Heterogeneous microvascular coronary vasodilation by adenosine and nitroglycerin in dogs

Medication and ECG Patterns That May Hinder SPECT Myocardial Perfusion Scans

Coronary artery disease (CAD) is the leading cause of death followed by cancer, in men and women. With risk factors being endemic and the increasing costs of healthcare for management and treatment, myocardial perfusion imaging (MPI) finds a central role in risk stratification and prognosis for CAD patients, but it comes with its limitations in that the referring clinician and managing team must be aware of and use at their advantage. This narrative review examines the utility of myocardial perfusion scans in the diagnosis and management of patients with ECG alterations such as atrioventricular block (AVB), and medications including calcium channel blockers (CCB), beta blockers (BB), and nitroglycerin which may impact the interpretation of the exam. The review analyzes the current evidence and provides insights into the limitations, delving into the reasons behind some of the contraindications to MPI.

Whole-body recruitment of glycocalyx volume during intravenous adenosine infusion

Adenosine-mediated recruitment of microvascular volume in heart and muscle has been suggested to include, in addition to vasodilation of resistance vessels, an increased accessibility of the endothelial glycocalyx for flowing plasma as a result of an impairment of its barrier properties. The aim of the current study was to investigate the effect of systemic intravenous administration of adenosine on the glycocalyx-dependent exclusion of circulating blood at a whole-body level. In anesthetized goats (N = 6), systemic blood-excluded glycocalyx volume was measured by comparing the intravascular distribution volume of the suggested glycocalyx accessible tracer dextrans with a molecular weight of 40 kDa (Dex-40) to that of circulating plasma, derived from the dilution of labeled red blood cells and large vessel hematocrit. Systemic glycocalyx volume was determined at baseline and during intravenous infusion of adenosine (157 ± 11.6 μg/kg min⁻¹). Blood-inaccessible glycocalyx volume decreased from 458.1 ± 95.5 to 18.1 ± 62.2 mL (P < 0.01) during adenosine administration. While circulating plasma volume did not change significantly (617.1 ± 48.5 vs. 759.2 ± 47.9 mL, NS), the decrease in blood-excluded glycocalyx volume was associated with a decrease in Dex-40 distribution volume (from 1075.2 ± 71.0 to 777.3 ± 60.0 mL, P < 0.01). Intravenous administration of adenosine is associated with a robust impairment of whole-body glycocalyx barrier properties, reflected by a greatly reduced exclusion of circulating blood compared to small dextrans. The observed decrease in Dex-40 distribution volume suggests that the reduction in glycocalyx volume coincides with a reduction in tracer-accessible vascular volume.

The hepatic microcirculation: mechanistic contributions and therapeutic targets in liver injury and repair

The complex functions of the liver in biosynthesis, metabolism, clearance, and host defense are tightly dependent on an adequate microcirculation. To guarantee hepatic homeostasis, this requires not only a sufficient nutritive perfusion and oxygen supply, but also a balanced vasomotor control and an appropriate cell-cell communication. Deteriorations of the hepatic homeostasis, as observed in ischemia/reperfusion, cold preservation and transplantation, septic organ failure, and hepatic resection-induced hyperperfusion, are associated with a high morbidity and mortality. During the last two decades, experimental studies have demonstrated that microcirculatory disorders are determinants for organ failure in these disease states. Disorders include 1) a dysregulation of the vasomotor control with a deterioration of the endothelin-nitric oxide balance, an arterial and sinusoidal constriction, and a shutdown of the microcirculation as well as 2) an overwhelming inflammatory response with microvascular leukocyte accumulation, platelet adherence, and Kupffer cell activation. Within the sequelae of events, proinflammatory mediators, such as reactive oxygen species and tumor necrosis factor-alpha, are the key players, causing the microvascular dysfunction and perfusion failure. This review covers the morphological and functional characterization of the hepatic microcirculation, the mechanistic contributions in surgical disease states, and the therapeutic targets to attenuate tissue injury and organ dysfunction. It also indicates future directions to translate the knowledge achieved from experimental studies into clinical practice. By this, the use of the recently introduced techniques to monitor the hepatic microcirculation in humans, such as near-infrared spectroscopy or orthogonal polarized spectral imaging, may allow an early initiation of treatment, which should benefit the final outcome of these critically ill patients.

