Hemodynamic effects of intermittent transdermal nitroglycerin in chronic congestive heart failure

Clinical and Research Considerations for Patients With Hypertensive Acute Heart Failure: A Consensus Statement from the Society for Academic Emergency Medicine and the Heart Failure Society of America Acute Heart Failure Working Group

Management approaches for patients in the emergency department (ED) who present with acute heart failure (AHF) have largely focused on intravenous diuretics. Yet, the primary pathophysiologic derangement underlying AHF in many patients is not solely volume overload. Patients with hypertensive AHF (H-AHF) represent a clinical phenotype with distinct pathophysiologic mechanisms that result in elevated ventricular filling pressures. To optimize treatment response and minimize adverse events in this subgroup, we propose that clinical management be tailored to a conceptual model of disease that is based on these mechanisms. This consensus statement reviews the relevant pathophysiology, clinical characteristics, approach to therapy, and considerations for clinical trials in ED patients with H-AHF.

Nitrate therapy for heart failure: benefits and strategies to overcome tolerance

Combination therapy with hydralazine and nitrates can improve outcomes in patients with heart failure and low ejection fraction. However, this combination is underused in clinical practice for several reasons, including side effects related to hydralazine and polypharmacy. Some of the benefits seen with hydralazine, including afterload reduction and attenuation of nitrate tolerance, have also been observed with angiotensin-converting enzyme inhibitors. Demonstrating similar clinical benefits with nitrates plus angiotensin-converting enzyme inhibitor therapy alone, in the absence of hydralazine, may represent an opportunity to improve heart failure care by increasing the use of nitrates. In this paper, we summarize data that support studying such an approach.

Application of pharmacodynamic modeling for designing time-variant dosing regimens to overcome nitroglycerin tolerance in experimental heart failure

PURPOSE

Prolonged continuous administration of nitroglycerin (NTG) leads to hemodynamic tolerance. We used a previously developed pharmacokinetic-pharmacodynamic (PK/PD) model of NTG tolerance in experimental heart failure to test whether dosage regimens, designed from this model, may allow avoidance of tolerance development upon continuous NTG infusion.

METHODS

Simulation experiments (using ADAPT II) were performed to evolve a time-variant infusion regimen that would theoretically provide sustained hemodynamic effect (30% reduction in left ventricular end-diastolic pressure, LVEDP) throughout 10 hours of drug dosing. A computer controlled infusion pump was utilized to deliver this time-variant input. Infusion experiments were then conducted in CHF rats to challenge the predictability of the applied PK/PD model.

RESULTS

Simulations showed that exponentially increasing input functions provided more sustained LVEDP effects when compared to linear or hyperbolic input functions delivering the same total NTG dose. A computer-selected infusion regimen of 6.56e0.00156 x minutes micrograms/min was anticipated to provide the desired hemodynamic profile in our animal model. Experiments conducted in rats with congestive heart failure (n = 4) confirmed the prediction of sustained hemodynamic effect without tolerance (28 +/- 4% reduction in LVEDP at 10 hrs).

CONCLUSIONS

These findings support the utility of our PK/PD model of NTG tolerance in predicting NTG action, and serve as an example of therapeutic optimization through PK/PD considerations.

Nitrates in the treatment of congestive heart failure

Nitrates have been widely used for the treatment of patients with chronic congestive heart failure. Although the use of these drugs has not been evaluated by large-scale studies traditionally used for evaluation of new therapy, multiple studies over the years have demonstrated their favorable effects. Organic nitrates have been shown to have a beneficial effect on ischemia, hemodynamic profile, magnitude of mitral regurgitation, endothelial function, and cardiac remodeling. These drugs alone or in combination with hydralazine have improved exercise capacity, maximal oxygen consumption, cardiac function, and survival. The use of nitrates in patients with heart failure has been limited by reduced responsiveness (resistance) and early development of tolerance. Nitrate resistance is due to reduced vascular response and results in the need to use a larger dose of any nitrate preparation when used for the treatment of patients with heart failure compared to patients without heart failure. Recent information suggests that nitrate tolerance is caused by increased levels of superoxide at the vascular wall, which leads to reduced nitric oxide level and to increased sensitivity to vasoconstrictive mechanisms, such as endothelin and angiotensin II. Intermittent dosing of nitrates allowing a 12-hour nitrate-free interval is effective in preventing nitrate tolerance and is, therefore, recommended. Recent information suggests that augmentation of nitrate dose by the use of an escalating dose regimen and a concomitant use of hydralazine can prevent or overcome the effect of nitrate tolerance in patients with heart failure.

