Simultaneous Carotid and Coronary Arteries Disease: Staged or Combined Surgical Approach?

Systematic review and meta-analysis of the treatment strategies for coronary artery bypass graft patients with concomitant carotid artery atherosclerotic disease

OBJECTIVE

Stroke is one of the devastating complications after coronary artery bypass graft (CABG). Underlying carotid artery atherosclerotic disease is reported to be an independent risk factor. The optimal treatment strategy for these patients remains under debate.

METHODS

We aimed to perform a network meta-analysis to evaluate the safety and efficacy of additional carotid interventions for patients with concomitant carotid artery atherosclerotic disease who require CABG by comparing perioperative adverse event rates. All articles through February 2022 were searched using MEDLINE and EMBASE to identify studies that investigated outcomes of CABG only as well as additional staged vs combined carotid interventions by both carotid endarterectomy (CEA) and carotid artery stenting (CAS).

RESULTS

Two randomized controlled trials and 23 observational studies were included, yielding a total of 32,473 patients who underwent combined CEA and CABG (n = 20,204), CEA and staged CABG (n = 6882), CABG and staged CEA (n = 340), CAS and CABG regardless of timing and sequences (n = 1224), and CABG only (n = 3823). No strategy showed a significant advantage over CABG only in all perioperative outcomes. CEA and staged CABG was associated with the lowest perioperative stroke/transient ischemic attack (TIA) rate, significantly lower compared with CAS and CABG (odds ratio [OR], 0.52; 95% confidence interval [CI], 0.36-0.76) as well as CABG and staged CEA (OR, 0.41; 95% CI, 0.23-0.74), but was also associated with the highest perioperative mortality (OR, 2.50; 95% CI, 1.67-3.85, vs CAS and CABG) and myocardial infarction rate (OR, 3.70 [95% CI, 1.16-12.5] and OR, 2.50 [95% CI, 1.35-4.55] vs CAS and CABG, vs combined CEA and CABG, respectively).

CONCLUSIONS

CEA and staged CABG are associated with low perioperative stroke/transient ischemic attack rates with a tradeoff of higher mortality and myocardial infarction rate. No strategy showed a significant advantage over the CABG-only strategy in all perioperative outcomes, outlining the importance of a tailored approach and determining proper indications for carotid intervention in these patients.

Patients with Polyvascular Disease: A Very High-risk Group

Polyvascular disease (PolyvascDis) with atherosclerosis occurring in >2 vascular beds (coronary, carotid, aortic, visceral and/or peripheral arteries) is encountered in 15-30% of patients who experience greater rates of major adverse cardiovascular (CV) events. Every patient with multiple CV risk factors or presenting with CV disease in one arterial bed should be assessed for PolyvascDis clinically and noninvasively prior to invasive angiography. Peripheral arterial disease (PAD) can be readily diagnosed in routine practice by measuring the ankle-brachial index. Carotid disease can be diagnosed by duplex ultrasound showing % stenosis and/or presence of plaques. Coronary artery disease (CAD) can be screened by determining coronary artery calcium score using coronary computed tomography angiography; further, non-invasive testing includes exercise stress and/or myocardial perfusion imaging or dobutamine stress test, prior to coronary angiography. Abdominal ultrasound can reveal an abdominal aortic aneurysm. Computed tomography angiography will be needed in patients with suspected mesenteric ischemia to assess the mesenteric arteries. Patients with the acute coronary syndrome and concomitant other arterial diseases have more extensive CAD and poorer CV outcomes. Similarly, PolyvascDis in patients with carotid disease and/or other PAD is independently associated with an increased risk for all-cause and CV mortality during long-term follow-up. Treatment of patients with PolyvascDis should include aggressive management of all modifiable risk factors by lifestyle changes and drug therapy, with particular attention to patients who are commonly undertreated, such as those with PAD. Revascularization should be reserved for symptomatic vascular beds, using the least aggressive strategy in a multidisciplinary vascular team approach.

Staged Versus Synchronous Carotid Endarterectomy and Coronary Artery Bypass Grafting: A Meta-Analysis and Systematic Review

BACKGROUND

There are several treatment options for patients with concomitant carotid and coronary artery disease, and it is difficult to identify an optimal treatment strategy that has consensus. Here, we performed a meta-analysis to compare the early and long-term outcomes of staged and synchronous carotid endarterectomy and coronary artery bypass grafting approaches.

