Pulmonary embolism (PE) is a dreaded condition which nearly every physician, in practice or training, is aware about. It is a leading and avoidable cause of sudden death . That PE is now overdiagnosed , merits reflection. PE’s journey from an acute emergency which physicians feared missing, to a frequently overdiagnosed entity leading to overtreatment, is instructive at multiple levels, showcasing the inevitability of overdiagnosis and exposing barriers to its reduction.
Embolism to the pulmonary arteries, if large, can lead to right heart strain, cardiovascular collapse, and sudden death . PE is in the differential for pulseless electrical activity, a type of cardiac arrest. PE, more often, presents less dramatically and such presentations can be so nonspecific that a range of symptoms is attributable to PE. Emboli that do not cause cardiovascular collapse at their first presentation are not important for their own sake but because they herald a much larger clot burden in the future. This means that the significance of PE lies in its recurrence. That the risk of recurrence can be reduced by anticoagulation makes recurrent PE an avoidable harm.
The suspicion for PE induces uncertainty exacerbated by the fact that death from recurrent PE is potentially avoidable. The imperative to diagnose PE is substantial, particularly in the young patient in the emergency department. With such a high degree of clinical uncertainty, responsibility for diagnosis is shared by imagers.
The gold standard for the diagnosis of PE was once catheter angiogram , at which thrombus in the pulmonary arteries is straightforward to detect, as it appears as a filling defect. However, catheter angiography is invasive and operationally difficult to perform in everyone suspected of PE. The first test for the diagnosis of PE used to be ventilation-perfusion (VQ) scan, in which PE was inferred, rather than anatomically depicted.
The efficacy of VQ scans in the diagnosis of PE was shown in the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) . Reporting of VQ scans is an exemplar of standardization. The imager grades the likelihood of PE as “high,” “intermediate,” and “low” probability based on objective criteria drawn from PIOPED. The imager then merges the findings on VQ with the prior probability of PE to give a context-dependent interpretation.
Paradoxically, and despite reporting of VQ scans being evidence-based, VQ increases uncertainty. Why the uncertainty is increased is because the “intermediate” category includes a wide bandwidth of probabilities and is a frequent finding. In PIOPED, nearly 40% of patients with suspected PE had an “intermediate” probability VQ scan. Of those with intermediate probability VQ, only one-third had PE .
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“Definitive evidence of overdiagnosis would, of course, be the finding that untreated patients never experience harm from the PE during the rest of their lives and died from another cause, but no such studies exist” and “we need to learn which small emboli need treatment.”
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