Preferential dilation of recipient coronary arteries of the collateral circulation by intracoronary administration of nitroglycerin

Vasodilator Therapy: Nitrates and Nicorandil

Nitrates have been used to treat symptoms of chronic stable angina for over 135 years. These drugs are known to activate nitric oxide (NO)-cyclic guanosine-3',-5'-monophasphate (cGMP) signaling pathways underlying vascular smooth muscle cell relaxation, albeit many questions relating to how nitrates work at the cellular level remain unanswered. Physiologically, the anti-angina effects of nitrates are mostly due to peripheral venous dilatation leading to reduction in preload and therefore left ventricular wall stress, and, to a lesser extent, epicardial coronary artery dilatation and lowering of systemic blood pressure. By counteracting ischemic mechanisms, short-acting nitrates offer rapid relief following an angina attack. Long-acting nitrates, used commonly for angina prophylaxis are recommended second-line, after beta-blockers and calcium channel antagonists. Nicorandil is a balanced vasodilator that acts as both NO donor and arterial K(+) ATP channel opener. Nicorandil might also exhibit cardioprotective properties via mitochondrial ischemic preconditioning. While nitrates and nicorandil are effective pharmacological agents for prevention of angina symptoms, when prescribing these drugs it is important to consider that unwanted and poorly tolerated hemodynamic side-effects such as headache and orthostatic hypotension can often occur owing to systemic vasodilatation. It is also necessary to ensure that a dosing regime is followed that avoids nitrate tolerance, which not only results in loss of drug efficacy, but might also cause endothelial dysfunction and increase long-term cardiovascular risk. Here we provide an update on the pharmacological management of chronic stable angina using nitrates and nicorandil.

Role of short-acting nitroglycerin in the management of ischemic heart disease

Nitroglycerin is the oldest and most commonly prescribed short-acting anti-anginal agent; however, despite its long history of therapeutic usage, patient and health care provider education regarding the clinical benefits of the short-acting formulations in patients with angina remains under-appreciated. Nitrates predominantly induce vasodilation in large capacitance blood vessels, increase epicardial coronary arterial diameter and coronary collateral blood flow, and impair platelet aggregation. The potential for the prophylactic effect of short-acting nitrates remains an under-appreciated part of optimal medical therapy to reduce angina and decrease myocardial ischemia, thereby enhancing the quality of life. Short-acting nitroglycerin, administered either as a sublingual tablet or spray, can complement anti-anginal therapy as part of optimal medical therapy in patients with refractory and recurrent angina either with or without myocardial revascularization, and is most commonly used to provide rapid therapeutic relief of acute recurrent angina attacks. When administered prophylactically, both formulations increase angina-free walking time on treadmill testing, abolish or delay ST segment depression, and increase exercise tolerance. The sublingual spray formulation provides several clinical advantages compared to tablet formulations, including a lower incidence of headache and superiority to the sublingual tablet in terms of therapeutic action and time to onset, while the magnitude and duration of vasodilatory action appears to be comparable. Furthermore, the sublingual spray formulation may be advantageous to tablet preparations in patients with dry mouth. This review discusses the efficacy and utility of short-acting nitroglycerin (sublingual spray and tablet) therapy for both preventing and aborting an acute angina attack, thereby leading to an improved quality of life.

Influence of oxidative stress on the development of collateral circulation in total coronary occlusions

OBJECTIVES

The purpose of this study was to investigate whether the levels of total antioxidant capacity (TAC), total peroxide and oxidative stress index (OSI) are associated with the development of collaterals in total coronary occlusions.

DESIGN AND METHODS

Our study group contained 176 consecutive men patients with single-vessel TCO, 94 of whom had poorly developed coronary collateral, while 82 had well-developed coronary collateral. TAC and total peroxide concentration were measured of plasma. The ratio of TAC to total peroxide was accepted as an indicator of oxidative stress.

RESULTS

The values of total peroxide and OSI in the Group I were significantly lower than that in Group II (p<0.001, for both). TAC levels were significantly higher in patients with poorly developed collaterals than in well-developed collateral group (p<0.001). OSI values were also significantly different among the Rentrop class-0, -1, -2 and -3 (ANOVA p<0.001). We found significant correlations between collaterals score and TAC, total peroxide and OSI levels (p<0.001 for all). In multiple linear regression analysis, total peroxide and OSI were independent predictors of collaterals score (p=0.006 and p<0.001 respectively).

CONCLUSION

This study clearly demonstrates that the level of OSI is independently and positively associated with the presence of collateral circulation in total coronary occlusion patients.

