D. Computed tomographic pulmonary angiography – Explanation
| Pulmonary embolism – CTPA is first-line investigation |
It is still common in UK hospitals, despite guidelines, for a ventilation-perfusion scan to be done first-
line
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