Nitrates in congestive heart failure

Nitrates are commonly used in the therapy of congestive heart failure (CHF). They exert beneficial hemodynamic effects by decreasing left ventricular filling pressure and systemic vascular resistance while modestly improving cardiac output. The improvement in left ventricular function caused by nitrates is the result of combined reduction in outflow resistance and mitral regurgitation, while decreased pericardial constraint and subendocardial ischemia may also contribute to the process. With continuous nitrate administration, complete arterial tolerance develops, while venous tolerance appears to be only partial. The major mechanism of tolerance is loss of vascular smooth muscle sensitivity to nitrates. An increase in total blood volume occurring during the first few hours of an acute administration may partly contribute to tolerance. The importance of reflex neurohumoral activation is controversial; although it may contribute to tolerance in CHF, its role does not appear to be major. Chronic continuous nitrate therapy in CHF improves submaximal and maximal exercise tolerance. In combination therapy with hydralazine, isosorbide dinitrate reduces mortality, although to a lesser extent than the angiotensin converting enzyme inhibitor enalapril. Intravenous or sublingual nitrates are first-line agents in the therapy of acute pulmonary edema. In severe CHF, refractory to standard medical therapy, a short course of intravenous nitroglycerin, with or without inotropic agents, can help break the vicious spiral of CHF. Because tolerance occurs without nitrate-free intervals and until an optimal schedule of administration is determined, it makes good sense to include a nightly nitrate-free interval when prescribing nitrates for CHF in order to maintain maximal benefit during the hours of activity.

S-nitrosothiol modulation of tolerance to glyceryl trinitrate in bovine isolated coronary artery

The aim of the study was to ascertain whether induction of tolerance to glyceryl trinitrate (GTN) was modified by nitrosothiols. Vasodilator responses to GTN and to two nitrosothiols, namely S-nitroso-N-acetylpenicillamine (SNAP) and S-nitroso-N-acetylcaptopril (SNOCAP) were measured in bovine coronary artery rings constricted with the thromboxane mimetic agent U46619. Pre-exposure of vessels to SNAP (10 microM) for 1 h resulted in cross-tolerance to GTN comparable with tolerance induced by GTN (1.0 microM). Tolerance was increased after pre-exposure to both agents. SNAP alone in lower concentrations (0.01, 0.1 and 1.0 microM) did not produce cross tolerance to GTN, but in combination with GTN (1.0 microM) caused a small but significant decrease in tolerance to GTN. This attenuating effect of SNAP on GTN tolerance was reproduced by SNOCAP (0.01 microM). We conclude that the nitrosothiols exert a biphasic effect on induction of tolerance to GTN.

Nitrate resistance and tolerance: potential limitations in the treatment of congestive heart failure

Organic nitrates are commonly used in the treatment of chronic congestive heart failure (CHF). These drugs have been shown to improve exercise capacity and reduce symptoms and in combination with hydralazine to prolong life in patients with mild-to-moderate symptoms of CHF. Recent investigations, however, have indicated that nitrate-mediated benefit to patients with CHF may be limited by resistance to their hemodynamic effects seen in many patients and by early development of tolerance. The incidence, potential mechanisms, and possible methods for prevention of nitrate resistance and tolerance in patients with chronic CHF are reviewed.

Use of nitrates in ischemic heart disease

The organic nitrates have remarkably diverse actions that are or should be beneficial in patients with ischemic heart disease. These drugs are effective in all the important ischemic syndromes. Preliminary data in patients with acute infarction suggest that the drugs may be truly cardioprotective, resulting in improved mortality. This review has not discussed the role of nitrates in congestive heart failure or LV dysfunction, a subject of great importance. The nitrates are useful adjunctive agents in these syndromes, and the two VeHfT trials support the concept that long-term nitrate administration, in conjunction with hydralazine, may favorably alter the natural history of heart failure. This cardioprotective effect is similar to that suggested for the post-MI patient. The data are not strong enough for definitive conclusions at this time. The clinical benefits of nitrates in decreasing subjective (angina) and objective indices of ischemia in stable and unstable angina, as well as limited data in asymptomatic myocardial ischemia, are unequivocal and are as favorable as those for beta blockers or calcium antagonists. Tolerance is an important problem that unfavorably influences the potential benefits of nitrate therapy. I believe that this problem can be avoided with well-designed dosing regimens. Current research into endothelial biology in health and disease has further supported a physiologic role for the organic nitrates in patients with ischemic heart disease. The nitrate-platelet story, while controversial, is promising and offers another positive rationale for nitrate administration. The concept of nitrates replenishing disordered EDRF release or action is an exciting one. Physicians should feel fortunate to have such a remarkable group of drugs available for their patients.

