Resolution rates of pulmonary embolism assessed by serial positron imaging with inhaled O-15-labeled carbon dioxide

Embolic obstruction of pulmonary blood flow results in delayed regional clearance of inhaled C ¹⁵O ₂. Focally retained C ¹⁵O ₂ appears as zones of increased O-15 activity on serial positron scintigrams, which show the locations of occluded pulmonary segments. Inhalation of C ¹⁵O ₂, with serial imaging by a multicrystal positron camera, was used to locate and assess the magnitude of occluded pulmonary segments in 8 patients with arteriographically documented pulmonary emboli. The imaging with C ¹⁵O ₂ inhalation was repeated after 1 wk of i.v. heparin therapy to evaluate the ability of this technique to determine resolution rates of pulmonary emboli during anticoagulant therapy. In all patients, zones of increased C ¹⁵O ₂ activity corresponded with sites of emboli identified arteriographically. After 1 wk of continuous heparin therapy, zones of focally retained C ¹⁵O ₂ were totally resolved in 3 patients, diminished in 4, and unchanged in 1. The regional pulmonary clearance rate of C ¹⁵O ₂ was delayed over embolized pulmonary segments in all patients (mean clearance half-time = 42.2 sec ± 11.2 s.e.m.) and improved after heparin therapy (13.9 ± 3.9 sec; p < 0.05). Serial C ¹⁵O ₂ inhalation imaging is a rapid noninvasive radionuclide technique for detection of pulmonary emboli. It can be repeated at frequent intervals to assess the resolution of emboli during anticoagulant therapy.