European Heart Journal Advance Access published online on January 16, 2007
European Heart Journal, doi:10.1093/eurheartj/ehl444
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Clinical and economic impact of stress echocardiography compared with exercise electrocardiography in patients with suspected acute coronary syndrome but negative troponin: a prospective randomized controlled study
1 Department of Cardiovascular Medicine, Northwick Park Hospital and Institute for Medical Research, Watford Road, Harrow, Middlesex HA1 3UJ, UK
2 Health Economics Research Group, Brunel University, Uxbridge, Middlesex, UK
Received 1 August 2006; revised 21 November 2006; accepted 30 November 2006.
* Corresponding author. Tel: +44 208 869 2547/8; fax: +44 208 864 0075. E-mail address: roxysenior{at}cardiac-research.org
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Abstract |
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Aims Patients attending hospital with suspected acute coronary syndrome (ACS), non-diagnostic electrocardiogram (ECG), and negative troponin present a diagnostic dilemma for admitting physicians. We sought to determine the clinical and economic impact of stress echocardiography (SEcho) when compared with exercise ECG (ExECG) in the assessment of these patients.
Methods and results Following pre-test assessment by (Thrombolysis in Myocardial Infarction) TIMI score, patients were randomized to ExECG (n = 218) or SEcho (n = 215). Subsequently, low-risk patients were discharged; those considered high risk were referred for coronary angiography. Patients were followed-up for cardiac events and a cost-analysis performed. SEcho was superior to ExECG in stratifying patients as low risk (77 vs. 33%, respectively, P < 0.0001) with no difference in cardiac event rate (5 vs. 3%, respectively). SEcho classified fewer patients as intermediate risk (3 vs. 39%, respectively, P < 0.0001) and fewer patients required further tests when compared with ExECG (3 vs. 47%, respectively, P < 0.0001). Costs for detection of coronary artery disease were significantly less in patients undergoing SEcho (£366.63 vs. £515.48, P = 0.004).
Conclusion SEcho is superior to ExECG in the risk stratification of patients with suspected ACS but negative troponin. SEcho resulted in less diagnostic uncertainty, fewer referrals for further investigation, and hence, a significant cost benefit over ExECG.
Key Words: Echocardiography • Prognosis • Coronary disease
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Introduction |
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Patients presenting to hospital with acute chest pain account for a significant proportion of patients seen in acute medical units in the UK.1 Evaluation of these patients usually involves a clinical assessment, resting electrocardiogram (ECG) interpretation, and the use of biochemical markers (troponins) of myocardial necrosis. Patients with ECG evidence of ischaemia and/or raised serum markers have high likelihood of prognostically significant coronary artery disease (CAD).2
However, patients presenting with non-diagnostic ECG and negative serum troponin but with risk factors for CAD, present a dilemma, i.e. whether to discharge such patients or admit them for further investigation. Around 6% of patients discharged with an initial negative troponin may have a final diagnosis of acute coronary syndrome (ACS) with an annual non-negligible hard cardiac event [death or acute myocardial infarction (AMI)] of 6.7%.3,4 However, to admit all of these patients would place significant demands on hospital resources. Current European Society of Cardiology (ESC) guidelines5 recommend pre-discharge stress testing to provide additional prognostic information. In the UK, the most commonly used stress test is exercise ECG (ExECG). Although it is inexpensive and there is extensive clinical experience with this test, it is less accurate than stress echocardiography (SEcho),6,7 which is a well recognized method for the detection of prognostically significant CAD.8–10
There have been no direct comparisons between the use of ExECG and SEcho in the investigation of patients presenting with suspected ACS and a negative troponin. We therefore designed a prospective randomized study to compare the clinical and economic impact of these two techniques as the initial investigation of this population.
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Methods |
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We conducted a prospective, randomized study on patients presenting to hospital with suspected ACS. The primary endpoint was the correct identification of patients at high risk of having underlying CAD as determined by a combined endpoint of cardiac death, non-fatal AMI, or coronary revascularization; the secondary endpoint was the cost to diagnosis. Written, informed consent was obtained from all patients and the study was approved by the local Ethics Committee.