Recent insights into human coronary collateral development

Enhancement of coronary collateral function is an intriguing approach to the preservation of ischaemic myocardium. Coronary collateral development consists of collateral recruitment and collateral growth. Collateral growth encompasses proliferation of capillaries in the ischaemic area (angiogenesis) and maturation of pre-existing collateral vessels (arteriogenesis), with the latter being more relevant in humans. Therefore, treatment intended directly for arteriogenesis of collateral vessels appears to be more effective. Promotion of coronary collateral growth has many attractive features, particularly in patients with angina who are not indicated for percutaneous coronary intervention or coronary artery bypass grafting surgery. A complete elucidation of the remaining practical and mechanistic questions of arteriogenesis may lead to a new remedy capable of developing collateral vessels more effectively.

Echocardiographic assessment of coronary blood flow velocity during controlled hypotensive anesthesia with nitroglycerin

OBJECTIVE

To determine the effect of nitroglycerin on coronary blood flow velocity during controlled hypotensive anesthesia in humans.

DESIGN

Internally controlled prospective study.

SETTING

Single university hospital.

PARTICIPANTS

Twenty American Society of Anesthesiologists class I and II patients undergoing general anesthesia for surgical resection of a malignancy.

INTERVENTIONS

General anesthesia was induced with thiopental, fentanyl, and succinylcholine and maintained with isoflurane and vecuronium. Transesophageal echocardiography was used to evaluate left ventricular wall motion and blood flow velocity in the left anterior descending coronary artery. Intravenous nitroglycerin was used to reduce systolic arterial pressure to 60 to 70 mmHg. Intravenous albumin 5% was administered to maintain pulmonary capillary wedge pressure >5 mmHg.

MEASUREMENTS AND MAIN RESULTS

The left anterior descending coronary artery was visualized clearly in 16 of 20 patients. At a mean nitroglycerin dose of 16+/-14 microg/kg/min, peak diastolic left anterior descending flow velocity increased significantly from 32.5+/-10.3 cm/sec to 44.7+/-14.6 cm/sec (p = 0.0103). None of the patients developed any ST-segment changes.

CONCLUSIONS

During nitroglycerin-induced hypotensive anesthesia, coronary blood flow as assessed by peak diastolic left anterior descending flow velocity is preserved or increased in most patients. Increases in left anterior descending flow velocity are predictably achieved only at nitroglycerin doses >5 microg/kg/min. Intraoperative transesophageal echocardiography is useful in monitoring coronary flow velocity responses to controlled hypotensive anesthesia.

Sustained reduction of exercise perfusion defect extent and severity with isosorbide mononitrate (Imdur) as demonstrated by means of technetium 99m sestamibi

BACKGROUND

The impact of long-acting nitrates on the extent and severity of stress-induced myocardial ischemia is not well described, especially after long-term treatment.

METHODS

Forty patients with chronic stable angina and reversible ischemia on an exercise stress myocardial perfusion single photon emission computed tomography (ex-SPECT) were prospectively studied in a 6-week period. At baseline, rest thallium-201/exercise stress technetium 99m sestamibi SPECT was performed, followed by treatment with extended-release isosorbide 5-mononitrate (5-ISMN, Imdur). Follow-up ex-SPECT was performed 5 days and 6 weeks after the initiation of therapy with extended-release 5-ISMN. The exercise treadmill testing (ETT) protocol and exercise duration of the follow-up studies were the same as that of the baseline ETT. Defect extent and severity were analyzed both by means of an automated quantitative method, with CEqual software, and visually, with a 20-segment scoring system (which was also used to derive a summed stress score [SSS]).

RESULTS

In the 6-week study period, significant reductions occurred in both the extent and the severity of exercise-induced ischemia by means of quantitative SPECT (13.8% [P<.0003] and 12.7% [P<.0003], respectively). There was no significant change in these variables between stages 2 (day 5) and 3 (6 weeks), indicating no development of tolerance to the nitrate effect. Similar reductions were noted by means of the visual analysis (SSS reduction of 13.0% [P<.002]) in the entire study period.

CONCLUSIONS

Patients with chronic-stable-angina treated with a long-acting nitrate demonstrate improvement in myocardial perfusion defect extent and severity in an extended period by means of both visual and quantitative analysis of sequential exercise testing to the same rate-pressure product end point.

