Right ventricular oxygen supply/demand balance in exercising dogs

Unraveling the Gordian knot of coronary pressure-flow autoregulation

The coronary circulation has the inherent ability to maintain myocardial perfusion constant over a wide range of perfusion pressures. The phenomenon of pressure-flow autoregulation is crucial in response to flow-limiting atherosclerotic lesions which diminish coronary driving pressure and increase risk of myocardial ischemia and infarction. Despite well over half a century of devoted research, understanding of the mechanisms responsible for autoregulation remains one of the most fundamental and contested questions in the field today. The purpose of this review is to highlight current knowledge regarding the complex interrelationship between the pathways and mechanisms proposed to dictate the degree of coronary pressure-flow autoregulation. Our group recently likened the intertwined nature of the essential determinants of coronary flow control to the symbolically unsolvable "Gordian knot". To further efforts to unravel the autoregulatory "knot", we consider recent challenges to the local metabolic and myogenic hypotheses and the complicated dynamic structural and functional heterogeneity unique to the heart and coronary circulation. Additional consideration is given to interrogation of putative mediators, role of K+ and Ca2+ channels, and recent insights from computational modeling studies. Improved understanding of how specific vasoactive mediators, pathways, and underlying disease states influence coronary pressure-flow relations stands to significantly reduce morbidity and mortality for what remains the leading cause of death worldwide.

Right ventricular myocardial energetic model for evaluating right heart function in pulmonary arterial hypertension

Background

Pulmonary arterial hypertension (PAH) increases right ventricular (RV) workload and decreases myocardial oxygen reserve, eventually leading to poor cardiac output. This study created and assessed a novel model of RV work output based on RV hemodynamics and oxygen supply, allowing new insight into causal mechanisms of RV dysfunction.

Methods

The RV function model was built upon an earlier, left ventricular model and further adjusted for more accurate clinical use. The model assumes that RV total power output (1) is the sum of isovolumic and stroke power and (2) is linearly related to its right coronary artery oxygen supply. Thus, when right coronary artery flow is limited or isovolumic power is elevated, less energy is available for producing cardiac output. The original and adjusted models were validated via data from patients with idiopathic PAH (n = 14) and large animals (n = 6) that underwent acute pulmonary banding with or without hypoxia.

Results

Both models demonstrated strong, significant correlations between RV oxygen consumption rate and RV total power output for PAH patients (original model, R² = 0.66; adjusted model, R² = 0.78) and sheep (original, R² = 0.85; adjusted, R² = 0.86). Furthermore, the models demonstrate a significant inverse relationship between required oxygen consumption and RV efficiency (stroke power/total power) (p < 0.001). Lastly, higher NYHA class was indicative of lower RV efficiency and higher oxygen consumption (p = 0.013).

Conclusion

Right ventricular total power output can be accurately estimated directly from pulmonary hemodynamics and right coronary perfusion during PAH. This model highlights the increased vulnerability of PAH patients with compromised right coronary flow coupled with high afterload.

Perioperative Management of Pulmonary Hypertension. a Review

Pulmonary hypertension is a rare and progressive pathology defined by abnormally high pulmonary artery pressure mediated by a diverse range of aetiologies. It affects up to twenty-six individuals per one million patients currently living in the United Kingdom (UK), with a median life expectancy of 2.8 years in idiopathic pulmonary hypertension. The diagnosis of pulmonary hypertension is often delayed due to the presentation of non-specific symptoms, leading to a delay in referral to specialists services. The complexity of treatment necessitates a multidisciplinary approach, underpinned by a diverse disease aetiology from managing the underlying disease process to novel specialist treatments. This has led to the formation of dedicated specialist treatment centres within centralised UK cities. The article aimed to provide a concise overview of pulmonary hypertension’s clinical perioperative management, including key definitions, epidemiology, pathophysiology, and risk stratification.

