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Original Research |
1 Harvard Ph.D. Program in Health Policy, 14 Story St., 4th Floor, Cambridge, MA
02138.
2 Department of Radiology, Massachusetts General Hospital, Boston, MA.
3 Harvard Medical School, Boston, MA.
4 Department of Emergency Medicine, Massachusetts General Hospital, Boston,
MA.
5 Department of Economics, Harvard University, Cambridge, MA.
6 National Bureau of Economic Research, Cambridge, MA.
7 Department of Health Policy and Management, Harvard School of Public Health,
Boston, MA.
8 Institute for Technology Assessment, Massachusetts General Hospital, Boston,
MA.
Received January 1, 2008;
accepted after revision March 4, 2008.
Funding for this study was provided through the Walker Fund of the Harvard
Ph.D. Program in Health Policy, which provided support for the design and
conduct of the study; collection, management, analysis, and interpretation of
the data; and preparation and review of the manuscript. Any errors or
omissions are the sole responsibility of the authors. Joseph A. Ladapo had
full access to all the data in the study and takes responsibility for the
integrity of the data and the accuracy of the data analysis.
Abstract
 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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MATERIALS AND METHODS. A microsimulation model was developed to compare costs and health effects of performing CT coronary angiography and either discharging, stress testing, or referring emergency department patients for invasive coronary angiography, depending on their severity of atherosclerosis, compared with a standard-of-care (SOC) algorithm that based management on biomarkers and stress tests alone.
RESULTS. Using CT coronary angiography to triage 55-year-old men with acute chest pain increased emergency department and hospital costs by $110 and raised total health care costs by $200. In 55-year-old women, the technology was cost-saving; emergency department and hospital costs decreased by $410, and total health care costs decreased by $380. Compared with the SOC, CT coronary angiography-based triage extended life expectancy by 10 days in men and by 6 days in women. This translated into corresponding improvements of 0.03 quality-adjusted life years (QALYs) and 0.01 QALYs, respectively. The incremental cost-effectiveness ratio for CT coronary angiography was $6,400 per QALY in men; in women, CT coronary angiography was cost-saving. Cost-effectiveness ratios were sensitive to several parameters but generally remained in the range of what is typically considered cost-effective.
CONCLUSION. CT coronary angiography-based triage for patients with low-risk chest pain is modestly more effective than the SOC. It is also cost-saving in women and associated with low cost-effectiveness ratios in men.
Keywords: acute chest pain • cardiac CT • cost-effectiveness • health policy • noninvasive angiography
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Patients who are at low risk for ACS because of negative initial cardiac biomarkers and normal or nondiagnostic ECG examinations, but in whom the source of chest pain is not readily identified, are a cohort whose management is notably inefficient [4–7]. The American College of Cardiology (ACC) and the American Heart Association (AHA) recommend further evaluation for these patients, but their hospitalizations typically result in benign diagnoses [5, 8, 9].
Because ACS is rare in the absence of coronary atherosclerotic plaque, several researchers have suggested that the incorporation of noninvasive CT coronary angiography may improve the triage of patients with acute chest pain and reduce health care costs [3, 10–14]. Many of these researchers have based their proposals on the capabilities of the relatively new 64-MDCT.
High spatial and temporal resolution is offered by 64-MDCT, which generates diagnostically useful images of the coronary artery lumen and wall [15]. Because these vessels are the site of atherosclerotic plaques that lead to ACS, 64-MDCT coronary angiography (CT coronary angiography), by excluding the presence of atherosclerosis, may facilitate the triage of patients with acute chest pain who are suspected of having ACS.
Our review of the literature identified a small but growing number of studies examining the usefulness of 64-MDCT in a clinical setting. In a prospective observational cohort study, Hoffmann et al. [10] found that 64-MDCT coronary angiography accurately distinguished a subset of patients with chest pain who could be safely sent home from the emergency department. Rubinshtein et al. [16] incorporated CT coronary angiography in a similar patient population and concluded that MDCT identified the presence of ACS with high sensitivity and was also moderately specific. Gallagher et al. [3] reached similar conclusions. Nonetheless, except for a small but novel randomized controlled trial conducted by Goldstein et al. [14], little is known about the impact of CT coronary angiography on health care costs, and larger studies evaluating its effectiveness in comparison with traditional triage strategies are still in their early stages. In the absence of this information, an economic evaluation of CT coronary angiography is warranted to help guide hospitals and policy makers who are considering acquiring 64-MDCT and reimbursing their use in an emergent chest pain setting. We constructed an analytic model to aid these policy makers in their decisions.