Controlled hypotension: a guide to drug choice

For half a century, controlled hypotension has been used to reduce bleeding and the need for blood transfusions, and provide a satisfactory bloodless surgical field. It has been indicated in oromaxillofacial surgery (mandibular osteotomy, facial repair), endoscopic sinus or middle ear microsurgery, spinal surgery and other neurosurgery (aneurysm), major orthopaedic surgery (hip or knee replacement, spinal), prostatectomy, cardiovascular surgery and liver transplant surgery. Controlled hypotension is defined as a reduction of the systolic blood pressure to 80-90 mm Hg, a reduction of mean arterial pressure (MAP) to 50-65 mm Hg or a 30% reduction of baseline MAP. Pharmacological agents used for controlled hypotension include those agents that can be used successfully alone and those that are used adjunctively to limit dosage requirements and, therefore, the adverse effects of the other agents. Agents used successfully alone include inhalation anaesthetics, sodium nitroprusside, nitroglycerin, trimethaphan camsilate, alprostadil (prostaglandin E1), adenosine, remifentanil, and agents used in spinal anaesthesia. Agents that can be used alone or in combination include calcium channel antagonists (e.g. nicardipine), beta-adrenoceptor antagonists (beta-blockers) [e.g. propranolol, esmolol] and fenoldopam. Agents that are mainly used adjunctively include ACE inhibitors and clonidine. New agents and techniques have been recently evaluated for their ability to induce effective hypotension without impairing the perfusion of vital organs. This development has been aided by new knowledge on the physiology of peripheral microcirculatory regulation. Apart from the adverse effects of major hypotension on the perfusion of vital organs, potent hypotensive agents have their own adverse effects depending on their concentration, which can be reduced by adjuvant treatment. Care with use limits the major risks of these agents in controlled hypotension; risks that are generally less important than those of transfusion or alternatives to transfusion. New hypotensive drugs, such as fenoldopam, adenosine and alprostadil, are currently being evaluated; however, they have disadvantages and a high treatment cost that limits their development in this indication. New techniques of controlled hypotension subscribe to the use of the natural hypotensive effect of the anaesthetic drug with regard to the definition of the ideal hypotensive agent. It must be easy to administer, have a short onset time, an effect that disappears quickly when administration is discontinued, a rapid elimination without toxic metabolites, negligible effects on vital organs, and a predictable and dose-dependent effect. Inhalation agents (isoflurane, sevoflurane) provide the benefit of being hypnotic and hypotensive agents at clinical concentrations, and are used alone or in combination with adjuvant agents to limit tachycardia and rebound hypertension, for example, inhibitors of the autonomic nervous system (clonidine, beta-blockers) or ACE inhibitors. When they are used alone, inhalation anaesthetics require high concentrations for a significant reduction in bleeding that can lead to hepatic or renal injury. The greatest efficacy and ease-of-use to toxicity ratio is for techniques of anaesthesia that associate analgesia and hypotension at clinical concentrations without the need for potent hypotensive agents. The first and oldest technique is epidural anaesthesia, but depending on the surgery, it is not always appropriate. The most recent satisfactory technique is a combination treatment of remifentanil with either propofol or an inhalation agent (isoflurane, desflurane or sevoflurane) at clinical concentrations. In light of the current literature, and because of their safety and ease of use, these two techniques are preferred.

Multidetector computed tomography myocardial perfusion imaging during adenosine stress

OBJECTIVES

The purpose of this study is to validate the accuracy of multidetector computed tomography (MDCT) to measure differences in regional myocardial perfusion during adenosine stress in a canine model of left anterior descending (LAD) artery stenosis, during first-pass, contrast-enhanced helical MDCT.

BACKGROUND

Myocardial perfusion imaging by MDCT may have significant implications in the diagnosis and treatment of coronary artery disease.