Assessment of the right ventricle in donation after circulatory death hearts

In donation after circulatory death heart transplantation, the donor heart is exposed to circulatory load. The right ventricle, due to its structure, has high compliance for volume load but is particularly vulnerable to increased pressure load. This study used a porcine model to conduct a functional assessment of the hemodynamics of the heart, with a focus on the right ventricle. Six pigs weighing 24.6 ± 1.4 kg were used. Circulatory death was induced by asphyxiation after median sternotomy. After 30 minutes in the state of global warm ischemia, the ascending aorta was clamped, followed by a 20-minute reperfusion of the heart with a 20°C blood cardioplegia solution. Systemic circulation was established by cardiopulmonary bypass after aortic cross-clamping. After initial reperfusion, the blood cardioplegia solution was replaced with blood. The blood was then rewarmed while the heart was still in a non-working state. Cardiac function was assessed twice in situ, first by the thermodilution method, and then, by the pressure-volume measurement both at preischemia and at three hours after initiation of reperfusion. The recovery rate of cardiac output was 75%. End-systolic elastance (P = .02) and pulmonary arterial elastance significantly increased (P = .03), but the ratio of arterial elastance to end-systolic elastance was preserved (P = .91) in the right ventricle. Despite a decrease in cardiac output after reperfusion from warm ischemia, the right ventricle had a potential to respond the elevated afterload. It is important that donations after circulatory death heart transplantation should be performed with attention to avoiding right ventricular distension.

Is the healthy respiratory system built just right, overbuilt, or underbuilt to meet the demands imposed by exercise?

In the healthy, untrained young adult, a case is made for a respiratory system (airways, pulmonary vasculature, lung parenchyma, respiratory muscles, and neural ventilatory control system) that is near ideally designed to ensure a highly efficient, homeostatic response to exercise of varying intensities and durations. Our aim was then to consider circumstances in which the intra/extrathoracic airways, pulmonary vasculature, respiratory muscles, and/or blood-gas distribution are underbuilt or inadequately regulated relative to the demands imposed by the cardiovascular system. In these instances, the respiratory system presents a significant limitation to O₂ transport and contributes to the occurrence of locomotor muscle fatigue, inhibition of central locomotor output, and exercise performance. Most prominent in these examples of an "underbuilt" respiratory system are highly trained endurance athletes, with additional influences of sex, aging, hypoxic environments, and the highly inbred equine. We summarize by evaluating the relative influences of these respiratory system limitations on exercise performance and their impact on pathophysiology and provide recommendations for future investigation.

Cardioprotection in right heart failure

Ischaemic and pharmacological conditioning of the left ventricle is mediated by the activation of signalling cascades, which finally converge at the mitochondria and reduce ischaemia/reperfusion (I/R) injury. Whereas the molecular mechanisms of conditioning in the left ventricle are well characterized, cardioprotection of the right ventricle is principally feasible but less established. Similar to what is known for the left ventricle, a dysregulation in signalling pathways seems to play a role in I/R injury of the healthy and failing right ventricle and in the ability/inability of the right ventricle to respond to a conditioning stimulus. The maintenance of mitochondrial function seems to be crucial in both ventricles to reduce I/R injury. As far as currently known, similar molecular mechanisms mediate ischaemic and pharmacological preconditioning in the left and right ventricles. However, the two ventricles seem to respond differently towards exercise‐induced preconditioning.

LINKED ARTICLES

This article is part of a themed issue on Risk factors, comorbidities, and comedications in cardioprotection. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.23/issuetoc

Right ventricular oxygen delivery as a determinant of right ventricular functional reserve during exercise in juvenile swine with chronic pulmonary hypertension

Assessing right ventricular (RV) functional reserve is important for determining clinical status and prognosis in patients with pulmonary hypertension (PH). In this study, we aimed to establish RV oxygen (O₂) delivery as a determinant for RV functional reserve during exercise in swine with chronic PH. Chronic PH was induced by pulmonary vein banding (PVB), with sham operation serving as control. RV function and RV O₂ delivery were measured over time in chronically instrumented swine, up to 12 wk after PVB at rest and during exercise. At rest, RV afterload (pulmonary artery pressure and arterial elastance) and contractility (E_es and dP/dt_max) were higher in PH compared with control with preserved cardiac index and RV O₂ delivery. However, RV functional reserve, as measured by the exercise-induced relative change (Δ) in cardiac index, dP/dt_max, and end-systolic elastance (E_es), was decreased in PH, and RV pulmonary arterial coupling was lower both at rest and during exercise in PH. Furthermore, the increase in RV O₂ delivery was attenuated in PH during exercise principally due to a lower systolic coronary blood flow in combination with an attenuated increase in aorta pressure while arterial O₂ content was not significantly altered in PH. Moreover, RV O₂ delivery reserve correlated with RV functional reserve, Δcardiac index (r² = 0.85), ΔdP/dt_max (r² = 0.49), and ΔE_es (r² = 0.70), all P < 0.05. The inability to sufficiently increase RV O₂ supply to meet the increased O₂ demand during exercise is principally due to the reduced RV perfusion relative to healthy control values and likely contributes to impaired RV contractile function and thereby to the limited exercise capacity that is commonly observed in patients with PH.NEW & NOTEWORTHY Impaired right ventricular (RV) O₂ delivery reserve is associated with reduced RV functional reserve during exercise in a swine model of pulmonary hypertension (PH) induced by pulmonary vein banding. Our data suggest that RV function and exercise capacity might be improved by improving RV O₂ delivery.