METHODS

We performed a meta-analysis that compared staged and synchronous carotid endarterectomy and coronary artery bypass grafting approaches between July 1976 and September 2021. PubMed, EMBASE, and the Cochrane Library were systematically searched for related articles.

RESULTS

Nineteen studies were identified with a total of 39,269 and 30,066 patients in the synchronous and staged groups, respectively. Early mortality was lower in the staged group than in the synchronous group (odds ratio OR 1.256, 95% confidence interval CI 1.006-1.569, P= P < 0.05, I² = 54.5%), and stroke rates were significantly higher in the synchronous group (OR 1.356, 95% CI 1.232-1.493, P < 0.05, I² = 33.3%). The rate of myocardial ischemia was significantly higher in the staged group than in the synchronous group (OR 0.757, 95% CI 0.635-0.903, P < 0.05, I² = 51.5%), and this meta-analysis also showed a significantly higher risk of transient ischemic attacks (TIAs) in the synchronous group (OR 1.335, 95% CI 1.055-1.688, P < 0.05, I² = 0.00%). The secondary outcomes, including the rate of reoperation, were significantly lower for the staged procedure than for the synchronous procedure (OR 1.177, 95% CI 1.052-1.318, P < 0.05, I² = 4.2%), and the rate of wound infection was significantly higher in the synchronous group than in the staged group (OR 0.457, 95% CI 0.403-0.519, P < 0.05, I² = 0.0%). There was no significant difference in the rate of cardiac arrhythmia between the two groups (OR 0.544, 95% CI 0.265-1.117, P > 0.05, I² = 12.7%). There was also no statistical significance in the long-term results regarding the incidence of stroke, myocardial ischemia, and mortality between the synchronous and staged groups (P > 0.05).

CONCLUSIONS

Patients treated with the synchronous approach had a significantly higher risk of early mortality, stroke, TIA, wound infection, and reoperation and a lower risk of myocardial ischemia than those treated with the staged approach. There was no significant difference in the long-term results between the 2 groups.

A Systematic Review Supporting the Society for Vascular Surgery Guidelines on the Management of Carotid Artery Disease

BACKGROUND

To support the development of guidelines on the management of carotid disease, a writing committee from the Society for Vascular Surgery has commissioned this systematic review.

METHODS

We searched multiple data bases for studies addressing 5 questions: medical management vs. carotid revascularization (CEA) in asymptomatic patients, CEA vs. CAS in symptomatic low surgical risk patients, the optimal timing of revascularization after acute stroke, screening high risk patients for carotid disease, and the optimal sequence of interventions in patients with combined coronary and carotid disease. Studies were selected and appraised by pairs of independent reviewers. Meta-analyses were performed when feasible.

RESULTS

Medical management compared to carotid interventions in asymptomatic patients was associated with better early outcome during the first 30 days. However, CEA was associated with significantly lower long-term rate of stroke/death at 5 years. In symptomatic low risk surgical patients, CEA was associated with lower risk of stroke, but a significant increase in MI compared to CAS during the first 30 days. When the long-term outcome of transfemoral CAS vs. CEA in symptomatic patients were examined using pre-planned pooled analysis of individual patient data from four randomized trials, the risk of death or stroke within 120 days of the index procedure was 5.5% for CEA and 8.7% for CAS, which lends support that over the long-term, CEA has superior outcome than transfemoral CAS. When managing acute stroke, the comparison of CEA during the first 48 hours to that between day 2 and day14 did not reveal a statistically significant difference on outcomes during the first 30 days. Registry data show good results with CEA performed in the first week, but not within the first 48 hours. A single risk factor, aside from PAD, was associated with low carotid screening yield. Multiple risk factors greatly increase the yield of screening. Evidence on the timing of interventions in patients with combined carotid and coronary disease was sparse and imprecise. Patients without carotid symptoms, who had the carotid intervention first, compared to a combined carotid intervention and CABG, had better outcomes.

CONCLUSIONS

This updated evidence summary will support the SVS clinical practice guidelines for commonly raised clinical scenarios. CEA was superior to medical therapy in long-term prevention of stroke/death over medical therapy. CEA was also superior to transfemoral CAS in minimizing long-term stroke/death for symptomatic low risk surgical patients. CEA should optimally be performed between 2-14 days from the onset of acute stroke. Having multiple risk factors increases the value of carotid screening.

