- Email: [email protected]
Using a stroke database to develop an optimal vascular imaging protocol for ischemic stroke
Using a Stroke Database to Develop an Optimal Vascular Imaging Protocol for Ischemic Stroke Kamakshi Lakshminarayan, MBBS, PhD,* David C. Anderson, and Raymond A. Gensinger, Jr, MD†
MD,*,†
Objective: There are many combinations of studies for imaging the cervicocranial vasculature for diagnosis of ischemic stroke mechanism. We demonstrate a simple approach for using data from a stroke database to determine an optimal sequence of imaging studies on the basis of cost minimization and diagnostic yield. Methods: Our study uses patient data from the Hennepin County Medical Center stroke database, Minneapolis, Minn, to compare two vascular imaging protocols. In protocol 1, carotid ultrasound was the first vascular investigation for clinically determined anterior circulation strokes. Additional imaging with cervical and intracranial magnetic resonance angiography was obtained in nonanterior circulation cases and in cases when carotid ultrasound did not explain the mechanism for anterior circulation events. In protocol 2, cervical and intracranial magnetic resonance angiography was the first vascular investigation irrespective of the presumed topography of the stroke. Results: The mean cost per patient for imaging workup with protocol 1 was $702. With protocol 2, the cost was $679. Protocol 2 also provided clinically relevant information regarding tandem lesions and variations in intracranial collateral patterns that would have been missed by strictly following protocol 1. Conclusions: Our results indicate that, given our hospital costs and patient population, obtaining a cervical and intracranial magnetic resonance angiogram after acute ischemic stroke leads to a lower average cost per patient than obtaining a carotid ultrasound first on presumed anterior circulation events. This result depends on the relative costs of imaging studies and could be different among hospitals serving different patient populations. Key Words: Carotid ultrasound—imaging protocol—stroke database. © 2004 by National Stroke Association
The purpose of the diagnostic workup of a patient with acute stroke is to characterize the lesion and, in the case of ischemic stroke, determine its mechanism. Diagnostic investigations include imaging of the cervical and intracranial (C&I) circulation to identify vascular lesions. In this article, we demonstrate how data from a stroke database can be
From the *Department of Neurology, University of Minnesota, and †Hennepin County Medical Center, Minneapolis. Received February 20, 2004; accepted April 2, 2004. Supported by a clinical research training fellowship from the American Academy of Neurology (Dr Lakshminarayan). The Hennepin County Medical Center Stroke database is maintained by grant MMRF 3434. Address correspondence to: Kamakshi Lakshminarayan, MBBS, PhD, Department of Neurology, University of Minnesota, MMC 295, 420 Delaware St SE, Minneapolis, MN 55455. E-mail: [email protected]. 1052-3057/$—see front matter © 2004 by National Stroke Association doi:10.1016/j.jstrokecerebrovasdis.2004.04.002
used to determine an optimal sequence of imaging studies on the basis of cost minimization and diagnostic yield. Prior studies have focused on efficacy and cost-effectiveness of imaging strategies as a prelude to carotid endarterectomy,1,2 rather than for diagnosis of stroke mechanism. Our focus in this article is on mechanism diagnosis after ischemic stroke with the understanding that explanatory vascular pathology may be found intracranially or in the cervical carotid vessels. Investigations of other stroke mechanisms such as cardioembolism are beyond the scope of this article.
Methods We used actual hospital costs of various imaging studies as a measure of resource use. All analyses were retrospective and did not interfere with patient care. Patient data from the Hennepin County Medical Center,
Journal of Stroke and Cerebrovascular Diseases, Vol. 13, No. 3 (May-June), 2004: pp 113-117
113
K. LAKSHMINARAYAN ET AL.
114
Figure 1. Two imaging protocols. Number of patients is indicated in parenthesis. Classification of strokes as belonging to anterior versus nonanterior circulation was obtained from attending neurologist’s admission dictation where presumed vascular territory of stroke is indicated. C&I, Cervical and intracranial; CUS, carotid ultrasound; MRA, magnetic resonance angiography.
Minneapolis, Minn, (HCMC) stroke database were used to compare expected cost per patient under two vascular imaging protocols. We also performed a sensitivity analysis on the results. Subsequently, we reviewed additional cases looking for differences in useful clinical information obtained by following one or the other imaging protocol. Permission was obtained from the HCMC internal review board for the use of the patient data discussed in this article.