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Population Characteristics
We conducted separate analyses for 55-year-old men and women with
"low-risk chest pain," defined as patients with negative initial
troponin measurements, a normal or nondiagnostic ECG, and no history of heart
disease. We did not include patients with very low-risk chest pain, who would
be discharged without cardiac evaluation, or patients with ST segment
elevation MIs. The diagnosis of coronary artery disease (CAD) was reserved for
patients with significant coronary disease, defined by either 
50%
stenosis in the left main coronary artery or 70% stenosis in any other
coronary artery. Patients whose atherosclerosis did not meet the definition
for CAD were considered to have "early heart disease" or mild
stenosis, and those with no atherosclerosis were considered healthy.
Overview of Model and Triage Strategies
Patients begin the model in the emergency department with acute chest pain,
negative initial biomarkers, a normal or nondiagnostic ECG, and no history of
heart disease. Chest pain is either ACS (with non-ST segment elevation MI or
unstable angina) or non-ACS (chronic stable angina or noncardiac chest pain)
in origin. Subsequent management loosely follows an algorithm proposed by
Goldstein et al. [14] in their
randomized controlled trial of CT coronary angiography in the emergency
department.
In the SOC pathway, patients are assumed to still have normal or nondiagnostic ECGs and await the results of their final troponin measurement. They are evaluated with a stress test if the enzyme is not found to be elevated (Fig. 1). Patients with elevated follow-up biomarkers or whose stress tests confirm ischemia are referred for invasive coronary angiography. Coronary catheterization is followed by percutaneous coronary intervention (for one- or two-vessel disease) or coronary artery bypass graft (CABG) surgery (for left main and three-vessel disease) if their angiograms reveal significant stenosis. Mild coronary stenosis is managed medically. If the stress test and follow-up troponin measurement are normal in the emergency department, patients are eligible for discharge.
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In the CT coronary angiography care pathway, patients are imaged, and those with evidence of normal coronary arteries are discharged from the emergency department with no further testing (Fig. 2). Patients with mild atherosclerotic lesions are evaluated with follow-up biomarker measurements and stress tests, as in the SOC algorithm. Further management also follows the SOC pathway, and patients may be discharged if their examinations are normal or if they are referred for coronary catheterization. Patients with evidence of significant stenosis on CT coronary angiography are also referred for coronary catheterization.
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Distribution of Disease in Patients with Acute Chest Pain
The distribution of ACS and non-ACS diagnoses in the initial emergency
department visit was primarily derived from a study of 317 patients with acute
chest pain who had no history of heart disease and were considered to be at
low risk for ACS on the basis of a clinical algorithm constructed by Goldman
et al. [21,
22] and other authors
[9,
23]. Data from a study of 817
emergency department patients with chest pain were also used
[24]. Low-risk patients with
non-ACS chest pain were assumed not to have a life-threatening condition
(Table 1). Although this is not
true in reality, because such patients may be experiencing aortic dissections,
pulmonary embolisms, or other conditions, our omission of any explicit
modeling of these health events likely does not affect incremental
cost-effectiveness because these patients are presumably evaluated similarly
under both management strategies.
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After the initial chest pain episode, the distribution of disease in subsequent emergency department visits was estimated using simplifying assumptions and bayesian methods. For example, a patient with CAD who returned to the emergency department with chest pain would have a probability of experiencing non-ST segment elevation MI equal to (0.8) (0.03) / [(0.8) (0.03) + (0.5) (0.97)], where 0.8, 0.03, 0.5, and 0.97 are, respectively, the probabilities, in the initial emergency department visit, of a patient having CAD conditional on experiencing non-ST segment elevation MI, the total probability of a patient experiencing non-ST segment elevation MI, the probability of a patient having CAD conditional on experiencing non-ST segment elevation MI chest pain, and the total probability of a patient experiencing non-ST segment elevation MI chest pain. In this example, the 0.8 and 0.5 figures are fictitious (the actual values vary by sex), and we have collapsed the conditional probabilities of CAD in the presence of unstable angina, stable angina, and noncardiac chest pain for illustrative purposes.