Pharmacodynamic and pharmacokinetic evaluation of a new transdermal delivery system with a time-dependent release of glyceryl trinitrate

The pharmacokinetics of glyceryl trinitrate (GTN) and its main metabolites as well as the hemodynamic effects of a new transdermal delivery system (TDS) were investigated in ten healthy male volunteers using a single blind, placebo-controlled study design with an application period of active drug of 4 successive days. The adhesive-type matrix system contains 20-mg GTN and released about 75% in a time-dependent manner. The plasma concentrations of GTN and its metabolites 1-2- and 1-3 glyceryl dinitrate reflected the time-dependent release with higher plasma concentrations during the first 12 hours than during the second 12 hours. Continuous administration of the TDS, which released 15 mg GTN/day, caused an accumulation of GTN in the plasma (about 70% greater AUC at the fourth day in comparison with the first day). The total effect per dose on the a/b-ratio of the digital pulse (height of the peak of the systolic wave divided by height of the peak of the dicrotic wave) and the reflex tachycardia were diminished by about 50% and 37%, respectively, at the fourth treatment day. The effect on systolic blood pressure measured under orthostatic conditions was blunted already 8 hours after the first application. The effect of sublingually administered GTN on digital pulse was attenuated during administration and also 1 hour after removal of the last TDS. The effect was restored 8 to 12 hours after removal of the TDS. Thus, the discontinuous release of GTN from the new system does not prevent the decline of hemodynamic efficacy during continuous therapy.

Randomized study to evaluate the relation between oral isosorbide dinitrate dosing interval and the development of early tolerance to its effect on left ventricular filling pressure in patients with chronic heart failure

BACKGROUND

Early development of nitrate tolerance has been shown in patients with chronic congestive heart failure (CHF) receiving continuous nitroglycerin therapy. The influence of dosing interval of oral isosorbide dinitrate (ISDN), the nitrate preparation most widely used for the treatment of CHF, has not been investigated.

METHODS AND RESULTS

We performed a prospective, randomized study to evaluate the effect of various regimens of oral ISDN on the development of early tolerance to its effect on left ventricular filling pressure in patients with moderate to severe CHF. Forty-four responders (20% or greater reduction in mean pulmonary artery wedge pressure lasting 1 hour or longer) were divided into four groups of 11 patients each, and randomized to receive their effective ISDN dose (40-120 mg) Q 4 hours, Q 6 hours, Q 8 hours, or t.i.d. (drug given at 0, 6, 12, and 24 hours allowing 12 hours of ISDN washout interval between the third and fourth doses). All groups demonstrated a significant and comparable reduction in LV filling pressure following administration of the first ISDN dose. Early attenuation of hemodynamic response was demonstrated with frequent dosing (Q 4 hours and Q 6 hours) ISDN. Tolerance was with a Q 8-hour regimen as demonstrated by preserved hemodynamic response to each dose. The effect of each dose, however, was short-term, with return of pulmonary artery wedge pressure to baseline level at 2 to 4 hours, resulting in an intermittent effect totaling no longer than 12 hours of the 30-hour study period. The use of a t.i.d. regimen resulted in marked attenuation of response after the third dose with complete restoration of nitrate effect following a 12-hour washout period between the third and fourth doses. ISDN plasma concentration was measured in five patients in each of the Q 4- and Q 8-hour groups. In the Q 4-hour group, plasma levels were significantly higher after administration of the last dose than after the first dose (area under the curve, 242 +/- 216 versus 123 +/- 130 ng/ml, p less than 0.05), and trough levels before administration of the second and the fifth dose (15 +/- 17 and 27 +/- 27 ng/ml, respectively) were both markedly higher than the baseline value of 2 +/- 4 ng/ml.

