Recent insights into coronary collateral circulation

Effect of hypoxia on the proliferation of retinal microvessel endothelial cells in culture

BACKGROUND

To determine if hypoxia stimulates the proliferation of retinal microvessel endothelial cells in culture.

METHODS

Bovine retinal microvessel endothelial cells were cultured in normoxic (95% air, 5% CO2) and hypoxic (2% O2, 5% CO2, 93% N2) conditions. Endothelial cells were identified by acetylated LDL and Factor VIII-related antigen immunocytochemical staining. Cells from passages three to eight were used in these experiments. Proliferation assays included cell counts by hemocytometer and autoradiographic analysis of incorporated 3H-thymidine (3H-TdR).

RESULTS

At day 4, cell counts of endothelial cells in hypoxia showed a 133% increase over those grown in normoxic conditions (N = 25, P < 0.01). Cell counts per day for 5 days were 121-181% greater in hypoxia. Autoradiography of endothelial cells exposed to 3H-TdR and counted every 12 hours for 60 hours exhibited labeling indices 112-118% higher in hypoxic conditions (P < 0.0001). Endothelial cells cultured under hypoxic conditions were smaller and spindle-shaped, whereas those grown under normoxic conditions were larger and more polygonal.

CONCLUSIONS

Hypoxia increases DNA synthesis and stimulates proliferation of retinal microvessel endothelial cells in vitro and induces alterations in morphology. These results may be relevant to microvessel angiogenesis, which occurs in vivo under ischemic conditions.

Adaptation to myocardial ischemia during coronary angioplasty demonstrated by clinical, electrocardiographic, echocardiographic, and metabolic parameters

It has been shown that brief episodes of myocardial ischemia can render the heart more resistant to a prolonged subsequent ischemic episode. This phenomenon, called "preconditioning," has been described in human beings during coronary angioplasty with the use of clinical, electrocardiographic (ECG), or metabolic parameters. The goal of this study was to assess this phenomenon further with the use of echocardiographic and metabolic parameters in addition to clinical and ECG parameters. Eighteen patients with isolated stenosis of the left anterior descending coronary artery and a normal left ventricular function were included. Angioplasty consisted of four consecutive balloon inflations. Sequential changes in clinical, ECG (intracoronary ECG), echocardiographic, and metabolic parameters of myocardial ischemia were compared between the first and the fourth balloon inflations. Improved tolerance to myocardial ischemia with repeated coronary occlusions was demonstrated by a significant reduction in the severity of angina, ST-segment elevation, wall motion abnormalities, and lactate production. This study confirms the adaptation of myocardial ischemia to repeated coronary occlusions through measurement of clinical, ECG, echocardiographic, and metabolic parameters.

Association of preceding angina with in-hospital life-threatening ventricular tachyarrhythmias and late potentials in patients with a first acute myocardial infarction

We studied 140 patients with a first acute myocardial infarction to examine the effect of preceding angina as a marker of ischemic preconditioning on clinical ventricular arrhythmias and late potentials. Preceding angina was defined as the presence of ischemic chest pain within 24 hours before onset of myocardial infarction lasting no longer than 30 minutes and seen three or more times per day or at rest. Clinical features, angiographic findings, and late potentials were compared between patients with and without preceding angina. Thirty-four (24%) patients had preceding angina. Although the incidence of life-threatening ventricular tachyarrhythmias significantly differed (p = 0.0219), other clinical findings, including presence of late potentials, were not different between the two groups. Of 14 patients with life-threatening ventricular tachyarrhythmias, five events were considered as reperfusion arrhythmias. In patients who had successful reperfusion therapy, the incidence of life-threatening ventricular tachyarrhythmias had a tendency to be lower in patients with preceding angina than in those without preceding angina (p = 0.0586). Severe angina within 24 hours of onset of acute myocardial infarction is suggested to reduce occurrence of life-threatening ventricular tachyarrhythmias mainly associated with reperfusion during hospitalization.

Fate of collateral vessels after successful coronary angioplasty in patients with effort angina

OBJECTIVES

The purpose of the present study was to evaluate whether severe restenosis after percutaneous transluminal coronary angioplasty (PTCA) promotes collateral development and whether successful dilation regresses collateral vessels.

