Low Tidal MEChanical Ventilation Against NO Ventilation During Cardiopulmonary Bypass Heart Surgery

BACKGROUND. Ventilator-acquired pneumonia (VAP) are a common postoperative complication and account for a large part of post-cardiac surgery morbidity and mortality. Incidence of VAP depends on numerous factors, some of which are pulmonary collapsus and atelectasis during cardiopulmonary bypass, a lowering of bronchial arterial blood flow and a systemic inflammation response syndrome during and after cardiopulmonary bypass (CPB).

On the one hand, CPB allows blood oxygenation during cardiac surgery, regardless of heartbeat and oscillations, allowing surgeon to operate without disturbance.

On the other hand, postoperative pulmonary complications appear to be more frequent when no mechanical ventilation is maintained while under CPB.

Recent meta-analysis found 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). Furthermore, maintaining mechanical ventilation would reduce the inflammation response and tissue damage. The design of these studies did not provide with clinical hard endpoints such as respiratory complications, death or length of stay, hence, an unquestionable standardized strategy of lung protection during CPB has not been evidenced at this time and there are no scientific recommendations on whether mechanical ventilation has to be maintained during cardiac surgery or not, notably between low-tidal volume ventilation (LTV) and no-ventilation (noV).

The investigators aim to prove superiority of LTV over noV strategy during CPB in cardiac surgery, in order to decrease postoperative respiratory complications.

DESIGN. The MECANO trial is a single-center, double-blind, non-pharmacological, randomized controlled trial comparing two mechanical ventilation strategies, LTV and noV, during cardiac surgery with CPB.

Population. All patients aged more than 18 years will be eligible planned for cardiac surgery with CPB will be eligible. All patients will provide written informed consent before their inclusion in the trial.

Endpoints. Primary endpoint will be composite of overall death, early respiratory failure defined as PaO2/FiO2 ratio <200 at one-hour after arrival in the ICU, heavy oxygenation support (defined as a patient requiring either non-invasive ventilation, mechanical ventilation or high flow oxygen) at 2 days after arrival in the ICU or ventilator acquired pneumoniae defined by Center of Disease Control. Secondary endpoints are those composing the primary endpoint with the addition of pneumothorax, CPB duration, volume of postoperative bleeding, red blood cells transfusions, requirements for revision surgery, length of stay in the ICU and in the hospital and total hospitalization costs. Patients will be followed until hospital discharge.

Randomization. Patients will be randomized by the anesthesiologist when arriving in the operating room, between LTV and noV arm, on a 1:1 ratio, using a web software. Only the anesthesiologist will be aware of the allocation arm in order for him to perform the necessary ventilation strategy, however, physicians outside the operating room (hence, study investigators) will not be aware of the treatment arm. Any deviation from the protocol will be recorded, as will be the reason of deviation.

Intervention. The noV group will receive no ventilation during CPB. The LTV group will receive 5 acts/minute with a tidal volume of 3 mL/kg and positive end-expiratory pressure of 5 cmH2O.

Lung recruitment maneuvers will be performed in both groups, at the end of surgery, and at the arrival in ICU with insufflation at +30 cmH20 during 5 seconds. In ICU, ventilation strategy will be lung-protective with: tidal volume=6 ml/kg of ideal body weight, PEEP=5 cmH2O, FiO2 set to obtain PaO2 between 200 and 250 mmHg, inspiration/expiration time ratio = 1:2. Other therapeutics will be left to the appreciation of the critical care medicine specialist.

Data collection. All data will be recorded on a dedicated CRF. Preoperative data will be collected prior to the surgery (age, height, weight, Euroscore 2, smoking status, diabetes, peripheral arterial disease, pulmonary comorbidity, forced expiratory volume in one second, pulmonary infection in the past 30 days, creatininemia). Variables linked to the surgery will be: type of surgery, duration of CPB, number of red blood cells transfusions, number and reasons of manual insufflations. Daily visit will record temperature, PaO2, FiO2, ventilation mode, hemoglobinemia, leucocytemia, quantity of bleeding and any of the endpoints listed above. Time to events will be recorded as well. Follow up will be maintained until hospital discharge.

Statistical considerations Sample-size calculation was based on a two-sided alpha error of 0.05 and a 80% power. On the basis of respiratory insufficiency incidence after cardiac surgery, the investigators anticipate at least 25% of patients presenting postoperative respiratory complications. A relative improvement in the incidence of primary outcome of 20% between the 2 arms (odd-ratio 0.8 in favor of LTV arm as compared to noV arm) is expected. The required sample size is then 720 patients per group, 1440 patients in total. Accounting for attrition ratio, 1500 patients will be included. Intermediary analysis will be performed.

Data analysis. Patients will be analyzed following intention to treat principle. Logistic regression will be performed for statistical analysis. Relative risks with 95% confidence intervals and differences between medians with 95% confidence intervals will be calculated when appropriate. Two-sided significance tests will be used throughout. The investigators will infer a subgroup effect if the interaction term of treatment and subgroup is statistically significant at P <0.05.