Physiology of One-Lung Ventilation

Ventilation and perfusion are matched under physiologic conditions for optimal gas exchange and oxygenation. Ventilation involves the entry and exit of air from the lungs, while perfusion involves the flow of blood through alveolar pulmonary capillaries. Both are necessary for the diffusion of oxygen into and carbon dioxide out of the bloodstream. During thoracic surgery, the ventilation/perfusion (V/Q) relationship is disrupted by induction of general anesthesia, lateral decubitus positioning, one-lung ventilation, and the open chest. Despite ventilation/perfusion alterations, severe hypoxia is usually avoided with the aid of several intrinsic and extrinsic factors. This chapter will delve into the changing V/Q relationship that occurs during thoracic surgery; modifying factors, including hypoxic pulmonary vasoconstriction and position, among others; and strategies to address hypoxia intraoperatively.