Adaptation to acute pulmonary hypertension in pigs

The extent of right ventricular compensation compared to the left ventricle is restricted and varies among individuals, which makes it difficult to define. While establishing a model of acute pulmonary hypertension in pigs we observed two different kinds of compensation in our animals. Looking deeper into the hemodynamic data we tried to delineate why some animals could compensate and others could not. Pulmonary hypertension (mean pressure 45 mmHg) was induced gradually by infusion of a stable thromboxane A₂ analogue U46619 in a porcine model (n = 22). Hemodynamic data (pressure‐volume loops, strain‐analysis of echocardiographic data and coronary flow measurements) were evaluated retrospectively for the short‐term right ventricular compensatory mechanisms and limits (Roehl et al. [2012] Acta Anaesthesiol. Scand., 56:449–58) 10 animals showed stable arterial blood pressures, whereas 12 pigs exhibited a significant drop of 16.4 ± 9.9 mmHg. Cardiac output and heart rate were comparable in both groups. In contrast, right ventricular contractility and coronary flow only rose in the stable group. The unchanging values in the decrease group correlated with an increasing ST‐segment depression and a loss of ventricular synchronism and resulted in a larger septum bulging to the right ventricle. Simultaneously, a reduced left‐ventricular end‐diastolic volume and a missing improvement in contractility in the posterior septal and inferior free wall of the left ventricle have been observed. Our findings suggest that right ventricular compensation during acute pulmonary hypertension is strongly dependent on the individual capability to increase coronary flow. The cause for inter‐individual variability could be the dimension and reactivity of the coronary system.

Right Ventricular Perfusion

Regulation of blood flow to the right ventricle differs significantly from that to the left ventricle. The right ventricle develops a lower systolic pressure than the left ventricle, resulting in reduced extravascular compressive forces and myocardial oxygen demand. Right ventricular perfusion has eight major characteristics that distinguish it from left ventricular perfusion: (1) appreciable perfusion throughout the entire cardiac cycle; (2) reduced myocardial oxygen uptake, blood flow, and oxygen extraction; (3) an oxygen extraction reserve that can be recruited to at least partially offset a reduction in coronary blood flow; (4) less effective pressure–flow autoregulation; (5) the ability to downregulate its metabolic demand during coronary hypoperfusion and thereby maintain contractile function and energy stores; (6) a transmurally uniform reduction in myocardial perfusion in the presence of a hemodynamically significant epicardial coronary stenosis; (7) extensive collateral connections from the left coronary circulation; and (8) possible retrograde perfusion from the right ventricular cavity through the Thebesian veins. These differences promote the maintenance of right ventricular oxygen supply–demand balance and provide relative resistance to ischemia-induced contractile dysfunction and infarction, but they may be compromised during acute or chronic increases in right ventricle afterload resulting from pulmonary arterial hypertension. Contractile function of the thin-walled right ventricle is exquisitely sensitive to afterload. Acute increases in pulmonary arterial pressure reduce right ventricular stroke volume and, if sufficiently large and prolonged, result in right ventricular failure. Right ventricular ischemia plays a prominent role in these effects. The risk of right ventricular ischemia is also heightened during chronic elevations in right ventricular afterload because microvascular growth fails to match myocyte hypertrophy and because microvascular dysfunction is present. The right coronary circulation is more sensitive than the left to α-adrenergic–mediated constriction, which may contribute to its greater propensity for coronary vasospasm. This characteristic of the right coronary circulation may increase its vulnerability to coronary vasoconstriction and impaired right ventricular perfusion during administration of α-adrenergic receptor agonists.

