We thank Dr. Fierro for his emphasis on tidal volume reduction in response to our recent article “Driving Pressure during Thoracic Surgery: A Randomized Clinical Trial.”1 The definition of driving pressure is: plateau pressure − positive end expiratory pressure. Another formula of driving pressure is: tidal volume / static lung compliance. Therefore, reduction of tidal volume can also reduce driving pressure. However, the key point is that reduction of tidal volume can increase driving pressure if it deceases lung compliance (as in atelectasis), or increased tidal volume can decrease driving pressure if it increases lung compliance (as in recruitment). Therefore, reduction of tidal volume would decrease driving pressure until it reaches to the point where lung compliance starts to decrease. No study ever tested tidal volume in terms of driving pressure and it would be another interesting study subject. We think optimal tidal volume would be different in each individual if it is based on the lowest driving pressure.
We thank Dr. Amar for his careful review of our study. 1 As he said, lung resection and esophagectomy are two different surgeries. However, our hospital has many esophageal cancer surgeries (more than 300 cases per year). All included patients underwent the Ivor Lewis operation which usually takes only 4 to 5 h. All patients had no preoperative adjuvant chemoradio therapy. We only studied complications until postoperative day 3, thus a lot of delayed complications (graft failure, aspiration pneumonia, among others) were not included. For this reason, we did not see inclusion of esophageal cancer surgery as a problem. The number of esophageal surgeries was small (control group n = 12 vs. driving pressure group n = 16) and the incidence of pulmonary complications diagnosed by Melbourne Group Scale was control group n = 3 and driving pressure group n = 4. Dr. Amar’s other concern was the use of statistics. As he said, it is correct to use the Fisher exact test when expected frequencies are less than 5. Our concern was that the Fisher exact test runs an exact procedure especially for small-sized samples and is more conservative than the chi-square test. Our institutional statistician advised that acute respiratory distress syndrome (ARDS) is a small part of our primary outcome (pulmonary complications); therefore, showing the incidence itself is enough (ARDS: control group n = 5, driving pressure group n = 0). P = 0.05 cut is a consensus, some argue P = 0.10, or P = 0.001 is meaningful. Our P value by two different statistics was 0.025 versus 0.060, and the difference mostly came from small incidence of ARDS. Dr. Amar questioned why pneumonia occurred more frequently in both operated and nonoperated lungs in the control group. We think direct surgical injury and one-lung ventilation are associated with a profound inflammatory cytokine release because of abundant immune cells on the lung endothelium and alveolus.2 Excessive neutrophils recruited in response to the proinflammatory cytokines increase pulmonary vascular permeability in both dependent and nondependent lungs.3 These reactions often precede systemic inflammatory response syndrome, ARDS, and pneumonia.4–6
The authors report no conflicts of interest.