Synchronous versus Staged Carotid Endarterectomy and Coronary Artery Bypass Graft for Patients with Concomitant Severe Coronary and Carotid Artery Stenosis: A Systematic Review and Meta-analysis

BACKGROUND

Due to the systemic nature of atherosclerosis, arteries at different sites are commonly simultaneously affected. As a result, severe coronary artery disease (CAD) requiring coronary artery bypass grafting (CABG) frequently coexists with significant carotid stenosis that warrants revascularization. To compare simultaneous carotid endarterectomy (CEA) and CABG versus staged CEA and CABG for patients with concomitant CAD and carotid artery stenosis in terms of perioperative outcomes.

METHODS

This study was performed according to the Preferred Reporting Items for Systematic reviews and Meta-Analyses guidelines. A meta-analysis was conducted with the use of a random effects model. The I² statistic was used to assess for heterogeneity.

RESULTS

Eleven studies comprising 44,895 patients were included in this meta-analysis (21,710 in the synchronous group and 23,185 patients in the staged group). The synchronous CEA and CABG group had a statistically significant lower risk for myocardial infarction (MI) (odds ratio [OR] 0.15, 95% CI 0.04-0.61, I² = 0%) and higher risk for stroke (OR 1.51, 95% CI 1.34-1.71, I² = 0%) and death (OR 1.33, 95% CI 1.01-1.75, I² = 47.8%). Transient ischemic attacks (TIAs) (OR 1.27, 95% CI 1.00-1.61, I² = 0.0%), postoperative bleeding (OR 0.82, 95% CI 0.22-3.05, I² = 0.0%), and pulmonary complications (OR 1.52, 95% CI 0.24-9.60, I² = 67.5%) were similar between the 2 groups.

CONCLUSIONS

Patients in the simultaneous CEA and CABG group had a significantly higher risk of 30-day mortality and stroke and lower risk for MI as compared to staged CEA and CABG group. The rates of TIA, postoperative bleeding, and pulmonary complications were similar between the 2 groups. Future randomized trials or prospective cohorts are needed to validate our results.

Contemporary Management of Patients with Concomitant Coronary and Carotid Artery Disease

The ideal management of concomitant carotid and coronary artery occlusive disease remains elusive. Although researchers have advocated the potential benefits of varying treatment strategies based on either concomitant or staged surgical treatment, there is no consensus in treatment guidelines among national or international clinical societies. Clinical studies show that coronary artery bypass grafting (CABG) with either staged or synchronous carotid endarterectomy (CEA) is associated with a high procedural stroke or death rate. Recent clinical studies have found carotid artery stenting (CAS) prior to CABG can lead to superior treatment outcomes in asymptomatic patients who are deemed high risk of CEA. With emerging data suggesting favorable outcome of CAS compared to CEA in patients with critical coronary artery disease, physicians must consider these diverging therapeutic options when treating patients with concurrent carotid and coronary disease. This review examines the available clinical data on therapeutic strategies in patients with concomitant carotid and coronary artery disease. A treatment paradigm for considering CAS or CEA as well as CABG and percutaneous coronary intervention is discussed.

Effect of hypothermia in patients undergoing simultaneous carotid endarterectomy and coronary artery bypass graft surgery

Purpose

We sought to determine whether hypothermia provided any benefit in patients undergoing simultaneous coronary artery bypass graft surgery (CABG) and carotid endarterectomy (CEA) using one of two different surgical strategies.

Methods

Group 1 patients (n = 34, 88.2% male, mean age 65.94 ± 6.67 years) underwent CEA under moderate hypothermia before cross clamping the aorta, whereas group 2 patients (n = 23, 69.6% male, mean age 65.78 ± 9.29 years) underwent CEA under normothermic conditions before initiating cardiopulmonary bypass (CPB). Primary outcome of interest was the occurrence of any new neurological event.

Results

The two groups were similar in terms of baseline characteristics. Permanent impairment occurred in one patient (2.9%) in group 1. One patient from each group (2.9 and 4.3%) had transient neurological events and they recovered completely on the sixth and 11th postoperative days, respectively. Overall, there was no statistically significant difference between the two groups with regard to occurrence of early neurological outcomes (n = 2, 5.8% vs n = 1, 4.3%, p = 0.12).

