Magnitude and pattern of breathing effort during spontaneous breathing trials after cardiac surgery: a physiological model of sustained breathing effort despite diaphragm dysfunction

Background: After cardiac surgery, patients are liberated from mechanical ventilation despite diaphragm dysfunction and atelectasis; understanding their breathing pattern can help interpreting conditions with diaphragm dysfunction and defining a tolerable range of effort under mechanical ventilation. Methods: Prospective physiological study describing the magnitude and pattern of breathing effort after cardiac surgery. Three spontaneous breathing trial modalities performed in random order, including two un-assisted (continuous positive airway pressure 0 cmH2O -CPAP0- and T-piece) and one assisted (pressure-support 5/PEEP 5 cmH2O- PS5PEEP5). Airway, esophageal, and gastric pressures were recorded and diaphragm ultrasound was performed. Airway occlusion pressure (P0.1) was also measured. Difference in magnitude of respiratory effort between conditions was explored through linear mixed-models with Tukey adjustment for pairwise comparisons. Association between pre-defined clinical variables (opioid dose, body mass index, and chest wall compliance) as well as measures of diaphragm function with expiratory muscle activity was explored through mixed-effects models. Results: Thirty patients were included. Maximum inspiratory pressure during a Mueller maneuver was − 29.8 ± -12.6 cmH2O. Estimated mean (CI95%) global inspiratory effort (pressure-time product per minute) during un-assisted modalities for the population was 138.5 (120.6,156.3) cmH2O•sec/min. With increasing support, drive decreased as well as pressure-time product per minute (median PS5PEEP5: 101, CPAP0 129, T-piece 135 cmH2O•sec/min, p < 0.001 for PS5PEEP5 vs. other modalities), and transdiaphragmatic pressure (PS5PEEP5 median 4.1, CPAP0 median 6.6, T-piece median 6.0 cmH2O respectively, p < 0.001 for PS5PEEP5 vs. other modalities). During mechanical ventilation, expiratory muscle effort (i.e., gastric pressure rise) contributed on average to 31–37% of the drop in esophageal pressure during inspiration, being lower after extubation. Gastric pressure rise was directly associated with a higher intraoperative dose of opioids (p = 0.004) and inversely with measures of respiratory muscle function (p = 0.001). Tension-time indices lied within the higher end of a non-fatiguing range. All patients were successfully extubated. Conclusions: Stable patients after cardiac surgery show an increase in respiratory drive and inspiratory effort with increasing load despite signs consistent with diaphragm dysfunction. Expiratory muscle use is common, it is associated with decreased diaphragm activity, higher intraoperative dose of opioids and decreases after extubation.