Effects of position and open chest on lung and chest wall resistance during thoracic surgery

Introduction: Airway-pressure (Paw) based respiratory mechanics guides mechanical ventilation during anesthesia. However, altered properties of the chest wall may significantly influence the interpretation of Paw, especially in settings, in which an alteration of chest physiology occurs. Aim of the present study was to investigate the resistive behavior of lung and chest wall during different phases of thoracic surgery. Methods: Subjects undergoing thoracotomic pulmonary resection were enrolled. Double-lung ventilation (DLV): tidal volume (TV) 8 ml/kg, PEEP 8 cmH2O, FiO2 and respiratory rate to maintain SaO2 94-98% and EtCO2 30-35 mmHg. One-lung ventilation (OLV): TV 5 ml/kg and respiratory rate increased accordingly. Esophageal pressure was measured as a surrogate for pleural pressure. Maximal respiratory system resistance (Rmax,rs): difference of peak and plateau airway pressure divided by inspiratory flow. Rmax,rs includes flow resistance of airways (Rmin,rs) and that caused by stress relaxation/time constant inequalities within the tissues (DR,rs). Each resistance was partitioned into their respective lung/chest wall component (Rmax,l; Rmin,l; DR,l; Rmax,cw). Measurements were repeated during DLV in supine and lateral decubitus, OLV in lateral decubitus during closed and open chest conditions. Results: 20 patients were enrolled, 13 males (65%), age 68±10 years. Table 1 shows the changes of the partitioned resistances of lung and chest wall. All measurements are in cmH2O/L/sec. * p<0.05 vs previous step; § p<0.05 vs baseline. Conclusion: Paw-based monitoring of resistance during the different phases of thoracic surgery does not allow to differentiate the cause of an alteration. In this context, we suggest that partitioning of respiratory mechanics may give a deeper insight into the resistive behavior of the lung and the chest wall.