Patient selection
Patients with suspected ACS but non-diagnostic ECG, and two or more risk factors (men aged >45 years, women aged >55 years, diabetes mellitus, smoking history, hyperlipidaemia, hypertension, previous history of CAD, a family history of premature CAD) for CAD were eligible for inclusion. Patients with ST-segment elevation or planar depression >1 mm in two contiguous leads on the presenting ECG; any significant rise in cardiac troponin (troponin I >0.03 or troponin T >0.1); known CAD awaiting revascularization; history and clinical examination suggesting a non-cardiac chest pain; absolute contra-indications for exercise testing (acute concurrent illness, serious arrhythmia, known significant obstructive hypertrophic cardiomyopathy, severe LV dysfunction, uncontrolled hypertension) and any physical condition precluding exercise were excluded from the study.
Cardiac troponin was measured at a minimum of 12 h after the onset of pain. Patient management, e.g. the decision to order stress testing, admission to hospital, and further therapy was at the discretion of the attending physician.
Randomization to either standard ExECG testing or SEcho was performed using computerized random number generation and undertaken within 24 h of admission to hospital. A pre-test risk of CAD disease was determined on the basis of individual (Thrombolysis in Myocardial Infarction) TIMI risk score.11 Patients were categorized as having a low (score 
1), intermediate (score 2–4), or high (score 
5) pre-test risk. Stress testing was carried out during the patient's hospital stay and a post-test risk was determined on the basis of the pre-test risk and the result of the test: a negative test conferred a low post-test risk and a positive test a high post-test risk irrespective of the pre-test risk. However, in patients with an inconclusive test, the pre-test risk remained unchanged as the test was of no incremental value.
All subsequent management decisions were taken by the attending physicians after the results of stress testing were made available. Generally, patients with a low post-test risk were discharged after admission. Patients with either a high or intermediate risk remained as inpatients, whereas those having a high post-test risk were referred for further management to the cardiologists. Patients with an intermediate risk were considered for further investigation at the discretion of the attending physician.
ExECG testing
Patients randomized to ExECG underwent symptom-limited Bruce (or modified Bruce) protocol treadmill testing. Endpoints were: fatigue, severe ischaemia (severe chest pain, >2 mm ST-segment depression), hypertension (systolic BP >220 mmHg), hypotension, pre-syncope, or arrhythmia.
Patients who achieved a work-load of 9 METS12 without haemodynamic compromise or ECG changes were considered to have a negative test. Patients who developed significant hypotension, arrhythmia, or 1 mm planar or downsloping ST-depression during exercise, or in recovery, were considered to have a positive test. All other patients were considered to have inconclusive tests.
Stress echocardiography
Patients randomized to SEcho underwent either treadmill or pharmacological testing at the discretion of the attending cardiologist. A two-dimensional echocardiogram was performed in the left lateral decubitus position using harmonic imaging. Digitized images of the left ventricle (LV) were obtained in the parasternal long-axis, short-axis, and apical four-, two-, and three-chamber views.
In the case of exercise SEcho, standard symptom-limited treadmill exercise testing was performed with images acquired immediately (usually within 60 s) after peak exercise. Early post-exercise images with the best endocardial definition were selected and displayed alongside the corresponding baseline images. In patients who were considered unsuitable for exercise testing, dobutamine was infused peripherally in 3 min dose increments, starting from 10 µg/kg/min and increased to 20, 30, and 40 µg/kg/min. If no endpoint was reached, atropine was added to the continuing 40 µg/kg/min dobutamine infusion in four divided doses of 0.3 mg up to a maximum of 1.2 mg.
Endpoints were the achievement of 85% of age-predicted maximum heart rate; development of ischaemia (i.e. WMA); achievement of peak dose (40 µg/kg/min of dobutamine IV + 1.2 mg atropine IV); or the occurrence of intolerable side effects. The stress images were then acquired and assessed in the same way as in the exercise stress protocol. In technically difficult patients, intravenous contrast (Optison®, Amersham Health, UK) was used to enhance endocardial definition. Bolus injections of 0.3 mL were administered through a peripheral cannula followed by a flush with 0.9% NaCl solution.
Image analysis
On-line digital images were interpreted qualitatively for the presence, extent, and location of segmental wall motion abnormality (WMA). An experienced observer (RS) blinded to patient history prospectively analysed the images for systolic wall thickening and endocardial wall motion according to a four-point score (1, normal; 2, reduced; 3, akinetic; and 4, dyskinetic motion) using a 17-segment LV model.13 Results were considered normal if all the segments that were normal at baseline showed a normal hyperdynamic response, with increased systolic wall thickening, after stress. Inducible ischaemia was defined as the development of new or worsening regional WMA during stress.
