European Heart Journal Advance Access published online on June 6, 2007
European Heart Journal, doi:10.1093/eurheartj/ehm166
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Straining to detect ischaemia
Department of Medicine, University of Queensland, Princess Alexandra Hospital, Ipswich Road, Brisbane, Queensland 4102, Australia
Corresponding author. Tel: +61 7 3240 5340; fax: +61 7 3240 5399. E-mail address: t.marwick{at}uq.edu.au
While stress echocardiography is a ‘mainstream’ test for the diagnosis and prognostic evaluation of coronary artery disease, the limitations of qualitative assessment of wall motion (WM) scoring are widely recognized.1 Strain rate imaging might provide a quantitative strategy that could reduce the need for expert interpretation and improve concordance between observers.
The validation of functional testing has traditionally been performed against stenosis severity at coronary angiography. Among many problems with this approach, the difficulty in defining physiologic significance from stenosis severity is perhaps the greatest. Stenoses >75% severity are usually flow-limiting under conditions of maximal hyperaemia, while those involving <45% of lumen diameter are rarely flow-limiting.2 Limited comparisons have been made between WM assessment and coronary physiology rather than stenosis severity.3 Existing studies of the accuracy of deformation imaging in combination with stress echocardiography have followed a traditional angiographic comparison,4,5 but the paper of Weidemann et al.6 compares tissue deformation results against the distinction of significant vs. non-significant coronary stenosis, based on fractional flow reserve (FFR). The investigators studied 30 patients with intermediate coronary stenosis and defined physiologically significant stenosis as a FFR <0.75 after intracoronary adenosine. Following stress, strain rate was the most reliable predictor of stenosis severity.
One of the challenges of applying strain-rate imaging for clinical decision-making relates to the selection of which of a number of potential parameters should be measured. Tissue velocity and strain give important information about tissue properties,7,8 but strain measurements do not account for the time taken to achieve deformation, and corresponds to regional ejection fraction. As LV volumes fall during dobutamine stress, strain reaches a plateau or may even fall, and this can compromise the detection of ischaemia or contractile reserve.9 However, the morphology of the strain curve may give useful clues about the temporal aspects of contraction. Post-systolic shortening (PSS) is a characteristic of ischaemic myocardium, identifiable on the strain curve as continuing thickening after the electrocardiographic T-wave, or other more sophisticated markers of end-systole.10 While animal models involving arterial occlusion and studies performed during angioplasty have proposed this parameter to be the most accurate marker of ischaemia11,12 and some human work has supported this finding;4 these findings are not uniform. The failure of PSS to exceed the accuracy of WM is an important negative result that corresponds with the experience of other groups.5 On the other hand, strain rate accounts for speed of contraction, and is an analogue of regional dP/dt, a marker of contractility that is reduced by ischaemia. This normally increases throughout dobutamine stress and has been shown in this and previous studies to be the most accurate marker of ischaemia. The optimal cut off for strain rate that gave the highest sensitivity and specificity for the definition of significant stenoses was an increment of <0.6 per second. The definition of this cut-point is a very important finding of this current study.
Strain rate imaging is not a useful or necessary adjunct to WM assessment in all subjects. Experience in the interpretation of stress echocardiography teaches that probably three quarters of studies are clearly normal (with no significant coronary disease) or clearly abnormal (usually with significant disease, often in multiple vessels). Patients without disease or with multiple occluded vessels are easy to recognize, evidenced by high levels of concordance between reviewers.1 At the extremes of flow reserve, the additional information provided by new modalities is probably superfluous. Deformation analysis has most to offer in patients with intermediate lesion severity, where WM changes are subtle and may be missed. In this respect, the authors' selection of patients with intermediate severity lesions reflects a highly appropriate study group to understand the value of this new technology. Nonetheless, the selection process (direct referral for diagnostic coronary angiography) likely engendered some referral bias that may limit the applicability of the findings to a less selected diagnostic group, among whom the more usual strategy would be to perform a functional test initially. This referral pattern likely reflects a high likelihood of CAD, although the pre-test probability of disease in these patients is unclear from the clinical information. These selection issues do not influence the validity of the results, but it may be important in relation to the transportability of the findings.
While the scientific value of these findings is great, the practical application of these data to clinical decision-making is less clear. Certainly, in the patient undergoing elective coronary angiography without a preceding functional test, proof of haemodynamic significance is warranted to justify intervention on intermediate (45–75% diameter) stenoses. Several methods have been developed to assess the physiologic significance of intermediate lesions. Non-invasive methods are usually geared towards measuring bloodflow (e.g. positron emission tomography, contrast echocardiography, and Doppler measurement of flow reserve), although clearly, the successful provocation of ischaemia—for example at dobutamine stress echocardiography—confirms that the lesion is haemodynamically significant. Compared to the traditional assessment of coronary flow reserve using a Doppler wire, the introduction of FFR, using a pressure wire simplified the process of showing stenoses to be flow-limiting,13 and is now a routine strategy for immediate evaluation during coronary angiography. The real value of deformation imaging during dobutamine stress is in patients being evaluated for angiography.
In conclusion, the paper of Weidemann et al.6 adds to an existing literature that shows that deformation imaging adds both incremental diagnostic and prognostic information to the interpretation of dobutamine stress echocardiography.4,5,14 Its particular strengths are that it shows incremental value in the most difficult circumstances (intermediate lesions) and identifies an appropriate cut off for the detection of functionally significant disease.
Acknowledgements
This program is supported in part by a Clinical Centre of Research Excellence award from the National Health and Medical Research Council of Australia.
Conflict of interest: The author receives research support (grants, equipment) from General Electric Medical Szystems, Philips Medical Systems and Siemens Medical Solutions.
Footnotes
The opinions expressed in this article are not necessarily those of the Editors of the European Heart Journal or of the European Society of Cardiology.
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