Prevalence of Atherosclerosis
The distribution of coronary atherosclerosis among patients with acute
chest pain is critical to the effectiveness of 64-MDCT as a triage and
diagnostic instrument. We derived this distribution from a large cohort of
patients with chest pain who underwent invasive angiography but were not
diagnosed with ACS [25]. These
patients were stratified by age, sex, and the likelihood that their chest pain
complaints were anginal in nature (definite angina, probable angina, or
nonspecific chest pain).
Patients with ACS were assigned a distribution of vessel disease similar to the "definite angina" chest pain group; patients with stable angina were assigned a distribution of vessel disease that averaged results from the definite angina and probable angina groups because we assumed these patients were healthier than patients with ACS; patients with noncardiac chest pain were assigned a distribution of vessel disease similar to the nonspecific chest pain group. Because patients in this group are unlikely to be experiencing cardiac chest pain, their vessel disease distribution was assumed to be representative of a random sample of U.S. adults. Note that our method of estimating vessel disease patterns permits the occurrence of ACS in the absence of significant atherosclerosis, a phenomenon that has been previously documented [26–28].
Diagnostic Tests and Procedures
No published studies have reported all 64-MDCT coronary angiography test
characteristics needed for our model on a per-patient basis, so we used data
that applied to individual segments of the coronary arteries
[29]
(Table 1). Note that this
method of reporting will, on average, underestimate the diagnostic power of CT
coronary angiography because many patients have multiple significant coronary
lesions. The diagnostic performance of other tests, including serial troponin
measurements, stress ECG, stress echocardiography, and SPECT myocardial
perfusion imaging (SPECT MPI), for identifying ACS or CAD (in patients without
ACS but with CAD) was derived from a meta-analysis
[30]. We assumed that patients
were equally likely to undergo evaluation with any of these stress test
techniques. Coronary CT angiography yielded nondiagnostic results at a rate of
4%, and patients with nondiagnostic examinations were subsequently evaluated
with a stress test [31]. The
risks and benefits of CABG and percutaneous coronary intervention were derived
from a previous decision analysis
[32].
Long-Term Survival
Mortality risk over time was based on age, sex, the existence and severity
of CAD, history of CABG or percutaneous coronary interventional treatment, and
history of ACS [26,
32–34]
(Table 2). Patients wrongfully
discharged from the emergency department with undiagnosed ACS faced an
elevated risk of mortality
[35]. The likelihood of
developing atherosclerosis was estimated using data from a previous study, and
the annual probability of transitioning from undiagnosed to diagnosed CAD was
assumed to be 5% [25].
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Health-Related Quality of Life
CAD may be associated with chronic chest discomfort; therefore,
health-related quality of life in this study was based on the existence and
severity of this discomfort. Utilities for various chest discomfort
classifications were derived from a study of 878 Canadian patients using the
standard gamble method and were combined with data on the impact of CABG and
percutaneous coronary intervention on chest pain
[36,
37]
(Table 3).
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Health Care Costs
Emergency department and inpatient costs were estimated using data from a
randomized controlled trial reported by Goldstein et al.
[14] and Medicare
reimbursement data [38]
(Table 3). Using the former, we
derived the implicit cost of care for patients discharged from the emergency
department and adjusted this cost using national Medicare reimbursement rates
for stress SPECT examinations. We then used the difference between the
reported cost of stress SPECT in the randomized controlled trial and the
average national Medicare reimbursement for this examination to estimate the
cost of CT coronary angiography. Note that Medicare's fee schedule is
considered a reasonable proxy for economic costs
[39]. The annual cost of
medications for CAD and angina was estimated using assumptions about typical
treatment doses for statins, aspirin, β-blockers, and isosorbide
mononitrate.
Model Analysis
We ran 1,000,000 first-order Monte Carlo microsimulations for 55-year-old
men and women and performed one-way sensitivity analyses on key parameters.
The model was built with TreeAge Pro Suite 2006 (TreeAge Software), and
results were analyzed using Excel 2003 (Microsoft) and Intercooled Stata 9.2
software (Stata).
