Carbon Dioxide, Blood Pressure, and Stroke: Comment

Carbon Dioxide, Blood Pressure, and Stroke: Reply

“…I n their article, Vlisides et al 1 present retrospective associations between hypotension, hypocarbia, and hypercarbia and clinically apparent postoperative strokes, with 122 used in the analysis. (Covert strokes are nearly 10 times as common, 2 but were not considered.)…”

“…these in a cohort of 197,474 surgical patients where higher intraoperative tidal volumes (greater than 8 ml/kg) were associated with increased risk of postoperative respiratory failure only in patients with a low compliance, and this effect was completely mediated by the resulting driving pressure. 5 In addition to lowering the applied tidal volume, modification of PEEP, which was the second factor in the trial by Turan et al, 1 can reduce driving pressure through shifting tidal ventilation toward the high-compliance part of the pressure or volume loop. However, even in patients with "healthy" lungs undergoing general anesthesia, PEEP requirements are heterogeneous, and a "standard" PEEP is often inadequate.…”

“…6,7 To better understand the effect of the study intervention, it would be helpful if the driving pressure per group could be provided by Turan et al to understand group differentiation regarding this crucial factor. Based on previous literature, 5,6 it may be hypothesized that the interventions in the present study 1 were effective when they resulted in a reduction in driving pressure, which might be tested in a post hoc analysis. In addition, it would be informative to analyze the effects of lowering tidal volume in patients with a lower respiratory system compliance.…”

“…We would like to add two important observations based on recent literature to the discussion of the study by Turan et al 1 First, Neto et al 4 and others have shown that it is not tidal volume per se that is the critical component of lung protection, but rather the resulting driving pressure, i.e., the interaction of the applied tidal volume with the individual patient's respiratory system compliance, which is a more accurate determinant of perioperative lung injury and postoperative pulmonary complications.…”