Functional magnetic nerve stimulation : the development of a method of generation of explosive expiratory flows in the intubated patient through abdominal muscle stimulation

• Main Author: Turnbull, David
• Other Authors: Mills, Gary
• Published: University of Sheffield 2012
• Subjects: 617.96
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.561075

A voluntary cough is an explosive expiratory manoeuvre where the larynx is closed during the early expiratory phase. Subsequent opening of the larynx generates high peak flows to facilitate the removal of mucus and inhaled material from the large airways. The objective of the thesis was to explore the mechanics of a voluntary cough and develop a surrogate voluntary cough with application to the intubated critical care patient. The thesis developed an understanding of voluntary cough mechanics through a variety of laboratory and clinical models. A modification of the classic Starling Resistor demonstrated that during a peak expiratory flow (PEF) manoeuvre, the addition of a surrogate larynx produced a significant reduction in the time to develop a peak flow, 0.2 to 0.04 seconds. In clinical trials of the surrogate larynx, cough mechanics were compared with a PEF. A large rise in esophageal pressure (Pes) (118cmH2O ±14cmH2O) was a signature of a voluntary cough when compared with Pes during a PEF (66cmH2O ±9cmH2O). The addition of a surrogate larynx during a PEF created an elevation in Pes and rapid rise in peak flow, comparable to a voluntary cough. Observation of the transdiaphragmatic pressure (Pdi) suggested that thoracic muscles contribute to the elevation in Pes during a voluntary cough. Though gastric pressure is applied as a surrogate marker of abdominal pressure, the validity of this was confirmed in a clinical trial when compared with actual abdominal pressure recorded with a laparoscope. The surrogate cough model considered for application to the critical care subject was the application of functional magnetic nerve stimulation of the abdominal muscles in intubated patients during sedation or anaesthesia. The development of this model needed to consider the deleterious effects on the force of muscle contraction following anaesthesia with Propofol, and the potential for abdominal muscle stimulation to provide the force driving a voluntary cough. A clinical trial observed a reduction in twitch strength of 14% - 28%, following magnetic nerve stimulation of the phrenic nerve with Propofol anaesthesia. The magnitude of the effect of the abdominal muscles upon expulsive manoeuvres was also considered. In a clinical trial, spinal anaesthesia, with the loss of abdominal muscle function, diminished maximum expiratory pressure compared with baseline value (P = 0.003), with no observed reduction in maximum inspiratory pressure. Cough function in subjects following a laryngectomy observed the changes in Pes during a volitional cough. The objective was to observe if the rise in Pes may or may not be related to laryngeal closure. The observation were that the volitional “cough” generated a large elevation in thoracic pressure with (145cmH2O) that exceeded the maximum abdominal pressure (126cmH2O), but there was no rapid rise in time to peak flow. The latter could be reversed with the addition of a surrogate larynx. Testing of the surrogate cough model in anaesthetised subjects demonstrated the potential of the model to reproduce some elements of a voluntary cough. However, the expiratory flow generated was limited even in the presence of a surrogate larynx. The surrogate larynx confirmed that it supports the rapid rise in expiratory flow but does not promote a rise in thoracic pressure. Some elements of the thesis objective were realised. A voluntary cough bears similarities to a forced expiratory flow manoeuvre. Thoracic muscles are actively recruited to support the rise in esophageal pressure characteristic of a voluntary cough. Laryngeal closure does not support the elevation in thoracic pressure but shortens the time to peak flow improving the force generated. The complex pattern of expiratory muscle recruitment observed during a voluntary cough is not easily reproducible through magnetic nerve stimulation of the abdominal muscles. The mechanics of delivery of a surrogate cough applying magnetic nerve stimulation is perhaps too complex to have practical application to intensive care respiratory physical therapy. The thesis developed a model of pressure and flow generation of a voluntary cough that could have application to the development of alternative physical therapy techniques. In particular a surrogate larynx may find practical applications to subjects following a laryngectomy or in critical care where the normal larynx is bypassed by an endotracheal tube.