In vivo validation of compensatory enlargement of atherosclerotic coronary arteries

Three-dimensional intravascular ultrasound assessment of plaque volume after successful atherectomy

The primary purpose of directional coronary atherectomy is the removal of intraluminal plaque. Angiography allows assessment of residual lumen narrowing but is limited in the assessment of residual plaque burden. Intravascular ultrasound has proven useful in assessing plaque size, but current use has been limited to a single, representative cross-sectional image rather than an evaluation of the entire plaque volume. To determine the volume of residual plaque after angiographically successful directional coronary atherectomy ( < or = 20% residual stenosis), we performed intravascular ultrasound in 19 patients before and after atherectomy. Only coronary lesions optimal for three-dimensional analysis (a single, discrete stenosis in a nontortuous, noncalcified native coronary artery) were selected. A 2.9F sheath-design intravascular ultrasound catheter with a motorized pullback device was used in all patients. The cross-sectional area of the artery (defined by the medial-adventitia border), the lumen, and the plaque were measured at 1 mm intervals over a 15 to 20 mm segment, which included the target lesion and a proximal reference segment (n = 362 cross-sections), before and after atherectomy. The volumes of the artery, vessel lumen, or plaque were calculated with a modified Simpson's equation and compared with standard area measurements at the point of maximal stenosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Value and limitations of intravascular ultrasound imaging in characterizing coronary atherosclerotic plaque

Intravascular ultrasound uses a high-frequency, miniaturized, ultrasound transducer positioned on the tip of a coronary catheter to provide detailed cross-sectional images of the coronary vessel wall. Unlike angiography, which details only luminal encroachment, this imaging technique has the unique potential to provide an image of the atherosclerotic plaque, characterize its composition, and assess stenosis severity. Lipid-filled "soft" plaque, dense fibrous "hard" plaque, calcification, and thrombosis have all been identified on intravascular ultrasound images. Individual lesion types have been associated with specific clinical ischemic syndromes and with different responses to catheter-based intervention. By means of analyzing tissue composition, intravascular ultrasound may permit identification of high-risk lesions that may eventually rupture and cause unstable coronary ischemic syndromes. This article reviews the current approaches to plaque characterization by two-dimensional intravascular ultrasound imaging and addresses clinical implications, technical limitations, and future promise of the technique.

Remodeling of De Novo atherosclerotic lesions in femoral arteries: impact on mechanism of balloon angioplasty

OBJECTIVES

Using 30-MHz intravascular ultrasound in the human femoral artery, we related the mode of arterial remodeling to the immediate result and the mechanism of balloon angioplasty.

BACKGROUND

The atherosclerotic femoral artery may undergo three modes of remodeling in response to plaque formation: compensatory enlargement, failure of compensatory enlargement and paradoxic shrinkage.

METHODS

In 83 patients an ultrasound catheter pullback maneuver was performed before and after balloon angioplasty. For each lesion (n = 121), the cross section with the narrowest lumen was selected for further analysis. For each cross section, the lumen area stenosis was expressed as percent of the lumen area at an adjacent reference site. Similarly, the media-bounded area was expressed as percent of the media-bounded area at the reference site. Cross sections were classified into one of three groups based on percent relative media-bounded area: 1) > 105% (group A, compensatory enlargement, n = 24); 2) 95% to 105% (group B, failure of compensatory enlargement, n = 26); and 3) < 95% (group C, arterial wall shrinkage, n = 71). The power of the present study was 99.3% to demonstrate a difference in lumen gain of 2.5 mm2 among groups.

RESULTS

The gain in lumen area induced by balloon angioplasty did not differ significantly among the three groups (group A, 7.0 +/- 4.0 mm2 [mean +/- SD]; group B, 8.6 +/- 4.8 mm2; group C, 8.9 +/- 4.9 mm2). Stretch of the media-bounded area was observed in all three groups, but it was significantly larger in group C (7.5 +/- 5.2 mm2) than in the other two groups (group A, 3.9 +/- 5.1 mm2; group B, 5.1 +/- 4.1 mm2). A significantly positive correlation between balloon/media-bounded area ratio and elastic recoil was observed for cross sections in groups A and B (r = 0.71 and r = 0.69, respectively). However, no correlation was observed between balloon/media-bounded area ratio and elastic recoil for cross sections in group C (r = 0.17).

