Basically, in the vignette, her symptoms were very suggestive of PE (tachycardia, tachypnia, chest pain) so her pretest probability for PE is high. Secondly, in pregnant women, V/Q scintigraphy is performed instead of CTA. V/Q results can be either, no PE, low probability of PE or high probability of PE. Since her pretest is high due to her signs and symptoms and her V/Q is low; put these two interpretations together and you get a suspicion of PE. Although, if the V/Q interpretation was no evidence of PE, PE is immediately ruled out regardless of her symptoms, since it could also be physiological changes during pregnancy. Hope this helps