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Both sexes experienced incremental improvements in unadjusted and quality-adjusted life expectancy (QALE) with CT coronary angiography, but the benefits were small. Compared with the SOC, CT coronary angiography-based triage extended life expectancy by 10 days in men and 6 days in women (Table 4). This translated into corresponding improvements of 0.03 and 0.01 QALYs, respectively. The modest improvements reflect the low incidence of ACS in our cohort. Compared with the SOC, the incremental cost-effectiveness ratio for CT coronary angiography management was $6,400 per QALY in men; in women, the strategy was cost-saving (Table 4).
Emergency department discharge rates in men and women were 69% and 82% in the MDCT algorithm and 62% and 65% in the SOC algorithm, respectively. These discharges were associated with a rate of missed ACS of 1% in men under both management algorithms. In women, the missed ACS rate was 1% under SOC management and 2% under CT coronary angiography management. Whereas only 13% of these missed cases occurred in the setting of cardiac syndrome X under SOC management, this figure approached 80% in men evaluated with CT coronary angiography. The corresponding percentages in women were 48% and 95%. Thus, most patients who were wrongfully discharged from the emergency department after being triaged with CT coronary angiography (with or without stress testing) faced a low risk of mortality from the missed diagnosis [40].
Sensitivity Analysis
We conducted sensitivity analyses using the data ranges specified in Tables
1,
2,
3. In general, varying key
parameters over plausible ranges changed incremental cost-effectiveness ratios
but left CT coronary angiography-based triage as the preferred strategy when
compared with the SOC.
Reducing by 25% the ability of CT coronary angiography to correctly classify a healthy patient as having normal coronary arteries increased its incremental cost-effectiveness ratio to $17,000 per QALY in men but made little difference in women. Minimizing the ability of CT coronary angiography to classify a patient with CAD as having significant stenosis marginally affected its cost-effectiveness in men, but CT coronary angiography remained a cost-saving strategy in women compared with the SOC. The QALY benefit also decreased modestly in men, primarily because fewer patients were diagnosed with CAD and treated. Reducing the time horizon over which patients could return to the emergency department to 1 year increased the incremental cost-effectiveness ratio of CT coronary angiography-based management to $15,000 per QALY in men, but this strategy remained cost-saving in women. The difference in total health care expenditures between SOC and CT coronary angiography-based management rose to $530 in men, and cost savings in women decreased to $80 in women.
Increasing the prevalence of nondiagnostic CT coronary angiography examinations to 10% also had little quantitative impact on the results. Maximizing the 30-day mortality risk associated with missed ACS widened the gap between QALY outcomes in the two strategies, and the incremental cost-effectiveness ratio in men decreased to $4,040 per QALY. Increasing the cost of CT coronary angiography to $1,000 per examination increased the incremental cost-effectiveness of CT coronary angiography to $25,300 per QALY in men and disrupted the cost-saving relationship between the two strategies in women (Fig. 4). The incremental cost-effectiveness of CT coronary angiography was $17,200 per QALY in this population.
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CT coronary angiography raised overall costs in men primarily because it was more likely to identify patients with CAD. These patients incurred costs related to a lifetime of cardiovascular treatment. However, they also gained risk reductions because of this treatment. Thus, though overall costs were higher, QALYs also rose, and this combination yielded a low (favorable) cost-effectiveness ratio. In contrast, CT coronary angiography lowered overall costs in women because savings from avoided admissions outweighed cost increases from a higher rate of detection and treatment of CAD. The balance swung in favor of cost savings in women because the prevalence of CAD in this population is relatively low, whereas in men, the costs rose because their CAD prevalence is relatively high. CT coronary angiography increased QALYs in women through a similar mechanism as it did in men, but it also did so by shielding appropriately discharged patients from the risks of diagnostic testing, particularly cardiac catheterization. Thus, no incremental cost-effectiveness ratio was calculated for women, and CT coronary angiography was deemed cost-saving.
The results of our analysis generally meet our a priori expectations of the impact of CT coronary angiography on costs and health outcomes: A technology with its diagnostic characteristics would be expected to be cost-saving in populations with suspected ACS but a low prevalence of CAD. In populations with a higher prevalence of CAD, we believe the net impact of CT coronary angiography is more difficult to predict: Admissions might increase because of the test's sensitivity, raising costs but potentially improving health outcomes, or costs could fall because of savings from emergency department discharges.
