Departments of Radiology and Cardiology, St. Luke's Episcopal Hospital, Houston, TX, USA
Department of Radiology, Baylor College of Medicine, Houston, TX, USA
* Correspondence to: Scott D. Flamm, M.D., Department of Radiology, MC 2-270, St. Luke's Episcopal Hospital, 6720 Bertner Avenue, Houston, TX 77030, USA. Tel.: +1-832-355-4201; fax: +1-832-355-2591
E-mail address: sflamm{at}sleh.com
This editorial refers to "Safety and feasibility of high-dose dobutamine-atropine stress cardiovascular magnetic resonance for diagnosis of myocardial ischaemia: experience in 1000 consecutive cases"1 by A. Wahl et al. on page 1230
Cardiovascular magnetic resonance imaging (CMRI) has had particularly rapid advances in the evaluation of ischaemic heart disease over the last five years. The introduction of steady-state free precession sequences has improved both spatial and temporal resolution in cine imaging for evaluation of global and regional left ventricular function,1 the advent of delayed-enhancement imaging has thrust CMRI into the forefront of myocardial viability assessment,2 and on-going advances on multiple fronts are bringing magnetic resonance angiography of the coronary arteries for stenosis detection closer to clinical reality.3 Nonetheless, it is perhaps in the area of pharmacological stress ischaemia evaluation that CMRI may show the broadest clinical utility and greatest promise.
In this edition of the European Heart Journal, Wahl et al.4 report a five-year experience in performing high-dose dobutamine (Dob) stress CMRI examinations for determination of myocardial ischaemia. This group of investigators brings extensive experience to this arena, having been the first to report a large study of Dob-CMRI with a head-to-head comparison with Dob-echocardiography.5 In the current study, Wahl et al. performed 1000 Dob-CMRI studies in 960 patients from a clinically referred pool of 1075 potential patient studies. Patients were not excluded from analysis for lack of imaging as a result of claustrophobia or obesity well-acknowledged limitations of MRI technology in clinical evaluations. Of the 1000 studies performed 10.5% were non-diagnostic: 1% for either poor ECG-triggering or motion artefacts, and 9.5% for failure to reach target heart rate (with limiting side-effects more frequent than pharmacologic failure). These figures are parallel to those in the echocardiography literature where similar percentages of patients are excluded from imaging as a result of inadequate acoustic windows, and, among those imaged, comparable percentages of patients have non-diagnostic studies. These investigators also demonstrated an impressive safety profile with only one patient suffering sustained ventricular tachycardia requiring emergent defibrillation, and no cases of death or myocardial infarction.
This study builds on the initial publication of Nagel et al.,5 where the diagnostic accuracy of Dob-CMRI was characterised with quantitative coronary angiography as the gold standard, and its relative accuracy determined in a head-to-head comparison with Dob-echocardiography. The results indicated that Dob-CMRI had clinically useful accuracy for detection of myocardial ischaemia, as well as superiority to Dob-echocardiography. The exclusion rate for both Dob-CMRI and Dob-echocardiography was equivalent, although Dob-echocardiography had a higher incidence of non-diagnostic images, while patients were excluded from Dob-CMRI more frequently for claustrophobia or obesity precluding imaging in the MRI scanner.
Despite these notable results, the study of Nagel et al. has been criticised as a result of Dob-echocardiography performing less well than recent literature would anticipate. However, this should not necessarily be unexpected as there is a predictable history of declining accuracy for established techniques in the face of technological improvements. For example, the percentage of non-diagnostic studies in the echocardiography literature has risen over the years providing the underpinnings for improved methodological implementations such as second harmonic imaging, and more recently intravenous contrast. It should not be surprising then that Dob-echocardiography results may become more translucent as CMRI clarifies what can be seen in wall motion imaging studies.