Patients with evidence of inducible ischaemia at stress, or a resting WMA without prior history of AMI, were considered to be at high risk of prognostically significant CAD. Patients undergoing exercise SEcho who achieved a workload >7 METS14 and 85% of maximum predicted heart rate without WMA were considered to have a low risk of prognostically significant CAD. Those undergoing pharmacological SEcho, who achieved target heart rate or the maximum dose of dobutamine and atropine with no evidence of WMA, were considered to have a low risk of prognostically significant CAD.9,15 All other patients including those with uninterpretable images were considered to have an inconclusive test.
Coronary angiography
The decision to perform coronary angiography was taken at the discretion of the patients attending clinician after the results of the non-invasive testing were made available. Standard techniques were used with the clinician managing the patient utilizing a visual quantitative scoring system for image analysis with CAD defined as a >50% luminal diameter narrowing in one or more epicardial arteries or their major branches.16 Disease was classified as single, two-, or three-vessel disease. In patients who had undergone previous coronary artery bypass surgery, a stenosis >50% was taken to demonstrate CAD within the graft.
Follow-up
Follow-up time was calculated from the initial test date to either the date of a cardiac event (cardiac death, AMI, or revascularization) or the date of last contact with the patient. During the period of follow-up, data was collected regarding further investigations. Cardiac death was defined as death associated with known or suspected AMI, life-threatening arrhythmia, or heart failure (based on clinical assessment, serum cardiac markers, ECG, or post-mortem findings). Non-fatal AMI was defined according to recommended criteria.5 Event data was collected by questionnaires sent at three monthly intervals with additional telephone follow-up or hospital record review where appropriate.
Cost analysis
The cost analysis was carried out from the UK NHS perspective. Resource use data was collected for all patients and unit costs were obtained from UK Government Department of Health figures (2003–4 financial year).17 Cost of diagnosis was defined as the sum of direct medical costs incurred up to the point of diagnosis. Only tests that could directly confirm or refute the diagnosis of CAD were considered.
Statistical analysis
Continuous variables are shown as mean±SD except for those that are not normally distributed, which are presented as medians with the interquartile range (IQR). All categorical variables are shown as proportions. Comparison of continuous data was made by independent t-test, one-way ANOVA, or Mann–Whitney U test, whichever was appropriate. For categorical variables, 
2 analysis was used. Survival curve analysis to compare event rates in the SEcho vs. ExECG groups was made using log-rank testing. A P-value <0.05 was considered significant. Statistical analyses were performed using Analyse-it version 1.62 (Leeds, UK) and StatsDirect version 2.5.7.
Our sample size calculation was based on the sensitivity rates of the two tests to detect CAD. Sensitivity values of 68% for ExECG and 85% for SEcho were used to calculate the number of patients required to demonstrate a difference between the two techniques in two independent groups. Using a significant value of 5 and 80% power, a sample size containing 108 patients with CAD in each group was required to give adequate statistical power to the study. An estimated disease prevalence of 50% was used to give a sample size of 432 patients in total.
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Patient characteristics
In total, 433 patients were enrolled (from January 2003 to April 2004) in the study, 218 were randomized to ExECG and 215 to SEcho (Figure 1). There was no difference in time from admission to testing between the two groups [median time to testing 1 day (IQR, 0–2 days) for both modalities, P = 0.35] with 298 (69%) patients being tested within 24 h of admission. Baseline characteristics of the study population is shown in Table 1. There were no significant differences in the drug therapy between ExECG and SEcho at randomization.
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Exercise ECG
Dynamic testing was not performed on nine patients; four were found to have new ECG changes suggestive of ischaemia immediately prior to testing and assigned a high post-test risk; five patients could not perform ExECG and therefore their post-test risk remained unchanged from their pre-test assessment. Of the 218 patients randomized to ExECG, 71 (33%) were classified as low post-test risk, 63 (29%) demonstrated high post-test risk, and 84 (39%) were classified as intermediate.