Different Imaging Protocols Two vascular imaging protocols were compared (Fig 1). In protocol 1, carotid ultrasound (CUS) was used as a first vascular investigation for clinically determined anterior circulation strokes. C&I magnetic resonance (MR) angiography (MRA) was obtained in nonanterior circulation cases and when the CUS did not explain the stroke mechanism in anterior circulation cases. At HCMC, the radiology imaging protocol always includes the intracranial MRA with the cervical MRA. Hence, intracranial MRA is never ordered alone. In protocol 2, C&I MRA was the first vascular investigation irrespective of presumed topography of the stroke. The classification of strokes as belonging to anterior versus nonanterior circulation was obtained from the attending neurologist’s
admission note where the presumed vascular territory of the stroke is indicated.
Cost Calculation The two protocols were compared using the mean cost per patient under each protocol. The calculation of this mean cost for each protocol is described in detail in the analysis section. Costs of all imaging studies were provided by the radiology department of HCMC. The total cost of an imaging study may have been calculated by adding direct and indirect costs of the particular imaging center where the study is performed. Direct costs consist of patient-related activity and material. Examples include minutes of labor and supplies used. Indirect costs of an imaging study are allotted in a formulaic manner on the basis of area in square feet of the imaging center, and the direct cost of the imaging study. Indirect costs include depreciation, housekeeping, overhead, billing and finance, security, and rent. The costs do not include “profees,” which are the fees charged by the radiologists for the interpretation of imaging studies. Hence, the costs represent the actual costs to HCMC of performing the imaging study and are not the charges billed to the patient. The costs discussed in this article pertain to the
STROKE DATABASE USE IN DEVELOPING IMAGING PROTOCOLS
year 2002 and are in US dollars. The cost of performing a CUS was $138.15 and that of a C&I MRA was $678.95.
our patient population the mean cost per patient with protocol 2 was $23 less than protocol 1.
Sensitivity Analysis
Patient Data The two protocols were compared using patient data in the HCMC stroke database. This database contained clinical information on 107 consecutive patients at the time of our analyses. Figure 1 shows the numbers of patients with anterior and nonanterior circulation strokes and the number with explanatory (⬎50% of lumen diameter) carotid stenosis by ultrasound. The use of 50% as the cutoff for explanatory stenosis is on the basis of the stroke subtype classification system used in the TOAST trial.3 The expected cost per patient for the entire imaging workup was calculated for each protocol. Under protocol 1, patients with presumed anterior circulation strokes who have significant stenosis on CUS will not receive a C&I MRA.
Results Results are presented as mean cost per patient and are rounded to the nearest US dollar after calculation. The mean cost per patient is the total cost for all patients divided by the total number of patients. For protocol 2 this will work out to be the cost of the C&I MRA. For protocol 1, the total cost for all patients is the sum of: (1) total costs of the all the CUS studies–as can be seen from Figure 1, only patients with anterior circulation symptoms will receive CUS evaluations; and (2) total costs of all the C&I MRA studies–patients who will obtain C&I MRA under protocol 1 include those with nonanterior circulation symptoms and those with anterior circulation symptoms but without explanatory stenosis on CUS. This sum divided by the total number of patients will yield the mean cost for protocol 1. The two equations below summarize the calculation of mean cost for each protocol. (Nant ⫻ Costcus) ⫹ ((Nant ⫺ Nant_stenosis ⫹ Nnonant) ⫻ Costmra) Protocol 1 ⫽ (1) N Protocol 2 ⫽
115
N ⫻ Costmra ⫽ Costmra N
(2)