METHODS

Eight dogs were prepared with a LAD stenosis, and contrast-enhanced MDCT imaging was performed 5 min into adenosine infusion (0.14 to 0.21 mg/kg/min). Images were analyzed using a semiautomated approach to define the regional signal density (SD) ratio (myocardial SD/left ventricular blood pool SD) in stenosed and remote territories, and then compared with microsphere myocardial blood flow (MBF) measurements.

RESULTS

Mean MBF in stenosed versus remote territories was 1.37 +/- 0.46 ml/g/min and 1.29 +/- 0.48 ml/g/min at baseline (p = NS) and 2.54 +/- 0.93 ml/g/min and 8.94 +/- 5.74 ml/g/min during adenosine infusion, respectively (p < 0.05). Myocardial SD was 92.3 +/- 39.5 HU in stenosed versus 180.4 +/- 41.9 HU in remote territories (p < 0.001). There was a significant linear association of the SD ratio with MBF in the stenosed territory (R = 0.98, p = 0.001) and between regional myocardial SD ratio and MBF <8 ml/g/min, slope = 0.035, SE = 0.007, p < 0.0001. Overall, there was a significant non-linear relationship over the range of flows studied (LR chi-square [2 degrees of freedom] = 31.8, p < 0.0001).

CONCLUSIONS

Adenosine-augmented MDCT myocardial perfusion imaging provides semiquantitative measurements of myocardial perfusion during first-pass MDCT imaging in a canine model of LAD stenosis.

The role of nitric oxide in the spatial heterogeneity of basal microvascular blood flow in the rat diaphragm

The effects of N omega-nitro-L: -arginine (L: -NOARG) and N(G)-monomethyl-L: -arginine (L: -NMMA) on the spatial distribution of diaphragmatic microvascular blood flow were assessed in anesthetized, mechanically ventilated rats. Microvascular blood flow was measured after different periods at either a fixed site (Q stat) or 25 different sites (Q scan) using computer-aided laser-Doppler flowmetry (LDF) scanning. The value of Q stat was unaffected after 15-20 min superfusion with any one of the following agents: L: -NOARG (0.1 mM), L: -NMMA (0.1 mM), L: -arg (10 mM). The cumulative frequency histogram of the Q scan value in the control group displayed a non-Gaussian distribution that was not significantly affected after 15 min superfusion with the vehicle of L: -NOARG. Superfusion with either L: -NMMA or L: -NOARG at 0.1 mM for 15 min displaced the histogram of cumulative frequency to the left, with the median value of blood flow decreasing by 10 to 20%. However, skewness and kurtosis of the distribution of basal Q(scan) were unaffected after superfusion of either of the L: -arg analogues. Pretreatment with L: -arg (10 mM), followed by co-administration of L: -arg (10 mM) with L: -NOARG (0.1 mM) only partially prevented L: -NOARG from exerting this inhibitory effect on the distribution of basal Q scan, while pretreatment with L: -arg in the same manner could prevent L: -NMMA from exerting its inhibitory effect. There was a weak but significant linear relationship between the magnitude of basal Q(scan) and normalized changes in basal Q scan after superfusion of either of the L: -arg analogues. In conclusion, a basal NO activity is present in the diaphragmatic microvascular bed of rats. LDF scanning rather may yield more vivid information about the extent of overall tissue perfusion than conventional LDF whenever basal NO activity is involved. Moreover, the parallel flow profiles after NO synthase blockade suggest that the spatial inhomogeneity of basal diaphragmatic microvascular blood flow is not dependent on basal NO formation.

Changes in myocardial blood volume over a wide range of coronary driving pressures: role of capillaries beyond the autoregulatory range

OBJECTIVE

To determine whether, when the vasomotor capacity of the coronary arterioles is exhausted at rest, myocardial blood volume decreases in order to maintain a normal capillary hydrostatic pressure, even at the expense of myocardial oxygen delivery.

METHODS

18 dogs were studied. In group 1 (n = 9), coronary driving pressure (CDP) was reduced by 10-80 mm Hg below normal by a stenosis; in group 2 (n = 9), it was increased 20-80 mm Hg above baseline by increasing aortic pressure with phenylephrine. Myocardial contrast echocardiography (MCE) was undertaken to measure the myocardial blood volume fraction and myocardial blood flow (MBF).