CONCLUSIONS

We conclude that lumen gain by balloon angioplasty is not related to the mode of atherosclerotic arterial remodeling. However, the mode of arterial remodeling affects the dilation mechanism.

Paradoxical arterial wall shrinkage may contribute to luminal narrowing of human atherosclerotic femoral arteries

BACKGROUND

This study was done to assess how local changes in vessel size, together with plaque load, determine luminal narrowing in atherosclerotic arteries. Fifty-one human femoral arteries were analyzed: 32 postmortem and 19 in vivo by 30-MHz intravascular ultrasound.

METHODS AND RESULTS

Histological and intravascular ultrasound cross sections were examined every 0.5 cm over an arterial segment 10 to 15 cm long. In each cross section we measured the lumen area and the area circumscribed by the internal elastic lamina (the IEL area). In each arterial segment, the cross section that contained the least amount of plaque was the reference site. For each cross section, the lumen area stenosis was expressed as percent of the lumen area in the reference site. Similarly, the IEL area was expressed as percent of the IEL area in the reference site (the relative IEL area). There was a significant negative correlation between the relative IEL area and the lumen area stenosis percentage (r = -.62, P < .001 for histology and r = -.66, P < .001 for intravascular ultrasound). When lumen area stenosis was less than about 25%, mainly compensatory enlargement was observed. When lumen area stenosis exceeded about 25%, however, mainly a decrease of the IEL area was observed, which is consistent with arterial wall shrinkage. Furthermore, the increase in plaque area does not account for the total loss of luminal area. There was a moderate correlation between an increase in plaque area and reduction of the corresponding lumen area (r = .49 and r = .56 for histology and intravascular ultrasound, respectively).

CONCLUSIONS

The decrease in luminal area cannot be attributed to plaque increase alone. Arterial wall shrinkage is a paradoxical mechanism that may contribute to severe luminal narrowing of the atherosclerotic human femoral artery.

The Rhizobium-plant symbiosis

Rhizobium, Bradyrhizobium, and Azorhizobium species are able to elicit the formation of unique structures, called nodules, on the roots or stems of the leguminous host. In these nodules, the rhizobia convert atmospheric N2 into ammonia for the plant. To establish this symbiosis, signals are produced early in the interaction between plant and rhizobia and they elicit discrete responses by the two symbiotic partners. First, transcription of the bacterial nodulation (nod) genes is under control of the NodD regulatory protein, which is activated by specific plant signals, flavonoids, present in the root exudates. In return, the nod-encoded enzymes are involved in the synthesis and excretion of specific lipooligosaccharides, which are able to trigger on the host plant the organogenic program leading to the formation of nodules. An overview of the organization, regulation, and function of the nod genes and their participation in the determination of the host specificity is presented.

Restenosis after percutaneous transluminal coronary angioplasty: have we been aiming at the wrong target?

Restenosis after percutaneous coronary balloon angioplasty remains a significant problem. Despite success with a variety of agents in animal models, no agent has proved clearly successful in reducing restenosis in humans. There are many potential reasons for this, but one possibility is that because of our incomplete understanding of the restenotic process, therapy has been directed at the wrong target. Arterial remodeling (changes in total vessel area or changes in area circumscribed by the internal elastic lamina) is well described in de novo atherosclerosis, and there is increasing evidence that this process occurs after angioplasty. Thus, restenosis can be thought of not merely as neointimal formation in response to balloon injury, but as arterial remodeling in response to balloon injury and neointimal formation. Arterial remodeling may consist of actual constriction of the artery, as has been described in some animal models and in preliminary fashion in humans, or of compensatory enlargement as has been described in de novo atherosclerosis and in the hypercholesterolemic rabbit iliac artery model. Arterial constriction can result in restenosis with minimal neointimal formation. Compensatory enlargement accommodates significant amounts of neointimal formation, with preservation of lumen area despite an increase in neointimal area adequate to cause restenosis in a noncompensated artery. This expanded paradigm of arterial remodeling and intimal formation may in part account for the lack of success in clinical trials to date, and therapy directed at arterial remodeling as well as intimal formation may be required to reduce restenosis after coronary interventions.

The safety of intracoronary ultrasound. A multicenter survey of 2207 examinations

BACKGROUND

Intracoronary ultrasound (ICUS) is increasingly used in clinical practice to study the natural history of coronary artery disease and to assess the effects of intracoronary, catheter-based interventions. However, the risk associated with the procedure is not well documented.