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

α-Adrenergic Vasoconstrictor Tone Limits Right Coronary Blood Flow in Exercising Dogs

In exercising dogs, increased myocardial O2 consumption (MVO2) of the left ventricle is met primarily by hyperemia, whereas increased O2 extraction makes a greater contribution to right ventricular (RV) O2 supply. We hypothesized that α-adrenergic vasoconstrictor tone limits right coronary (RC) blood flow during exercise, forcing increased O2 extraction. This tone might also contribute to lesser RC vascular conductance at rest. Accordingly, RV O2 balance was examined at rest and during graded treadmill exercise before and during α-adrenergic blockade with phentolamine (1 mg/kg, iv, n = 6). The transmural distribution of RC flow was measured with radiolabeled microspheres in 4 additional dogs. At rest, α-adrenergic receptor blockade did not significantly increase RC flow or conductance. During exercise, α-adrenergic blockade increased RC flow and conductance responses to increased RV MVO2 by 25% and 60%, respectively. The transmural distribution of RC flow was not altered by exercise or by α-adrenergic blockade. Before α-adrenergic blockade, hyperemia provided 39%–66% of the additional O2 consumed by the right ventricle during graded exercise; after α-adrenergic blockade, hyperemia contributed 74%–85%. After α-adrenergic blockade, the slope of the relationship between RC venous PO2 and RV MVO2 became less steep, reflecting less O2 extraction due to enhanced hyperemia. Additional experiments were conducted on 5 anesthetized, open-chest dogs with constant RC perfusion pressure and β-adrenergic blockade. The RC flow response to intracoronary norepinephrine was shifted to the left compared with that measured in the left coronary circulation, consistent with observations in the conscious exercising dogs. In conclusion, α-adrenergic vasoconstrictor tone does not restrict resting RC blood flow, but during exercise, this tone transmurally blunts RC hyperemia and forces the right ventricle to mobilize its O2 extraction reserve. This effect is more pronounced than has been reported for the left ventricle.

Kv7 channels critically determine coronary artery reactivity: left-right differences and down-regulation by hyperglycaemia

AIMS

Voltage-gated potassium channels encoded by KCNQ genes (Kv7 channels) are emerging as important regulators of vascular tone. In this study, we analysed the contribution of Kv7 channels to the vasodilation induced by hypoxia and the cyclic AMP pathway in the coronary circulation. We also assessed their regional distribution and possible impairment by diabetes.

METHODS AND RESULTS

We examined the effects of Kv7 channel modulators on K+ currents and vascular reactivity in rat left and right coronary arteries (LCAs and RCAs, respectively). Currents from LCA were more sensitive to Kv7 channel inhibitors (XE991, linopirdine) and activators (flupirtine, retigabine) than those from RCA. Accordingly, LCAs were more sensitive than RCAs to the relaxation induced by Kv7 channel enhancers. Likewise, relaxation induced by the adenylyl cyclase activator forskolin and hypoxia, which were mediated through Kv7 channel activation, were greater in LCA than in RCA. KCNQ1 and KCNQ5 expression was markedly higher in LCA than in RCA. After incubation with high glucose (HG, 30 mmol/L), myocytes from LCA, but not from RCA, were more depolarized and showed reduced Kv7 currents. In HG-incubated LCA, the effects of Kv7 channel modulators and forskolin were diminished, and the expression of KCNQ1 and KCNQ5 was reduced. Finally, vascular responses induced by Kv7 channel modulators were impaired in LCA, but not in RCA, from type 1 diabetic rats.

CONCLUSION

Our results reveal that the high expression and function of Kv7 channels in the LCA and their down-regulation by diabetes critically determine the sensitivity to key regulators of coronary tone.

State of the art: advanced imaging of the right ventricle and pulmonary circulation in humans (2013 Grover Conference series)

Pulmonary arterial hypertension (PAH) is a progressive disease characterized by remodeling and vasoconstriction of the pulmonary vasculature, ultimately leading to right ventricular (RV) failure and death. Recent developments in echocardiography, cardiovascular magnetic resonance imaging, computed tomography, and positron emission tomography allow advanced, noninvasive, in vivo assessment of the RV and have contributed to the identification of risk factors, prognostic factors, and monitoring of therapeutic responses in patients with PAH. Although far from reaching its future potential, these techniques have not only provided global RV assessment but also allowed evaluation of changes in cellular and molecular tissue processes, such as metabolism, oxygen balance and ischemia, angiogenesis, and apoptosis. Integrated application of these techniques could provide full insights into the different pathophysiological aspects of a failing RV in the setting of PAH. Recent advances in hybrid imaging have implemented simultaneous measurements of myocardial and vascular interactions and will be one of the most important potential future developments.