BACKGROUND

It is well known that in the presence of severe coronary stenosis, native collateral arterioles mature to small coronary arteries with several layers of smooth muscle cells. However, it remains unclear whether well developed collateral vessels regress after removal of coronary stenosis.

METHODS

The study group comprised 41 patients who underwent elective PTCA for effort angina due to single-vessel disease, followed by repeat PTCA to treat restenosis. We classified the patients into three groups depending on the change in baseline Thrombolysis in Myocardial Infarction (TIMI) flow grade of the ischemia-related artery at initial and repeat PTCA, and we compared the extent of ST segment elevation at 1 min of the first balloon inflation between the two procedures. The average interval from initial to repeat PTCA was 125 days.

RESULTS

The three patient groups comprised group A, 12 patients with decreased flow grade because of severe coronary restenosis; group B, 12 patients with increased flow grade who had severe initial stenosis and relatively mild restenosis; and group C, 17 patients with unchanged flow grade. In the presence of comparable rate-pressure products at initial and repeat PTCA, patients in group A had significantly greater ST segment elevation (p < 0.01) at initial than at repeat PTCA (mean +/- SD 0.42 +/- 0.31 vs. 0.13 +/- 0.22 mV). In group B, ST segment elevation was significantly less at initial than at repeat PTCA (0.13 +/- 0.25 vs. 0.19 +/- 0.17 mV, p < 0.05), and in group C, it was comparable at the two procedures (0.37 +/- 0.32 vs. 0.35 +/- 0.33 mV, p = 0.50).

CONCLUSIONS

These findings indicate that severe restenosis after PTCA promotes collateral development and that successful dilaton regresses collateral vessels during a relatively short period of time.

Exercise training and heparin pretreatment in patients with coronary artery disease

The purpose of this study was to evaluate whether combined treatment with a cardiovascular exercise rehabilitation program and low doses of heparin can induce changes in ergometric parameters of ischemia in patients with coronary artery disease (CAD). Heparin may potentiate the development of new vessels promoted by ischemia and therefore may produce important clinical improvement. Thirty-six patients with stable CAD and evidence of myocardial ischemia on exercise testing were randomized into three groups: a control group (n = 11) received the usual medical treatment; another group (n = 11) underwent three exercise sessions per week during 12 weeks; and a third group (n = 14) undertook this exercise program and also received calcium heparin 12,500 IU subcutaneously 20 to 30 minutes before each exercise session. Pretreatment and posttreatment exercise tests were compared. Patients who underwent the rehabilitation program had an increase in exercise duration and workload at the onset of 1 mm ST-segment depression, but only patients who received calcium heparin showed a significant increase in rate-pressure product at the ST-segment ischemic threshold (p = 0.035). This result suggests that higher levels of myocardial oxygen consumption were now tolerated, a change that may be related to an improvement in myocardial perfusion.

Association between coronary occlusions and myocardial infarcts. The INTACT investigators. International Nifedipine Trial on Antiatherosclerotic Therapy

The number of angiographically documented coronary occlusions and the incidence of Q-wave myocardial infarcts were retrospectively compared in 348 patients with moderate coronary artery disease from the INTACT study (International Nifedipine Trial on Antiatherosclerotic Therapy). In only 68 out of 118 infarcts (58%) an occlusion of the respective coronary artery was found, suggesting a spontaneous recanalization rate of 42%. On the other hand, only 68 out of 150 coronary occlusions (45%) had resulted in a Q-wave infarct. Considering the high spontaneous recanalization rate of the occlusions, it seemed possible that roughly only every fourth coronary occlusion might result in a myocardial infarct. This hypothesis was confirmed in the prospective 3 years follow-up of the identical patients during which 41 new occlusions developed causing only 10 myocardial infarcts (24%). These findings might contribute to explain the relatively low incidence of clinically apparent coronary heart disease in the general population despite a high prevalence of coronary artery disease.