METHODS AND RESULTS

ICUS studies performed in 28 centers were retrospectively included; these centers agreed to contribute to the study among a total of 60 centers initially invited. Among the 2207 ICUS studies, 505 (23%) were performed in heart transplant recipients and 1702 (77%) in nontransplant patients. Indication for ICUS was diagnostic imaging in 915 (41%), drug testing in 244 (11%), and guidance for intracoronary interventions in 1048 patients (47%). There were no complications in 2034 patients (92.2%). In 87 patients (3.9%), complications occurred but were judged to be "not related" to ICUS by the operator. In 63 patients (2.9%), spasm occurred during ICUS imaging. In 9 patients (0.4%), complications other than spasm were judged to have a "certain relation" to ICUS, including acute procedural events in 6 (3 acute occlusion, 1 embolism, 1 dissection, and 1 thrombus) and major events in 3 patients (2 occlusion and 1 dissection; all resulting in myocardial infarction). In 14 patients (0.6%), complications with "uncertain relation" to ICUS were recorded, including acute procedural events in 9 (5 acute occlusion, 3 dissection, and 1 arrhythmia) and major events in 5 patients (2 myocardial infarction and 3 emergency coronary artery bypass surgery). The incidence of acute procedural or major complications judged to be associated with ICUS (uncertain relation or certain relation to ICUS) was compared in different patient groups. The complication rate was higher in patients with unstable angina or acute myocardial infarction (2.1% events) as compared with patients with stable angina pectoris and asymptomatic patients (0.8% and 0.4%, respectively; chi 2 = 10.9, P < .01). These complications were also more frequent in patients undergoing interventions (1.9%) as compared with transplant and nontransplant patients undergoing diagnostic ICUS imaging (0% and 0.6%, respectively; chi 2 = 13.5, P < .001). Adverse events were few, and no association was detected between these events and the size or type of ICUS catheter used.

CONCLUSIONS

ICUS is associated with (but not necessarily the direct cause of) a minor acute clinical risk. Vessel spasm is the most frequent event occurring during ICUS. Other complications predominantly occur in patients with acute coronary syndromes and during guidance for intervention.

Differences in compensatory vessel enlargement, not intimal formation, account for restenosis after angioplasty in the hypercholesterolemic rabbit model

BACKGROUND

In de novo human atherosclerosis, compensatory vessel enlargement limits the effect of intimal plaque formation on lumen narrowing. We hypothesized that arterial remodeling may also play an important role in determining the chronic lumen size after angioplasty and tested this hypothesis using the hypercholesterolemic rabbit iliac artery angioplasty model.

METHODS AND RESULTS

Morphometric analysis of histological cross-sectional areas of vessels from animals killed immediately after angioplasty (acute group, n = 11) were compared with the same areas from animals killed 4 weeks after the procedure (chronic group, n = 37), when restenosis occurs in this model. The area circumscribed by the internal elastic lamina (IEL) increased by 20% from acute to 4 week follow-up after angioplasty (acute group, 2.36 +/- 0.45 mm2, chronic group, 2.84 +/- 0.89 mm2). Over the same time period, intimal area increased by 0.82 mm2. Despite this increase in intimal area, lumen area decreased by only 0.34 mm2 because of the compensatory enlargement of the IEL area. In the chronic group, polynomial regression analysis revealed a quadratic relation between intimal area and lumen area (R2 = .35, P < .001). A lumen area of 0.45 mm2 (the nadir of the quadratic relation) was used to divide the chronic group into two subgroups: restenotic (n = 21; lumen area, < 0.45 mm2) and nonrestenotic (n = 16; lumen area, > 0.45 mm2). By definition, there was a significant difference in lumen area between the two subgroups (0.15 +/- 0.15 mm2 for restenotic; 0.73 +/- 0.18 mm2 for nonrestenotic). Surprisingly, the intimal areas in the two subgroups were virtually identical (2.41 +/- 0.92 mm2 for restenotic, 2.49 +/- 0.69 mm2 for nonrestenotic, P = NS). The difference in the lumen area between restenotic and nonrestenotic vessels was a result of the significantly greater IEL area in the nonrestenotic subgroup (3.22 +/- 0.83 mm2 for nonrestenotic, 2.56 +/- 0.84 mm2 for restenotic, P < .05). In both restenotic and nonrestenotic vessels, the IEL area increased with increases in intimal area. In the restenotic arteries, the slope of this correlation was < 1, showing inadequate compensatory enlargement for the intimal plaque. In the nonrestenotic vessels, the slope was > 1, limiting the effect of intimal plaque on luminal narrowing.