The role of capillary transit time heterogeneity in myocardial oxygenation and ischemic heart disease

Ischemic heart disease (IHD) is characterized by an imbalance between oxygen supply and demand, most frequently caused by coronary artery disease (CAD) that reduces myocardial perfusion. In some patients, IHD is ascribed to microvascular dysfunction (MVD): microcirculatory disturbances that reduce myocardial perfusion at the level of myocardial pre-arterioles and arterioles. In a minority of cases, chest pain and reductions in myocardial flow reserve may even occur in patients without any other demonstrable systemic or cardiac disease. In this topical review, we address whether these findings might be caused by impaired myocardial oxygen extraction, caused by capillary flow disturbances further downstream. Myocardial blood flow (MBF) increases approximately linearly with oxygen utilization, but efficient oxygen extraction at high MBF values is known to depend on the parallel reduction of capillary transit time heterogeneity (CTH). Consequently, changes in capillary wall morphology or blood viscosity may impair myocardial oxygen extraction by preventing capillary flow homogenization. Indeed, a recent re-analysis of oxygen transport in tissue shows that elevated CTH can reduce tissue oxygenation by causing a functional shunt of oxygenated blood through the tissue. We review the combined effects of MBF, CTH, and tissue oxygen tension on myocardial oxygen supply. We show that as CTH increases, normal vasodilator responses must be attenuated in order to reduce the degree of functional shunting and improve blood-tissue oxygen concentration gradients to allow sufficient myocardial oxygenation. Theoretically, CTH can reach levels such that increased metabolic demands cannot be met, resulting in tissue hypoxia and angina in the absence of flow-limiting CAD or MVD. We discuss these predictions in the context of MVD, myocardial infarction, and reperfusion injury.

11C-Acetate clearance as an index of oxygen consumption of the right myocardium in idiopathic pulmonary arterial hypertension: a validation study using 15O-labeled tracers and PET

Idiopathic pulmonary arterial hypertension (IPAH) results in increased right ventricular (RV) workload and oxygen demand. It has been shown that myocardial oxygen consumption (MVO2) of the hypertrophied right ventricle of IPAH patients can be measured using PET and (15)O-labeled tracers. This method is, however, not very suitable for routine clinical practice. The purpose of the present study was to assess whether MVO2 can also be determined in the right ventricle of IPAH patients from the clearance of (11)C-acetate, a simple method that is in use for MVO2 measurements of the left myocardium.

METHODS

Seventeen of 26 IPAH patients performed the total PET study. Nine other patients were scanned only for (11)C-acetate. (15)O-H2O, (15)O-O2, and (15)O-CO scans were used to derive RV flow, oxygen extraction fraction, and blood volume, respectively, from which RV MVO2 was calculated. The rate of clearance determined by monoexponential curve fitting (K(mono)) and the efflux rate constant k2 were derived from the (11)C-acetate scan. The RV rate-pressure product was also determined by means of right heart catheterization, as an index of the RV MVO2, and was calculated as the product of systolic pulmonary artery pressure and heart rate.

RESULTS

Both (11)C-acetate clearance rates, K(mono) (R(2) = 0.41, P = 0.006) and k2 (R(2) = 0.45, P = 0.003), correlated with RV MVO2. They also correlated with RV rate-pressure product (K(mono), R(2) = 0.41, P = 0.0005; k2, R(2) = 0.48, P < 0.0001).

CONCLUSION

(11)C-acetate clearance rates correlated moderately with quantitative RV MVO2 measurements in IPAH. Therefore, (11)C-acetate PET can be used only as an index of RV oxidative metabolism in IPAH patients.

Homogenous protein programming in the mammalian left and right ventricle free walls

Despite identical cardiac outputs, the right (RV) and left ventricle (LV) have very different embryological origins and resting workload. These differences suggest that the ventricles have different protein programming with regard to energy metabolism and contractile elements. The objective of this study was to determine the relative RV and LV protein expression levels, with an emphasis on energy metabolism. The RV and LV protein contents of the rabbit and porcine heart were determined with quantitative gel electrophoresis (2D-DIGE), mass spectrometry, and optical spectroscopy techniques. Surprisingly, the expression levels for more than 600 RV and LV proteins detected were similar. This included proteins many different compartments and metabolic pathways. In addition, no isoelectric shifts were detected in 2D-DIGE consistent with no differential posttranslational modifications in these proteins. Analysis of the RV and LV metabolic response to work revealed that the metabolic rate increases much faster with workload in the RV compared with LV. This implies that the generally lower metabolic stress of the RV actually approaches LV metabolic stress at maximum workloads. Thus, identical levels of energy conversion and mechanical elements in the RV and LV may be driven by the performance requirements at maximum workloads. In summary, the ventricles of the heart manage the differences in overall workload by modifying the amounts of cytosol, not its composition. The constant myocyte composition in the LV and RV implies that the ratio of energy metabolism and contractile elements may be optimal for the sustained cardiac contractile activity in the mammalian heart.