Conclusions

This study could not provide evidence regarding benefit of hypothermia in simultaneous operations for carotid and coronary artery disease because of the low occurrence rate of adverse outcomes. The single-stage operation is safe and completion of the CEA before CPB may be considered when short duration of CPB is required.

Surgical treatment for cervical carotid artery stenosis in the elderly: importance of perioperative management of ischemic cardiac complications

Ischemic cardiac complication is one of the major perioperative complications of surgical treatment for cervical carotid stenosis, carotid endarterectomy (CEA), and carotid artery stenting (CAS), and may greatly affect surgical outcome, especially in elderly patients aged ≥ 80 years. We retrospectively analyzed the records of 259 patients (34 patients aged ≥ 80 years) treated by CEA and 61 patients (12 patients aged ≥ 80 years) treated by CAS at Aizu Chuo Hospital from January 2000 to September 2010. Preoperative ischemic heart disease screening was performed in all patients. If high risk of coronary atherosclerotic stenosis was detected, treatment for coronary lesion was performed prior to CEA or CAS. There was no preoperative ischemic cardiac complication in both the CEA and CAS groups. Perioperative complications (morbidity + mortality) occurred in 2.9% of patients aged ≥ 80 years and 1.7% of patients aged ≤ 79 years in the CEA group, and 8.3% and 8.1% of patients, respectively, in the CAS group. There was no statistically significant difference by age in either group. CEA could be safely performed with tolerable complication rates even in elderly patients. However, the complication rate in the CAS group was relatively high. New ischemic lesion on diffusion-weighted magnetic resonance imaging, both symptomatic and asymptomatic, tended to occur at a higher rate in the CAS group, especially in the elderly patients. Thorough perioperative management may minimize ischemic cardiac complications even in elderly patients. Efforts must be continued to minimize surgical complications, especially for CAS. Noninvasive medical treatment should also be considered for elderly patients.

A comparative study of risk factors and the occurrence rate of coronary atherosclerosis in extra- and intracranial atherosclerotic lesions

BACKGROUND

The risk factors and epidemiology of extracranial and intracranial atherosclerotic lesions may be different. We evaluated the importance of perioperative management of coronary atherosclerotic lesions in carotid endarterectomy (CEA) or carotid artery stenting (CAS) for extracranial cervical carotid artery stenosis and superior temporal artery (STA)-middle cerebral artery (MCA) bypass for intracranial severe MCA stenosis/occlusion.

METHODS

The medical records of patients who underwent cerebrovascular surgery at Aizu Chuo Hospital between January 2000 and September 2010 were retrospectively analyzed. Preoperative cardiovascular screening was performed for all patients to prevent perioperative ischemic heart disease-related complications. The number of patients requiring preoperative treatment of the coronary artery was compared.

RESULTS

A total of 320 patients underwent surgery for cervical carotid stenosis (IC group; 259 patients with CEA and 61 patients with CAS), and 92 patients underwent STA-MCA bypass for MCA stenosis/occlusion (MC group). Treatment for coronary lesions was required in 35 of 320 patients (10.9%) in the IC group and 14 of 92 patients (15.2%) in the MC group. Surgery was safely performed in both groups without any ischemic heart disease-related complications during the perioperative period.

CONCLUSIONS

This study shows the importance of perioperative management of coronary atherosclerotic lesions for STA-MCA bypass, similar to that for CEA/CAS.

Long-term outcomes of synchronous carotid endarterectomy and coronary artery bypass grafting versus solely carotid endarterectomy

OBJECTIVE

To compare the effect of combined carotid endarterectomy (CEA) and coronary artery bypass grafting (CABG) with solely CEA.

METHODS

During a five-year period ending December 2009, 25 consecutive patients received concomitant CEA and CABG, whereas, 62 consecutive patients underwent only CEA. They were followed at the median for 64.5 months. The Kaplan-Meier method was used to evaluate the survival rate of patients in both groups.