CONCLUSIONS

Our data demonstrate the development of tolerance and early attenuation of effect on left ventricular filling pressure with frequent oral dosing (Q 4 and Q 6 hours) with ISDN in patients with chronic CHF, which may be related to persistently elevated trough blood levels of ISDN. The development of tolerance can be reversed after a washout period of 12 hours and can be prevented with a Q 8-hour administration. These regimens, however, are limited by an inconsistent effect. Although long-term implications of these findings need further evaluation, the present study demonstrates the difficulty of maintaining a persistent ISDN-mediated reduction in left ventricular filling pressure in patients with chronic, moderate to severe CHF. These results suggest the need to use intermittent ISDN therapy allowing a daily nitrate washout interval and the rationale for combined vasodilator therapy in patients with CHF.

Haemodynamic effects of glyceryl trinitrate following repeated application of a transdermal delivery system with a phasic release profile

The haemodynamic effects and plasma concentrations of glyceryl trinitrate (GTN) and its dinitrate metabolites were investigated in 8 healthy male volunteers during 5 days of application of a new transdermal delivery system (TDS) with time-dependent release characteristics, which were considered to prevent or to diminish development of nitrate tolerance. On the first and fifth day of administration the following haemodynamic parameters were determined: digital pulse ratio of height of systolic peak to height of dicrotic wave (i.e. a/b-ratio), heart rate and systolic blood pressure under orthostatic conditions. Peak plasma concentrations of GTN were 139 and 155 pg.ml-1 on the first and fifth day of treatment, and the corresponding trough concentrations (i.e. 24 h after administration) were 52.5 and 36.6 pg.ml-1, respectively. Compared to placebo, the area under the effect curve of the a/b-ratio of the digital pulse was increased on the first (25.6%) and fifth day (13%). A significant increase of heart rate and a decrease of systolic blood pressure were seen only on the first day of treatment. The haemodynamic effects of sublingual GTN 0.8 mg were reduced by 69% (a/b-ratio) and 52% (standing heart rate) on the fifth day compared to the pretreatment values. Thus, the phasic release of GTN from the new TDS can be demonstrated by the time course of the plasma concentrations of GTN and its metabolites. Nevertheless, following repeated administration the hemodynamic effects are blunted.

Management of myocardial ischemia: role of intermittent nitrate therapy

Tolerance to long-term nitrate therapy has now been documented in a large number of clinical studies. These trials have demonstrated attenuation of effect over time but a return of responsiveness with the start of nitrate-free intervals. Intermittent therapy prevents attenuation or tolerance when used as the initial treatment regimen. This strategy is appropriate for use with isosorbide dinitrate, as well as intravenous or transdermal nitroglycerin. Transmucosal nitroglycerin has not been associated with tolerance; the lack of such an effect may be a result of rapid increases and decreases in drug levels. In the future, sulfhydryl donors may also prove useful in prevention or reversal of nitrate tolerance.

Duration and reproducibility of initial hemodynamic effects of flosequinan in patients with congestive heart failure

The duration and reproducibility of hemodynamic effects of flosequian, a direct-acting, balanced-type vasodilator, were studied in 19 heart failure patients (NYHA class 3.0 +/- 0.7) receiving 100 mg orally (day 1), placebo (day 2), and again 100 mg (day 3). Flosequinan immediately reduced systemic and pulmonary resistance (23% and 35%, respectively, at 60-90 minutes postdrug) and decreased pulmonary wedge, right atrial, mean pulmonary artery, and mean arterial pressure by 38%, 50%, 25%, and 7%, respectively. Concomitantly, cardiac output, and stroke volume and work increased by 26%, 20%, and 22%, respectively. Most hemodynamic effects persisted for 48 hours. In contrast, changes in pulmonary wedge and arterial pressures, stroke volume, and stroke work only lasted for 2-12 hours. Maximum absolute changes on day 3 were generally comparable with first-dose effects with, again, long-lasting effects on systemic resistance and cardiac output. However, changes in pulmonary artery, wedge, and resistance were significantly shorter than after first dose administration. These data indicate sustained and reproducible arterial dilating effects of flosequinan, but less pronounced and shorter lasting pulmonary arterial and venodilator properties.