Regulation of Coronary Blood Flow During Exercise

Exercise is the most important physiological stimulus for increased myocardial oxygen demand. The requirement of exercising muscle for increased blood flow necessitates an increase in cardiac output that results in increases in the three main determinants of myocardial oxygen demand: heart rate, myocardial contractility, and ventricular work. The approximately sixfold increase in oxygen demands of the left ventricle during heavy exercise is met principally by augmenting coronary blood flow (∼5-fold), as hemoglobin concentration and oxygen extraction (which is already 70–80% at rest) increase only modestly in most species. In contrast, in the right ventricle, oxygen extraction is lower at rest and increases substantially during exercise, similar to skeletal muscle, suggesting fundamental differences in blood flow regulation between these two cardiac chambers. The increase in heart rate also increases the relative time spent in systole, thereby increasing the net extravascular compressive forces acting on the microvasculature within the wall of the left ventricle, in particular in its subendocardial layers. Hence, appropriate adjustment of coronary vascular resistance is critical for the cardiac response to exercise. Coronary resistance vessel tone results from the culmination of myriad vasodilator and vasoconstrictors influences, including neurohormones and endothelial and myocardial factors. Unraveling of the integrative mechanisms controlling coronary vasodilation in response to exercise has been difficult, in part due to the redundancies in coronary vasomotor control and differences between animal species. Exercise training is associated with adaptations in the coronary microvasculature including increased arteriolar densities and/or diameters, which provide a morphometric basis for the observed increase in peak coronary blood flow rates in exercise-trained animals. In larger animals trained by treadmill exercise, the formation of new capillaries maintains capillary density at a level commensurate with the degree of exercise-induced physiological myocardial hypertrophy. Nevertheless, training alters the distribution of coronary vascular resistance so that more capillaries are recruited, resulting in an increase in the permeability-surface area product without a change in capillary numerical density. Maintenance of α- and ß-adrenergic tone in the presence of lower circulating catecholamine levels appears to be due to increased receptor responsiveness to adrenergic stimulation. Exercise training also alters local control of coronary resistance vessels. Thus arterioles exhibit increased myogenic tone, likely due to a calcium-dependent protein kinase C signaling-mediated alteration in voltage-gated calcium channel activity in response to stretch. Conversely, training augments endothelium-dependent vasodilation throughout the coronary microcirculation. This enhanced responsiveness appears to result principally from an increased expression of nitric oxide (NO) synthase. Finally, physical conditioning decreases extravascular compressive forces at rest and at comparable levels of exercise, mainly because of a decrease in heart rate. Impedance to coronary inflow due to an epicardial coronary artery stenosis results in marked redistribution of myocardial blood flow during exercise away from the subendocardium towards the subepicardium. However, in contrast to the traditional view that myocardial ischemia causes maximal microvascular dilation, more recent studies have shown that the coronary microvessels retain some degree of vasodilator reserve during exercise-induced ischemia and remain responsive to vasoconstrictor stimuli. These observations have required reassessment of the principal sites of resistance to blood flow in the microcirculation. A significant fraction of resistance is located in small arteries that are outside the metabolic control of the myocardium but are sensitive to shear and nitrovasodilators. The coronary collateral system embodies a dynamic network of interarterial vessels that can undergo both long- and short-term adjustments that can modulate blood flow to the dependent myocardium. Long-term adjustments including recruitment and growth of collateral vessels in response to arterial occlusion are time dependent and determine the maximum blood flow rates available to the collateral-dependent vascular bed during exercise. Rapid short-term adjustments result from active vasomotor activity of the collateral vessels. Mature coronary collateral vessels are responsive to vasodilators such as nitroglycerin and atrial natriuretic peptide, and to vasoconstrictors such as vasopressin, angiotensin II, and the platelet products serotonin and thromboxane A2. During exercise, ß-adrenergic activity and endothelium-derived NO and prostanoids exert vasodilator influences on coronary collateral vessels. Importantly, alterations in collateral vasomotor tone, e.g., by exogenous vasopressin, inhibition of endogenous NO or prostanoid production, or increasing local adenosine production can modify collateral conductance, thereby influencing the blood supply to the dependent myocardium. In addition, vasomotor activity in the resistance vessels of the collateral perfused vascular bed can influence the volume and distribution of blood flow within the collateral zone. Finally, there is evidence that vasomotor control of resistance vessels in the normally perfused regions of collateralized hearts is altered, indicating that the vascular adaptations in hearts with a flow-limiting coronary obstruction occur at a global as well as a regional level. Exercise training does not stimulate growth of coronary collateral vessels in the normal heart. However, if exercise produces ischemia, which would be absent or minimal under resting conditions, there is evidence that collateral growth can be enhanced. In addition to ischemia, the pressure gradient between vascular beds, which is a determinant of the flow rate and therefore the shear stress on the collateral vessel endothelium, may also be important in stimulating growth of collateral vessels.

