Ventilatory muscle recruitment and work of breathing in patients with respiratory failure after thoracic surgery

Objectives: Increased work of breathing (WOB) and respiratory muscle weakness have been identified as major causes of respiratory failure after thoracic surgery. This study was undertaken firstly to characterize the mechanical impairment in patients with respiratory failure after cardio- thoracic surgery, and secondly, to determine how diaphragmatic paralysis affects deterioration in the ventilatory mechanics. Methods: We evaluated the respiratory mechanics of 24 patients following cardiac and thoracic surgery. Ten patients without respiratory problems were examined as control subjects. There were nine patients with phrenic nerve injury and five patients without phrenic nerve injury who required mechanical ventilation for more than 7 days. Phrenic nerve injury was assessed with a phrenic nerve stimulation test. We measured the respiratory variables, the esophageal, gastric and transdiaphragmatic pressure swing (ΔPes, ΔPga and ΔPdi, respectively), and the work of breathing during quiet tidal breathing. Results: Both the groups requiring mechanical ventilation exhibited abnormally negative ΔPga/ΔPes values, compared with the control subjects. A significant increase in WOB with the normal generation of ΔPdi was seen in the patients without phrenic nerve injury. In contrast, the poor generation of ΔPdi with a slight increase in work of breathing was noted in patients with phrenic nerve injury. Conclusions: These results demonstrated two different types of respiratory failure in thoracic surgery patients, focusing on the impact of phrenic nerve paralysis. Diaphragmatic dysfunction should not be overlooked in postoperative care, and the amelioration of this compromise in respiratory mechanics is an important aspect of good patient management.