Emerging Technologies for the Electrophysiology Lab

New technologies are emerging to integrate the 3D volume information obtained with multi-detector computed tomography (MDCT) scanning to the electrophysiology lab. These offer a major improvement in the ability of the electrophysiologist to accurately and safely target and treat atrial fibrillation. Atrial fibrillation is the most common cause of sustained cardiac arrhythmia and a major cause of stroke.¹ The pharmacologic treatment of atrial fibrillation has significant iatrogenic morbidity. It is associated with the costs of long-term medical treatment, including lifetime monitoring of clotting factors (INR), hospital admissions for bleeding complications, as well other needed therapy while on long-term anticoagulates. Pioneering work by Haissaguerre and colleagues^2,3 has documented that the vast majority of triggering foci for atrial fibrillation occur in the pulmonary veins, and radiofrequency ablation techniques have produced consistently high cure rates. The addition of pre-procedure cross-sectional imaging to identify important anatomic structures offers the opportunity to improve the safety and speed of the ablation procedure. This new concept in electrophysiology (EP) labs combines the latest MDCT and proven EP and cath interventional lab technologies with a number of innovative instruments to help make them more convenient and efficient, thereby creating a more intuitive EP lab working environment and integrating data management across the EP care cycle. CT Pulmonary Venography Recent advances in non-invasive imaging modalities like MDCT have facilitated the availability of clinical knowledge earlier in the care cycle, thereby providing opportunities of creating new navigation capabilities and fusion of morphological and physiological information. Primarily a diagnostic modality, CT can also play a vital role in the therapy planning portion of the care cycle. While most of the recent literature using MDCT has focused on the coronary anatomy, MDCT can also be useful in guiding AF ablation by providing a pre-procedural assessment of the pulmonary vein anatomy. First, CT pulmonary venography enables assessment of the left atrium and the location and anatomy of the pulmonary veins, including congenital anatomic variants. Additionally, mass effects from adjacent structures, such as the aorta and esophagus, can be assessed on the CT scan. Second, CT venography can be used for both pre-procedure planning and direct guidance of the procedure. CT data can be transmitted to the electrophysiology lab and used to construct 3D modeling. Finally, CT venography is useful for assessment of both immediate and delayed complications. The protocol for CT pulmonary venography is similar to that used for CT coronary angiography, with a few key changes. The scan can be performed at a lower radiation dose because thicker slices than those used for coronary angiography are typically used to assess the coronary venous anatomy. Further reduction of dose can be achieved by using prospectively-gated axial acquisitions (Step & Shoot Cardiac, Philips Medical Systems, Cleveland, Ohio), which can reduce the scan dose by up to 80% compared to a spiral retrospective scan. In our institution, scanning is performed in a caudocranial direction (cardiac apex to cardiac base) to minimize any artifact from residual contrast in the superior vena cava. To obtain the most reliable contrast enhancement in these patients, who have variable cardiac function, we routinely use bolus tracking and position the trigger point within the left atrium to initiate scanning. As with coronary computed tomography angiogram (coronary CTA), all of our scans are performed with a dual injector, a saline chaser and dose-reduction software, (Cardiac DoseRight, Philips Medical Systems, Cleveland, Ohio) and they are all reconstructed at a standard 75% window in the R-to-R interval. Although there may be more gating artifacts in the patient population than the population on whom coronary CTA is typically performed, these gating artifacts have not been problematic. This is likely due to a combination of the increased slice thickness, which minimizes artifact, as well as beta-blockade, which yields a lower heart rate and less gating artifacts, even with patients who are not in normal sinus rhythm at the time of the examination. Anatomical Variants Performing pre-procedural, non-invasive imaging provides critical information to manage morphological differences. Although most patients have 4 pulmonary veins, the most common variant is a fifth branch of the pulmonary vein arising from the right middle lobe. Other congenital variants include ostial veins, which are branches off the V1 segment within 5 mm of the ostium, and accessory pulmonary veins, which are additional branches terminating directly into the left atrium. Less commonly, patients present with a common pulmonary trunk on 1 or both sides. In our laboratory, we have also seen 2 cases of partial anomalous pulmonary venous return in the past 3 years. There have also been reports from the literature of absence of the inferior pulmonary vein.⁴ A variety of structures can indent the posterior wall of the left atrium, including the aorta and esophagus.⁵ Given the reports in the literature of fistulization⁶ between the left atrium and esophagus as a result of catheter ablation procedures, we carefully evaluate the location of the esophagus prior to it. Viewing the CT Data Advanced visualization techniques, like the automated whole heart segmentation are now available to augment traditional CT visualization tools, especially in the presence of anatomical variations. Our center offers an identical solution for both the radiology department with EP Planning software on the Extended Brilliance Workspace (Philips Medical Systems, Cleveland, Ohio) and the cardiology suite/EP lab as EP Navigator (Philips Medical Systems, Cleveland, Ohio). The CT data is visualized using a dedicated workstation (Extended Brilliance Workspace, Philips Medical Systems, Cleveland, Ohio). These reconstructions include thick-slab maximum intensity projection (MIP) reconstructions along the long axis of the V1 segments of the pulmonary veins and along the axial and coronal planes; 3D surface reconstructions of the posterior wall of the atrium with and without the esophagus and descending aorta; and CT venoscopy of the posterior wall of the left atrium and the ostia visualized from an intra-atrial perspective. In addition to the standard reconstructions, we also perform a single thick-slab MIP reconstruction along the long axis of the combined ostia on the left and right side of the atrium to measure the span of the combined ostial openings. Integrated EP lab Using the same automated, whole heart segmentation, this time on the EP Navigator (Philips Medical Systems, Best, the Netherlands), we are able to segment and color code the heart structures, including segmentation of the left atrium and distal pulmonary veins. This modeling provides a link to the superimposition of the segmented model over the live fluoroscopy in the electrophysiology lab, which is one of the latest tools to facilitate catheter navigation during ablation procedures. This registration and overlay allows for much more accurate positioning of the catheters with a minimum of fluoroscopy and contrast administration and gives the operator a live 3D CT image of the left atrial anatomy superimposed on 2D fluoroscopy. Confirmation of the correct positioning of the ablation catheters can easily be performed using the overlaid segmented model. A variety of complications have been reported from radiofrequency ablation procedures.^7-9 These include perforation into adjacent structures, such as the esophagus, pericardium, pleural space, embolic strokes, edema of the left atrium or pulmonary vein and, rarely, coronary occlusion. While many of these complications are immediate and are visible in the electrophysiology laboratory, these are also easily diagnosed using contrast-enhanced CT scanning. Because the radiologists review all of the pre-procedure CT scans, we have noted a variety of incidental but clinically significant pathologies that required further evaluation, including a small lung cancer, as well as a patient with clinically occult active tuberculosis. Lastly, patients can present with stenosis of the pulmonary veins as a late complication of radiofrequency ablation, and this finding is also easily assessed when comparing the pre- and delayed post-procedure CT scans. Electrophysiologists carefully evaluate pre-procedural images together with a radiologist on a routine basis and appreciate the input from the radiology department. Close collaboration between the departments of radiology and cardiology at our institution has made our CT pulmonary venography program highly successful. Philips Medical Systems offers the flexibility of realizing these emerging technologies. Reprinted with permission from Cath Lab Digest 2007;10(15 Suppl):16-19.