The role of NF-kappaB in the angiogenic response of coronary microvessel endothelial cells

The activation of nuclear factor (NF)-kappaB by 12(R)-hydroxyeicosatrienoic acid [12(R)-HETrE], an arachidonic acid metabolite with potent stereospecific proinflammatory and angiogenic properties, was examined and its role in the angiogenic response was determined in capillary endothelial cells derived from coronary microvessels. Electrophoretic mobility-shift assay of nuclear protein extracts from cells treated with 12(R)-HETrE demonstrated a rapid and stereospecific time- and concentration-dependent increase in the binding activity of NF-kappaB, which was inhibitable by the antioxidants N-acetylcysteine, butylated hydroxyanisole, and pyrrolidine dithiocarbamate and was partially attenuated by the protein kinase C inhibitors, staurosporine and calphostin C. Neither 12(S)-HETrE nor other related eicosanoids--e.g., 12(R)-HETE, 12(S)-HETE, and leukotriene B4--stimulated the activation of NF-kappaB relative to 12(R)-HETrE, substantiating the claim for a specific receptor-mediated mechanism. 12(R)-HETrE stimulated the formation of capillary-like cords of microvessel endothelial cells distinguishable from a control; this effect was comparable to that observed with basic fibroblast growth factor (bFGF). Inhibition of NF-kappaB activation resulted in inhibition of capillary-like formation of endothelial cells treated with 12(R)-HETrE by 80% but did not affect growth observed with bFGF. It is suggested that 12(R)-HETrE's angiogenic activity involves the activation of NF-kappaB, possibly via protein kinase C stimulation and the generation of reactive oxygen intermediates for downstream signaling.

Relation between collateral flow assessed by Doppler guide wire and angiographic collateral grades

We investigated the relation between the angiographic collateral grade (Rentrop's classification) and the collateral flow velocity pattern in 43 patients with angina pectoris. Collateral flow velocity was measured with a Doppler guide wire during balloon occlusion in coronary angioplasty. Collateral flow was detected in 21 of the 43 patients. In 6 of the 21 patients, collateral vessels were not seen angiographically before angioplasty. The direction of collateral flow was classified as forward, backward, or bidirectional. Forward and backward collateral flows were seen in all angiographic grades. Bidirectional collateral flows were observed only in grades 0 to 2. The peak collateral flow velocity was not correlated with the angiographic grades, but the ratio of the collateral flow duration to a cardiac cycle length was correlated with them (grade 0, 44% +/- 15%; grade 1, 70% +/- 16%; grade 2, 84% +/- 11%; and grade 3, 93% +/- 3%; p < 0.0005, analysis of variance). The peak velocity integral was also correlated with the angiographic collateral grades (p < 0.05; analysis of variance). The peak velocity integral was also correlated with the angiographic collateral grades (p < 0.05; analysis of variance). Electrocardiographic signs of ischemia were less observed in patients with unidirection and longer duration of collateral flow pattern (p < 0.05, respectively). A Doppler guide wire may be useful in assessing collateral flow grade.

Angiogenesis: mechanistic insights, neovascular diseases, and therapeutic prospects

This review of angiogenesis aims to describe (a) stimuli that either elicit or antagonize angiogenesis, (b) the response of the vasculature to angiogenic or anti-angiogenic stimuli, i.e., processes required for the formation of new vessels, (c) aspects of angiogenesis relating to tissue remodeling and disease, and (d) the potential of angiogenic or antiangiogenic therapeutic measures. Angiogenesis, the formation of new vessels from existing microvessels, is important in embryogenesis, wound healing, diabetic retinopathy, tumor growth, and other diseases. Hypoxia and other as yet ill-defined stimuli drive tumor, inflammatory, and connective tissue cells to generate angiogenic molecules such as vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), transforming growth factor-beta (TGF-beta), platelet-derived growth factor (PDGF), and others. Natural and synthetic angiogenesis inhibitors such as angiostatin and thalidomide can repress angiogenesis. Angiogenic and antiangiogenic molecules control the formation of new vessels via different mechanisms. VEGF and FGF elicit their effects mainly via direct action on relevant endothelial cells. TGF-beta and PDGF can attract inflammatory or connective tissue cells which in turn control angiogenesis. Additionally, PDGF may act differently on specific phenotypes of endothelial cells that are engaged in angiogenesis or that are of microvascular origin. Thus phenotypic traits of endothelial cells committed to angiogenesis may determine their cellular responses to given stimuli. Processes necessary for new vessel formation and regulated by angiogenic/antiangiogenic molecules include the migration and proliferation of endothelial cells from the microvasculature, the controlled expression of proteolytic enzymes, the breakdown and reassembly of extracellular matrix, and the morphogenic process of endothelial tube formation. In animal models some angiogenesis-dependent diseases can be controlled via induction or inhibition of new vessel formation. Life-threatening infantile hemangiomas are a first established indication for antiangiogenic therapy in humans. Treatment of other diseases by modulation of angiogenesis are currently tested in clinical trials. Thus the manipulation of new vessel formation in angiogenesis-dependent conditions such as wound healing, inflammatory diseases, ischemic heart and peripheral vascular disease, myocardial infarction, diabetic retinopathy, and cancer is likely to create new therapeutic options.

