The Relationship Between Diaphragmatic Force Output And Lung Volume At Different Magnetic Stimulation Intensity

Objective To investigate the influence of lung volume on diaphragm force output under different intensity of the cervical magnetic stimulation (CMS) and various airway occlusion conditions, and determine the lung volume corresponding to optimal diaphragm muscle length, which provide objective proves for the reference range of volume reduced in lung volume reduction surgery (LVRS) in clinics.Methods In 7 normal subjects, changes in the twitch transdiaphragmatic pressure (Pdi(t)) and the twitch esophageal pressure(Peso(t)) elicited by CMS under different magnetic stimulation intensity(100%, 80%, 60% and 40% maximum stimulator output), various levels of lung volume (RV, FRC-1/2ERV, FRC, FRC+1/5IC, FRC+2/5IC ,FRC+3/5IC,RC+4/5IC,TLC),and different airway occlusion conditions (Occlusion or Unocclusion) were observed. The diaphragmatic force output was reflected by above parameters.Results At RV or FRC-1/2ERV, Pdi(t) and Peso(t) reached to peak, and decreased linearly as the lung volume increased, with its minimum at TLC under different intensity of the cervical magnetic stimulation and various airway occlusion conditions. Besides, Pdi(t) increased linearly following the magnetic stimulation intensity increased, with its maximal slope at RV. No significant difference (P>0.05) in Pdi(t) was observed under the same lung volume and magnetic stimulation intensity, regardless of airway occlusion conditions and additional devices.Conclusions1. The optimal initial length of diaphragm muscle is between RV and FRC, which is also below FRC in normal subjects. The force output of the human diaphragm decreases as lung volume increases above the lung volume corresponding to the optimal initial length; 2. The diaphragm force output increases as the magnetic stimulation intensity(as neural respiratory drive) increases at given lung volume;3. The efficiency of diaphragmatic muscle force output reaches to its peak at RV,and declines as the lung volume increases;4. At the optimal initial length, the work efficiency of diaphragmatic muscle reaches to its peak and diaphragmatic fatigue is the least prone to occur.