This has led to criticism of the overuse of CTA and questions regarding the significance of peripheral filling defects when the findings don't fit the clinical picture (see here and here).
It doesn't mean, however, that subsegmental defects are never important. What if the clinical findings do suggest PE? A recent study was cited by the bloggers at Academic Life in Emergency Medicine. From the paper (SSPE stands for subsegmental PE):
We analyzed 3728 consecutive patients with clinically suspected PE. SSPE patients were contrasted to patients with more proximal PE and to patients in whom suspected PE was ruled out, in regards of the prevalence of thromboembolic risk factors and the 3-month risks of recurrent venous thromboembolism (VTE) and mortality. PE was confirmed in 748 patients, of whom 116 (16%) had SSPE; PE was ruled out in 2980 patients. No differences were seen in the prevalence of VTE risk factors, the 3-month risk of recurrent VTE (3.6% vs 2.5%; P = .42), and mortality (10.7% vs 6.5%; P = .17) between patients with SSPE and those with more proximal PE. When compared with patients without PE, aged greater than 60 years, recent surgery, estrogen use, and male gender were found to be independent predictors for SSPE, and patients with SSPE were at an increased risk of VTE during follow-up (hazard ratio: 3.8; 95% CI: 1.3-11.1). This study indicates that patients with SSPE mimic those with more proximally located PE in regards to their risk profile and clinical outcome.
The key phrase in the above abstract is clinically suspected PE. If the peripheral filling defect fits the clinical picture it matters. If it doesn't fit, or is an incidentaloma, it's a different ballgame.
This paper raises broader questions about whether PE anatomy as assessed by CTA (i.e. clot burden, location) matters at all. Certainly we know from prior studies that hemodynamic findings and physiologic data (RV function, biomarkers) mean more than anatomy. A study from 2012 in Chest, for example, found no correlation between overall anatomic clot burden and clinical outcome. It did, however, in contrast to the Blood paper referenced above, find a correlation (though barely statistically significant) between location (proximal versus distal) and clinical outcome in the subset of hemodynamically stable patients (not in the overall cohort). More discussion on that paper can be found at PulmCCM.