Effects Of Continuous Positive Airway Pressure Level On Respiratory Mechanics And Central Drive During Dynamic Hyperinflation

BACKGROUND: Expiratory flow limitation(EFL) is the main mechanism leading to dynamic pulmonary hyperinflation(DPH)and intrinsic positive end-expiratory pressure(PEEPi).DPH and PEEPi lead to increased inspiratory threshold load and impaired inspiratory muscle function (especially the diaphragm), resulting in increased inspiratory muscle efforts and impaired central drive mechanical and ventilation coupling, which is an important mechanism of dyspnea. Appropriate setting of extrinsic PEEP(PEEPe) can decrease inspiratory effort and work of breathing, improve patient-ventilator synchrony in severe COPD patients with PEEPi and treated with mechanical ventilation. The effects of level of PEEPe on respiratory mechanics, especially on central drive mechanical and ventilation coupling, demand further investigation.OBJECTIVE: To investigate the levels of PEEPe on end expiratory lung volume, central drive mechanical and ventilation coupling purpose, dyspnea and other respiratory mechanics parameters on the COPD model of induced with expiratory flow limitation in normal volunteerer.METHODS: The static pressure volume curves of the lung and chest wall (Campbell diagram) were established with slow deep respiratory manoeuvre. Expiratory flow limitator was employed to induce COPD model with expiratory flow limitation and dynanmic hyperinflation. After the model establishment, the changes of end expiratory lung volume, respiratory flow, diaphragm electromyogram, central drive mechanical and ventilation coupling, as well as other respiratory mechanics parameters were evaluated to elucidate the effects of COPD model on respiratory mechanics. Upon this model, incremental CPAP level was applied to investigate the effects of CPAP level on the above mentioned respiratory mechanics parameters.RESULTS:1. The establishment of pathophysiological model of COPD.After the application of flow restrictor, the subjects become dyspnic, and develop the pathophysiological changes similar to COPD, such as flow limitation,dynamic pulmonary hyperinflation (inspiratory capacity(IC) decrease,P<0.001), static PEEPi(PEEPi,stat were 4.94±1.33 cmH₂O), increased use of accessory respiratory muscle(elevated ratio of tidal Pes to Pdi swing(△Pes/△Pdi),P<0.05). There were also increase of central driver(root mean square of diaphragm electromyogram, RMSdi),percentage of RMSdi to maximal RMSdi (RMSdi%) (P<0.01)and deterioration of central drive mechanical and ventilation coupling (decreased VT/RMS,VT/△Pes,P<0.01;△Pdi/RMS decrease,P<0.05). However, there were also some changes different from those in COPD, such as the tendency of elevated tidal volume (VT)(increased from 0.74±0.11L to 1.03±0.27L, P>0.05), decreased respiratory rate(RR) (P< 0.01).2. Effects of CPAP on respiratory mechanics in COPD.In this COPD model, as the increment of CPAP level, the change of EELV (ΔEELVx) and its percentage to inspiratory capacity (ΔEELVx/ICx%) increases in a S-shaped curve. In the range of CPAP level less than 73.95% of PEEPi,stat, there is slight impact on EELV. However, when CPAP level is over this critical value, the impact on EELV increased markedly. The parameters representing central drive mechanical and ventilation coupling including VT/RMS, VE/RMS,ΔPdi/RMS, VT/ΔPdi, VT/ΔPes all were shown to have the tendency of decreasing with no significance difference among groups.3. The correlation and relationship between different indexes.As the increase of CPAP level, there were increased in parameters representing respiratory central drive, including RMSdi% and mean inspiratory flow(VT/Ti), which are positively related to Borg index and negatively related to IC. The parameters representing central drive mechanical and ventilation coupling, including VT/ΔPes,ΔPdi/RMS,VT/RMS were negatively related to Borg index and positively to IC. The parameter representing diaphragmatic muscle activity to achieve inspiratory flow trigger of common ventilators,the integrals of RMS from the beginning to the time of inspiratory flow of 40ml/s normalized by volume(ΣRMS/V)was negatively related to IC,VT/ΔPes,ΔPdi/RMS and VT/RMS and positively related to the RMSdi% and the Borg index, indicating this parameter in the early inspiration can also adequately reflect the changes of respiratory central drive. Let the Borg index as the dependent variable, stepwise multiple factors regression analysis revealed five related factors, including RMSdi%,ΔPdi/RMS,VT/RMS,ΔPes,VT/Ti,with coefficient of determination(R²)of 0.614. Let theΔEELVx or IC as the dependent variables, it was shown that CPAP/PEEPi,stat and Te were related factors with R² of 0.666 and 0.718 respectively.CONCLUSION:1. The COPD model was successfully established with flow restrictor in normal volunterers, with the presentation of dynamic pulmonary hyperinflation and PEEPi. It was observed in this model that there were increased respiratory effort, respiratory central drive, use of accessory respiratory muscle and dyspnea, as well as the deterioration of central drive mechanical and ventilation coupling.2. In this COPD model, as the increment of CPAP level, the change of EELV (ΔEELVx) and its percentage to inspiratory capacity (ΔEELVx/ICx%) increases in a S-shaped curve. In the range of CPAP level less than 73.95% of PEEPi,stat, there is slight impact on EELV. However, when CPAP level is over this critical value, the impact on EELV increased markedly. CPAP can not improve the central drive mechanical ventilation coupling. These results did not support the use of central drive mechanical and ventilation coupling to guide the rational setting CPAP level.More attention should be paid to the monitoring EELV and the patients'dyspnea in the titration of CPAP (PEEP) level in COPD.