In future studies, we hope researchers will analyze the cost-effectiveness of CT coronary angiography in the context of other management algorithms. For example, although we referred patients with mild stenosis on CT coronary angiography for stress testing, these patients could plausibly be discharged. Managing them in this manner may have significant health and economic consequences. Similarly, although we assumed that patients undergoing stress tests were equally likely to undergo stress ECG, stress echocardiography, or SPECT, the choice of diagnostic test could be better tailored to a patient's demographic and clinical characteristics. As one recent review of nuclear imaging suggests, the primary test choice for some patients may be CT coronary angiography, with or without SPECT, whereas other patients may benefit more from the anatomic and functional information gleaned from cardiac MRI [41]. These patient-tailored algorithms could be incorporated into a decision tree.
Emergency department, hospital, and total health expenditures, along with the estimated effectiveness of the two management strategies, were sensitive to several parameters, including the rate at which patients with low-risk chest pain returned to the emergency department, the test characteristics of 64-MDCT, and the cost of CT coronary angiography. Nonetheless, 64-MDCT coronary angiography-based triage was generally still associated with improved health benefits in both sexes and cost savings in women when compared with the SOC.
Our results may not be generalizable to other countries, where the demographics and epidemiology of CAD vary and the economic value of resources used in care also differ [42, 43]. Furthermore, in the United States, the generalizability of our results may be limited to regions in which disease distribution, clinical practices, and economic costs are similar to those we evaluated. However, the sensitivity analyses suggest that our results are applicable under many different conditions.
Our study has several limitations. We modeled a clinical situation in which the population of interest has several key unknowns, including the distribution of diagnoses and the extent of arterial disease. Because of the lack of data in this area, we assumed age- and sex-independent distributions of diagnoses for different demographic groups and ascribed vessel disease distributions using a parallel patient population. Data on these parameters would have improved the analysis but were not available in the literature. We also did not incorporate information on whether patients had a history of diabetes, hypertension, hypercholesterolemia, or smoking for similar reasons. These inputs would have made our study more robust.
We also assumed that patients with noncardiac chest pain did not face an increased risk of death from being discharged instead of hospitalized. Although they may theoretically be suffering from a multitude of fatal conditions, including pulmonary embolism, esophageal rupture, and other conditions, we assumed that these patients could be differentiated from patients who can safely be followed up in a primary care setting.
Also, we did not consider the economic or health impact of incidental findings discovered on CT coronary angiography. These findings would almost certainly increase the costs associated with CT coronary angiography-based strategies for triaging patients with acute chest pain but may also be associated with improved longevity, depending on the distribution of undiagnosed conditions in that population. This is an important area of uncertainty that warrants further investigation.
Finally, in the randomized controlled trial conducted by Goldstein et al. [14], patients with mild coronary atherosclerosis (< 25% stenosis) on CT coronary angiography were discharged, whereas we submitted these patients to stress testing in our model. This difference highlights the general issue of differential interpretation and application of test results, and the impact of MDCT on patient outcomes is likely to be sensitive to the treatment thresholds applied by clinicians.
Related to this issue are the implications of CT coronary angiography for patient resource utilization in the outpatient setting. Negative CT coronary angiography examinations may provide clinicians and patients with reassurance and extinguish any intentions of performing further CAD diagnostic studies in an outpatient setting. In contrast, some CT coronary angiography examinations will identify incidental findings that may require regular follow-up long after the patient is discharged from the hospital. We conjecture that the overall impact of CT coronary angiography on outpatient resource utilization is ambiguous and will require further investigation.
Despite these limitations, our results suggest that 64-MDCT coronary angiography may be a safe addition to the management of low-risk patients with acute chest pain and is likely to be cost-saving in women and marginally more expensive than the SOC in men. Its high sensitivity for coronary atherosclerosis compared with conventional diagnostic technologies may also hasten the identification and treatment of CAD in patients with an unknown history of CAD. Clinical trials evaluating this technology are under way and may ultimately illuminate a more efficient and cost-effective management approach to low-risk patients with chest pain in the emergency department.
Acknowledgments
We thank Claudia Chae for her helpful comments on the structure of the
simulation model.
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