Mechanisms of nitrate tolerance: a review

With the increased use of long-acting nitroglycerin preparations, there has been greater recognition of the problem of nitrate tolerance. In recent years extensive research has broadened our understanding of the mechanisms of nitroglycerin action and the mechanisms of drug attenuation. This paper reviews the current state of knowledge regarding nitroglycerin tolerance, with an emphasis on the concepts of cellular and neurohumoral mechanisms of drug attenuation. The discussion includes potential approaches to prevent nitrate tolerance, including the introduction of a nitrate-free interval, or concomitant administration of sulfhydryl donors or neurohumoral blocking agents.

Nitrates: why and how should they be used today? Current status of the clinical usefulness of nitroglycerin, isosorbide dinitrate and isosorbide-5-mononitrate

Nitrates are highly effective both in terminating acute attacks of angina pectoris and in the prophylaxis of symptomatic and asymptomatic myocardial ischemia. Preload reduction by venodilatation is the prevailing mechanism of nitrates in patients with chronic stable angina and is the unique feature distinguishing them from beta and calcium-channel blockers. Nitrates dilate coronary arteries not only in pre- and poststenotic vessels, but also in eccentric lesions. In patients with endothelial dysfunction, nitrates seem to be the physiological substitute for endothelium-derived relaxing factor. During the past decade, however, there has been substantial evidence of a clinically relevant loss of the anti-ischemic effects ("nitrate tolerance"). Many studies with oral dosing of isosorbide dinitrate or isosorbide-5-mononitrate at least three times daily have proven nitrate tolerance in patients with coronary artery disease and/or congestive heart failure. Complete loss of anti-ischemic effects after repetitive, continuous patch attachments has also been found. As we first showed in 1983, intermittent therapy with once-daily ingestion of high-dose sustained-release isosorbide dinitrate was successful in preventing the development of tolerance. Similarly, tolerance to isosorbide-5-mononitrate also does not develop when it is ingested once daily. It is now generally accepted that a daily low-nitrate interval is required to prevent tolerance development. Although the minimal patch-free interval required to prevent tolerance needs further investigation, a 12-h patch-free interval should prevent tolerance in most patients. The prolonged duration of action of once-daily high-dosage administration of sustained-release formulations, the improved patient compliance with a single daily administration, and the increased likelihood of maximal anti-ischemic effects are important reasons for recommending high single daily doses of isosorbide dinitrate or isosorbide-5-mononitrate.

Vasodilator therapy without converting-enzyme inhibition in congestive heart failure--usefulness and limitations

Despite a well-established rationale for pharmacologically induced arterial and venous vasodilatation in congestive heart failure, the clinical usefulness of long-term vasodilator therapy without concomitant converting-enzyme inhibition generally has been disappointing. With the exception of nitrates and, possibly, the combination of nitrates and hydralazine, the use of converting-enzyme inhibitors in many aspects appears preferable in the majority of patients. This article reviews the pathophysiology of inappropriate vasoconstriction in heart failure, the cellular mode of action of the various vasodilators, hemodynamic effects with respect to the peripheral site of action, clinical usefulness and limitations of different vasodilators, and the various determinants of clinical efficacy. Finally, an attempt is made to assess when and how to introduce vasodilator treatment with and without concomitant ACE inhibition.

Effect of intermittent exposure and drug-free intervals on the in vitro vascular tolerance to nitroglycerin

Recent reports suggest that intermittent nitroglycerin (NTG) treatment, incorporating a daily nitrate-free interval, can avoid the tolerance associated with continuous NTG therapy. This study has investigated whether an in vitro model of NTG tolerance could be used to examine the mechanisms of tolerance avoidance (by intermittent NTG exposure) and tolerance reversal. Isolated rat abdominal aortic rings were exposed to 55 microM NTG at varying intervals over a 60 min period, and the concentration-relaxation curves to NTG were subsequently determined. Intermittent NTG exposure (either 12 x 0.5 min or 3 x 2 min) significantly reduced NTG tolerance compared to continuous exposure over the same period of time (60 min). The diminished tolerance was apparently due to the reduced total exposure time, since the NTG responsiveness of aortic rings exposed to NTG intermittently or continuously for 6 min of the incubation period was not significantly different. Under the present conditions, in vitro NTG tolerance could be reversed if sufficient washout time was allowed. Thus, aortic rings exposed to NTG for the initial, but not the final, 6 min of incubation were not tolerant to NTG. In addition, rings exposed to NTG for 3 min exhibited near-maximal tolerance after 5 min washout, but no tolerance after 60 or 120 min washout. It appears, therefore, that the isolated vessel retains the "repair" mechanism responsible for tolerance reversal under the present conditions. This study suggests that the in vitro model of NTG tolerance may be useful for investigating the characteristics and mechanisms of tolerance avoidance and reversal, as well as tolerance induction.