RESULTS

In group 1 dogs, as CDP was reduced, both coronary blood flow (CBF) and MBF decreased. Myocardial blood volume fraction also decreased and myocardial vascular resistance increased, while coronary sinus PO2 decreased. In group 2 dogs, as CDP was increased, epicardial CBF increased but MBF remained unchanged because of a decrease in myocardial blood volume fraction. Myocardial vascular resistance decreased, however, implying the presence of coronary arteriovenous shunting, which was supported by a progressive increase in the coronary sinus PO2.

CONCLUSIONS

When arteriolar tone is exhausted so that CBF becomes dependent on CDP, myocardial blood volume decreases in order to maintain a constant capillary hydrostatic pressure, which takes precedence over myocardial oxygen delivery. These novel findings implicate capillaries in the regulation of CBF beyond the autoregulatory range.

The coronary microcirculation in health and disease

Apart from being the site of nutrient and gas exchange for the myocardium, the capillary bed is a dynamic participant in the regulation of coronary and myocardial blood flow. MBV also responds to changes in myocardial oxygen demand. Because MCE can be used to assess MBF and MBV in vivo, it can be used to provide unique insights into tissue perfusion.

Endothelial and Microvascular Function

The endothelium is more than just a passive vessel lining. New advances have revealed and expanded the multifactorial role of the endothelium in the homeostatic regulation of the microvasculature, including control of primary hemostasis, blood coagulation and fibrinolysis, platelet and leukocyte interactions with the vessel wall, lipoprotein metabolism, presentation of histocompatibility antigens, regulation of vascular tone and growth, and regulation of blood pressure. It possesses numerous receptors and releases compounds that affect the regulation of vascular tone and contribute to vascular permeability. Many crucial vasoactive endogenous compounds are formed in the endothelial cells to control the functions of vascular smooth muscle cells and circulating blood cells. Gap junctions facilitate the exchange of metabolites, ions, and other messenger molecules among endothelial cells and smooth muscle cells, and regulate cell growth. Among the numerous regulatory systems affecting microvascular function are the cholinergic and adrenergic (α1, α2, and β) systems. Flow-metabolism coupling is affected by a variety of signaling systems, including adenosine, oxygen, carbon dioxide, lactate, nitric oxide, and others. Agents such as the angiotensin system and endothelin, as well as others, play a role in autoregulation (maintenance of constant flow in the face of changing pressure). All of these are discussed in detail.

Relation between myocardial oxygen consumption and myocardial blood volume: a study using myocardial contrast echocardiography

Myocardial blood volume (MBV) is the volume of blood residing in myocardial vessels, 90% of which is in capillaries. MBV can be measured in vivo using myocardial contrast echocardiography (MCE). It has been shown that when increases in coronary blood flow (CBF) are not associated with increase in myocardial oxygen consumption (MVO(2)), MBV does not increase. We hypothesized that MBV would increase when increases in CBF are associated with an increase in MVO(2). The atrioventricular node was ablated in 18 dogs and dual-chamber pacing was instituted. In group 1 dogs (n = 9), heart rate was altered from 50 to 150 bpm(-1) in increments of 20 bpm(-1) in random order. In group 2 dogs (n = 9), heart rate was kept constant, and dobutamine was infused at doses of 5, 10, 20, 30, and 40 microg/kg(-1)/min(-1). During each intervention, hemodynamic parameters and MVO(2) were measured, and MCE was performed. MVO(2) increased more (P <.01) with inotropic compared with chronotropic stimulation, resulting in a parallel increase in CBF. MBV fraction and MCE-derived myocardial blood flow increased significantly with increases in MVO(2) (P <.05 and P <.001, respectively) when dobutamine was infused, but remained unchanged when heart rate alone was increased. We conclude that when MVO(2) is increased substantially, the resulting increase in CBF and MCE-derived myocardial blood flow is mediated, in part, by an increase in MBV. Thus, capillary recruitment plays an important role in the physiologic regulation of CBF. Lack of increase in MBV during dobutamine stress may indicate the presence of coronary stenosis or microvascular disease.

