Positive end expiratory pressure will be maintained at the level facilitating optimal oxygenation during the off-pump period. Partial pressure of carbon dioxide monitoring is employed to determine the tidal volume and respiratory rate. Inspiration/expiration pattern is adjusted based on the preoperative small airway condition. This parameter is subject to modification upon cessation of ventilation, ICU admission, and every morning throughout the ventilation period. During cardiopulmonary bypass, mechanical ventilation is maintained using the low-level parameters.

A Pressure regulated volume control mode is used in the patients before extubation. The ventilation target are: (1) a pulse oximetry ≥ 90% or a partial pressure of arterial oxygen ≥ 60mm Hg; (2) a partial pressure of arterial carbon dioxide: 35 ~ 50 mm Hg and (3) a pondus hydrogenii (pH) value > 7.20. The ventilation parameters are: (1) tidal volume: 6 ~ 8 mL/Kg predictive body weight; (2) respiratory rate 10 ~ 15 cycles per minute; (3) inspiratory/expiratory ratio: 1:1.5 (1:2.5 - 1:3 in the patients with chronic obstructive pulmonary disease); positive end expiratory pressure: 0 ~ 8 cm centimeter water column. On-pump ventilation parameters are: (1) tidal volume: 4 mL/Kg predictive body weight; (2) respiratory rate: 4 circles per minute; (3) positive end-expiratory pressure: 4 cm centimeter water column; (4) inspiratory oxygen fraction: 21%.

Management of hypoxemia will be initiated immediately through the following steps: (1) carefully checking anaesthesia apparatus malfunction, airway normality, and monitoring accuracy; (2) improving cardiac function, correcting fluid overload, and alleviating systemic inflammation; (3) performing alveolar recruitment manoeuvres as described above; (4) increasing the tidal volume and positive end expiratory pressure within the upper limits; (5) increasing the respiratory rate while addressing concurrent hypercapnia; (6) titrating the fraction of inspiratory oxygen until the pulse oximetry reaches or exceeds 90%; and (7) considering the use of extracorporeal membrane oxygenation if any following situations occurred 14: (a) a partial pressure of arterial oxygen < 50 mm Hg for more than 3 hours; (b) a partial pressure of arterial oxygen to inspiratory oxygen fraction ratio < 80 mm Hg for more than 6 hours; or (c) a critical respiratory acidosis for more than 6 hours.

Postoperative pulmonary complications are defined as any postoperative respiratory system complication that occurs from admission to the intensive care unit to 7 days post-surgery, encompassing (1) respiratory infection, (2) respiratory failure, (3) bronchospasm, (4) atelectasis, (5) pleural effusion, (6) pneumothorax, and (7) aspiration pneumonitis.

Early death was defined as any death occurring within 72 hours of surgery, while late death was considered for deaths occurring within 30 days after the surgery was scheduled.

The vasoactive-inotropic score is the sum of the dosages of frequently used vasoactive or inotropic agents according to the weighted values of the enrolled patients. Vasoactive-Inotropic Score = dopamine dose (μg·kg-1·min-1) + dobutamine dose (μg·kg-1·min-1) + 100 × epinephrine dose (μg·kg-1·min-1) + 10 × phosphodiesterase inhibitor (milrinone or olprinone) dose (μg·kg-1·min-1) + 100 × norepinephrine dose (μg·kg-1·min-1) +10000 × vasopressin dose (U·kg-1·min-1).

The postoperative adverse cardiovascular events include new-onset lethal arrhythmias (supraventricular/ventricular tachycardia, ventricular fibrillation, or Adams-Stokes syndrome), acute myocardial infarction, cardiogenic shock, and thrombotic or embolic events.

The duration from admission to discharge of the ICU

Duration from the end of the surgery to extubation.

Postoperative extrapulmonary complications include tracheotomy, rethoracotomy for exploration, wound infection, sepsis, gastrointestinal haemorrhage, and neurological complications (such as delayed recovery, delirium, cognitive dysfunction, coma, new-onset stroke, and syncope)