Stress echocardiography
Of the 215 patients randomized to SEcho, five patients did not undergo dynamic testing. Of these five, four had WMA at baseline in the absence of prior history of AMI. The final patient, with a high pre-test risk of CAD, developed a broad complex tachycardia immediately prior to testing. All five patients were therefore assigned a high post-test risk. Intravenous contrast was used in 18 patients (8%) in order to improve endocardial definition. Of the 80 patients undergoing pharmacological SEcho, 55 (69%) achieved 85% of age-adjusted maximum heart rate. Of the remaining 25, four developed inducible WMA; nine achieved the maximum allowed doses of dobutamine and atropine; and one achieved 83% of maximum heart rate. In the remaining 11, the study was terminated before target heart rate could be achieved because of adverse symptoms. Of the 215 patients undergoing SEcho, 165 (77%) were classified as low post-test risk, 44 (20%) as high post-test risk, and six (3%) as intermediate.
Risk stratification
The proportion of patients having a low, intermediate, and high pre-test risk was comparable for both ExECG and SEcho [21 vs. 22% (P = 0.76); 69 vs. 69% (P = 0.92); 10 vs. 8% (P = 0.65), respectively]. Significantly more patients undergoing SEcho compared with ExECG were classified as having a low post-test risk (77 vs. 33%, respectively; P < 0.0001) and could therefore be discharged. SEcho was considerably more effective than ExECG reducing the proportion of patients considered to be at intermediate risk (3 vs. 39%, respectively; P < 0.0001). More patients in the ExECG group were classified as having a high post-test risk when compared with SEcho (29 vs. 20%, P = 0.04) (Figure 2). There were 21 patients who did not develop inducible WMA and who did not achieve 85% of maximum heart rate during dobutamine SEcho. If these patients were reclassified to retain the pre-test risk, SEcho still classified more patients as low-risk when compared with ExECG (70 vs. 33%, respectively, P < 0.0001), classified less as intermediate (9 vs. 39%, respectively, P < 0.0001) and high risk (21 vs. 29%, respectively, P = 0.07) when compared with ExECG.
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Given that the abnormal baseline ECG and poor workload are confounding factors in ExECG interpretation, further analysis accounting for these factors was undertaken. Patients with an abnormal baseline ECG were excluded, giving a total patient cohort of 247 (127 in ExECG group and 120 in SEcho group). At follow-up, 60/247 (24%) had coronary angiography performed (32% of the ExECG group and 16% of the SEcho group, P = 0.0042). Those patients who performed ExECG and achieved a workload of 7 METS or more had no significant chest pain or ECG changes and were reclassified as having a low post-test risk. In this subgroup analysis, SEcho still classified more patients as low risk (85 vs. 59%, P < 0.0001) with similar cardiac event rates at follow-up (2 vs. 3%, P = 0.97). Moreover, patients undergoing ExECG were still more likely to be classified as having an intermediate post-test risk when compared with those undergoing SEcho (19 vs. 3%, P = 0.0003).
Further diagnostic investigations
In total, 144/433 (33%) patients required further tests to confirm or refute the diagnosis of CAD of which significantly more were in the ExECG group when compared with SEcho group (47 vs. 20%, respectively, P < 0.0001). Of the patients undergoing further non-invasive tests, 31 were investigated with myocardial perfusion imaging of which 11 (35%) subsequently underwent coronary angiography (Table 2). Similarly, 14 underwent SEcho, of which three (21%) went on to coronary angiography. At follow-up, 113 coronary angiograms were performed of which 74 (67%) were in those patients classified as high risk by either ExECG or SEcho; 25 (22%) as intermediate risk; and 14 (12%) as low risk. Flow-limiting CAD was demonstrated in 76 (67%) patients of which 36, 26, and 14 patients demonstrated single vessel, two-vessel, and three-vessel disease, respectively.
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A positive stress test predicting a high likelihood of CAD was seen in 86 patients of which 76 underwent coronary angiography. Of these, SEcho correctly predicted the presence of CAD in 24 out of 31 (77%) patients when compared with 20 out of 33 (61%) patients by ExECG (P = 0.17). Thus the rate of normal angiography in patients with a positive ExECG was higher than that seen in patients with a positive SEcho (39 vs. 23%, P = 0.17). This was maintained when patients with abnormal baseline ECG were removed (45 vs. 8% for ExECG and SEcho, respectively; P = 0.08).