Vasodilator and inotropic drugs for the treatment of chronic heart failure: distinguishing hype from hope

During the past 10 years, more than 80 orally active vasodilator and inotropic agents have been tested in the clinical setting to evaluate their potential utility in the treatment of chronic heart failure. Although the initial reports of all of these drugs suggested that each represented a major therapeutic advance, only three agents--digoxin, captopril and enalapril--have produced consistent long-term hemodynamic and clinical benefits in these severely ill patients. Most of the other drugs that have been tested have not (to date) distinguished themselves from placebo therapy in large-scale, controlled trials, even though these agents produce hemodynamic effects that closely resemble those seen with digitalis and the converting-enzyme inhibitors. These observations suggest that the hemodynamic derangements that characteristically accompany the development of left ventricular dysfunction cannot be considered to be the most important pathophysiologic abnormality in chronic heart failure. Although cardiac contractility is usually depressed in this disease, positive inotropic agents do not consistently improve the clinical status of these patients. Similarly, although the systemic vessels are usually markedly constricted, drugs that ameliorate this vasoconstriction do not consistently relieve symptoms, enhance exercise capacity or prolong life. Hence, correction of the central hemodynamic abnormalities seen in heart failure may not necessarily provide a rational basis for drug development, and future advances in therapy are likely to evolve only by attempting to understand and modify the basic physiologic derangements in this disorder.

Quality of life on angina therapy: a randomised controlled trial of transdermal glyceryl trinitrate against placebo

In a randomised controlled trial in 427 men with chronic stable angina continuous use of 5 mg transdermal glyceryl trinitrate (GTN) showed no advantage over placebo in terms of efficacy (anginal attack rates and sublingual GTN consumption) or quality of life (as measured with the sickness impact profile and a health index of disability). Patients on the active drug reported headaches more frequently than patients on placebo, and a higher proportion of them withdrew from the trial because of headache. Quality-of-life measurements showed a significant adverse effect of active treatment, principally in the social interaction dimension of the sickness impact profile. A similar effect was observed in placebo patients when crossed to active treatment in a 4-week single-blind period. The results suggest no benefit in the relief of chest pain from 5 mg transdermal GTN when used continuously.

Prevention and reversal of nitrate tolerance in patients with congestive heart failure

To evaluate possible mechanisms underlying the development of nitrate tolerance, we treated 35 patients who had severe chronic heart failure with a prolonged (48-hour) intravenous infusion of nitroglycerin (6.4 micrograms per kilogram of body weight per minute) given either continuously or intermittently (12-hour infusions separated by intervals of 12 hours). Intravenous nitroglycerin produced immediate hemodynamic benefits in all patients, but the magnitude of this improvement was greatly diminished after 48 hours of continuous therapy with the drug. This attenuation was accompanied by cross-tolerance to oral isosorbide dinitrate and by an increase in heart rate, plasma renin activity, and body weight. In contrast, intermittent therapy with intravenous nitroglycerin was not associated with a loss of hemodynamic efficacy or cross-tolerance to oral nitrates and was not accompanied by changes in neurohormonal activity or body weight. In eight patients in whom nitrate tolerance developed during continuous intravenous therapy, the administration of the sulfhydryl-containing compound N-acetylcysteine (200 mg per kilogram orally) restored the hemodynamic state toward that observed at the start of the infusion of nitroglycerin (partial reversal of tolerance). In contrast, N-acetylcysteine had little hemodynamic effect in patients who were not receiving nitroglycerin. These data support the hypothesis that neurohormonal activation and depletion of sulfhydryl groups may interact to cause the loss of hemodynamic efficacy that occurs during prolonged treatment with intravenous nitroglycerin in patients with heart failure. Evaluation of the suggested role of sulfhydryl depletion in the development of tolerance will, however, require direct studies of vascular tissue.