will be studied using 3 different clearance strategies low pressure MIE, higher pressure MIE and standard airway techniques. The project will assess differences in regional ventilation following augmented airway clearance and inform potential endpoints and feasibility for a larger trial regarding speed of recovery from respiratory infections in patients with neuromuscular disease.

There is a paucity of data regarding the physiological efficacy of MIE devices and although clinical consensus regards them as a beneficial adjunct to the management of patients with respiratory muscle weakness causing a reduction in effective secretion clearance it is less clear the optimum protocol to achieve secretion clearance. The aim of chest physiotherapy is to achieve clearance of airway secretions and allow improved gas exchange by enhancing ventilation and perfusion matching within the lung. The data suggesting optimum pressures for MIE are based around early work on achieving enhanced cough peak expiratory flow. More recently higher pressures have been suggested to achieve enhanced airway clearance. There may be potential beneficial effects with high pressure MIE by enhancing airway opening during the insufflation pressure achieving a degree of lung volume recruitment. Conversely, high insufflation pressures may adversely affect both pulmonary and cardiac physiology which may be further impacted by the large pressure swing during exsufflation. This is particularly relevant given the increasing co-morbid cardiomyopathy that occurs in many congenital neuromuscular diseases.

The initial phase of the project would be to generate pilot data with a comprehensive physiological evaluation of patients with neuromuscular disease and requirement of cough augmentation with MIE devices. The patients will undergo a randomised protocol of chest clearance with standard physio, low pressure and high pressure MIE. Data from this trial will allow an optimal treatment strategy to be developed and if data suggested would provide sufficient data to power an interventional trial to examine important clinical outcomes.