The Direction and Magnitude of Cardiac Contractile Motion: Considerations for Stereotactic Radio-Ablation of Ventricular Tachycardia

Introduction: Stereotactic ablative radiotherapy is an emerging therapy for ventricular tachycardia (VT), but it is unknown whether its precision may be limited by cardiac motion from the beating heart. Hypothesis: The magnitude of cardiac contractile motion may vary between patients and across different directions in 3D. Methods: Patients who underwent 4D cardiac computed tomography (4DCT) prior to a catheter ablation procedure for atrial or ventricular arrhythmias at 2 medical centers were studied retrospectively. 4DCT was acquired using cardiac-gated Siemens Force or GE Revolution scanners. The displacement of transvenous right atrial (RA) and right ventricle (RV) ICD lead tips across the cardiac cycle (divided into 10 time phases) were measured in orthogonal 3D views on a maximal-intensity projection CT reconstruction (Fig 1) using imaging analysis software (Horos Project). Displacement was compared using 1-way ANOVA. Results: In 14 patients (age 65±11.7 years), contractile motion along the left-right axis, craniocaudal axis, and anteroposterior axis were significantly different for both the RV (0.7±0.4, 0.4±0.2, 0.4±0.2cm, p=0.017) and RA (0.7±0.5, 0.4±0.2, 1.1±0.7cm, p=0.008). The greatest variability between patients in the displacement of the RV lead tip was along the left-right axis (range 0.16cm to 1.52cm) and of the RA lead tip was along the anteroposterior axis (0.2cm to 2.3cm). Clinical characteristics (i.e. ejection fraction) were not associated with the magnitude of motion. Conclusions: Contractile motion was greatest in the left-right direction in the ventricle (up to 1.52cm), but there was a large variation in motion between patients (ranging up to 1.4cm). The magnitude of contractile motion should be assessed on a patient-specific basis. Further studies are underway to develop optimal strategies to account for contractile motion, such as patient-tailored planned target volume expansions and cardiac-gated radiotherapy delivery.