N, Nant, Nant_stenosis, and Nnonant refer to total number of patients, number of patients with anterior circulation symptoms, number of patients with anterior circulation symptoms plus explanatory carotid stenosis on CUS, and number of patients with nonanterior circulation symptoms, respectively. Costcus and Costmra denote costs of CUS and C&I MRA, respectively. The cost per patient with protocol 2 was $679, which is lower than the cost with protocol 1, which was $702. Hence, on the basis of
Sensitivity analysis was done to examine the effects of varying the probability of anterior circulation events, the probability of stenosis on CUS, and the imaging costs. Substituting probabilities in the two equations above the expected cost for each protocol is calculated as: Protocol 1 ⫽ Pant ⫻ Costcus ⫹ (Pnonant
(3)
⫹ (Pant ⫺ Pant_stenosis))Costmra Protocol 2 ⫽ Costmra
(4)
Pant and Pnonant denote the probabilities of anterior and nonanterior circulation events, respectively, and Pant_stenosis denotes the probability of anterior circulation events with explanatory stenosis on CUS. These two equations imply that protocol 2 will be less expensive than protocol 1 as long as: Cost mra ⬍ P ant ⫻ Cost cus ⫹ (P nonant
(5)
⫹ (P ant ⫺ P ant_stenosis ))Cost mra Simplifying this we see that protocol 2 will remain less expensive than protocol 1 when: Cost cus P ant_stenosis ⬎ . Cost mra P ant
(6)
In other words, protocol 2 will incur less mean cost per patient when the ratio of the cost of CUS to that of the C&I MRA is greater than the ratio of the observed probabilities of anterior circulation cases with explanatory stenosis to that of anterior circulation cases alone. The above relationship is represented by the graph in Figure 2. Using the data from the HCMC database as shown in Figure 1 and costs of imaging studies as described in the previous section, we see that the cost ratio of the CUS to MRA is 138.15/678.95, which is 0.204. The ratio of the proportion of cases with anterior circulation symptoms and explanatory stenosis to the proportion of anterior circulation cases alone is (11/107)/(72/107), which is 0.153. This value is shown on the graph and falls in the area where protocol 2 costs less.
Discussion Better Resource Use Through Cost Minimization The results indicate that obtaining a C&I MRA on all patients with acute stroke leads to a lower cost than obtaining a CUS first on presumed anterior circulation events. This may seem surprising at first glance because a CUS is less expensive than an MRA. However, many
116
K. LAKSHMINARAYAN ET AL.
Figure 2. Sensitivity analysis. Probabilities can be determined from local stroke databases. Hennepin County Medical Center (HCMC), Minneapolis, Minn, data fall in area where protocol 2 is less expensive than protocol 1. Costmra, cost of magnetic resonance angiography; Costcus, costs of carotid ultrasound; Pant, probabilities of anterior circulation events; Pant_stenosis, probability of anterior circulation events with explanatory carotid stenosis.
patients with anterior circulation symptoms go on to undergo an MRA after the CUS because the proportion of these patients who have significant carotid stenosis is small. Further, because of radiology protocols at HCMC, the C&I vessels are always imaged together by MR. Because CUS is inadequate for imaging high bifurcations or distal internal carotid artery stenosis, the protocol includes a cervical MRA in protocol 1 in those cases where the CUS did not uncover explanatory stenosis. Sensitivity analysis finds that a preliminary CUS can provide savings (i.e., protocol 1 becomes less expensive than protocol 2) when the proportion of cases with significant carotid stenosis exceeds a threshold. This threshold will depend on the proportion of anterior circulation cases, the proportion with significant carotid stenosis, and costs of imaging studies. Hence, the relative costs of the protocols will vary among hospitals serving different populations. The analyses presented in this article shows that ordering investigations sequentially from the least to most costly may hurt overall cost-efficiency when the least costly tests are ultimately redundant or uninformative with high probability.
