PE with a negative CT: Basic & Bonus lessons.

 A healthy male came to the ED complaining of chest pain and exertional dyspnea, worsening over the past few days. He mentioned that it felt like “the time I had a PE.” (That PE had been provoked by a lower-extremity fracture, and he had completed 6 months of anticoagulation without problems.) 

While the vitals and exam were unremarkable, the ECG suggested RV strain:

A bedside echo also supported RV strain,with the apical 4-chamber showing notable RV dilation and septal bowing.

A CT for PE was ordered. Even before he had left for the CT suite, however, enoxaparin was ordered. The patient, who had been reading quite a bit about diagnosing and treating PEs in the past few days (quite educated!), reasonably asked why treatment was being ordered prior to the test.

The Basic Lesson:

If the patient has a high probability of having a PE, there is still a 40% chance of PE if the CTA is negative.

In this case, the ED echo, along with the clinical and ECG data, assuredly defined this patient as very high risk for PE.  

Despite the high probability, the CTA was negative. However, a V/Q done the next day was interpreted as high probability for PE, with multiple areas of mismatch.  

So, the CTA is not the gold standard we often take it to be, and we need to be careful to avoid ruling-out VTE prematurely. Of course, patients who are truly high probability for PE are infrequently seen (only 6% of subjects in PIOPED II).

The Bonus Lesson:

Why the discrepancy? The answer was likely on the ED echo as well.

Color and continuous wave Doppler, were used to interrogate the tricuspid valve. Only a moderate degree of regurgitation was seen, but with impressive velocities. The RV inflow view is shown here:

The maximum velocity of the TR jet exceeded 4 m/s, suggesting a RV systolic pressure over 70 mm Hg. This is a bit high for an acute PE, and suggests instead that a chronic process is involved. And indeed, chronic PE is known to be poorly visualized by CTA, but well demonstrated on V/Q scans.