Quantification of recruitable coronary collateral blood flow in conscious humans and its potential to predict future ischemic events

OBJECTIVES

The present study was designed to evaluate the applicability of a pressure-flow equation for quantitative calculation of recruitable collateral blood flow at coronary artery occlusion in conscious patients and to investigate the value of that index to predict future ischemic events.

BACKGROUND

Recent experimental studies have indicated that recruitable collateral blood flow at coronary artery occlusion can be expressed as a fraction of normal maximal myocardial blood flow by simultaneous recordings of mean arterial, coronary wedge and central venous pressures, respectively. This index is called the pressure-derived fractional collateral flow and is independent of hemodynamic loading conditions.

METHODS

In 120 patients undergoing elective coronary angioplasty, mean arterial, coronary wedge and central venous pressures were measured at balloon inflations of 2 min. All patients had a recent exercise electrocardiogram (ECG) with positive findings showing clearly distinguishable, reversible ECG abnormalities, enabling recognition of ischemia at balloon inflation. Fractional collateral blood flow at angioplasty was calculated by coronary wedge pressure minus central venous pressure divided by mean arterial pressure minus central venous pressure and correlated to the presence or absence of ischemia at balloon inflation. Ischemic events were monitored during a follow-up period of 6 to 22 months.

RESULTS

In 90 of the 120 patients, ischemia was present at balloon inflation, and in 82 of these patients, fractional collateral blood flow was < or = 23%. By contrast, in 29 patients, no ischemia was present, and fractional collateral blood flow was > 24% in all 29. During the follow-up period, 16 patients had an ischemic event. Fifteen of these 16 patients were in the group with insufficient collateral flow (p < 0.05).

CONCLUSIONS

To our knowledge, this study presents the first method for quantitative assessment of recruitable collateral blood flow in humans in the catheterization laboratory. Sufficient and insufficient collateral circulation can be reliably distinguished by this method. Use of this method can also help to provide more insight into the extent and behavior of the collateral circulation for investigational purposes and may have potential clinical implications.

Prospective, randomized, placebo-controlled, double-blind, multicenter study of exercise with enoxaparin pretreatment for stable-effort angina

In this double-blind, placebo-controlled, multicenter trial, we examined the combined effects of repeated exercise and intravenous enoxaparin (low-molecular-weight heparin) on treadmill exercise capacity and angiographic collateral growth and compared them with the effect of repeated exercise with placebo. Fifty-two patients with stable-effort angina were randomly assigned to receive one of two doses of enoxaparin (40 or 60 mg) or placebo. In each patient, 20 treadmill exercise sessions were performed with the pretreatment of enoxaparin or placebo for 2 to 3 weeks. Before and after treatment, coronary cineangiography was repeated to evaluate the changes in coronary and collateral circulation. Improvement of rate-pressure product (RPP) at the onset of angina was taken as an index of enhanced collateral flow reserve. Although the mean differences in the magnitude of increase in RPP were not significantly different between the 3 groups, a heterogeneous response was observed: 1620 beats/min.mm Hg in 40 mg (p = 0.12), 3060 beats/min.mm Hg in 60 mg (p = 0.02), and 1090 beats/min.mm Hg in placebo (p = 0.44). The end-points of the exercise test were changed from chest discomfort to leg fatigue or dyspnea in 10 (28%) of 36 enoxaparin-treated patients but in only 1 (6%) of 16 placebo patients (p = value not significant (NS)). Similarly, the extent of coronary and collateral circulation to the completely obstructed coronary artery was increased in 17 (47%) of 36 enoxaparin-treated patients but only in 4 (25%) of 16 placebo patients (p = NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Damage to the collateral circulation by PTCA of an occluded coronary artery

