Pulmonary
- Critical Care Associates
of East Texas
Jeffrey M.
Shea, M.D., F.C.C.P.
Venkatesh Donty, M.D.
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Jeffrey M.
Shea, M.D., F.C.C.P. |
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Purpose Ventilation and perfusion scans are used for two major reasons: detection of pulmonary emboli and assessment of regional lung function. When a blood clot blocks a pulmonary artery, blood flow ceases to the lung region normally supplied by that vessel, and a corresponding "perfusion defect" results. Ventilation and perfusion lung scanning to assess regional lung function is often performed before surgery involving resection of a part of the lung, usually one or more lobes. By visualizing which areas of lung receive ventilation and perfusion, the physician can determine how much the area to be resected is contributing to the overall lung function. When the scanning techniques are used in conjunction with pulmonary function testing, the physician can also predict postoperative pulmonary function, which is a guide to postoperative respiratory problems and impairment. Technique Scanning of gamma radiation produced by the chemicals inhaled and injected into the vein results in a picture showing the distribution of blood flow and ventilation throughout both lungs. For lung perfusion scanning, the most common technique involves injecting clusters of human albumin with a radioactive particle into a vein. These clusters travel through the right side of the heart, enter the pulmonary vasculature, and become lodged in small pulmonary vessels. Only areas of the lung receiving perfusion from the pulmonary arterial system demonstrate uptake of tracer, while regions that are blocked by blood clots show no uptake of the chemical. For ventilation scanning, a gaseous radioactive gas, usually xenon, is inhaled and sequential pictures are obtained showing how the gas distributes within the lung. Pictures at different times after inhalation reveal information about gas distribution after the first breath (wash-in phase), after a longer time of breathing the gas (equilibrium phase), and after the patient again breathes air to eliminate the radioisotope (wash-out phase). Ventilation scanning shows which regions of the lungs are being ventilated, and whether there are significant localized problems with expiratory airflow and "gas trapping" of the radioisotope during the washout phase.
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