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    Interventional procedure overview of percutaneous thrombectomy for massive pulmonary embolism

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    Outcome measures

    The main outcome measures included right ventricular (RV)/left ventricular (LV) ratio, pulmonary artery pressure, modified Miller score, cardiac index, RV systolic pressure, and CT obstruction index. The measures used are detailed in the following paragraphs.

    RV/LV ratio

    RV/LV ratio can be measured by CT, echocardiography or as a composite measure. A meta-analysis found that, across all CT parameters reviewed as potential predictors of outcome in acute PE, RV/LV diameter ratio had the strongest predictive value for adverse clinical outcomes and mortality (Meinel 2015). It has also been used in multiple studies of various PE treatments as a marker of treatment effectiveness. For comparison, a meta-analysis of CDT reported a mean reduction of RV/LV ratio of 0.34 (Bloomer 2017).

    Pulmonary artery pressure

    Usually reported as mean pulmonary artery pressure (mPAP), this outcome measure is used by multiple studies of the effectiveness of treatment in acute PE. Echocardiography is generally used to measure systolic pulmonary artery pressure and raised mPAP is a key feature of acute PE. Pre- and post-procedure mPAP is commonly reported, and some studies include later follow-up of this outcome measure.

    Modified Miller score

    The modified Miller score is a measure of thrombus burden according to CT pulmonary angiogram (CTPA) imaging which is used in acute PE. The extent of thrombus in each part of the pulmonary arteries is scored from 0 (none) to total occlusion (3), out of a maximum of 16. There is also a refined modified Miller score which has a maximum score of 40.

    CT obstruction index

    The CT obstruction index is another scoring system for thrombus burden on CT imaging (Qanadli 2001). The system assigns a heavier weighting to full vessel occlusion than to partial occlusion. However, there have been conflicting results on the ability of this scoring system to predict mortality in acute PE(Vedovati 2013).

    Cardiac index

    The cardiac index is a haemodynamic parameter that is a measure of cardiac function. Specifically it is a measure of cardiac output that normalises the cardiac output value according to body size. The normal range for this measure is 2.6 to4.2 L/min/m2. The equation for calculating this measure is:

    Cardiac index = cardiac output/body surface area = (stroke volume x heart rate)/body surface area

    Major adverse event (MAE) rate

    The MAE rate is a composite measure used in studies to detail the rate of MAE, the components of which vary by study.