E. Computed tomographic pulmonary angiography – Explanation
| CTPA is the first line investigation for PE according to current BTS guidelines |
This is a difficult question to answer as both computed tomographic pulmonary angiography (CTPA)
and ventilation-perfusion scanning are commonly used in UK clinical practice. The 2003 British
Thoracic Society (BTS) guidelines, however, recommended that CTPA is now used as the initial lung
imaging modality of choice. Pulmonary angiography is of course the ‘gold standard’ but this is not
what the question asks for
Pulmonary embolism: investigation
2012 NICE guidelines
All patients with symptoms or signs suggestive of a PE should have a history taken, examination
performed and a chest x-ray to exclude other pathology.
If a PE is still suspected a two-level PE Wells score should be performed:
| Clinical feature | Points |
| Clinical signs and symptoms of DVT (minimum of leg swelling and pain with palpation of the deep veins) | 3 |
| An alternative diagnosis is less likely than PE | 3 |
| Heart rate > 100 beats per minute | 1.5 |
| Immobilisation for more than 3 days or surgery in the previous 4 weeks | 1.5 |
| Previous DVT/PE | 1.5 |
| Haemoptysis | 1 |
| Malignancy (on treatment, treated in the last 6 months, or palliative) | 1 |
Clinical probability simplified scores
- PE likely – more than 4 points
- PE unlikely – 4 points or less
If a PE is ‘likely’ (more than 4 points) arrange an immediate computed tomography pulmonary
angiogram (CTPA). If there is a delay in getting the CTPA then give low-molecular weight heparin
until the scan is performed.
If a PE is ‘unlikely’ (4 points or less) arranged a D-dimer test. If this is positive arrange an immediate
computed tomography pulmonary angiogram (CTPA). If there is a delay in getting the CTPA then
give low-molecular weight heparin until the scan is performed.
If the patient has an allergy to contrast media or renal impairment a V/Q scan should be used
instead of a CTPA.
CTPA or V/Q scan?
The British Thoracic Society (BTS) published guidelines back in 2003 on the management of
patients with suspected pulmonary embolism (PE). Key points from the guidelines include:
- computed tomographic pulmonary angiography (CTPA) is now the recommended initial lung-
imaging modality for non-massive PE. Advantages compared to V/Q scans include speed, easier toperform outof- hours, a reduced need for further imaging and the possibility of providing an
alternative diagnosis if PE is excluded - if the CTPA is negative then patients do not need further investigations or treatment for PE
- ventilation-perfusion scanning may be used initially if appropriate facilities exist, the chest x-ray is
normal, and there is no significant symptomatic concurrent cardiopulmonary disease
Some other points
D-dimers
- sensitivity = 95-98%, but poor specificity
ECG
- the classic ECG changes seen in PE are a large S wave in lead I, a large Q wave in lead III and an
inverted T wave in lead III – ‘S1Q3T3’. However this change is seen in no more than 20% of patients - right bundle branch block and right axis deviation are also associated with PE
- sinus tachycardia may also be seen
V/Q scan
- sensitivity = 98%; specificity = 40% – high negative predictive value, i.e. if normal virtually excludes
PE - other causes of mismatch in V/Q include old pulmonary embolisms, AV malformations, vasculitis,
previous radiotherapy - COPD gives matched defects
CTPA
- peripheral emboli affecting subsegmental arteries may be missed
Pulmonary angiography
- the gold standard
- significant complication rate compared to other investigations