Follow-up
Of the 433 patients randomized, 17 (4%) were lost to follow-up. Of the remainder, follow-up was performed at a median of 8.7 (5.6–13.0) months. A total of eight patients suffered AMI (2%), and 54 patients (13%) underwent a revascularization procedure. Combined cardiac endpoint was reached by 62 patients (15%). Two patients died from non-cardiac causes, one in each group. There was no significant difference in either the overall combined endpoint of cardiac death, AMI, and revascularization, or in the incidence of hard cardiac endpoints (cardiac death and non-fatal AMI) between SEcho and ExECG group (Table 3). There were 223 patients with a low post-test risk of CAD at follow-up (68 in the ExECG group; 155 in the SEcho group). The incidence of events in the ExECG group was 2/68 (3%) when compared with 7/155 (5%) in the SEcho group with no significance difference in the event rates (P = 0.63). However, there was a significant difference (P = 0.01) in the combined endpoint with those in the SEcho group at high risk having more cardiac events, particularly revascularization (P = 0.01), than those in the equivalent ExECG group (Figure 3). Figure 4 shows the Kaplan–Meier survival curve analysis of overall cardiac events in all patients (A), in the high post-test risk patients (B), and in the low post-test risk groups in the SEcho vs. ExECG arms, respectively.
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Cost analysis
The total use of resources and unit costs for the two groups are summarized in Table 4. The mean cost to diagnosis of CAD was significantly less in the SEcho group when compared with the ExECG group (£366.63 vs. £515.48, respectively; P = 0.004). This difference was maintained irrespective of the pre-test probability of CAD but did not reach significance in the intermediate TIMI group (Table 5). If patients with an abnormal baseline ECG were excluded, the overall cost to diagnosis was still significantly less if SEcho was used as the first-line test when compared with ExECG (£325.98 vs. £495.00, respectively; P = 0.01).
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Patients undergoing SEcho were selected to do so primarily because they were considered able to exercise to an adequate workload (
7 METS). In order to provide an equivalent population in the ExECG group, patients who had an inconclusive test due to poor workload (i.e. <7 METS) were excluded. Patients were again reclassified as being at low risk if they achieved 7 METS workload with no significant ECG changes. If both of these groups had patients with abnormal baseline ECGs excluded, SEcho would be still cheaper than ExECG (£341.71 vs. £486.30; P = 0.07).
Discussion
The present study is the first prospective randomized trial comparing the relative value of ExECG and SEcho in predicting the outcome and their impact on cost in patients with risk factors for CAD presenting to hospital with suspected ACS, but with non-diagnostic ECG changes, and negative troponin. These patients are referred by treating physicians for stress testing for further management. The study confirmed previous reports,18,19 that this group of patients have a moderately high cardiac event rate (15% over a 9-month follow-up period). Both ExECG and SEcho identified high- and low-risk group effectively, but SEcho was more effective than ExECG for the correct identification of higher risk patients with more patients in this group having cardiac events at follow-up (51 vs. 29%, P = 0.01). This is because proportionately more revascularization procedures were carried out in the high-risk SEcho group when compared with equivalent ExECG group, probably because of the ability of SEcho to correctly identify more patients with prognostically significant CAD when compared with ExECG. Furthermore, despite ExECG classifying more patients as high risk compared with SEcho (29 vs. 20%), the rate of normal coronary angiography in the ExECG group was nearly twice when compared with SEcho (39 vs. 23%, respectively). Thus, more patients in the ExECG group underwent unnecessary invasive investigation which has both outcome and cost implications.
On the other hand, SEcho classified significantly more patients in the low-risk group when compared with ExECG group (77 vs. 33% respectively), which allowed a larger proportion of patients to be discharged with no significant difference in cardiac events (5 vs. 3%) at a mean follow-up period of 9 months. Hence, more patients in the ExECG arm stayed in the hospital unnecessarily prolonging bed-occupancy.
As a consequence of these factors, the number of patients undergoing further tests to confirm or refute the diagnosis of CAD was significantly higher in the ExECG arm (47%) vs. SEcho (20%). As a result of these additional downstream costs, ExECG was more expensive than SEcho in the investigation of this population despite its lower initial cost (secondary endpoint). Cost to diagnosis with SEcho was cheaper than ExECG across all classes of pre-test TIMI risk reaching statistical significance in low and high pre-test risk patients.
