The imaginary line method is not reliable for identification of prosthetic heart valves on AP chest radiographs

To examine the utility of four criteria for distinguishing aortic from mitral valve prostheses on supine anteroposterior (AP) chest x rays in critically ill patients.

Two reviewers independently examined the post operative chest X-rays (CXR) of all patients undergoing either an aortic or mitral valve replacement over a 32 month period, in a blinded fashion. They applied four criteria to each film. For each criterion a sensitivity and specificity of differentiating the valve positions correctly was calculated for each reviewer, as well as a kappa statistic for inter-observer agreement between the two reviewers.

Two hundred and twenty seven CXR's were evaluated by each of the reviewers. There were 174 aortic and 53 mitral valve replacements. There was a high level of inter-observer agreement for all four criteria applied (kappa values 0.785 to 0.966). Criterion one (imaginary line method) could be applied by both reviewers to less than 50% of CXR's, and when applied was specific but not sensitive. The other three criteria could be applied by both reviewers to approximately 80% of films. Criterion 2 (orientation method) was sensitive but not specific. Criteria 3 (valve orifice method) and 4 (perceived direction of blood flow method) were both highly sensitive and specific and are therefore the best methods.

The well known imaginary line method is of limited value when identifying prosthetic valve positions on supine AP CXR's. We advocate the use of the "valve orifice" method or the "perceived direction of blood flow" method to gain valuable information regarding the presence and position of prosthetic heart valves.