Stroke Risk in Atrial Fibrillation. Is there a Missing Piece? (QF_FR_086)

A 58-year-old male presented in September 2015 for palpitations. The patient was diagnosed with paroxysmal atrial fibrillation (AF) which was confirmed with an event monitor. The patient underwent examinations which ruled out any intracranial abnormality. Echocardiography revealed normal left atrium (LA) size and E/A ratio of 0.84. He was discharged without anticoagulation given a CHA₂DS₂-VASc score of 0.

In January 2016, the patient presented to the emergency department for an episode of transient ischemic attack thought to be embolic. We present this case to highlight the need for improved stroke risk stratification in AF; namely, the potential for 4D flow MRI in providing personalized information on changes in LA and left atrial appendage (LAA) blood flow dynamics implicated in risk for cardioembolic events.

Diagnostic Techniques and Their Most Important Findings:

This patient’s low CHA₂DS₂-VASc score and E/A ratio at baseline indicated low stroke risk category^1-3. CHA₂DS₂-VASc is the main clinical tool used for stroke risk assessment but has limited predictive value for therapy management (c-statistic 0.6-0.7). This is because the score is based on upstream clinical factors rather than the physiologic mechanisms, such LA and LAA blood flow changes that cause thrombus formation^4-5. This is also suggested by the example in figure 1 of two AF patients, other than the patient presented in this case, with identical CHA2DS2-VASc score of 0 but substantially different LA and LAA blood flow. Thus, a deeper understanding of underlying physiologic factors leading to stroke is crucial.

4D flow MRI allows the comprehensive qualitative and quantitative volumetric in-vivo blood flow assessment of the LA and the LAA over the cardiac cycle^6-7. 4D flow MRI assessment of AF patients has found reduced peak velocities and increased blood stasis in the LA and LAA, indicating AF-associated atrial flow impairment that is thought to be associated with increased thromboembolism risk⁸.

In this case, 4D flow MRI could have provided the attending physician with insights of underlying hemodynamic derangements that favor thrombus formation prompting anticoagulant therapy.

In 2020, this patient underwent CMR, including 4D flow MRI, which showed substantially increased LA volume (83mL vs. 33mL) compared to echo findings in 2015. 4D flow analysis (figure 2) demonstrated reduced peak velocities and higher blood flow stasis compared to controls⁸ favoring thrombus formation. It has been suggested that the atrial myopathy due to AF could start even before the diagnosis of AF⁹. This implies that physiologic blood flow derangements could well precede structural changes (LA volume) at the time of diagnosis.

Learning Points from this Case:

This case illustrates the potential of 4D flow MRI for improved stroke risk stratification in AF. It is a non-invasive tool that provides useful physiologic information allowing physicians to make informed therapeutic decisions.

The modest predictive value of the clinical CHA₂DS₂-VASc stoke risk score may be due, in part, to its sole reliance on clinical factors and not physiological ones.