Nitroglycerin dilates coronary collateral vessels during exercise after blockade of endogenous NO production

In a previous study nitroglycerin failed to dilate coronary collateral vessels during exercise. This study tested the hypothesis that failure of nitroglycerin to increase collateral flow occurred because endogenous nitric oxide (NO) had activated the guanylate cyclase vasodilator pathway so that additional NO from nitroglycerin could have no additional effect. Six dogs were collateralized using intermittent 2-min occlusions of the left anterior descending coronary artery followed by permanent occlusion. One week after permanent coronary occlusion, dogs were exercised on a treadmill (heart rate 202 +/- 5 beats/min), while blood flow was measured with radioactive microspheres. Blood flow to the collateral zone during control exercise was 1.90 +/- 0.11 ml. min(-1). g(-1) compared with 2.28 +/- 0.15 ml. min(-1). g(-1) in the normal zone (P < 0.05); systolic wall thickening was 23 +/- 3% in the collateral zone compared with 27 +/- 2% in the normal zone. When N(G)-nitro-L-arginine (L-NNA; 20 mg/kg iv) was administered to block NO production, collateral zone flow during exercise decreased to 1. 43 +/- 0.20 ml. min(-1). g(-1) (P < 0.05), and systolic wall thickening decreased to 12 +/- 4% (P < 0.05). A subsequent infusion of nitroglycerin (2 microg. kg(-1). min(-1) iv) increased collateral zone blood flow to 1.65 +/- 0.16 ml. min(-1). g(-1) (P < 0.05) and increased systolic wall thickening to 22 +/- 5% (P < 0.05). These findings demonstrate that endogenous NO contributes to collateral zone blood flow during exercise. If endogenous NO synthesis is blocked, then nitroglycerin is effective in improving collateral zone blood flow and contractile function during exercise.

Nitroglycerin fails to dilate coronary collateral vessels during exercise

This study was performed to determine whether nitroglycerin can increase blood flow to collateral-dependent myocardium during exercise. Intermittent repetitive occlusions of the left circumflex coronary artery (LCX) were used to stimulate growth of coronary collateral vessels in seven adult mongrel dogs. Coronary pressure distal to the occluder was measured with a long-term implanted microcatheter. When sufficient collateral growth had occurred to increase distal coronary pressure to >40 mm Hg during occlusion, the artery was permanently occluded. Dogs were returned to the laboratory 1 week later for study. Measurements were obtained at rest and during treadmill exercise during control conditions and after a 300-microg bolus of nitroglycerin (i.v.). Aortic and coronary pressures were measured with fluid-filled catheters, whereas myocardial blood flow was measured with radioactive microspheres. During control conditions, exercise caused significant increases of blood flow in the normal and collateral zones with significant decreases in vascular resistance. However, nitroglycerin failed to cause a further increase in blood flow to either the normal or the collateral-dependent myocardial regions during exercise. Furthermore, neither calculated transcollateral resistance (TCR) nor small-vessel resistance (SVR) changed significantly in response to nitroglycerin (TCR, 27 +/- 9 mm Hg/ml/min/g before nitroglycerin and 27 +/- 6 mm Hg/ml/min/g after; SVR, 43 +/- 5 mm Hg/ml/min/g before nitroglycerin and 49 +/- 7 mm Hg/ml/min/g after). The finding that the collateral vessels failed to dilate in response to nitroglycerin suggests that the nitric oxide system is already maximally recruited during exercise.

Pharmacological modulation of the human collateral vascular resistance in acute and chronic coronary occlusion assessed by intracoronary blood flow velocity analysis in an angioplasty model

BACKGROUND

The pharmacological responsiveness of the coronary collateral circulation in humans has been studied only by indirect means.

METHODS AND RESULTS

Patients with one-vessel disease and recruitable (n = 14) or spontaneously visible (n = 24) collateral vessels were studied during coronary angioplasty. Collateral flow in the recipient coronary artery was determined with a 0.014-in Doppler guide wire during balloon coronary occlusion and expressed as the diastolic blood flow velocity integral (dVi). Collateral blood flow velocity, mean aortic pressure (Pao), and coronary wedge pressure (Pw) were used to calculate the collateral vascular resistance index: Rcoll = (Pao-Pw)/ dVi (mm Hg/cm) and the peripheral vascular resistance index of the recipient coronary artery: R4 = Pw/dVi (mm Hg/cm). Adenosine (12 to 18 micrograms) and nitroglycerin (0.2 mg) were injected as a bolus in the donor coronary artery during subsequent balloon inflations to assess their effect on these hemodynamic variables. The administration of adenosine or nitroglycerin in patients with recruitable collateral vessels did not induce a change in dVi and Pw/Pao ratio. In patients with spontaneously visible collateral vessels, dVi increased from 8.0 +/- 4.5 to 10.8 +/- 8.0 cm (P = .01) after adenosine and from 7.4 +/- 4.5 to 10.3 +/- 6.9 cm (P = .003) after nitroglycerin. The Pw/Pao ratio remained unchanged after adenosine and nitroglycerin. Rcoll decreased from 10.3 +/- 9.5 to 8.6 +/- 8.5 mm Hg/cm (P = .01) after adenosine and from 11.6 +/- 10.4 to 8.3 +/- 8.9 mm Hg/cm (P < .001) after nitroglycerin. R4 decreased from 7.7 +/- 5.5 to 5.9 +/- 5.1 mm Hg/cm (P < .001) after adenosine and from 8.4 +/- 6.6 to 7.1 +/- 7.2 mm Hg/cm (P = .01) after nitroglycerin.

