The Diagnosis of Pulmonary Embolism-Reply

In Reply.—  As reported in our discussion of study methods, the PIOPED ventilation scans included left and right posterior oblique views in addition to the posterior views. The PIOPED investigators considered the use of aerosol and of xenon 127 for ventilation scans during the planning phase for PIOPED. In comparison to xenon 133, aerosol has not been shown to be a superior diagnostic agent.¹ Because of aerosol's greater cost, the limitations Dr Boxen describes, and the lack of evidence of diagnostic superiority, aerosol was not used.Although xenon 127 has some desirable characteristics, at the time of the PIOPED planning phase it was much more expensive than xenon 133 and was distributed by only one commercial supplier, which made it impractical for use in PIOPED. We do not know of any large series conducted to establish whether xenon 127 is indeed superior as a diagnostic agent. The PIOPED investigators took