Increasing CPAP (Continuous Positive Airway Pressure) leads to increasing trans-pulmonary pressure with increased activity of the abdominal wall muscles to aid Expiration

Abstract

Introduction: Increasing CPAP makes breathing out difficult leading to air trapping. This can be uncomfortable and limit CPAP device use. We have studied lung mechanics pressures with CPAP and non-invasive ventilation (NIV) and abdominal muscle activity (EMG) to identify if there are changes with CPAP. Methods: We studied 10 normal male subjects sitting at rest and measured mouth pressure Pmo, oesophageal pressure Poes, gastric pressure Pga, trans-diaphragmatic pressure Pdi and ΔPdi, and abdominal EMG with surface electrodes during and after CPAP at 5,10, and 15 cmH2O and NIV,15/5cm H2O with a NIPPY3 ventilator. Results: Mean Pmo increases with CPAP, less than indicated on NIPPY3 settings, 5=4.2cmH2O, 10=8.8cmH2O, 15=13.1cmH2O, NIV 4.3cmH2O. Poes and Pga increase with CPAP and NIV and fall after CPAP/NIV ceases: 10-8.9cmH2O P Conclusion: We have shown CPAP increases trans-pulmonary pressure. Abdominal expiratory muscle activity increased with CPAP of 15 may assist expiration. This is similar to changes in expiratory muscle activity with external expiratory load (EJM Campbell ) J. Physiol. 1957,136,563. It is unlikely that fall in Pga when CPAP is removed versus before CPAP is due to abdominal wall relaxation as EMG activity is similar. Increased abdominal EMG may show the point at which CPAP leads to active expiration to counter air trapping non-invasively.