In this report, a patient is described with an occluded left circumflex artery, in whom the corresponding myocardium was protected at rest by sufficient collateral circulation. Because of angina pectoris class III, a PTCA of that occluded vessel was performed, complicated by a large dissection. Recruitable collateral flow, assessed from pressure calculations by a new technique, suddenly decreased at the very moment of dissection. This was accompanied by resting pain and ischemia on the ECG. This case report confirms the hypothesis that the collateral circulation can be damaged by PTCA and emphasizes that every PTCA implies a definite risk, even in case of an occluded coronary artery filled by collaterals.

Effect of coronary collateral circulation on myocardial ischemia and ventricular dysfunction

Although a functional role of coronary collaterals has been continuously debated, we observed the following facts in our studies during intracoronary thrombolytic therapy: (a) Myocardial ischemia is important for the development of collateral circulation, (b) collaterals can perfuse the infarcted myocardium, and (c) the presence of collaterals prevents the left ventricular aneurysm formation in acute myocardial infarction, even when the amount of the salvaged tissue is small. Thus, coronary collaterals are not merely markers of severe ischemia but help to preserve the functional integrity of the myocardium in the presence of coronary obstruction. We then attempted to promote collateralization to treat patients with angina pectoris. Patients with chronic stable effort angina were treated with heparin followed by treadmill exercise twice a day for 10 days. Treadmill capacity was found to improve in association with an increase in coronary collateral circulation. Heparin treatment of ischemic patients was found to be a noninvasive alternative to percutaneous transluminal coronary angioplasty and coronary bypass surgery for patients who are not candidates for invasive procedures.

Coronary arterial calcification as an active process: a new perspective on an old problem

The mechanism and purpose of coronary atherosclerotic calcification remain unknown. However, evidence reviewed here suggests that calcification is not passive precipitation or adsorption, but instead is organized and regulated. Gla containing proteins and other proteins normally associated with bone metabolism appear to play an important role in this process. A variety of studies are currently in progress in our laboratory which we hope will provide a more comprehensive understanding of processes leading to coronary calcification as well as prognostic data useful in clinical cardiologic practice. A clearer understanding of the nature and significance of coronary calcification may well pave the way toward new interventions to protect myocardium and minimize the morbidity and mortality associated with coronary artery disease.

Chronic stimulation enhances extramyocardial collateral blood flow after a cardiomyoplasty

We have previously demonstrated that collateral blood flow can be established between skeletal muscle and myocardium in animals that have undergone a latissimus dorsi cardiomyoplasty. We have also shown that 5 minutes after the thoracodorsal nerve is electrically stimulated at 1.25 Hz, there is a sixfold increase in the collateral blood flow between the latissimus dorsi and the heart. In this experiment, we hypothesized that chronic stimulation of a latissimus dorsi cardiomyoplasty would result in a sustained increase in the latissimus-derived collateral blood flow. In 24 adult male goats, an ameroid constrictor was placed around a branch of the circumflex coronary artery, and a latissimus dorsi cardiomyoplasty was performed. After a rest period of about 1 week, the latissimus dorsi cardiomyoplasties were stimulated continuously at a 2-Hz frequency for 6 weeks. Collateral blood flow between the muscle and the heart was then measured with colored microspheres. Sixteen animals survived to the final experiment, and collaterals developed in 10. In these 10 animals, the latissimus collaterals continuously delivered 0.17 +/- 0.03 mL.g-1 x min-1 (mean +/- the standard error) of blood to ischemic myocardium. This flow represents 24.0% +/- 3.9% of the flow measured to normal myocardium. These results demonstrate that in an animal model of coronary artery disease, chronic electrical stimulation of a latissimus dorsi cardiomyoplasty maintains an elevated level of latissimus-derived collateral blood flow to the myocardium.

Heparin accelerates coronary collateral development in a porcine model of coronary artery occlusion

BACKGROUND

Coronary collaterals develop in response to an ischemic stimulus. However, collateral growth is not sufficient to result in the complete recovery of coronary reserves. Using a porcine model of gradual coronary artery occlusion, we investigated the effect of continuous heparin infusion on coronary collateral development.