Improving Patient Treatment The sensitivities and specificities of a CUS and MRA at various levels of stenosis are considered comparable.4 At HCMC, neither the CUS nor MRA is sufficient investigation for carotid endarterectomy and this position is consistent with reported studies.5,6 Hence, surgical candi-
dates with explanatory stenosis of the carotid vessels typically go on to undergo an invasive angiogram, and obtaining a CUS versus an MRA will not change this for the patient. This may, however, change as some centers are using a combination of CUS and contrast-enhanced MRA to make endarterectomy decisions.7 Aside from cost considerations, the clinical information obtained from C&I MRA is greater than from a CUS because the intracranial circulation is visualized with the former and not the latter. A patient may have what seems to be an explanatory stenosis on CUS and also have a tandem intracranial stenosis that can be missed by protocol 1. In a fresh series of 50 patients with ischemic stroke from the HCMC stroke database, we found that 3 patients had tandem extracranial and intracranial lesions. Protocol 2 can also provide useful, practical information regarding intracranial collateral variations influencing decisions on volume therapy, blood pressure thresholds, and speed of mobilization after stroke. In addition, intracranial vascular connections are relevant for assessing area at risk for future strokes. For example, fetal origin of posterior cerebral arteries was present in 4 of the series of 50 cases mentioned above. One of these would have been missed with clinical consequences by strictly following protocol 1. In this one case, the patient had bilateral anterior circulation strokes and bilateral carotid stenosis. Protocol 1 would have stopped after the CUS because the strokes are explained by the results of the CUS. However, the patient had fetal origin of the posterior cerebral artery
STROKE DATABASE USE IN DEVELOPING IMAGING PROTOCOLS
on one side, which placed the posterior circulation on that side at risk from the carotid stenosis. In contrast, protocol 2 would have obtained an intracranial MRA regardless of explanatory carotid stenosis and would not have missed this clinically relevant information. One could also argue that in clear anterior circulation strokes absence of extracranial stenosis by CUS should be a stopping criterion for vascular investigation and that further intracranial evaluation is unnecessary because there are no clear evidence-based interventions for intracranial stenosis yet. This argument is valid if the goal of stroke workup is only immediate management. However, we would argue that it is important to diagnose mechanism because the early recurrence rates and functional outcomes are influenced by stroke mechanism.8-10 A drawback of our vascular protocol is that MR imaging cannot be used in all patients. For example, pacemakers are not MR compatible. In such patients CUS will be necessary. We have also not addressed the use of a transcranial Doppler study for examining the intracranial circulation. Where that option is available vascular imaging protocols incorporating transcranial Doppler can be examined using real data from stroke databases and the same reasoning that we have demonstrated above. A factor of importance to smaller hospitals is the time lag for getting a MR imaging study. In settings unable to obtain MR studies quickly, length of hospital stay must be taken into account while calculating relative costs of imaging protocols. In conclusion, various combinations of imaging studies can be chosen to determine stroke mechanism. We have shown how stroke databases or registries at individual hospitals can be used to develop imaging protocols optimizing cost and quality (diagnostic yield). Our strategy of doing a C&I MRA on all patients with ischemic stroke is appropriate for our hospital and patient population. However, the optimal strategy may differ among hospitals, and stroke databases can help determine the best protocol for each hospital. This type of retrospective analysis is inexpensive and easy because the relevant data are already being compiled for other reasons. Hav-
117
ing a fixed imaging protocol saves time, avoids duplicate efforts, and provides uniform care. Acknowledgment: The authors thank Mary DuPlessisTchida for assistance with data and Susan Bubany for information on cost and billing practices. The authors also thank Michele Gerhartz, Dawn Mayasich, David Berg, Nancy Bushek, and Diane Kelly for help with costs of imaging studies and for explaining the structure of various radiology protocols.
References 1. Carriero A, Ucchino S, Magarelli N, et al. Carotid bifurcation stenosis: A comparative study between MR angiography and duplex scanning with respect to digital subtraction angiography. J Neuroradiol 1995;22:103-111. 2. Vanninen R, Manninen H, Soimakallio S. Imaging of carotid artery stenosis: Clinical efficacy and cost-effectiveness. AJNR Am J Neuroradiol 1995;16:1875-1883. 3. Adams HP, Bendixen BH, Kapelle LJ, et al, and TOAST investigators. Classification of subtype of acute ischemic stroke: Definitions for use in a multicenter clinical trial. Stroke 1993;24:35-41. 4. Blakeley DD, Oddone EZ, Hasselblad V, et al. Noninvasive carotid testing: A meta-analytic review. Ann Intern Med 1995;122:360-367. 5. Johnston DC, Eastwood JD, Nguyen T, et al. Contrastenhanced magnetic resonance angiography of carotid arteries: Utility in routine clinical practice. Stroke 2002; 33:2834-2838. 6. Johnston DC, Goldstein LB. Clinical carotid endarterectomy decision making. Neurology 2001;56:1009-1015. 7. Friese S, Krapf H, Fetter M, et al. Ultrasonography and contrast-enhanced MRA in ICA-stenosis: Is conventional angiography obsolete? J Neurol 2001;248:506-513. 8. Petty GW, Brown RD, Whisnant JP, et al. Ischemic stroke subtypes: A population-based study of functional outcome, survival and recurrence. Stroke 2000;31:1062-1068. 9. Yamamoto H, Bogousslavsky J. Mechanisms of second and further strokes. J Neurol Neurosurg Psychiatry 1998; 64:771-776. 10. Murat SM, Erturk O. Ischemic stroke subtypes: Risk factors, functional outcome and recurrence. Neurol Sci 2002;22:449-454.
