Aspects of lung mechanics during mechanical ventilation

Abstract

Background: One of the most common diagnoses in the intensive care unit is acute respiratory failure. In its most severe form it is called acute respiratory distress syndrome and often requires mechanical ventilation. The main challenge for physicians is to provide mechanical ventilation that treats the patient´s hypoxia, which can be due to a variety of causes, without damaging the lung. Method: In paper I computed tomography scans were acquired in ten anesthetized surfactant depleted pigs. The volume of gas and atelectasis were correlated with transpulmonary pressure as the pressure support and PEEP were lowered. In paper II a non-invasive method for measuring transpulmonary driving pressure was validated in 31 mechanically ventilated intensive care patients. In paper III external expiratory resistors were added to the expiratory limb of the ventilator while calculating expiratory time constant, respiratory compliance, driving pressure and intrinsic PEEP in 12 anesthetized pigs. In paper IV transpulmonary pressure was calculated from esophageal pressure in supine and prone position in 10 anesthetized lung healthy patients. Results: Gradual decrease in transpulmonary pressure causes a proportional increase in atelectasis and decrease in gas content while the work of breathing increases. There is a good statistical agreement between the conventional and the non-invasive method for measuring transpulmonary driving pressure. Increasing the expiratory resistance increases the expiratory time constant and increases intrinsic PEEP in healthy lungs. There is a great variability in esophageal pressure in the part of the esophagus 22 – 44cm from the nostrils in both supine and prone position. Depending on method the transpulmonary pressure either increases or decreases when patients are turned in prone position. Conclusion: There is no transpulmonary pressure threshold, where atelectasis with desaturation or cyclic collapse suddenly occurs during gradual decrease in the ventilator support. The PEEP-step method is comparable to the traditional esophageal balloon method for measuring transpulmonary driving pressure. The application of expiratory resistors could be useful during weaning from mechanical ventilation. The mean end-expiratory esophageal pressure changes which affect the calculation of transpulmonary pressure but uncertainties about the use of absolute esophageal pressure remains.