Maintaining mechanical ventilation during CPB was not associated in our study with better preservation of EELV. Despite less accurate than chest tomography imaging, N2 washin-washout techniques have been widely studied and showed good correlations and/or agreements with pulmonary volumes, allowing their use at the bench side in various settings, including critical cares16–18. We can therefore strongly believe that post-operative FRC was not modified by per-CPB ventilation in our cohort.
This study approaches the effect of per-CPB ventilation from an original angle. Previous studies have focused on its effect on oxygenation, postoperative respiratory complications or systemic inflammation8,10,19. To date there is no study specifically studying the impact of maintaining mechanical ventilation per-CPB on lung volumes during adult cardiac surgery, whereas most of the modern ventilation strategies in ICU or operative room are based on this concept of “lung protection”, using reduced tidal volume, sufficient PEEP and recruitment maneuvers to prevent lung collapse20,21.
Different studies have investigated the effect of maintaining ventilation during CPB on different clinical or biological parameters. A meta-analysis of 15 randomized trials conducted between 1993 and 2016, explored mechanical ventilation or continuous positive airway pressure during CPB on the evolution of the alveolar-arterial oxygen gradient, oxygenation, duration of mechanical ventilation and length of stay22. No significant difference was found with regard to the criteria studied. However, a significant improvement of the PaO2/FiO2 ratio in favor of maintaining ventilation can be noted in one of the 4 trials studying this parameter, as well as a decrease of the alveolo-arterial oxygen gradient in 2 trials. One of the limits of this meta-analysis was the heterogeneity of ventilation parameters among studies. More recently, the PROVECS study compared the effect of two ventilation strategies on the occurrence of postoperative respiratory complications. A total of 494 patients was randomized between "conventional" ventilation strategy and "protective" strategy, which included the maintenance of ventilation during CPB but also higher intraoperative PEEP and recruitment maneuvers. No significant difference was found in the occurrence of postoperative pulmonary complications, hypoxemia or use of respiratory support.
Different physiological hypotheses may explain the lack of effect in our study. The duration of CPB was limited with a mean duration of 60 min, and thus we can hypothesize that this period was too short to induce a significant difference in FRC. Another element may be the non-selected nature of the included patients, notably with the absence of patients with pulmonary underlying conditions. Indeed, in the IMPROVE study which demonstrated a benefit of protective ventilation in major abdominal surgery, the included patients presented a preoperative risk index for pulmonary complications of more than two20. Thus, it is possible that the association of a short duration of aortic clamping and the absence of preoperative risk factors may have mitigated the potential beneficial effects of per-CPB ventilation strategy. Another hypothesis concerns the choice of mechanical ventilation parameters during CPB. In our study, the ventilation was maintained with low tidal volume (Vt 2.6 ± 0.6ml/kg of IBW), which may be close to the theoretical dead-space volume (≈1.5mL/kg), and thus insufficient to keep the alveoli opened. However, higher volumes hamper the surgeon, potentially decreasing the quality of surgical exposure. Another element, specific to cardiac surgery under CPB with aortic clamping, is the exclusion of pulmonary vascularization. Indeed, experimental studies suggested that pulmonary vascularization is involved in maintaining the mechanical and architectural properties of the lung. Gibney et al, observed in animal models a decrease in pulmonary compliance if vascularization is stopped23. Thus, the mere maintenance of mechanical ventilation may not be able to counteract this “ischemic” closure of the lung.
Concerning the secondary endpoints, maintaining mechanical ventilation during CPB does not appear to be associated with any significant effect in our population. Thus, we found no significant difference either on clinical criteria (occurrence of pneumonia, duration of invasive mechanical ventilation, non-invasive ventilation or high flow oxygen therapy) or other criteria (oxygenation parameters, driving pressure, static compliance, postoperative atelectasis). Nevertheless, the small size of the population does not allow us to definitively conclude on this point.
Some limitations of our study must be discussed. First, the lack of randomization limits the impact of conclusion, despite great similarities between groups, notably concerning euroscore-2, obesity, age… Even if we observed a trend to an unbalance concerning the type of surgery, with more combined procedures in the not-ventilated group, this difference was not significant. Furthermore, if we suggest that the complexity of the procedure may alter pulmonary function, thus this difference may be more susceptible to worsen the EELV in the not-ventilated group, which has not been observed. Moreover, because the choice to maintain or not ventilation during CPB was left to the discretion of the medical team, it may be a source of selection bias. Finally, we cannot rule out a lack of power related to the limited sample size.