MD,*,†
Objective: There are many combinations of studies for imaging the cervicocranial vasculature for diagnosis of ischemic stroke mechanism. We demonstrate a simple approach for using data from a stroke database to determine an optimal sequence of imaging studies on the basis of cost minimization and diagnostic yield. Methods: Our study uses patient data from the Hennepin County Medical Center stroke database, Minneapolis, Minn, to compare two vascular imaging protocols. In protocol 1, carotid ultrasound was the first vascular investigation for clinically determined anterior circulation strokes. Additional imaging with cervical and intracranial magnetic resonance angiography was obtained in nonanterior circulation cases and in cases when carotid ultrasound did not explain the mechanism for anterior circulation events. In protocol 2, cervical and intracranial magnetic resonance angiography was the first vascular investigation irrespective of the presumed topography of the stroke. Results: The mean cost per patient for imaging workup with protocol 1 was $702. With protocol 2, the cost was $679. Protocol 2 also provided clinically relevant information regarding tandem lesions and variations in intracranial collateral patterns that would have been missed by strictly following protocol 1. Conclusions: Our results indicate that, given our hospital costs and patient population, obtaining a cervical and intracranial magnetic resonance angiogram after acute ischemic stroke leads to a lower average cost per patient than obtaining a carotid ultrasound first on presumed anterior circulation events. This result depends on the relative costs of imaging studies and could be different among hospitals serving different patient populations. Key Words: Carotid ultrasound—imaging protocol—stroke database. © 2004 by National Stroke Association
The purpose of the diagnostic workup of a patient with acute stroke is to characterize the lesion and, in the case of ischemic stroke, determine its mechanism. Diagnostic investigations include imaging of the cervical and intracranial (C&I) circulation to identify vascular lesions. In this article, we demonstrate how data from a stroke database can be
From the *Department of Neurology, University of Minnesota, and †Hennepin County Medical Center, Minneapolis. Received February 20, 2004; accepted April 2, 2004. Supported by a clinical research training fellowship from the American Academy of Neurology (Dr Lakshminarayan). The Hennepin County Medical Center Stroke database is maintained by grant MMRF 3434. Address correspondence to: Kamakshi Lakshminarayan, MBBS, PhD, Department of Neurology, University of Minnesota, MMC 295, 420 Delaware St SE, Minneapolis, MN 55455. E-mail: [email protected]. 1052-3057/$—see front matter © 2004 by National Stroke Association doi:10.1016/j.jstrokecerebrovasdis.2004.04.002
used to determine an optimal sequence of imaging studies on the basis of cost minimization and diagnostic yield. Prior studies have focused on efficacy and cost-effectiveness of imaging strategies as a prelude to carotid endarterectomy,1,2 rather than for diagnosis of stroke mechanism. Our focus in this article is on mechanism diagnosis after ischemic stroke with the understanding that explanatory vascular pathology may be found intracranially or in the cervical carotid vessels. Investigations of other stroke mechanisms such as cardioembolism are beyond the scope of this article.
Methods We used actual hospital costs of various imaging studies as a measure of resource use. All analyses were retrospective and did not interfere with patient care. Patient data from the Hennepin County Medical Center,
Journal of Stroke and Cerebrovascular Diseases, Vol. 13, No. 3 (May-June), 2004: pp 113-117
113
K. LAKSHMINARAYAN ET AL.
114
Figure 1. Two imaging protocols. Number of patients is indicated in parenthesis. Classification of strokes as belonging to anterior versus nonanterior circulation was obtained from attending neurologist’s admission dictation where presumed vascular territory of stroke is indicated. C&I, Cervical and intracranial; CUS, carotid ultrasound; MRA, magnetic resonance angiography.
Minneapolis, Minn, (HCMC) stroke database were used to compare expected cost per patient under two vascular imaging protocols. We also performed a sensitivity analysis on the results. Subsequently, we reviewed additional cases looking for differences in useful clinical information obtained by following one or the other imaging protocol. Permission was obtained from the HCMC internal review board for the use of the patient data discussed in this article.
