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Paradoxical Embolism

The pulmonary circulation provides a large filter for any thrombi returning from the systemic circulation. A small microthrombus to the lungs is probably insignificant but if the pulmonary circulation can be bypassed and shunted then there is a risk of thrombi entering the systemic arterial circulation. This can happen when there are areas allowing right to left shunting in the heart such as a patent foramen ovale or Atrial septal defect or Ventricular septal defect or other form of congenital heart disease. However clots bypassing the lungs can be seen in those with pulmonary arteriovenous malformations.

To reliably diagnose paradoxical embolism I would suggest you need to try to prove

  • A stroke that is Radiologically cardioembolic - i.e. large artery and perhaps multiple territories
  • Evidence of a DVT at the time - Doppler USS or at least a raised Dimer
  • Evidence of a shunt with Right to left flow
  • It helps if there is an underlying venous procoagulant state e.g. malignancy, pregnancy etc.

Management is difficult. First of all any PFO which is seen in 25% of the population may simply be a bystander and association does not mean causation. Closure of PFOs is not risk free and may even increase long term risk of (P)AF and so limits any reduction, cancels or even increases any risks of stroke. Unfortunately the interventionist have not proven their case. All those having PFO closure should be done as part of a proper clinical trial.

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