Cardiopulmonary bypass (CBP) during cardiac surgery induces a systemic inflammatory response associated with an immune dysregulation and a significant pulmonary dysfunction. First, the inflammatory response, usually attributed to surgical trauma, contact of blood with artificial surfaces, and ischemia reperfusion injury, is responsible for a postoperative immunodepression. For instance, an early impairment of lung cellular immune response after CPB, which could promote the development of postoperative pneumonia, has been found. Along these lines, a downregulation of human leukocyte antigen-DR antigen (HLA-DR) expression on monocytes and an increase in plasma interleukin 10 (IL-10) associated with the occurrence of nosocomial infections have been reported. Second, CPB induces a pulmonary dysfunction, which ranges from a temporary and clinically insignificant reduction in arterial oxygenation to a life-threatening injury manifested as acute respiratory distress syndrome (ARDS). This phenomenon is of multifactorial sources, but one of the main mechanisms is the occurrence of atelectasis during surgery. Atelectasis has been associated with lung injury and release of cytokines by shear forces on alveoli and small airways. However, it is not clear whether this injury is due to a recruitment/derecruitment phenomenon (i.e., atelectrauma) or whether it might by itself lead to the release of cytokine. Since CPB mechanically circulates and oxygenates blood bypassing the heart and lungs, usual procedure during CPB is to stop mechanical ventilation (MV) (apnea). Nevertheless, maintaining MV with positive expiratory pressure (PEEP) during CPB diminished the occurrence of atelectasis and the postoperative inflammatory response. Thus, we investigated the effects of maintaining MV during CPB for cardiac surgery on postoperative immunodepression and found that maintaining MV during CPB decreased postoperative immune dysfunction and could be an interesting strategy to diminish the occurrence of postoperative infection (nosocomial infection) without hampering the surgical procedure. However, these findings have to be confirmed in a clinical trial using the incidence of nosocomial infection as an endpoint.