Different Imaging Protocols Two vascular imaging protocols were compared (Fig 1). In protocol 1, carotid ultrasound (CUS) was used as a first vascular investigation for clinically determined anterior circulation strokes. C&I magnetic resonance (MR) angiography (MRA) was obtained in nonanterior circulation cases and when the CUS did not explain the stroke mechanism in anterior circulation cases. At HCMC, the radiology imaging protocol always includes the intracranial MRA with the cervical MRA. Hence, intracranial MRA is never ordered alone. In protocol 2, C&I MRA was the first vascular investigation irrespective of presumed topography of the stroke. The classification of strokes as belonging to anterior versus nonanterior circulation was obtained from the attending neurologist’s
admission note where the presumed vascular territory of the stroke is indicated.
Cost Calculation The two protocols were compared using the mean cost per patient under each protocol. The calculation of this mean cost for each protocol is described in detail in the analysis section. Costs of all imaging studies were provided by the radiology department of HCMC. The total cost of an imaging study may have been calculated by adding direct and indirect costs of the particular imaging center where the study is performed. Direct costs consist of patient-related activity and material. Examples include minutes of labor and supplies used. Indirect costs of an imaging study are allotted in a formulaic manner on the basis of area in square feet of the imaging center, and the direct cost of the imaging study. Indirect costs include depreciation, housekeeping, overhead, billing and finance, security, and rent. The costs do not include “profees,” which are the fees charged by the radiologists for the interpretation of imaging studies. Hence, the costs represent the actual costs to HCMC of performing the imaging study and are not the charges billed to the patient. The costs discussed in this article pertain to the
STROKE DATABASE USE IN DEVELOPING IMAGING PROTOCOLS
year 2002 and are in US dollars. The cost of performing a CUS was $138.15 and that of a C&I MRA was $678.95.
our patient population the mean cost per patient with protocol 2 was $23 less than protocol 1.
Sensitivity Analysis
Patient Data The two protocols were compared using patient data in the HCMC stroke database. This database contained clinical information on 107 consecutive patients at the time of our analyses. Figure 1 shows the numbers of patients with anterior and nonanterior circulation strokes and the number with explanatory (⬎50% of lumen diameter) carotid stenosis by ultrasound. The use of 50% as the cutoff for explanatory stenosis is on the basis of the stroke subtype classification system used in the TOAST trial.3 The expected cost per patient for the entire imaging workup was calculated for each protocol. Under protocol 1, patients with presumed anterior circulation strokes who have significant stenosis on CUS will not receive a C&I MRA.
Results Results are presented as mean cost per patient and are rounded to the nearest US dollar after calculation. The mean cost per patient is the total cost for all patients divided by the total number of patients. For protocol 2 this will work out to be the cost of the C&I MRA. For protocol 1, the total cost for all patients is the sum of: (1) total costs of the all the CUS studies–as can be seen from Figure 1, only patients with anterior circulation symptoms will receive CUS evaluations; and (2) total costs of all the C&I MRA studies–patients who will obtain C&I MRA under protocol 1 include those with nonanterior circulation symptoms and those with anterior circulation symptoms but without explanatory stenosis on CUS. This sum divided by the total number of patients will yield the mean cost for protocol 1. The two equations below summarize the calculation of mean cost for each protocol. (Nant ⫻ Costcus) ⫹ ((Nant ⫺ Nant_stenosis ⫹ Nnonant) ⫻ Costmra) Protocol 1 ⫽ (1) N Protocol 2 ⫽
115
N ⫻ Costmra ⫽ Costmra N
(2)
N, Nant, Nant_stenosis, and Nnonant refer to total number of patients, number of patients with anterior circulation symptoms, number of patients with anterior circulation symptoms plus explanatory carotid stenosis on CUS, and number of patients with nonanterior circulation symptoms, respectively. Costcus and Costmra denote costs of CUS and C&I MRA, respectively. The cost per patient with protocol 2 was $679, which is lower than the cost with protocol 1, which was $702. Hence, on the basis of
Sensitivity analysis was done to examine the effects of varying the probability of anterior circulation events, the probability of stenosis on CUS, and the imaging costs. Substituting probabilities in the two equations above the expected cost for each protocol is calculated as: Protocol 1 ⫽ Pant ⫻ Costcus ⫹ (Pnonant
(3)
⫹ (Pant ⫺ Pant_stenosis))Costmra Protocol 2 ⫽ Costmra
(4)
Pant and Pnonant denote the probabilities of anterior and nonanterior circulation events, respectively, and Pant_stenosis denotes the probability of anterior circulation events with explanatory stenosis on CUS. These two equations imply that protocol 2 will be less expensive than protocol 1 as long as: Cost mra ⬍ P ant ⫻ Cost cus ⫹ (P nonant
(5)
⫹ (P ant ⫺ P ant_stenosis ))Cost mra Simplifying this we see that protocol 2 will remain less expensive than protocol 1 when: Cost cus P ant_stenosis ⬎ . Cost mra P ant
(6)
In other words, protocol 2 will incur less mean cost per patient when the ratio of the cost of CUS to that of the C&I MRA is greater than the ratio of the observed probabilities of anterior circulation cases with explanatory stenosis to that of anterior circulation cases alone. The above relationship is represented by the graph in Figure 2. Using the data from the HCMC database as shown in Figure 1 and costs of imaging studies as described in the previous section, we see that the cost ratio of the CUS to MRA is 138.15/678.95, which is 0.204. The ratio of the proportion of cases with anterior circulation symptoms and explanatory stenosis to the proportion of anterior circulation cases alone is (11/107)/(72/107), which is 0.153. This value is shown on the graph and falls in the area where protocol 2 costs less.
