Cardiac CT: Looking Beyond the Coronaries
CRT & LVADsUp to 30 percent of patients who receive a cardiac resynchronization therapy (CRT) device do not respond appropriately. Researchers have been trying to determine anatomical and/or physiological characteristics that are predictive of device failure and CT imaging could play a prominent role.
Uebleis et el used PET/CT to determine that lead placement in relation to scar tissue was the best predictor of device success. The next step, they said at the 2010 SNM meeting, is to evaluate patients prior to therapy to determine the optimal location for lead placement.
Quynh A. Truong, MD, a cardiologist specializing in CT imaging at Massachusetts General Hospital (MGH) in Boston, and colleagues found that CT-derived changes in left ventricular (LV) wall thickness correlated best with the extent of dyssynchrony seen in heart failure patients with wide and narrow QRS, followed by LV wall motion and then LV volume (J Am Coll Cardiol Img 2008;1:772-781).
They have since begun a trial to determine which metric correlates best with clinical outcomes for device therapy. “The ideal CT exam would be done before implantation and would assess the pulmonary veins, evaluate the ventricles for dyssynchrony to target the area that is most delayed and identify scar tissue to help facilitate lead placement,” says Truong.
CT also can help assess the position and orientation of left ventricular assist devices (LVADs) after implantation, says Wm. Guy Weigold, MD, director of cardiac CT at the Washington Hospital Center (WHC) in Washington, D.C. “Patients can experience problems as to how the device is situated in the heart and CT can help us determine if it is perhaps rubbing against the LV wall or if there is a flow obstruction.”
In patients with longer-term LVADs who are going onto a heart transplant, CT can reveal whether there are adherent structures at risk for trauma when opening the chest. “CT allows us to look at the devices moving in real time during the cardiac cycle and detect evidence of adhesions,” he says.
Atrial fibrillationPatients scheduled for atrial fibrillation (AF) ablation at WHC undergo either CT or MRI prior to the procedure to ascertain variances in pulmonary veins and related structural anatomy, Weigold says. Having the option to use either modality allows referrers and imagers to choose the appropriate technique based on patient characteristics. Patients with devices that are not MRI compatible can undergo CT, while those with compromised kidney function who are unable to tolerate iodinated contrast media can undergo MRI. Recently, however, the risk of nephrogenic systemic fibrosis from gadolinium contrast media puts imagers on alert for utilizing MRI in certain patients with severely compromised kidney function.
The use of CT to image the pulmonary veins is quite mature, Weigold offers. In fact, most coronary artery postprocessing programs have an EP package for pulmonary vein analysis. Pre-procedural CT imaging can reveal variances in the pulmonary veins, as well as in the atrial septum, which would inform left atrium access. EPs can then integrate these images with electrical signal mapping and fluoroscopy images, giving them a 3D view of the heart during ablation.
Using CT prior to AF ablation typically includes viewing 3D volumetric images, in contrast to coronary CT, which often utilizes axial and curved reformatted images, says Jeffrey M. Schussler, MD, an interventional cardiologist at Baylor University Medical Center in Dallas. “The relatively larger pulmonary veins are beautifully displayed in 3D volumetric images, giving electrophysiologists an excellent overview of their relationship to other cardiac structures.”
AF ablation patients can sometimes present postoperatively with respiratory symptoms that frequently mimic more common diseases. “The most frequent cause, however, is pulmonary vein stenosis and CT is an excellent test to detect or exclude stenosis,” Schussler says. “While 3D volumetric images provide an overview of the pulmonary anatomy, it is the axial and curved reformatted views that are generally used to measure the degree of stenosis and the lesion size.”
Left atrial appendagePromising preliminary data suggest that CT imaging can be useful to measure the left atrial appendage (LAA) for placement of an occlusion device, such as the Watchman (Atritech, being acquired by Boston Scientific), which is used to help prevent stroke in patients with AF, says Suhny Abbara, MD, director of the cardiovascular imaging section at MGH. CT also has shown promise in detecting leaks post-device placement, he says.
A study presented at the 2010 Society of Cardiovascular Computed Tomography (SCCT) conference by Passen et al found CT sizing of the LAA orifice was significantly more accurate than that of transesophageal echo (TEE). Researchers said the elliptical geometry of the LAA results in underestimation by TEE and that CT imaging might improve the success rate and decrease the complication rate associated with occluder placement.
Typically, AF ablation patients undergo a TEE exam just prior to the procedure to look for LAA thrombus. Several presentations at the 2010 SCCT meeting explored this theme. Marques et al concluded that “cardiac CT is able to provide all imaging related information needed to exclude left atrial thrombus, precluding the need of TEE.” Furthermore, of the 130 cases, 11 percent had inhomogeneous contrast enhancement and required a second scan, but researchers performed it without more contrast administration and within five minutes of the first acquisition. All cases were “confidently assessed.”
Wilson et al in 145 patients found an LAA-ascending aorta attenuation ratio of less than 0.75 provided a sensitivity for thrombus detection comparable to qualitative visual inspection. Stone et al described a case where TEE was unable to sufficiently rule out thrombus in a 47-year-old man prior to AF ablation. Researchers suspected the mobile, echo-bright structure on TEE was an imaging artifact, given the patient’s adequate anticoagulation. Their suspicion was confirmed by late-phase CT imaging, which showed no thrombus.
“The results of these small studies are intriguing, but the field is not moving just yet to replace TEE with CT,” says Abbara.
Lead perforationWhile lead perforations are considered rare complications (between 0.1 and 0.8 percent for pacemakers and 0.6 and 5.2 percent for defibrillators) and rarely have dramatic consequences, they can cause pericardial tamponade, which can lead to death (Pacing Clin Electrophysiol 2002;25:1155-1158). In fact, Seegers et al said that lead perforation “should be considered as one of the potential mechanisms responsible for the high rate of sudden death observed in paced patients during the first year after pacemaker implantation” (Clin Res Cardiol 2009;98:517–520).
Numerous case studies of symptomatic patients attest to the efficacy of CT alone or in combination with echocardiography to confirm lead perforation. But asymptomatic patients also can have perforations. Hirschl et al with the aid of CT found that lead perforation was more common than expected (15 percent of 100 patients) in asymptomatic patients with implantable devices (Pacing Clin Electrophysiol 2007;30:28-32). They found that atrial leads perforated more frequently than ventricular leads, and ventricular implantable cardioverter-defibrillator (ICD) leads perforated more frequently than ventricular pacemaker leads.
Most lead perforations are acute, occurring within five to seven days after implantation, but a review of the literature by Venkata et al identified 38 patients with delayed (beyond 30 days) lead perforation (Cardiol Res Prac 2010;online July 25). They noted that the increasing use of cardiac rhythm management devices, especially in the elderly, warrants attention to the lifelong risk of complications, even years after implantation.
While CT also has found a niche in pre-procedural evaluation of atrial septal defects and aortic and mitral valve repairs and replacements, the next big hurdle, according to Weigold, is cardiac function assessment, in particular, to identify inducible ischemia. “CT perfusion already is being performed in other organs and it is only a matter of time before stress CT will be used routinely to assess cardiac function.”