METHODS AND RESULTS

We placed ameroid constrictors on the left circumflex coronary artery of 16 minipigs; the ameroid constrictors completely occluded the left circumflex coronary artery at 10 +/- 1 days. Half of the animals also were instrumented with subcutaneously placed osmotic pumps and catheters that delivered heparin (300 units/h) into the external jugular vein. At 2, 3, and 4 weeks, we assessed blood flow at rest and during vasodilation using radioactive microspheres. Our results indicate that the animals receiving heparin restored resting myocardial blood flow to normal levels at or before 2 weeks; in contrast, we did not see normal resting myocardial blood flow levels in the untreated-ameroid animals until 3 weeks. Under vasodilated conditions, untreated-ameroid animals experienced a severe loss of coronary reserves at 2 weeks. Although this improved with time, these animals still were significantly underperfused at 4 weeks. In contrast, in the heparin-treated animals, coronary reserves returned to near-normal levels between 3 and 4 weeks. In addition, infarct size was significantly smaller in the heparin-treated animals.

CONCLUSIONS

These experiments suggest that the administration of heparin in the early phases of gradual coronary occlusion accelerates the rate of return of normal blood flow under resting conditions, substantially increases the recovery of coronary reserve, and reduces the risk of infarction.

Experimental basis of determining maximum coronary, myocardial, and collateral blood flow by pressure measurements for assessing functional stenosis severity before and after percutaneous transluminal coronary angioplasty

BACKGROUND

Severity of coronary artery stenosis has been defined in terms of geometric dimensions, pressure gradient-flow relations, resistance to flow and coronary flow reserve, or maximum flow capacity after maximum arteriolar vasodilation. A direct relation between coronary pressure and flow, however, may only be presumed if the resistances in the coronary circulation are constant (and minimal) as theoretically is the case during maximum arteriolar vasodilation. In that case, pressure measurements theoretically can be used to predict maximum flow and assess functional stenosis severity.

METHODS AND RESULTS

A theoretical model was developed for the different components of the coronary circulation, and a set of equations was derived by which the relative maximum flow or fractional flow reserve in both the stenotic epicardial artery and the myocardial vascular bed and the proportional contribution of coronary arterial and collateral flow to myocardial blood flow are calculated from measurements of arterial, distal coronary, and central venous pressures during maximum arteriolar vasodilation. To test this model, five dogs were acutely instrumented with an epicardial, coronary Doppler flow velocity transducer. Distal coronary pressures were measured by an ultrathin pressure-monitoring guide wire (0.015 in.) with minimal influence on transstenotic pressure gradient. Fractional flow reserve was calculated from the pressure measurements and compared with relative maximum coronary artery flow measured directly by the Doppler flowmeter at three different levels of arterial pressure for each of 12 different severities of stenosis at each pressure level. Relative maximum blood flow through the stenotic artery (Qs) measured directly by the Doppler flowmeter showed an excellent correlation with the pressure-derived values of Qs (r = 0.98 +/- 0.01, intercept = 0.02 +/- 0.03, slope = 0.98 +/- 0.04), of the relative maximum myocardial flow (r = 0.98 +/- 0.02, intercept = 0.26 +/- 0.07, slope = 0.73 +/- 0.08), and of the collateral blood flow (r = 0.96 +/- 0.04, intercept = 0.24 +/- 0.07, slope = -0.24 +/- 0.06). Moreover, the theoretically predicted constant relation between mean arterial pressure and coronary wedge pressure, both corrected for venous pressure, was confirmed experimentally (r = 0.97 +/- 0.03, intercept = 9.5 +/- 13.3, slope = 4.4 +/- 1.2).

CONCLUSIONS

These results provide the experimental basis for determining relative maximum flow or fractional flow reserve of both the epicardial coronary artery and the myocardium, including collateral flow, from pressure measurements during maximum arteriolar vasodilation. With a suitable guide wire for reliably measuring distal coronary pressure clinically, this method may have potential applications during percutaneous transluminal coronary angioplasty for assessing changes in the functional severity of coronary artery stenoses and for estimating collateral flow achievable during occlusion of the coronary artery.