The use of intravenous contrast agents to enhance endocardial definition has become more widespread and therefore adds to the cost of performing SEcho. The use of such agents in our study was relatively low at 8%, when compared with up to 30% in some studies, which may be a reflection of the population studied with reasonable overall BMI. Nevertheless, even if 30% of the patients undergoing SEcho required contrast enhancement, SEcho remained cheaper than ExECG as a first-line investigation (£373.19 vs. £515.48, P = 0.006).
Clinical implications
A variety of strategies have been employed in order to better risk stratify patients presenting to hospital with suspected ACS. These range from the estimation of biochemical markers, imaging techniques such as resting echocardiography, and radionuclide perfusion imaging20 and ECG monitoring in chest pain observation units21 to improve risk stratification and prevent unnecessary hospital admission. However, current recommendations5 still require some form of rapid provocative testing in patients with coronary risk factors but normal troponin in order to exclude significant CAD.
ExECG remains the most widely used stress testing technique for this purpose and yet 60% of patients undergoing this test have inconclusive or intermediate risk studies.7 As a result, patients undergo further investigations, or remain uncertain of their diagnosis. Over the years, SEcho has matured to provide reliable diagnosis and risk-stratification of patients with known and suspected CAD.10 With the advent of contrast agents to improve endocardial delineation, non-diagnostic images are rare and, in our study, all patients had diagnostic images. SEcho is equivalent to radionuclide perfusion imaging which is another widely used technique for the assessment of CAD.10 However, it has the advantage of providing further information on myocardial and valvular structure and function. SEcho does not require ionizing radiation and has the advantage of a cheaper initial cost as well as being readily available using existing echocardiographic machines present in all district general hospitals.
When compared with ExECG, the ability of SEcho to provide definitive and accurate results in the risk stratification of patients presenting with troponin negative chest pain has been shown in a previous study comprising only female population22 and as part of the current study population.23 However, the present larger study establishes and confirms previous work in a non-acute setting24 in demonstrating that despite its greater initial cost, SEcho has significant clinical and cost-benefit over ExECG in assessing patients for CAD. It provides safe, rapid, provocative testing, reducing diagnostic uncertainty, and enabling discharge of patients from hospital, thereby reducing bed-occupancy. It also reduces costs from inappropriate drug treatment, inappropriate referral for coronary arteriography, and potentially reduces the number of subsequent outpatient visits at follow-up when compared with ExECG.
Study limitations
There could be a potential bias against ExECG in that patients were randomized to this arm despite baseline ECG changes and the patients needed to exercise 9 METS when compared with 7 METS with SEcho before classifying them as low risk. It has been shown that patients who achieve a workload 7 METS with SEcho without WMA have an excellent outcome,14 whereas patients undergoing ExECG require a higher workload for the same prognosis.12 However, when patients with baseline ECG changes were excluded and workload similar to SEcho was used to reclassify patients as low risk, SEcho maintained its superiority over ExECG both in terms of classification of patients as low risk (85 vs. 59%, P < 0.0001), and in reducing the number of patients classified as intermediate risk (3 vs. 19%, P = 0.0003).
In addition, when patients who were considered unable to exercise adequately (i.e. those undergoing pharmacological SEcho or patients with inconclusive ExECG due to poor workload) or those patients with abnormal baseline ECGs are excluded, SEcho was still cheaper than ExECG (£341.70 vs. £486.66). When patients with baseline ECG changes were excluded, the positive predictive value of ExECG for detecting CAD did not improve when compared with SEcho.
A single experienced reader, who was blinded to patient details, interpreted all SEcho images. There is a degree of interobserver variability with interpretation of these images and ideally all images should have been assessed by more than one blinded expert. With regard to the costs reported in this study, the figures used were based on UK NHS unit costs and, as a result, may not apply to other settings and countries.
Conclusions
SEcho was superior to ExECG in the risk stratification of patients presenting to hospital with suspected ACS and non-diagnostic ECG. Furthermore, SEcho resulted in cost-saving when compared with ExECG in the assessment of CAD.
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The study was funded by Cardiac Research Fund and North West London Hospital Trust.
Conflict of interest: none declared.
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References |
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[Abstract/Free Full Text]
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