Hypoxia-induced vasodilation in the right coronary circulation of conscious dogs: role of adrenergic activation

The role of adrenergic activation in the right coronary (RC) flow response to hypoxia has not been previously delineated, and limited information from left coronary studies is inconsistent. Seven dogs were instrumented with catheters implanted in the aorta and in the right ventricle to measure aortic pressure and right ventricular (RV) pressure, respectively. A flow transducer was placed around the RC artery to measure RC flow. After recovery from surgery, the dogs were exposed to systemic hypoxia in a Plexiglas chamber ventilated with N(2). Percent O(2) in the chamber was monitored, and blood samples and hemodynamic data were collected as chamber O(2) was progressively reduced to approximately 6%. The chamber was then opened, and the dog breathed room air. Phentolamine, 1 mg/kg, and propranolol, 2 mg/kg, were then administered via the RV catheter to achieve adrenergic blockade, and the hypoxia protocol was repeated. During hypoxia, arterial PO(2) progressively fell from 87+/-3 to 25+/-1 mmHg during untreated control condition and from 90+/-4 to 23+/-1 mmHg during adrenergic blockade. In the unblocked condition, hypoxia caused increases in aortic pressure, heart rate, RV pressure, and RV dP/dt(max). After adrenergic blockade, normoxic aortic pressure was reduced; heart rate and RV dP/dt(max) tended to be lower. Aortic pressure rose during hypoxia, but to lesser values than before blockade. Heart rate and RV dP/dt(max) also increased, but only at more severe hypoxia, and these values were less than before blockade. Normoxic flow and hypoxia-induced increases in RC flow and conductance were not altered by blockade. The relationship between RC conductance and RV triple product, an index of RV O(2) demand, was steeper after blockade. These findings indicate that in the normal, unblocked condition, RC flow during hypoxia is restrained by an adrenergic-mediated increase in RC vasomotor tone.

Partitioning locomotor energy use among and within muscles Muscle blood flow as a measure of muscle oxygen consumption

Linking the mechanics and energetics of locomotion in vertebrates has been hampered by a lack of information regarding the energy use of individual skeletal muscles in vivo. Here, we present a review of the available data concerning the relationship between the rates of skeletal muscle blood flow and oxygen consumption(V̇O2). In active muscle, during aerobically supported exercise, there is a linear relationship between these variables, irrespective of the muscle fiber type and intensity of exercise through most of the aerobic exercise range. We conclude that the rate of blood flow is the best available indicator of aerobic metabolic rate in multiple individual muscles or regions of muscles during locomotion. The practical considerations of using the injectable microsphere technique to measure muscle blood flow in this context are discussed.

Mechanisms of oxygen demand/supply balance in the right ventricle

Few studies have investigated factors responsible for the O2 demand/supply balance in the right ventricle. Resting right coronary blood flow is lower than left coronary blood flow, which is consistent with the lesser work of the right ventricle. Because right and left coronary artery perfusion pressures are identical, right coronary conductance is less than left coronary conductance, but the signal relating this conductance to the lower right ventricular O2 demand has not been defined. At rest, the left ventricle extracts approximately 75% of the O2 delivered by coronary blood flow, whereas right ventricular O2 extraction is only ~50%. As a result, resting right coronary venous PO2 is approximately 30 mm Hg, whereas left coronary venous PO2 is approximately 20 mm Hg. Right coronary conductance does not sufficiently restrict flow to force the right ventricle to extract the same percentage of O2 as the left ventricle. Endogenous nitric oxide impacts the right ventricular O2 demand/supply balance by increasing the right coronary blood flow at rest and during acute pulmonary hypertension, systemic hypoxia, norepinephrine infusion, and coronary hypoperfusion. The substantial right ventricular O2 extraction reserve is used preferentially during exercise-induced increases in right ventricular myocardial O2 consumption. An augmented, sympathetic-mediated vasoconstrictor tone blunts metabolically mediated dilator mechanisms during exercise and forces the right ventricle to mobilize its O2 extraction reserve, but this tone does not limit resting right coronary flow. During exercise, right coronary vasodilation does not occur until right coronary venous PO2 decreases to approximately 20 mm Hg. The mechanism responsible for right coronary vasodilation at low PO2 has not been delineated. In the poorly autoregulating right coronary circulation, reduced coronary pressure unloads the coronary hydraulic skeleton and reduces right ventricular systolic stiffness. Thus, normal right ventricular external work and O2 demand/supply balance can be maintained during moderate coronary hypoperfusion.