CONCLUSIONS

Coronary collateral blood flow can be increased with adenosine and nitroglycerin in patients with one-vessel disease and spontaneously visible collateral vessels, which is in contrast to patients with recruitable collateral vessels. This effect is the result of a reduction in the collateral vascular resistance and peripheral vascular resistance of the recipient coronary artery.

Achieving sustained improvement in myocardial perfusion: role of isosorbide mononitrate

The efficacy of antianginal agents in the treatment of patients with chronic stable angina has traditionally been evaluated by performance measures, such as the exercise treadmill test (ETT). Although reliable and reproducible, ETT is not a sensitive measure of changes in myocardial ischemia. The effects of antianginal agents on coronary blood flow and myocardial perfusion have been less frequently studied. Angiographic studies have demonstrated that nitrates may operate by preferentially directing blood flow to ischemic regions of the myocardium. These investigations have been limited, however, by the invasive nature of the evaluation. Measurements of regional myocardial perfusion may also be made with noninvasive tests. Both quantitative single-photon emission computed tomography (SPECT) and positron emission tomography (PET) have been employed, but few studies have used these techniques to assess the effects of antianginal drugs (in general) and nitrates (in particular) on changes in reversible myocardial perfusion defects. Studies that have evaluated the direct effects of nitrate treatment on coronary blood flow and myocardial perfusion defects in patients with chronic stable angina are reviewed, and preliminary data from a study of the effects of long-term nitrate treatment on myocardial perfusion are discussed.

Salutary effects of myocardial ischemia in coronary artery disease

Although myocardial ischemia in patients with coronary artery disease should be eliminated with medical and surgical treatment, it paradoxically contributes to the preservation of compromised myocardium through various mechanisms. First, ischemic vasodilation of coronary and collateral vessels resulting from the activation of the ATP-sensitive K+ channel maximizes a blood supply to the area having imbalance between myocardial oxygen supply and demand. Second, myocardial ischemia secondary to severe coronary stenosis develops functionally significant collateral circulation, which alleviates the deleterious sequelae of coronary obstructive disease. Finally, myocardial preconditioning with ischemia attenuates the subsequent ischemic insult. Particularly if combined with early reperfusion of the infarct-related coronary artery, the infarct size is decreased to one fourth of the permanent occlusion in dogs. A thorough understanding of the mechanisms of self-protecting benefits of myocardial ischemia would be useful in the care of patients with coronary artery disease.

Redistribution of myocardial blood flow with topical nitroglycerin in patients with coronary artery disease

BACKGROUND

Unlike nonselective coronary vasodilators, nitroglycerin (GTN) is said to exert its primary vasodilatory effect on epicardial conductance vessels. Thus, in experimental models of coronary occlusion GTN appears to preferentially direct blood flow to poststenotic zones of ischemia. This phenomenon has, to date, not been tested in humans. Using positron emission tomography we examined the effect of transdermal GTN on global and regional myocardial perfusion in patients with angiographically proven coronary artery disease.

METHODS AND RESULTS

Myocardial perfusion with [13N]ammonia was estimated from dynamic time-activity curves at baseline and 3 hours following application of either a 0.4 mg/h GTN skin patch (n = 10) or a placebo patch (n = 10) in a double-blind parallel design. From resliced cross-sectional images, regional flow, expressed as [13N]ammonia retention, was estimated from 216 myocardial sectors. Ischemia was defined as a significant reduction (> 2 SDs from average counts/pixel in maximally perfused zones) in [13N]ammonia retention within 10 contiguous myocardial sectors coupled with an increase or no change in counts derived from [18F]fluorodeoxyglucose. There was no change in global myocardial blood flow as expressed by [13N]ammonia retention following either placebo (0.61 +/- 0.14 to 0.62 +/- 0.12 min-1) or GTN (0.75 +/- 0.22 to 0.74 +/- 0.19 min-1). Conversely, there was a significant increase in the proportion of blood flow to the ischemic zones with GTN (73.9 +/- 12.6% to 94.9 +/- 17.8%; P < .05). No change in the distribution of blood flow to either ischemic or nonischemic zones was observed with placebo. A slight but insignificant decrease in [13N]ammonia retention in nonischemic zones was observed with GTN (1.01 +/- 0.31 to 0.93 +/- 0.26 min-1).

CONCLUSIONS

This study suggests that under resting conditions topical GTN alters myocardial perfusion by preferentially increasing flow to areas of reduced perfusion with little or no change in global myocardial perfusion in patients whose angina is responsive to GTN.