Hypoxia during OLV can also be defined as an arterial oxygen tension (PaO 2) of less than 60 mmHg when the patient is being ventilated at an inspired oxygen fraction (FiO 2) of 1.0. Hypoxia during OLV is defined as a decrease in oxygen saturation (SaPO 2) measured by pulse oximetry of less than 85% to 90% range value usually episodes last a few minutes during OLV. This hypoxemic event will activate hypoxic pulmonary vasoconstriction (HPV), which leads to the contraction of vascular smooth muscle in the pulmonary circulation in response to a low regional partial pressure of alveolar oxygen, thus decreasing the shunt by redirecting pulmonary blood flow to the well-oxygenated and dependent lung.During OLV, an obligatory shunt may produce hypoxemia secondary to the collapse of the nondependent lung and increased atelectatic areas in the dependent lung. One-lung ventilation (OLV) is used to facilitate surgical exposure in the thoracic surgical patient using a double-lumen endotracheal tube (DLT) or a bronchial blocker.
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