Regulation of flow and wall shear stress in arteriolar networks of the hamster cheek pouch

Our purpose was to define arteriolar network hemodynamics during moderate increases in interstitial adenosine or nitric oxide in the hamster (n = 34, pentobarbital sodium 70 mg/kg) cheek pouch tissue. The network consists of a feed arteriole (approximately 12-microm diameter, approximately 800-microm length) with three to six branches. Observations of diameter, red blood cell flux, and velocity were obtained at the feed before the branch and within the branch. A comparison of baseline with suffused adenosine or sodium nitroprusside (SNP) 10(-9) to 10(-5) M showed the following. First, diameter change was heterogeneous by agonist, did not reflect the expected dilatory response, and was related to location within the network. With adenosine, upstream branch points constricted and those downstream dilated, even at 10(-5) M. With SNP, upstream branch points dilated, whereas those downstream constricted. Second, with adenosine, changes in diameter, flux, and velocity together resulted in no change in wall shear stress until 10(-5) M. Wall shear stress was not maintained at a constant level with Nomega-nitro-L-arginine (10(-5) M), suggesting a role for flow-dependent diameter changes with adenosine. With SNP, diameter change correlated with the baseline (before SNP) shear stress conditions.

Effect of oral nitroglycerin and cold stress on myocardial perfusion in areas subtended by stenosed and nonstenosed coronary arteries

Physical obstruction and coronary vasoconstriction mediated by adrenergic stress are believed to be responsible for episodes of myocardial hypoperfusion and angina. Nitroglycerin relieves symptoms by reducing preload and dilating epicardial vessels. The net perfusion change and relation to stenosis severity of nitroglycerin and adrenergic stress have been debated. This study aimed to evaluate whether oral nitroglycerin and adrenergic stress alters perfusion in myocardial segments subtended by stenosed and nonstenosed coronary arteries. Myocardial perfusion was quantified (using N-13-ammonia positron emission tomography [PET]) at rest, after oral nitroglycerin 400 microg, and after cold stress in 25 patients with coronary artery disease (62 +/- 9 years, 21 men) and in 30 controls (34 +/- 9 years, 22 men). Myocardial perfusion was quantified in areas supplied by stenosed (>70%) and nonstenosed (<30%) coronary arteries. The cold pressor test did not significantly alter myocardial perfusion in any of the groups. However, when normalized for rate-pressure product, the response in stenosed areas showed a significantly more pronounced reduction compared with nonstenosed areas (0.78 +/- 0.18 vs 0.64 +/- 0.19 ml/g/min, p <0.005 and 0.86 +/- 0.19 vs 0.73 +/- 0.24 ml/g/min, p <0.05, p <0.05) for intergroup comparison. In both stenosed areas and nonstenosed areas nitroglycerin increased perfusion (0.51 +/- 0.14 vs 0.60 +/- 0.17 ml/g/min, p <0.05 and 0.56 +/- 0.14 vs 0.61 +/- 0.17 ml/g/min, p <0.05). Nitroglycerin did not alter myocardial perfusion in the control group. There was a negative correlation between the cold pressor test response and stenosis severity (r(2) = 0.17, p <0.046), whereas this was not the case for nitroglycerin. In patients with coronary artery disease, myocardial segments supplied by stenosed coronary arteries showed an altered perfusion response to adrenergic stress. Oral nitroglycerin increased myocardial perfusion irrespective of the presence of a stenosis.

