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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 5 of 12

Wang et al. [2] 2018 Systematic NA Different strategies used CPAP between 5-15 cm H 2 O during CPB may be associated
Review and for mechanical ventilation with short-term benefits such as improved gas exchange and
Meta-analysis during CPB and oxygenation. However, no significant differences in these
postoperative outcomes variables were found when comparing patients undergoing
mechanical ventilation during CPB and those non-ventilated
CPB: cardiopulmonary bypass; RCT: randomized clinical trial; CCL: chemokine ligand; POD: postoperative day; CPAP: continuous
positive airway pressure; VT: tidal volume; TNF-α: tumor necrosis factor alpha; IL: interleukin; ICU: intensive care unit; LOS: length of
stay; IBW: ideal body weight; PEEP: positive end-expiratory pressure; FiO2: inspired fraction of oxygen; PPCs: postoperative pulmonary
complications; CABG: coronary artery bypass graft; ARDS: acute respiratory distress syndrome

DISCUSSION
Multifactorial mechanisms affecting pulmonary function during and after CPB
Postoperative respiratory dysfunction is the most common postoperative complication in patients
undergoing cardiac surgery under CPB affecting 10% to 25% of these patients and also associated with high
mortality rates [17-19] . Patient-specific, anesthesia- and surgery-related factors contribute to the onset of a
complex mosaic of pathophysiological events that result in severe respiratory mechanics and gas exchange
impairment ensuing postoperative pulmonary dysfunction [18,20-23] .

Patient-specific factors
Chronic obstructive pulmonary disease, low ventricular ejection fraction (EF) (i.e., EF < 30%),
hypertension, blood transfusions, emergency surgery, previous cardiac surgery, combined procedures (i.e.,
cardiac and aortic procedures), active endocarditis, age > 70 are some of the patient-related risk factors
associated with respiratory insufficiency after cardiac surgery [24-26] .

Anesthesia-related factors
Several reports identified a strong association between general anesthesia and impaired postoperative
pulmonary function. Prolonged time in supine position and muscle relaxation have been linked to a
significant reduction in both, functional residual capacity (FRC) and lung volume, resulting from a
cephalic displacement of the diaphragm and the loss of balance between the elastic recoil of the lung and
the outward forces of the chest wall. This reduction in FRC promotes alveolar collapse (i.e., atelectasis)
and increases airway resistance with subsequent increased resistance to thoracic blood flow circulation.
Furthermore, the volatile agents inhibit pulmonary hypoxic vasoconstriction whereas intravenous agents
may decrease the hypoxic and hypercapnic ventilatory response. Intubation along with the aforementioned
mechanisms may result in ventilation-perfusion mismatch, abnormal shunt fraction, and wider AaDO 2 [27,28] .

Surgery-related factors
Sternotomy incision, sternosynthesis, and left internal mammary artery dissection
Numerous reports describe the association between the surgical technique and changes in respiratory
mechanics and lung function [22,29-33] . Median sternotomy disrupts sternum integrity, provokes chest wall
instability (i.e., uncoordinated rib cage expansion, decrease compliance), and reduces lung volumes with
subsequent impaired pulmonary mechanics [29,31] . The combination of sternotomy and dissection of the left
internal mammary artery (LIMA) has a significant impact on respiratory mechanics [29,31] . LIMA harvesting
maneuvers not only interfere with sternum stability but also may affect blood supply to the sternum,
intercostal muscles, and left phrenic nerve functionality. Moreover, instillation of saline slush in the
pericardial cavity has been also associated with phrenic nerve injury or dysfunction during cardiovascular
[22]
surgery .Therefore, chest wall mechanics and diaphragm mobility impairment results in significant
changes from pre-sternotomy breathing patterns (abdominal) to an upper thoracic pattern with reduced
[34]
lung volumes thereby, promoting atelectasis . Retraction of the chest wall during LIMA harvesting
produces additional trauma to the costovertebral joints ensuing an unstable rib cage with impaired
diaphragm contraction [34,35] . Likewise, altered thoracic wall mechanics and diaphragmatic dysfunction have
been associated to a reduced postoperative abdominal motion [22,29,31,34,36] . Nevertheless, disruption of the
[37]
anterior insertions of the diaphragm seems to recover shortly after surgery .