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Echeverria-Villalobos et al. Vessel Plus 2019;3:33 I http://dx.doi.org/10.20517/2574-1209.2019.12 Page 7 of 12

underwent CABG under CPB. Moreover, absence of ventilation has been associated with hydrostatic
pulmonary edema, poor pulmonary compliance, and higher incidence of lung infections [61,62] .

Ischemia-reperfusion injury
Under physiologic conditions, bronchial circulation represents 3%-4% of the pulmonary blood flow and
[63]
may decrease during CPB . The ischemic phase depletes the energy stores (i.e., ATP), increasing lactate
levels in the pulmonary blood flow [53,64] .The reperfusion and re-oxygenation phase after aortic cross-clamp
release stimulates the production of reactive oxygen species (ROS) resulting in dysregulation of intracellular
and mitochondrial calcium transport, increased inflammatory response (i.e., cytokines, complement
and activation of neutrophils), endothelial cell damage, and increased vascular permeability [60,65-67] . The
systemic and pulmonary inflammatory states originated during and after CPB generate a compensatory
anti-inflammatory response characterized by the production of anti-inflammatory cytokines (IL-10)
and leukocytes [68,69] . Monocytes downregulation follows this chain of events resulting in an increased
susceptibility to postoperative infections [52,70].

Hyperoxia
Increased oxygen concentrations are commonly administered during CPB in order to avoid cellular
[71]
hypoxia, reduce gaseous micro-embolism, and improve neutrophils’ functionality . Nonetheless, enhanced
production of ROS, cardiovascular dysregulation, and increased injury due to ischemia-reperfusion are
some of the systemic effects linked to hyperoxia [72,73] .

Could MV be a mechanistic strategy to protect the lungs during CPB?
Different strategies such as CPAP with and without PEEP have been implemented during MV under CPB.
Current evidence about the use of MV as a mechanistic strategy for lung protection during CPB remains
controversial. Early studies examined the effects of CPAP during CPB without showing any significant
beneficial effects on oxygenation [74,75] . Nevertheless, recent studies have reported that CPAP pressures of
10 cmH O were more effective in achieving and maintaining better postoperative PaO /FIO ratio than
2
2
2
[61]
lower CPAP pressures in patients undergoing cardiac surgery with CPB .

Even though only a small amount of patients undergoing cardiac surgery may develop acute respiratory
distress syndrome (ARDS), the reported mortality rates may reach up to 50% [17,24] . An increased body of
evidence supports the benefits of PMV (low V , FiO , and PEEP) in patients with ARDS [76-80] . The rationale
T
2
for using PMV during CPB lies in the fact that postoperative pulmonary dysfunction in cardiac surgery is
characterized by alterations in lung mechanics and gas exchange abnormalities, which may resemble some
of the ARDS physiologic and clinical features. Although many surgeons prefer the lung collapsed during
CPB in order to improve the surgical field, recent published reports suggest that PMV may be associated
with a significant reduction of pathophysiological events and pulmonary dysfunction after cardiac
surgery [81-89] . However, MV also entails some risk of pulmonary damage such as alveolar over distension
(resulting from high V ), alveolar rupture (due to cyclic opening), inactivation of surfactant, and excessive
T
lung stress inducing elevated transpulmonary pressure [90-93] .

[84]
John and Ervine randomized patients undergoing CABG under CPB to either MV with low V /no-PEEP
T
(ZEEP) or non-ventilation. Patients who were ventilated during CPB presented lower extravascular lung
water content and shorter extubation times when compared to the non-ventilation group (530 ± 50 mL
vs. 672 ± 32 mL, P = 0.028 and 3.60 ± 0.3 h vs. 4.8 ± 0.4 h, P = 0.038 respectively). Paradoxically, the cyclic
expansion of the lungs may further reduce the bronchial blood flow during the pulmonary exclusion phase
[94]
of extracorporeal circulation .