Discussion Better Resource Use Through Cost Minimization The results indicate that obtaining a C&I MRA on all patients with acute stroke leads to a lower cost than obtaining a CUS first on presumed anterior circulation events. This may seem surprising at first glance because a CUS is less expensive than an MRA. However, many
116
K. LAKSHMINARAYAN ET AL.
Figure 2. Sensitivity analysis. Probabilities can be determined from local stroke databases. Hennepin County Medical Center (HCMC), Minneapolis, Minn, data fall in area where protocol 2 is less expensive than protocol 1. Costmra, cost of magnetic resonance angiography; Costcus, costs of carotid ultrasound; Pant, probabilities of anterior circulation events; Pant_stenosis, probability of anterior circulation events with explanatory carotid stenosis.
patients with anterior circulation symptoms go on to undergo an MRA after the CUS because the proportion of these patients who have significant carotid stenosis is small. Further, because of radiology protocols at HCMC, the C&I vessels are always imaged together by MR. Because CUS is inadequate for imaging high bifurcations or distal internal carotid artery stenosis, the protocol includes a cervical MRA in protocol 1 in those cases where the CUS did not uncover explanatory stenosis. Sensitivity analysis finds that a preliminary CUS can provide savings (i.e., protocol 1 becomes less expensive than protocol 2) when the proportion of cases with significant carotid stenosis exceeds a threshold. This threshold will depend on the proportion of anterior circulation cases, the proportion with significant carotid stenosis, and costs of imaging studies. Hence, the relative costs of the protocols will vary among hospitals serving different populations. The analyses presented in this article shows that ordering investigations sequentially from the least to most costly may hurt overall cost-efficiency when the least costly tests are ultimately redundant or uninformative with high probability.
Improving Patient Treatment The sensitivities and specificities of a CUS and MRA at various levels of stenosis are considered comparable.4 At HCMC, neither the CUS nor MRA is sufficient investigation for carotid endarterectomy and this position is consistent with reported studies.5,6 Hence, surgical candi-
dates with explanatory stenosis of the carotid vessels typically go on to undergo an invasive angiogram, and obtaining a CUS versus an MRA will not change this for the patient. This may, however, change as some centers are using a combination of CUS and contrast-enhanced MRA to make endarterectomy decisions.7 Aside from cost considerations, the clinical information obtained from C&I MRA is greater than from a CUS because the intracranial circulation is visualized with the former and not the latter. A patient may have what seems to be an explanatory stenosis on CUS and also have a tandem intracranial stenosis that can be missed by protocol 1. In a fresh series of 50 patients with ischemic stroke from the HCMC stroke database, we found that 3 patients had tandem extracranial and intracranial lesions. Protocol 2 can also provide useful, practical information regarding intracranial collateral variations influencing decisions on volume therapy, blood pressure thresholds, and speed of mobilization after stroke. In addition, intracranial vascular connections are relevant for assessing area at risk for future strokes. For example, fetal origin of posterior cerebral arteries was present in 4 of the series of 50 cases mentioned above. One of these would have been missed with clinical consequences by strictly following protocol 1. In this one case, the patient had bilateral anterior circulation strokes and bilateral carotid stenosis. Protocol 1 would have stopped after the CUS because the strokes are explained by the results of the CUS. However, the patient had fetal origin of the posterior cerebral artery
STROKE DATABASE USE IN DEVELOPING IMAGING PROTOCOLS
