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least likely culprit lesion, due in part to its geometry (resulting in different shear wall stress) and/or due to probability. These studies support the trends noted in this CENSTEMI research.
Of the 55 patients, 13 (24%) patients had no significant CAD noted with angiography.
Previous studies have assessed the prevalence of false-positive AMI diagnosis, determined by coronary angiography in the STEMI population [150-152] , however, little research has been performed in the NSTEMI population. Perugini et al. [152] suggested the main causes for a false-positive STEMI diagnosis prior to coronary angiography were ‘absent or minimal coronary lesions, Tako-Tsubo, or chronic ischaemic heart disease without culprit lesion, repolarisation, left bundle branch block, pericarditis, left ventricular hypertrophy and
myocarditis’. These syndromes may explain the 24% of patients in this study diagnosed with NSTEMI who had no significant coronary lesions identified with coronary angiography.
Similarly, a study published in 2010 by Larson et al. [150] found of 1335 patients with suspected STEMI who underwent coronary angiography, 14% had no culprit coronary lesion while 9.5% had no significant CAD. Differential biomarker elevation provides another explanation for ‘false-positive’ NSTEMI diagnosis. Guidelines suggest the use of the 99th percentile as a reference limit of an elevated TnI level [1], however, TnI is not exclusive for ACS. Possible cardiac causes include acute heart failure, aortic dissection,
pericarditis/myocarditis or tachycardia.
Of the 24% of patients with no CAD, 11 had mitral regurgitation (all cases were mild), while six patients had raised RVSP. No other obvious cardiac causes for TnI elevation level were identified with echocardiography, which provides a quick, non-invasive, diagnostic tool to rule out many those alternate diagnoses. Causes for non-cardiac TnI elevation include pulmonary embolism, stroke, sepsis or renal failure [153]. It is rare that a TnI elevation cannot be explained; further testing is required to elucidate the condition behind a significant biomarker elevation.
105 4.5.1 RWMA Assessment and CAD
Regional wall motion assessment was possible in all 55 patients. 23 (42%) patients had regional dysfunction noted on echo. A retrospective study published in 2007 by Bridgman et al. [46] utilising NSTEMI patients at Christchurch Hospital, New Zealand found 60% of patients had RWMA identified with echocardiography. Of these patients, 89% correctly identified the culprit coronary lesion. This CENSTEMI research had a marginally lower number of patients with regional dysfunction visualised with echocardiography, and a marginally lower number of correctly identified culprit lesions (74%) which may be due to the small patient cohort, patient window quality or chance.
Of the 23 patients with regional dysfunction, two had no associated CAD. Of the remaining 21 patients, the cardiologist correctly identified 47 of 63 regions with abnormal function corresponding to CAD. Three regions from three patients were identified as
abnormal (RCA in all cases) when there was no corresponding CAD - while another region in the heart was affected by a culprit coronary artery (LAD in all cases) with no corresponding regional dysfunction. Coronary artery anatomy variations between patients, difficulty in interpreting the cardiac myocardial thickening (due to artefacts, motion etc.) and patient window quality may explain these misinterpretations.
Of the 55 patients, 19 had CAD, but no corresponding regional dysfunction. This means that there was no visual necrosis of cardiac tissue around or downstream of the culprit coronary lesion. Regional wall motion assessment is a subjective measure that requires significant expertise and experience. It is often complicated by difficult echocardiography windows and a lack of “baseline” for comparison. The presence of RWMAs is, however, in itself an interesting finding in an echocardiography examination as it is likely indicative of a degree of cardiac necrosis from the NSTEMI event itself.
106 4.5.2 Sensitivity versus Specificity
Of the 55 patients, RWMAs were identified in two male patients, both of whom had no significant CAD identified with angiography. In patient A, a RWMA was identified in the RCA region with no corresponding CAD: LVEF was 68% and there was a minor TnI rise, with no other significant echocardiography abnormalities. On re-review of the
echocardiogram, mild hypokinesis in the RCA territory is confirmed, suggesting there may be an underlying, undiagnosed disease process present, or perhaps this is a normal variant for this patient.
Patient B had an LVEF of 43% and a small peak TnI rise. On review of the
echocardiogram, the left ventricle was dilated, with mild-moderate mitral regurgitation, and raised filling pressures. Appearances suggest overall heart failure. The sensitivity and specificity of using echocardiography to determine RWMA association to CAD using coronary angiography was assessed by Medina et al. in 1985 [154]. They found that the predictive accuracy of detecting RWMA in patients with a dilated left ventricle had a sensitivity of 83% and a specificity of 57%, while the accuracy of RWMA detecting
significant CAD had a sensitivity of 95%, a specificity of 100% and a predictive accuracy of 95%. This suggests that echocardiography is highly suited for detecting RWMA.
The risk of a false positive RWMA identified by echocardiography is a diagnostic angiogram to investigate the cause. Angiography is regarded as a semi-invasive procedure and carries increased patient risk. The risk of a false-positive diagnosis is out-weighed by the benefit of performing coronary angiography. The risk of performing a coronary angiogram in a non-compromised patient and finding nothing outweighs the risk of not performing the test at all, with the risk of missing something important. This mantra is echoed in previous literature [150-152], all of which determined the prevalence of false-positive STEMI diagnosis with coronary angiography. Perugini et al. [152] found a false-positive diagnosis was an ‘acceptable and unavoidable price to pay to guarantee the lowest possible frequency of false negatives’. These studies are reassuring when faced with the possibility of false- positive RWMA diagnoses with echocardiography.
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