Briefly outline the role of each of the following in the diagnosis of pulmonary embolism in the critically ill:
b) CT pulmonary angiogram (CTPA)
c) Serum troponin
d) D-dimer levels
Bedside test, rapid.
Avoids transport, radiation exposure, IV contrast.
Only 30-40% with PE have suggestive changes. Changes more likely if massive.
Signs of right heart failure with shock and known PE may be an indication for thrombectomy / thrombolysis.
Spiral CT scan with IV contrast.
Ability to also detect alternative pulmonary abnormalities.
Issues of transport, radiation exposure, IV contrast (versus bedside tests (e.g. leg duplex U/S, ECHO) and/or empiric anticoagulation).
PIOPED II – suggested that CTPA requires concomitanTrt pre-test probability assessment (Wells) to be effective tool in diagnosing or excluding. Positive and negative predictive values differed significantly at different pre-test probabilities.
Positive predictive value varies with extent of PE and pre-test probability – v good (97%) with main or lobar, falling with smaller; v good with high pre-test probability (96%), falling with lower (NEJM 2006).
More recent studies with newer generation scanners suggest CTPA better at excluding PE than in PIOPED II - if good quality negative CTPA in an experienced centre, representation with thromboembolism is 1 – 2% at 3 months. In high risk patients, closer to 5TTE%. (J Thromb Haemost 2009).
Not useful for diagnosis of PE.
Elevated in 30 – 50% with moderate/large PE.
Presumably from acute RV strain/overload.
Associated with poorer prognosis.
Degradation product of cross-linked fibrin.
Detected in serum (ELISA or agglutination assay).
Multitude of causes of raised D-dimer other than PE.
Good negative predicative value – increased further if use clinical pre-test probability (e.g. Wells).
Poor specificity and positive predictive value.
Main role is to exclude PE if low pre-test probability and negative D-dimer.
No use in the critically ill population as elevated in elderly, post-op, infection, trauma.
Examiners' comments: Candidates did not answer the question as asked.
It was not a "compare and contrast" question, but judging by the examiner's comments it was treated as one by many candidates. However, the college answer to this question still discusses the advantages and disadvantages of these investigations. So, here is a "compare and contrast" sort of table, which incorporates both the model college answer and the 2014 ESC guidelines.
|Test||Rationale and advantages||Limitations and disadvantages|
|History and clinical examination||
|ECG features of RV strain||
|Lung scintigraphy (V/Q scan)||
Oh's Intensive Care manual: Chapter 34 (pp. 392) Pulmonary embolism by Andrew R Davies and David V Pilcher
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