on one side, which placed the posterior circulation on that side at risk from the carotid stenosis. In contrast, protocol 2 would have obtained an intracranial MRA regardless of explanatory carotid stenosis and would not have missed this clinically relevant information. One could also argue that in clear anterior circulation strokes absence of extracranial stenosis by CUS should be a stopping criterion for vascular investigation and that further intracranial evaluation is unnecessary because there are no clear evidence-based interventions for intracranial stenosis yet. This argument is valid if the goal of stroke workup is only immediate management. However, we would argue that it is important to diagnose mechanism because the early recurrence rates and functional outcomes are influenced by stroke mechanism.8-10 A drawback of our vascular protocol is that MR imaging cannot be used in all patients. For example, pacemakers are not MR compatible. In such patients CUS will be necessary. We have also not addressed the use of a transcranial Doppler study for examining the intracranial circulation. Where that option is available vascular imaging protocols incorporating transcranial Doppler can be examined using real data from stroke databases and the same reasoning that we have demonstrated above. A factor of importance to smaller hospitals is the time lag for getting a MR imaging study. In settings unable to obtain MR studies quickly, length of hospital stay must be taken into account while calculating relative costs of imaging protocols. In conclusion, various combinations of imaging studies can be chosen to determine stroke mechanism. We have shown how stroke databases or registries at individual hospitals can be used to develop imaging protocols optimizing cost and quality (diagnostic yield). Our strategy of doing a C&I MRA on all patients with ischemic stroke is appropriate for our hospital and patient population. However, the optimal strategy may differ among hospitals, and stroke databases can help determine the best protocol for each hospital. This type of retrospective analysis is inexpensive and easy because the relevant data are already being compiled for other reasons. Hav-
117
ing a fixed imaging protocol saves time, avoids duplicate efforts, and provides uniform care. Acknowledgment: The authors thank Mary DuPlessisTchida for assistance with data and Susan Bubany for information on cost and billing practices. The authors also thank Michele Gerhartz, Dawn Mayasich, David Berg, Nancy Bushek, and Diane Kelly for help with costs of imaging studies and for explaining the structure of various radiology protocols.
References 1. Carriero A, Ucchino S, Magarelli N, et al. Carotid bifurcation stenosis: A comparative study between MR angiography and duplex scanning with respect to digital subtraction angiography. J Neuroradiol 1995;22:103-111. 2. Vanninen R, Manninen H, Soimakallio S. Imaging of carotid artery stenosis: Clinical efficacy and cost-effectiveness. AJNR Am J Neuroradiol 1995;16:1875-1883. 3. Adams HP, Bendixen BH, Kapelle LJ, et al, and TOAST investigators. Classification of subtype of acute ischemic stroke: Definitions for use in a multicenter clinical trial. Stroke 1993;24:35-41. 4. Blakeley DD, Oddone EZ, Hasselblad V, et al. Noninvasive carotid testing: A meta-analytic review. Ann Intern Med 1995;122:360-367. 5. Johnston DC, Eastwood JD, Nguyen T, et al. Contrastenhanced magnetic resonance angiography of carotid arteries: Utility in routine clinical practice. Stroke 2002; 33:2834-2838. 6. Johnston DC, Goldstein LB. Clinical carotid endarterectomy decision making. Neurology 2001;56:1009-1015. 7. Friese S, Krapf H, Fetter M, et al. Ultrasonography and contrast-enhanced MRA in ICA-stenosis: Is conventional angiography obsolete? J Neurol 2001;248:506-513. 8. Petty GW, Brown RD, Whisnant JP, et al. Ischemic stroke subtypes: A population-based study of functional outcome, survival and recurrence. Stroke 2000;31:1062-1068. 9. Yamamoto H, Bogousslavsky J. Mechanisms of second and further strokes. J Neurol Neurosurg Psychiatry 1998; 64:771-776. 10. Murat SM, Erturk O. Ischemic stroke subtypes: Risk factors, functional outcome and recurrence. Neurol Sci 2002;22:449-454.