Investigations

• Arterial blood gas ^a  
  
  
• Chest X-ray ^a  
  
  Look for any abnormality particularly
  • atelectasis or pulmonary parenchymal abnormality 
  • pleural effusion 
  • pleural-based opacity 
  • decreased pulmonary vasculature 
  • pulmonary oedema 
    
    
• ECG ^a  
  
  Look for 
  • new S₁, Q₃, and T₃ (S wave in lead I, Q wave in lead III, T wave inversion in lead III) 
  • new right bundle-branch block 
  
  These three tests are not individually very helpful in diagnosing PE, but should be done to rule out other diseases. ^a   
  
  
• blood count ^a  
• d-dimer ^a in clinically low-risk cases. ^a Use a whole-blood agglutination test ^a
  Low-risk patients with a normal d-dimer are very unlikely have a PE. ^a
•  
• baseline clotting ^d
• factor V Leiden and other thrombophilia studies if indicated.
  (a family history of thrombophilia and a first thromboembolic event aged < 41) c ^c
  Consider a haematology referral if abnormal. ^d
  
  
• ventilation-perfusion scan ^a  
  High risk if at least 1 segmental perfusion defect or 2 large subsegmental defects > 75% of a segment

Patients with low or intermediate probability scans need further testing ^c, so base further investigations on the following plan ^a  

V/Q scan result	PE probability	next step	d-dimer positive	d-dimer negative
normal	low and + d-dimer, moderate or high	no PE	-	-
low or intermediate	low and + d-dimer	Bilateral leg vein ultrasound positive: PE negative: no PE	-	-
low or intermediate	moderate	Bilateral leg vein ultrasound positive: PE negative: see d-dimer	Bilateral leg vein ultrasound in 1 week or venogram positive: PE negative: no PE	no PE
low or intermediate	high	Bilateral leg vein ultrasound positive: PE negative: see d-dimer	Pulmonary angiography : positive: PE;  negative: no PE	Bilateral leg vein ultrasound in 1 week or venogram positive: PE negative: no PE
high	low and + d-dimer, moderate or high	PE	-	-

Other tests that may be helpful include

• capnogram. ^a  Measure the waveform area (area under curve produced by one expiration)
• impedence plethysmography
• helical CT ^b  
• MRI ^b