Analysis of non-invasive ventilation effects on gastric inflation using a non-linear mathematical model

A non-linear mathematical model of the oesophagus was developed to study the effects of non-invasive ventilation variables on the severity of gastric inflation. The model was based on the non-linear physical characteristics of biological tissue. The model simulated oesophageal mechanical function during non-invasive ventilation in cardiac arrest (2:30 ventilations/chest compressions cycles) and respiratory arrest (1:5 ventilations/s) as recommended by the European Resuscitation Council (ERC) in its 2005 guidelines for adult basic and advanced life support. Model predictions establish a strong correlation between the expiratory time and the occurrence of gastric inflation. For cardiac arrest, when using ventilation pressure lower than 12 cmH2O, expiratory time between consequent ventilations and time until the occurrence of gastric inflation were linearly dependent (r = 0.98). This linear correlation changed abruptly when airway pressure exceeded the threshold pressure of 12 cmH2O, indicating that air had entered the stomach during the first ventilation. The interval at which the pressure at the distal section of the oesophagus was above the lower oesophageal sphincter (LES) opening pressure was significantly prolonged in the model of cardiac arrest (approximately 5.5 s compared to 3 s in respiratory arrest), thus allowing a greater amount of air to enter the stomach at relatively low airway pressures. During cardiac arrest, the mean pressure at the distal section of the oesophagus and the amplitude of air backflow were higher compared to the mean pressure and amplitude during respiratory arrest. This is also due to the shorter expiratory intervals in the 2:30 ventilations/chest compressions technique. The model indicates that the time required for the air trapped in the oesophagus to completely deflate is approximately 2 s. This may be longer than the expiratory time recommended by the 2005 guidelines. Model predictions support the 2005 guidelines regarding the decrease in the tidal volume and in the inspiratory pressure in an effort to minimise gastric inflation.