Pharmacology of adenosine receptors in the vasculature

Adenosine is widely distributed in mammals. One of the primary roles of adenosine within the cardiovascular system is to directly control the functions of both cardiac and vascular tissues. Recently, there has been considerable interest in the subclassification of adenosine receptors. Characterization of a heterogeneous population of receptors for adenosine could provide an opportunity for the development of novel compounds of therapeutic value. Adenosine is released from cells as a result of metabolism, and its release can be increased dramatically from cells that are metabolically stressed. This implies that adenosine can be released from a variety of cells throughout the body, as a result of increased metabolic rates, in concentrations that can have a profound impact on blood vessel function and, consequently, blood flow. It is recognized that the actions of this nucleoside on the vasculature are most prominent when oxygen demand is high and there is a reduction in oxygen tension at the site in question. Therefore, it is not surprising that adenosine has been shown to be an important regulator of blood vessel tone under hypoxic conditions. Furthermore, the activation of adenosine receptors on blood vessels can result in relaxation and/or contractions. The nature of the response subsequent to the activation of adenosine receptors is primarily dependent on the type of blood vessel involved and basal tone. This review will focus on the characterization of subtypes of adenosine receptors in blood vessels, as well as the effect of the stimulation of adenosine receptors on the peripheral circulation.

Mechanism of reversible (99m)Tc-sestamibi perfusion defects during pharmacologically induced vasodilatation

Reversible perfusion defects on (99m)Tc-sestamibi imaging during hyperemia are thought to occur due to myocardial blood flow (MBF) "mismatch" between regions with and without stenosis. We have recently shown that myocardial blood volume (MBV) distal to a stenosis decreases during hyperemia, resulting in a reversible perfusion defect on myocardial contrast echocardiography (MCE). In this study, we hypothesized that a reversible perfusion defect on (99m)Tc-sestamibi imaging during hyperemia results from the same mechanism. We tested our hypothesis under the following conditions: 1) increases in MBF in the absence of changes in MBV by using direct intracoronary infusion of adenosine (group I, n = 10 dogs); 2) decrease in MBV despite an increase in MBF by left main infusion of adenosine proximal to a noncritical coronary stenosis placed on either coronary artery (group II, n = 13 dogs); and 3) reduction in both resting MBF and MBV by placement of a severe stenosis (group III, n = 7 dogs). In group I dogs, no difference in MBV or (99m)Tc-sestamibi uptake was found between the two coronary beds despite an up to fourfold increase in MBF in one bed with adenosine. In group II dogs, MBV distal to the stenosis decreased during hyperemia despite a twofold increase in mean MBF. A good correlation was found between (99m)Tc-sestamibi uptake and MBV ratios from the stenosed versus normal bed (r = 0.91, P < 0.001). In group III dogs, both MBF and MBV were decreased in the stenosed bed at rest with a good correlation noted between (99m)Tc-sestamibi uptake and MBV ratios from the stenosed versus normal bed (r = 0.92, P = 0.004). We conclude that reversible defects on (99m)Tc-sestamibi during vasodilator stress imaging are related to decreases in MBV distal to a stenosis and not to "flow mismatch" between beds. The decrease in MBV results in reduced (99m)Tc-sestamibi uptake during hyperemia.

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.

Role of capillaries in determining CBF reserve: new insights using myocardial contrast echocardiography

To define the role of capillaries in the control of coronary blood flow (CBF) reserve, we developed a model of the coronary circulation and evaluated experimental data in its context. Our model comprised three compartments connected in series (arterial, capillary, and venous), each with its own resistance. The resistance in each vascular compartment was derived from the model based on hemodynamic data obtained in nine dogs during baseline and stenosis, both at rest and during hyperemia. The capillary hydrostatic pressure was assumed to be constant in all stages. Although in the absence of stenosis, the contribution of capillaries to total myocardial vascular resistance was only 25 +/- 5% at rest, it increased to 75 +/- 14% during hyperemia, despite the total myocardial vascular resistance decreasing by 51 +/- 13%. In the presence of a noncritical stenosis, total myocardial vascular resistance decreased by 22 +/- 10% at rest, with no change in capillary resistance. During hyperemia, total myocardial vascular resistance increased by 58 +/- 50% in the presence of the noncritical stenosis. In this situation, because arteriolar and venular resistances were already minimal, the increase in myocardial vascular resistance was due to increased capillary resistance, making it the predominant source (84 +/- 8%) of total myocardial vascular resistance. Myocardial video intensity (VI) on myocardial contrast echocardiography (MCE), which reflects capillary blood volume, decreased distal to the stenosis during hyperemia. In the presence of a flow-limiting stenosis at rest, myocardial VI also decreased, indicating that decrease in CBF was associated with an increase in capillary resistance. Our findings also provide an alternative explanation for the critical coronary closing pressure. Thus, contrary to previously held notions, capillaries play a vital role in the regulation of CBF.