RESULTS

There was no significant difference in terms of age, proportion of gender, risk factors of coronary artery disease and carotid artery stenosis. The degree of carotid artery stenosis was identical in both study groups. One patient in CEA/CABG group had 60% stenosis of carotid artery with ulcerative plaque. There was no early death in the short postoperative period. Restenosis was found on ultrasonography in 4 patients in the CEA/CABG group, and 12 patients in the CEA group; no statistical difference was found between both groups (P = 0.952). The intubation time, ICU stay, and hospital stay in CEA/CABG group were longer than in solely CEA group (P <0.001). The median duration of follow-up was 64.5 months (IQR 24-84 months). The survival rate was 88 %(22/25) in CEA/CABG group and 80.6 %(50/62) in CEA group, product-limit analysis showed that there was no significant difference in survival rates between two groups (P >0.05).

CONCLUSION

concomitant carotid endarterectomy and CABG can be safely performed, it could prevent stroke and would not increase the overall risk of surgery.

Combined carotid and cardiac surgery: improving the results

BACKGROUND

Aim of this study was to analyze our experience in the last 5 years of combined carotid and cardiac surgery.

METHODS

During a 5-year period (January 2002-December 2006), 111 patients underwent combined carotid endarterectomy (CEA) and coronary artery bypass grafting (CABG) (group 1), while 1,446 patients underwent isolated CEA (group 2). Perioperative outcomes in the two groups were compared using chi(2) and Fisher's exact tests to analyze neurological deficits, cardiac events, and death at 30 days. Results during follow-up were analyzed using Kaplan-Meier survival curves, and both groups were compared using the log-rank test.

RESULTS

Immediate postoperative neurological deficits occurred more frequently in group 1 patients (2.5 vs. 0.4%, p = 0.002), with a higher incidence of transient ischemic attacks in group 1; however, there was no difference in the incidence of stroke (1% group 1 vs. 0.6% group 2, p = n.s.). Mortality rate was increased in the combined surgery group (3.5 vs. 0.5%, p < 0.001). Combined stroke/myocardial infarction/death rate at 30 days was 6.3% in group 1 compared with 1.4% in group 2, p = 0.001. Perioperative stroke/myocardial infarction/death rate was much improved in the 55% (61/111) of patients undergoing CABG off-pump (3.3 vs. 10%, p = 0.001). Mean follow-up was 18.7 months (range, 1-60). Survival at 24 months was significantly higher in patients of group 2 compared with group 1 (99.4 vs. 91.3% respectively, p < 0.001). At 24 months, there was no significant difference between the two groups in the risk of developing ipsilateral or contralateral neurologic events (3.1% group 1 vs. 1.7% group 2).

CONCLUSION

In our experience, combined CEA and cardiac surgery carries a higher risk of perioperative mortality than patients undergoing isolated CEA. Whenever possible, CEA combined with off-pump CABG seems to be the therapeutic strategy of choice.

Treatment strategy for combined carotid artery stenosis and coronary artery disease: staged or simultaneous surgical procedure?

Patients with combined carotid and coronary arterial diseases pose a high risk of cerebrovascular events, and the treatment of choice with either a simultaneous or a staged surgical procedure remains controversial. The literature of combined carotid and coronary arterial diseases of a recent decade in English was retrieved. Totally 41,901 patients undergoing simultaneous or staged carotid and coronary procedures from 53 reports were included. As a result, carotid endarterectomy plus coronary artery bypass remained the most commonly used procedure for the intervention of combined carotid artery stenosis and coronary artery disease, and was associated with higher incidences of perioperative transient ischemic attack, stroke and hospital mortality, but with less perioperative myocardial infarction comparing with the staged procedures. Patients with a simultaneous carotid endarterectomy and coronary artery bypass were generally related more to an advanced atherosclerotic coronary artery disease, so that a pure comparison between the two strategies was not always possible. To compare the efficacy of different surgical methods for combined carotid and coronary arterial diseases is of pronounced importance. The new hybrid approach consisting of the simultaneous carotid artery stenting and subsequent on-pump coronary artery bypass can be a safe approach, with the aim to reduce the surgical trauma as compared to surgical procedures, and to reduce the risk of myocardial infarction in the interval period required for the staged operations. Thus, for patients with combined carotid artery stenosis and coronary artery disease, the simultaneous surgical procedure, rather than the staged procedure, is recommended.

Surgery Insight: carotid endarterectomy--which patients to treat and when?