Matching coronary blood flow to myocardial oxygen consumption

At rest the myocardium extracts approximately 75% of the oxygen delivered by coronary blood flow. Thus there is little extraction reserve when myocardial oxygen consumption is augmented severalfold during exercise. There are local metabolic feedback and sympathetic feedforward control mechanisms that match coronary blood flow to myocardial oxygen consumption. Despite intensive research the local feedback control mechanism remains unknown. Physiological local metabolic control is not due to adenosine, ATP-dependent K(+) channels, nitric oxide, prostaglandins, or inhibition of endothelin. Adenosine and ATP-dependent K(+) channels are involved in pathophysiological ischemic or hypoxic coronary dilation and myocardial protection during ischemia. Sympathetic beta-adrenoceptor-mediated feedforward arteriolar vasodilation contributes approximately 25% of the increase in coronary blood flow during exercise. Sympathetic alpha-adrenoceptor-mediated vasoconstriction in medium and large coronary arteries during exercise helps maintain blood flow to the vulnerable subendocardium when cardiac contractility, heart rate, and myocardial oxygen consumption are high. In conclusion, several potential mediators of local metabolic control of the coronary circulation have been evaluated without success. More research is needed.

Nitric oxide contributes to right coronary vasodilation during systemic hypoxia

As arterial partial pressure of O(2) (Pa(O(2))) is reduced during systemic hypoxia, right ventricular (RV) work and myocardial O(2) consumption (MVo(2)) increase. Mechanisms responsible for maintaining RV O(2) demand/supply balance during hypoxia have not been delineated. To address this problem, right coronary (RC) blood flow and RV O(2) extraction were measured in nine conscious, instrumented dogs exposed to normobaric hypoxia. Catheters were implanted in the right ventricle for measuring pressure, in the ascending aorta for measuring arterial pressure and for sampling arterial blood, and in an RC vein. A flow transducer was placed around the RC artery. After recovery from surgery, dogs were exposed to hypoxia in a chamber ventilated with N(2), and blood samples and hemodynamic data were collected as chamber O(2) was reduced progressively to approximately 8%. After control measurements were made, the chamber was opened and the dog was allowed to recover. N(omega)-nitro-L-arginine (L-NNA) was then administered (35 mg/kg, via RV catheter) to inhibit nitric oxide (NO) production, and the hypoxia protocol was repeated. RC blood flow increased during hypoxia due to coronary vasodilation, because RC conductance increased from 0.65 +/- 0.05 to 1.32 +/- 0.12 ml x min(-1) x 100 g(-1) x L-NNA blunted the hypoxia-induced increase in RC conductance. RV O(2) extraction remained constant at 64 +/- 4% as Pa(O(2)) was decreased, but after L-NNA, extraction increased to 70 +/- 3% during normoxia and then to 78 +/- 3% during hypoxia. RV MVo(2) increased during hypoxia, but after L-NNA, MVo(2) was lower at any respective Pa(O(2)). The relationship between heart rate times RV systolic pressure (rate-pressure product) and RV MVo(2) was not altered by l-NNA. To account for L-NNA-mediated decreases in RV MVo(2), O(2) demand/supply variables were plotted as functions of MVo(2). Slope of the conductance-MVo(2) relationship was depressed by L-NNA (P = 0.03), whereas the slope of the extraction-MVo(2) relationship increased (P = 0.003). In summary, increases in RV MVo(2) during hypoxia are met normally by increasing RC blood flow. When NO synthesis is blocked, the large RV O(2) extraction reserve is mobilized to maintain RV O(2) demand/supply balance. We conclude that NO contributes to RC vasodilation during systemic hypoxia.