Obesity is an ongoing worldwide problem, and as such, weight-related issues arise especially
in the intensive care unit (ICU). Morbid obesity adversely affects physiology; restricted
thoracic movement and increased intra-abdominal pressure leads to reduced lung volumes and
compliance resulting frequently in atelectasis, ventilation-perfusion mismatch.
Cardiac surgical patients are at particular risk for PPCs. The use of cardiopulmonary bypass
(CPB) is responsible for a systemic inflammatory response and oxidative stress, leading to
pulmonary ischemia-reperfusion injury. Mechanical ventilation frequently is disrupted during
CPB, thus inducing atelectasis. Some proposed mechanisms are leukocyte activation, fluid
accumulation in the lung and atelectasis.
Atelectasis is a highly prevalent pulmonary complication in patients undergoing cardiac
surgery with cardiopulmonary bypass (CPB) and an important cause of postoperative hypoxemia.
Pulmonary collapse occurs early after the induction of anesthesia and persists for several
days postoperatively. Studies based on thoracic computed tomography (CT) have shown that
pulmonary collapse is mainly distributed to the dependent regions close to the diaphragm and
may encompass up to 35% of the overall lung parenchyma which predispose to pneumonia.
This impairment of lung function, particularly oxygenation of the blood, is a common and
potentially serious complication after cardiac surgery. It has been shown to persist for as
long as one week after the operation.
In obese patients, the functional residual capacity (FRC) and lung compliance are reduced,
and thus the elastic work of breathing (WOB) is elevated. By applying continuous positive
airway pressure (CPAP), the FRC is restored, and inspiration starts from a more favorable
pressure-volume relationship, facilitating spontaneous ventilation, and improves oxygenation.
APRV was originally described as a mode to treat acute lung injury in patients and attempt to
maintain the level of airway pressure without reducing cardiac function, delivering
mechanical breaths without excessive airway pressure and allowing unrestricted spontaneous
ventilation. 6 APRV is essentially a high-level continuous positive airway pressure (CPAP)
mode that is terminated for a very brief period of time. It is this short release period that
allows carbon dioxide to be cleared. The lengthy time during which the high-level CPAP is
present results in substantial recruitment of alveoli of markedly different regional time
constants, at rather low gas flow rates and lower airway pressures. The establishment of
intrinsic PEEP by the short release time enhances oxygenation. Carbon dioxide clearance is
aided by recruitment of the patient's lung at close to total lung capacity; elastic recoil
creates large volume gas flow during the release period.
Recent advances in the understanding of the respiratory physiologic alterations in the obese
patient suggest that ''open lung'' ventilation approaches such as APRV recruit lung alveolar
volumes without exacerbating lung injury and maintain acceptable gas exchange. Given the
spontaneous nature of the mode, it is hypothesized that there should be no need for
continuous infusions of neuromuscular blocking agents in patients placed on this mode of
ventilation.
This may result in a shorter duration of intensive care unit (ICU) stay. Furthermore, because
patients may be ventilated at lower airway pressures than are required with cyclic
ventilation, there may be a reduced need for pressor support of hemodynamics to ensure oxygen
delivery which is favorable in cardiac surgical patients.
So, the investigators hypothesize that APRV may be a preferred mode of ventilation in
morbidly obese patients after cardiac surgery.
