Background
Ventilator-acquired pneumonia (VAP) is a common postoperative complication and accounts for a large part of post-cardiac surgery morbidity and mortality. Incidence of VAP depends on numerous factors, including pulmonary collapse and atelectasis during cardiopulmonary bypass (CPB), lowering of bronchial arterial blood flow and systemic inflammation response syndrome during and after CPB [
1‐
6].
To date, the impact of mechanical ventilation during CPB is unknown. On the one hand, CPB allows blood oxygenation during cardiac surgery, regardless of heartbeat and oscillations, allowing the surgeon to operate without disturbance [
7]. On the other hand, postoperative pulmonary complications appear to be more frequent when no mechanical ventilation is maintained while under CPB [
8].
A recent meta-analysis identified oxygenation improvement after the weaning from CPB when low tidal volume (LTV) ventilation was maintained or after lung recruitment maneuvers (LRM), as compared to when there was no ventilation (noV) [
9]. Furthermore, maintaining mechanical ventilation may reduce the inflammation response and tissue damage [
10,
11]. As the design of previous studies did not include hard clinical endpoints such as respiratory complications, death or length of stay, there is as yet no evidence for an unquestionable standardized strategy of lung protection during CPB and there are no scientific recommendations on whether mechanical ventilation has to be maintained during cardiac surgery or not, notably between LTV ventilation and noV [
12]. Last, the nature of the intervention makes it hard for protocol investigators to blind the investigators to the intervention, explaining why all trials assessing mechanical ventilation are open-labeled [
13].
The trial - low tidal mechanical ventilation against no ventilation during cardiopulmonary bypass heart surgery (“MECANO”) - aims to prove the superiority of the LTV compared to the noV strategy during CPB in cardiac surgery, to decrease postoperative respiratory complications, assessed by hard clinical endpoints using a double-blind design.
Discussion
Postoperative pulmonary complications (PPCs) are common and serious complications after cardiac surgery [
15], despite continuing improvements in CPB techniques and postoperative intensive care. They are broadly defined as conditions affecting the respiratory tract that can significantly impact on patient outcomes and health economics [
16,
17].
During CBP, the lungs are under perfused, non-ventilated or supplied with low continuous ventilation, depending on the center protocol [
18]. Clinical trials have suggested that preventive lung-protective ventilation may improve outcomes in patients undergoing high-surgery [
16].
The goal of the MECANO trial is to compare the effects of no ventilation during CBP and LTV ventilation of 3 mL/kg with a PEEP of 5 cmH2O during CPB in cardiac surgery. We believe that the present study has several strengths. First, the number of patients to be included (n = 1500) is ambitious. Several previous studies aimed to prove the beneficial effect of protective ventilation in cardiac surgery [
16]. However, most trials were insufficiently powered or biased, leading to high heterogeneity and lack of conclusive results [
13,
19‐
31]. The CPBVENT trial (NCT02090205) aims to answer a similar question. Although multicenter by design, it is a single-blind study and focuses on indirect outcomes (PaO2/FiO2 ratio only) instead of harder clinical endpoints such as hospital-acquired pneumonia or death. This explains why the number of patients to be included in CPBVENT is smaller (n = 720 vs. n = 1500 in our study). Moreover, CPBVENT compares three ventilation strategies (no ventilation, continuous positive airway pressure (CPAP) and LTV ventilation), which may decrease the power of the study [
13]. Finally, the inclusion criteria are stricter, decreasing the possibility to generalize the results.
Second, the MECANO trial addresses observer bias by using a double-blind design, with investigators only involved in the post-surgery setting. As such, they are never aware of the allocation arm of the included patients. Randomization ensures equity and balance between the two treatment strategies. In-hospital follow up allows for complete follow up of all patients and is sufficiently pertinent, as postoperative pulmonary complications are expected to happen within the hospital stay after surgery. Third, data on all variables that may account for the increased risk of postoperative pulmonary complications are collected at baseline, ensuring equivalence between the two treatment strategies.
Limitations of the MECANO trial include its single-center design, although the number of physicians accounts for a wide scope of practices, all in line with current guidelines. Second, the surgeon has the final say in the type of mechanical ventilation, i.e. he can stop any type of ventilation strategy during CPB as he sees fit. This bias is addressed by systematically collecting the reason for and number of times that this may happen. Analyses will be performed on an intention-to-treat and per-protocol basis. Moreover, this will generally show how feasible or not, a maintained LTV ventilation might be and guide clinical practice accordingly.
In conclusion, the MECANO trial should help determine whether low-tidal ventilation is superior to no-ventilation, during cardiac surgery with CPB. It is the first double-blind trial of this kind, with a large population and focusing on hard clinical endpoints (Table
2, Additional file
1).
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