Physicians have yet to find a simple, inexpensive test to screen for the risk of sudden cardiac death. But they are learning how cardiac MR imaging may help stratify risk and guide treatment for conditions that can strike without warning.
Research reported in the opening session of the 2006 Society for Cardiovascular Magnetic Resonance meeting in Miami is showing that CMR is relevant to diagnosing and characterizing the causes of sudden cardiac death. The role of delayed-enhancement MR for predicting the likely success of revascularization is well established. Now ejection fraction measured with functional CMR and evaluations of myocardial scar with DE-MRI may improve the selection of candidates for expensive implantable cardioverter defibrillators.
Dr. Nathaniel Reichek of St. Francis Hospital in Roslyn, NY, has shown that CMR produces more measures of ejection fraction, volume, and masses than echocardiography, the first-line instrument of choice for many heart conditions that cause sudden mortality. CMR's greater precision leads to better prediction of inducibility for ventricular tachyarrhythmias and early arrhythmic events that indicate ICD implantation, said Dr. Katherine C. Wu, an assistant professor of medicine at Johns Hopkins University.
Dr. Raymond Kwong of Brigham and Women's Hospital reported that delayed enhancement of the inward borders of the myocardium and left ventricular ejection fraction are powerful, complementary predictors of sudden death. His findings were based on a 10-month follow-up study of 136 patients with previous myocardial infarction.
Wu's group is zeroing in on the relationship between delayed-enhancement signal intensity and ventricular arrhythmia risk. She discovered that signal intensity falls when necrotic and viable tissues are intermingled. In a study of 47 patients, intermediate signal intensities, indicating a heterogeneous mix, generated the most powerful marker for inducibility.
CORRELATING RISK FACTORS
CMR is playing a more important role in the characterization of hypertropic cardiomyopathy, another leading cause of sudden death, said Dr. Sanjay K. Prasad, a consultant cardiologist at Royal Brompton Hospital in London. In 2005, Prasad's colleague Dr. James Moon established a relationship between the percentage of delayed enhancement and myocardial wall thickness. The combination correlated directly with the presence of two or more other risk factors for sudden death.
The Royal Brompton group is also making connections between widespread patchy fibrosis detected with DE-MR and death risk. A clinical trial the group presented at the SCMR meeting indicates that evidence of myocardial midwall fibrosis observed with delayed-enhancement CMR is better than ejection fraction or ventricular volume measures for predicting future death and hospitalization from cardiovascular disease.
Findings were drawn from tracking death and hospitalization of 101 consecutive patients with dilated cardiomyopathy. Consistent with previous reported studies, DE-CMR of 30% of the patients revealed a midwall pattern of circumferential fiber, according to principal investigator Dr. Ravi Assomull. Patients with midwall fibrosis had significantly larger end-diastolic and end-systolic volume and lower left and right ejection fractions than the 70% of patients without evidence of fibrosis. Follow-up outcome analysis showed fibrotic patients were significantly more likely to die or be hospitalized because of their disease than patients without fibrosis.
The results add to growing evidence about the role of fibrosis as a major cause of death and morbidity from dilated myocardiopathy, said Dr. Dudley Pennell, director of cardiac MRI at Royal Brompton.
"This supports an overarching hypothesis that fibrosis, especially homogeneously distributed fibrosis, is a predictor of adverse remodeling and arrhythmia, which are the two things that produce all the other events," he said.
Researchers at the Robert Bosch Medical Center in Stuttgart, Germany, have uncovered a relationship between hyperenhancement and the viral causes of infiltrative cardiomyopathy and myocarditis. From DE-MR and biventricular biopsies performed on 134 patients, Dr. Heiko Mahrholdt concluded that intralateral wall enhancement is usually associated with parovirus B19 infection. Herpesvirus 6 is present mainly when hyperenhancement is localized in the septum. Coinfection with both viruses is also observed with enhancement in both regions. Serial imaging found that contrast enhancement in patients infected with parovirus B19 disappears over time, said Dr. Claudia Deliuigi, who substituted for Mahrholdt at the meeting.
ALTERNATIVE TO ECHO
Although echocardiography is usually the first choice for examining young athletes with suspected anomalous coronary arteries, cardiac MR is diagnostically more sensitive, albeit more expensive than echo, said Dr. Christina Basso, an associate professor of cardiovascular pathology at the University of Padua in Italy.
Anomalous coronaries lie between the aorta and pulmonary arteries, placing them at risk of compression when these vessels expand during exercise. If such compression occurs, it may lead to occlusion, ischemia, and sudden death.
About 20% of sudden deaths in young adults are attributed to arrhythmogenic right ventricular dysplasia (ARVD), a genetically linked cardiomyopathy characterized by fibrofatty infiltration of the right ventricular myocardium. Cine MR is well suited for detecting fatty and fibrotic deposits indicative of ARVD and identifying characteristic hemodynamic abnormalities in the diaphragmatic, apical, and infundibular regions, Basso said. Family members of newly diagnosed cases are frequently screened with CMR in Italy to prevent sudden deaths.