Computer-aided video angiometry in isolated rabbit hearts: a new method assessing epicardial coronary selectivity

The clinical value of coronary vasodilators in antianginal therapy depends on the ratio of their epicardial versus resistance coronary vessel actions. The coronary flow measured in standard isolated heart preparations, however, does not allow any conclusion about the function of epicardial vessels. Thus, we established a new technique assessing the epicardial coronary diameter directly by video angiometry. Pictures from the cardiac surface were taken by a videocamera mounted on a long-distance microscope. The video signal was digitized for computer-aided evaluation. An area of interest (AOI) was laid over the vascular section to be measured. The gray values of the pixels across the epicardial vessel were registered, and a mean curve of distribution was obtained by averaging the gray values from all video lines within the AOI. The inner epicardial coronary diameter resulted from the distance between the points of inflection of this mean curve of distribution. Experiments with NO-vasodilators and adenosine showed that epicardial coronary arteries of isolated perfused rabbit hearts have no appreciable tone. Pretreatment of the hearts with a combination of histamine (10[-6] mol/l), cimetidine (10[-5] mol/l), and adenosine (10[-7] mol/1), however, caused a marked contraction of the conductive vessels. NO-donors selectively dilated epicardial vessels in such pretreated hearts whereas adenosine increased both epicardial coronary diameter and coronary flow, with only a slight tendency toward preferential action on resistance vessels in low concentrations. Simultaneous registration of coronary flow and epicardial coronary diameter in isolated rabbit hearts pretreated with a spasmogenic drug combination (histamine, cimetidine, and adenosine) may be a feasible method assessing epicardial selectivity of coronary vasodilators.

Comparative effects of L-NOARG and L-NAME on basal blood flow and ACh-induced vasodilatation in rat diaphragmatic microcirculation

1. The effects of N omega-nitro-L-arginine (L-NOARG) and N omega-nitro-L-arginine methyl ester (L-NAME) on diaphragmatic microcirculation in male Sprague-Dawley rats were assessed under basal conditions and after acetylcholine (ACh) stimulation. In addition, L-arginine (L-arg) was used with the aim of preventing L-NOARG and L-NAME from inhibiting ACh-induced vasodilatation, in order to explore the possibility that L-NOARG is not only a nitric oxide (NO) synthase inhibitor but also a muscarinic receptor antagonist. 2. Male Sprague-Dawley rats were anaesthetized with urethane and mechanically ventilated. The left hemi-diaphragm of each rat was prepared and microvascular blood flow was recorded during continuous superfusion with bicarbonate-buffered prewarmed Ringer solution by using laser-Doppler flowmetry. The drugs were topically applied to the surface of the hemi-diaphragm. 3. Baseline microvascular blood flow was unaffected after 15 min superfusion with any one of the following agents: L-NOARG (0.1 mM). L-NAME (0.1 mM), L-arg (10 mM). 4. ACh (0.03 mM, 0.1 mM and 0.3 mM) elicited a significant increase of microvascular blood flow (171 +/- 16%, 214 +/- 55%, and 323 +/- 68% of baseline values, respectively), via interaction with the muscarinic receptor, for the vasodilator response was severely inhibited by 15 min superfusion with atropine (0.3 mM). 5. Following 15 min superfusion with either of the L-arg analogues (0.1 mM), the ACh-induced vasodilator response was significantly inhibited. Pretreatment with L-arg (10 mM) for 5 min, followed by co-administration of L-arg (10 mM) and L-NOARG (0.1 mM) for another 15 min significantly prevented the inhibitory effect of L-NOARG or ACh-induced vasodilatation. However, a similar pretreatment schedule with L-arg failed to prevent L-NAME from exerting its inhibitory effect. 6. Neither of the L-arg analogues potentiated sodium nitroprusside (10 microM and 30 microM)-induced vasodilatation. However, adenosine (0.1 mM)-induced vasodilatation was slightly but significantly attenuated by either L-NOARG (0.1 mM) or L-NAME (0.1 mM), an effect which was prevented by L-arg (10 mM). 7. In conclusion, an increase in endothelium-dependent blood flow stimulated by ACh may occur in diaphragmatic microcirculation of anaesthetized rats independently of low baseline NO activity. The results also suggest that L-NAME has muscarinic receptor antagonist action in addition to its ability to inhibit NO synthase. Thus, we suggest that L-NAME should not be used as a specific NO synthase inhibitor in the rat diaphragm in situations in which there is potential for muscarinic receptors to be stimulated.