Following quickly on the publication of Nagel et al., Hundley et al.6 performed complimentary work in patients with non-diagnostic Dob-echocardiograms despite the use of second harmonic imaging. In these studies by Nagel et al. and Hundley et al., image quality of Dob-CMRI was superior to Dob-echocardiography, and in Hundley's work patients with inadequate acoustic windows and without contraindications to MRI had newfound options for stress wall motion assessment of ischaemia.
While Nagel et al. demonstrated that Dob-CMRI was superior to Dob-echocardiography overall, this was most striking in those patients with moderate image quality (as judged by the performing echocardiographers). It is important to note that only slightly more than half of patients had echocardiographic image quality judged to be good or better, while 40% were of moderate image quality, and 8% were non-diagnostic. In those patients with moderate image quality, overall accuracy was only marginally better than chance. Conversely, with Dob-CMRI, over 80% of patients had image quality judged to be good to very good, while 16% were of moderate image quality, and only 2% were non-diagnostic. Furthermore, there was no significant difference in diagnostic accuracy between the moderate image quality and good to very good image quality groups.7
The work of Nagel et al. and Hundley et al. reasonably suggest that Dob-CMRI is the preferred technique for stress wall motion analysis in those patients amenable to CMRI. And in those patients with only moderate image quality Dob-CMRI may well be the compulsory imaging technique, where available.
Dob-CMRI is clearly a potentially important clinical tool for ischaemia detection, but where does it fit within the ischaemic "toolbox" of techniques that CMRI offers? At present the direct identification of coronary artery stenoses with coronary magnetic resonance angiography has not yet developed into a clinically robust technique for routine use, although stress perfusion-CMRI has been making claims for such a role. The current work by Wahl et al. demonstrates diagnostic accuracy for Dob-CMRI that is at least comparable to recent published reports on stress perfusion-CMRI. However, it is notably different in one critical aspect compared to results reported for stress perfusion-CMRI studies namely that Wahl et al. performed their study in an unselected patient population. A significant percentage had prior interventions, including angioplasty/stenting and/or coronary artery bypass grafting, as well as a significant percentage of prior myocardial infarctions. Alternatively, recently published work in stress perfusion-CMRI has excluded patients with prior myocardial infarction,8 hypertension, hypercholesterolaemia, and diabetes.9 Thus, results in such highly selected patient populations raise concern for extrapolation to unselected patients. This factor alone suggests a likely superiority of Dob-CMRI over stress perfusion-CMRI techniques once studies in comparable patient populations have been performed.
Even so, performance of stress perfusion-CMRI is more common than Dob-CMRI, and only a few centres (in both Europe and the US) routinely perform Dob-CMRI. The rationale for this is uncertain although, as Wahl et al., note it may reflect a general trepidation for performance of Dob stress testing in the MRI environment. Certainly physiological electrocardiographic monitoring is less capable in the CMRI scanner, though rate and rhythm and blood pressure monitoring are equivalent to that available outside the CMRI scanner. Nagel et al., 10 make the necessity of available emergency equipment and trained personnel for the performance of such studies clear, although their results indicate a safety profile similar to that of Dob-echocardiography despite the potentially more problematic imaging environment. The lack of centres performing Dob-CMRI may also reflect structural and/or philosophical differences between CMRI centres that are run primarily by cardiologists and those run by radiologists. However, the increasing number of CMRI centres that are primarily cardiology-driven have not correspondingly increased the numbers of those performing Dob-CMRI studies. Thus, there may be other factors at play.
Dob-CMRI's time has certainly come, but it is not at all certain that the CMRI community is keenly altering the restricted penetrance among CMRI clinicians. The experience of Wahl et al., should serve as robust evidence of the clinical feasibility of Dob-CMRI in CMRI centres with appropriately trained and capable providers, and a strong impetus to furthering the technique as a routine component of CMRI ischaemic evaluations. Greater adoption of Dob-CMRI at high clinical volume CMRI centres, however, is critical for its continuing use in, and support by, clinical cardiac care providers.
Footnotes
1 doi:10.1016/j.ehj.2003.11.018
References
Related articles in EHJ:
|