Over the past 15 years, we have witnessed a resurgence of surgery for prevention of ischemic stroke. Landmark trials including the North American Symptomatic Carotid Endarterectomy Trial and the European Carotid Surgery Trial have explored the role of carotid endarterectomy in this context, comparing the procedure with best medical treatment in patients with high-grade stenosis of the internal carotid artery and transient ischemic attack or minor nondisabling stroke in the same territory. Here, we discuss the lessons learnt from these trials, and review the Asymptomatic Carotid Atherosclerosis Study and the Asymptomatic Carotid Surgery Trial, which attempted to resolve the rather vexing issue of surgical treatment for patients with asymptomatic internal carotid artery stenosis. We also review the best medical treatment for patients undergoing carotid endarterectomy in the perioperative period, and examine the risk of ischemic stroke after CABG surgery, both when this procedure is performed alongside endarterectomy and when CABG surgery and endarterectomy are performed as a two-staged procedure.

Management of Concomitant Coronary and Bilateral Carotid Artery Disease: A Case Report

Patients with severe coexistent coronary and carotid artery stenosis represent a difficult and high-risk population. Herein we describe management of a patient with concomitant coronary artery and bilateral carotid artery disease. Firstly, left carotid artery stenting was done using a self-expandable monorail stent and a neurological protective device. Post-stent angiogram revealed satisfactory dilatation in the left carotid artery. Later, coronary artery bypass grafting to the four coronary arteries was done. Then right carotid endarterectomy was done. He had no neurological complication during or after any of the operation and he remains in good health since his last operation. We think the staged treatment, consisting of carotid artery stenting plus coronary artery bypass grafting plus carotid endarterectomy, in a patient with concomitant severe coronary artery and bilateral carotid artery disease is feasible, safe, and may be an alternative to combined coronary artery bypass grafting plus carotid endarterectomy.

Is There Any Benefit From Staged Carotid and Coronary Revascularization Using Carotid Stents?

Background and Purpose— To assess the benefits of carotid artery stenting before coronary artery bypass surgery to reduce the risk of stroke occurring during the cardiac procedure.

Methods— A prospective cohort study was performed in patients undergoing carotid artery stenting before coronary artery bypass surgery, or combined bypass and valve replacement procedures, to assess the procedures effectiveness in stroke prevention. Outcome measures including 30-day post stenting and cardiac surgery neurological complication and all-cause mortality rates were assessed.

Results— A total of 52 patients were included. Two patients underwent aortic valve replacements at the same time as coronary revascularization. No neurological complications occurred because of the stenting procedure. One cardiac death not related to coronary artery bypass surgery occurred in the 30-day follow-up period for the stent procedure. An additional 6 (11.5%) outcome events (3 strokes and 3 deaths) occurred in the 30-day follow-up period after the cardiac procedure. Three patients died of cardiac causes while awaiting their cardiac bypass procedure.

Conclusions— Our results are comparable to those in patients that undergo staged or combined carotid endarterectomy before cardiac surgery. Our small cohort study adds to the limited world literature on the subject but is not sufficiently powered to recommend alterations in practice.

Carotid Endarterectomy: Is It Still a Gold Standard?

Extracranial internal carotid artery stenosis accounts for 15-20% of ischemic strokes. Carotid endarterectomy has high efficacy in stroke prevention in selected patients with symptomatic (age <80 years) and asymptomatic carotid stenosis (age <75 years). Randomized clinical trials demonstrated that carotid endarterectomy reduces the stroke risk, compared to medical therapy alone, for patients with 70-99% symptomatic stenosis with 16% absolute risk reduction at 5 years. The benefit for patients with 50-69% symptomatic stenosis is lower i.e. absolute risk reduction 4.6% at 5 years. Endarterectomy is not indicated for symptomatic patients with <50% stenosis. There is no need for time-delay for surgery in patients after transient ischemic attack or minor stroke. Patients with more extensive strokes or hemorrhage should undergo surgery after 4-6 weeks following initial symptoms. Carotid endarterectomy for asymptomatic stenosis reduces the risk of ipsilateral stroke, and any stroke, by approximately 30% over 3 years. However, the absolute risk reduction is small over the first few years and decision should be based on individual institutional experience. In all situations, the best medical therapy should accompany surgery. In the recent years, appearance of angioplasty, stenting, and distal protection procedures provides competitive alternatives to classical endarterectomy. However, long-term benefits of carotid angioplasty should be confirmed by bigger, randomized, comparative clinical trials.