Endothelial cell dilatory pathways link flow and wall shear stress in an intact arteriolar network

Our purpose was to determine whether the endothelial cell-dependent dilatory pathways contribute to the regulation of flow distribution in an intact arteriolar network. Cell flow, wall shear stress (T omega), diameter, and bifurcation angle were determined for four sequential branches of a transverse arteriole in the superfused cremaster muscle of pentobaribtal sodium (Nembutal, 70 mg/kg)-anesthetized hamsters (n = 51). Control cell flow was significantly greater into upstream than into downstream branches [1,561 +/- 315 vs. 971 +/- 200 (SE) cells/s, n = 12]. Tissue exposure to 50 microM N omega-nitro-L-arginine + 50 microM indomethacin (L-NNA + Indo) produced arteriolar constriction of 14 +/- 4% and decreased flow into the transverse arteriole. More of the available cell flow was diverted to downstream branches, yet flow distribution remained unequal. Control T omega was higher upstream than downstream (31.3 +/- 6.8 vs. 9.8 +/- 1.5 dyn/cm2). L-NNA + Indo decreased T omega upstream and increased T omega downstream to become equal in all branches, in contrast to flow. To determine whether constriction in general induced the same changes, 5% O2 (8 +/- 4% constriction) or 10(-9) M norepinephrine (NE; 4 +/- 3% constriction) was added to the tissue (n = 7). With O2, flow was redistributed to become equal into each branch. With NE, flow decreased progressively more into the first three branches. The changes in flow distribution were thus predictable and dependent on the agonist. With O2 or NE, the spatial changes in flow were mirrored by spatial changes in T omega. Changes in diameter and in cell flux were not related for L-NNA + Indo (r = 0.45), O2 (r = 0.07), or NE (r = 0.36). For all agonists, when the bifurcation angle increased, cell flow to the branch decreased significantly, whereas if the angle decreased, flow was relatively preserved; thus active changes in bifurcation angle may influence red cell distribution at arteriolar bifurcations. Thus, when the endothelial cell dilatory pathways were blocked, the changes in flow and in T omega were uncoupled; yet when they were intact, flow and T omega changed together.

Does nitrous oxide affect coronary microcirculation? An intravital microscopic study in the canine heart

The safe use of nitrous oxide, in particular in patients with coronary artery disease, has been questioned. This study was designed to determine whether nitrous oxide directly affects global coronary hemodynamic variables and coronary arteriolar microvessels in the absence of changes of myocardial oxygen consumption. In dogs the effects of nitrous oxide were evaluated during normotension (NT, intravenous [IV] piritramid, nitrogen/oxygen; and NT/N2O, IV piritramid, nitrous oxide/oxygen) and during hypotension (MAP 60 mm Hg) (HT, IV piritramid, halothane, nitrogen/oxygen; and HT/N2O, IV piritramid, halothane, nitrous oxide/oxygen). The diameter of coronary arteriolar microvessels (range, 20-450 microns) was assessed by intravital fluorescence microscopy. Myocardial blood flow was determined by radioactive microspheres. Systemic and coronary hemodynamics, as well as arteriolar microvessel diameters, were comparable between NT and NT/N2O. During HT, nitrous oxide (HT/N2O) affected neither systemic nor coronary hemodynamics. Moreover, there was no obvious difference in the diameters of coronary microvessels between HT and HT/N2O. In conclusion, nitrous oxide, whether at normotensive or hypotensive conditions, neither influences coronary arteriolar tone nor reduces or redistributes myocardial blood flow.