CONCLUSIONS

These data indicate that the iliac artery in an atherosclerotic rabbit model compensates for intimal formation after angioplasty by vessel enlargement. Furthermore, the degree of vessel enlargement is more important than intimal area in determining the chronic lumen size.

Focal compensatory enlargement of human arteries in response to progressive atherosclerosis. In vivo documentation using intravascular ultrasound

BACKGROUND

Previous postmortem studies have demonstrated compensatory enlargement of atherosclerotic arteries in animal models and patients. Conclusions regarding these changes were drawn based on a comparison of the dimensions of diseased arteries in one group of subjects with the dimensions of normal arteries in another group. This method admits potential confounding variables, such as demographics and other disease states, which might also have an impact on arterial size.

METHODS AND RESULTS

Using intravascular ultrasound, we studied a total of 62 paired, adjacent normal and diseased sites in the superficial femoral arteries of 20 patients undergoing peripheral vascular interventions. Morphological assessment was performed using a computer-based image analysis system. Measurements were made of the cross-sectional area of the arterial lumen, the atherosclerotic plaque, and the outer border of the artery. These dimensions were then compared to determine the effects of progressive atherosclerosis on arterial morphology. Luminal cross-sectional area decreased from 21.1 +/- 2.2 mm2 in normal segments to 16.7 +/- 0.8 mm2 (P = .0001) in adjacent atherosclerotic segments. Similarly, minimal luminal diameter decreased from 5.7 +/- 0.2 to 5.0 +/- 0.1 mm2, and maximal luminal diameter decreased from 6.2 +/- 0.2 to 5.7 +/- 0.2 mm2. At these same sites, total arterial area was 32.9 +/- 1.6 and 37.9 +/- 1.9 mm2 (P = .0001) in normal and diseased segments, respectively. Minimal and maximal arterial diameters demonstrated similar increases (7.3 +/- 0.2 to 7.7 +/- 0.2 mm2 [P = .0015] and 7.6 +/- 0.2 to 8.3 +/- 0.2 mm2 [P = .0001], respectively). Regression analysis disclosed correlation of the cross-sectional area of plaque to the total arterial area (R = .70, P = .0001).

CONCLUSIONS

Human arteries enlarge in response to progressive atherosclerosis. This compensatory mechanism results in an increase in arterial size that is proportionate to the cross-sectional area of plaque that has accumulated in the vessel. Intravascular ultrasound demonstrates that this process is focal compensatory enlargement at discrete sites of atherosclerotic narrowing immediately adjacent to more normal areas in which arterial size is smaller.

Extent of atherosclerosis and remodeling of the left main coronary artery determined by intravascular ultrasound

This study used intravascular ultrasound (IU) to assess the incidence and extent of left main coronary artery (LMCA) disease and the effects of arterial remodeling. Sixty-nine patients undergoing cardiac catheterization were imaged with a 20 MHz rotational-tip IU device. Nine of the 69 studies (13%) could not be analyzed because of technical (n = 2) or anatomic (n = 7) reasons. Of the remaining 60 patients, 38 (63%) had at least 1 lesion in the left coronary artery perfusion territory by angiography; significant LMCA stenosis was present in 2 patients (3%). Intravascular ultrasonography demonstrated plaques in 27 of 60 LMCAs (45%), 6 of them in patients with normal angiograms. Twenty-four plaques (89%) were eccentric and calcium was present in 4 (15%). The mean minimal lumen diameter was 4.9 +/- 0.8 mm, the maximal lumen diameter was 5.6 +/- 0.8 mm, the planimetered lumen area was 22.6 +/- 6.0 mm2, the plaque area was 3.9 +/- 5.8 mm2, the vessel area was 26.5 +/- 5.9 mm2, and the area stenosis was 13 +/- 19%. In the 27 patients with plaque, plaque area was 8.7 +/- 5.7 mm2 and the area stenosis was 30 +/- 17%. The vessel area was significantly larger in diseased LMCAs (p < 0.001) and correlated with plaque area (r = 0.46). IU examination of the LMCA was feasible in 87% of patients and was more reliable for delineating plaques than angiography.