Studies on the regulation of myocardial blood flow in man. I.: Training effects on blood flow and metabolism of the healthy heart at rest and during standardized heavy exercise

Cardiometabolic determinants of cardiorespiratory fitness at rest, during exercise and post-exercise periods

We aimed to assess the relationship between cardiorepiratory fitness (CRF) and cardiometabolic parameters among young Nigerian adults. 100 young adults (50 males, 50 females) aged 20-30 years, selected from College of Health Sciences, Nnamdi Azikiwe University, Nigeria, participated in the study. Subjects’ demographic data and medical information were obtained through the use of structured pre-exercise health and lifestyle screening questionnaire, physical examination and morphometric measurements. Exercise test was carried out using a mechanically braked magnetic ergometer bicycle at an incremental workload of 30 W every 2 min until the subject reached a volitional exhaustion. Blood pressure (BP) and heart rate (HR) were measured at rest, during exercise and at post-exercise periods. Data indicated a significantly (Ρ<0.05) lower resting HR and rate pressure product (RPP), but higher targeted HR reserve, %RPP increase, peak oxygen pulse, cardiac output, exercise duration and work rate compared with the intermediate and unfit groups in both sexes. Age and BMI adjusted correlation test also indicated significant associations between peak oxygen consumption (VO2) and resting HR, resting RPP, targeted HR reserve, oxygen pulse, cardiac output, % RPP increase, actual HR reserve, exercise duration, and work rate. In contrast, resting BP, resting pulse pressure, peak systolic blood pressure (SBP), peak HR, percentage maximum HR, SBP recovery and HR recovery did not correlate with peak VO2. The present findings suggest that a multiple approach involving both metabolic and cardiovascular interventions might be appropriate when implementing strategies to enhance CRF and improve general well-being.

Modeling the oxygen transport to the myocardium at maximal exercise at high altitude

Exposure to high altitude induces a decrease in oxygen pressure and saturation in the arterial blood, which is aggravated by exercise. Heart rate (HR) at maximal exercise decreases when altitude increases in prolonged exposure to hypoxia. We developed a simple model of myocardial oxygenation in order to demonstrate that the observed blunting of maximal HR at high altitude is necessary for the maintenance of a normal myocardial oxygenation. Using data from the available scientific literature, we estimated the myocardial venous oxygen pressure and saturation at maximal exercise in two conditions: (1) with actual values of maximal HR (decreasing with altitude); (2) with sea-level values of maximal heart rate, whatever the altitude (no change in HR). We demonstrated that, in the absence of autoregulation of maximal HR, myocardial tissue oxygenation would be incompatible with life above 6200 m-7600 m, depending on the hypothesis concerning a possible increase in coronary reserve (increase in coronary blood flow at exercise). The decrease in maximal HR at high altitude could be explained by several biological mechanisms involving the autonomic nervous system and its receptors on myocytes. These experimental and clinical observations support the hypothesis that there exists an integrated system at the cellular level, which protects the myocardium from a hazardous disequilibrium between O₂  supply and O₂ consumption at high altitude.

Physiological Function during Exercise and Environmental Stress in Humans-An Integrative View of Body Systems and Homeostasis

Claude Bernard’s milieu intérieur (internal environment) and the associated concept of homeostasis are fundamental to the understanding of the physiological responses to exercise and environmental stress. Maintenance of cellular homeostasis is thought to happen during exercise through the precise matching of cellular energetic demand and supply, and the production and clearance of metabolic by-products. The mind-boggling number of molecular and cellular pathways and the host of tissues and organ systems involved in the processes sustaining locomotion, however, necessitate an integrative examination of the body’s physiological systems. This integrative approach can be used to identify whether function and cellular homeostasis are maintained or compromised during exercise. In this review, we discuss the responses of the human brain, the lungs, the heart, and the skeletal muscles to the varying physiological demands of exercise and environmental stress. Multiple alterations in physiological function and differential homeostatic adjustments occur when people undertake strenuous exercise with and without thermal stress. These adjustments can include: hyperthermia; hyperventilation; cardiovascular strain with restrictions in brain, muscle, skin and visceral organs blood flow; greater reliance on muscle glycogen and cellular metabolism; alterations in neural activity; and, in some conditions, compromised muscle metabolism and aerobic capacity. Oxygen supply to the human brain is also blunted during intense exercise, but global cerebral metabolism and central neural drive are preserved or enhanced. In contrast to the strain seen during severe exercise and environmental stress, a steady state is maintained when humans exercise at intensities and in environmental conditions that require a small fraction of the functional capacity. The impact of exercise and environmental stress upon whole-body functions and homeostasis therefore depends on the functional needs and differs across organ systems.

Casein Kinase-2 Interacting Protein-1 Regulates Physiological Cardiac Hypertrophy via Inhibition of Histone Deacetylase 4 Phosphorylation

Different kinds of mechanical stimuli acting on the heart lead to different myocardial phenotypes. Physiological stress, such as exercise, leads to adaptive cardiac hypertrophy, which is characterized by a normal cardiac structure and improved cardiac function. Pathological stress, such as sustained cardiac pressure overload, causes maladaptive cardiac remodeling and, eventually, heart failure. Casein kinase-2 interacting protein-1 (CKIP-1) is an important regulator of pathological cardiac remodeling. However, the role of CKIP-1 in physiological cardiac hypertrophy is unknown. We subjected wild-type (WT) mice to a swimming exercise program for 21 days, which caused an increase in myocardial CKIP-1 protein and mRNA expression. We then subjected CKIP-1 knockout (KO) mice and myocardial-specific CKIP-1-overexpressing mice to the 21-day swimming exercise program. Histological and echocardiography analyses revealed that CKIP-1 KO mice underwent pathological cardiac remodeling after swimming, whereas the CKIP-1-overexpressing mice had a similar cardiac phenotype to the WT controls. Histone deacetylase 4 (HDAC4) is a key molecule in the signaling cascade associated with pathological hypertrophy; the phosphorylation levels of HDAC4 were markedly higher in CKIP-1 KO mouse hearts after the swimming exercise program. The phosphorylation levels of HDAC4 did not change after swimming in the hearts of CKIP-1-overexpressing or WT mice. Our results indicate that swimming, a mechanical stress that leads to physiological hypertrophy, triggers pathological cardiac remodeling in CKIP-1 KO mice. CKIP-1 is necessary for physiological cardiac hypertrophy in vivo, and for modulating the phosphorylation level of HDAC4 after physiological stress. Genetically engineering CKIP-1 expression affected heart health in response to exercise.

Contribution of oxygen extraction fraction to maximal oxygen uptake in healthy young men

We analysed the importance of systemic and peripheral arteriovenous O₂ difference ( a-v¯O2 difference and a‐v_fO₂ difference, respectively) and O₂ extraction fraction for maximal oxygen uptake ( V˙O2max). Fick law of diffusion and the Piiper and Scheid model were applied to investigate whether diffusion versus perfusion limitations vary with V˙O2max. Articles (n = 17) publishing individual data (n = 154) on V˙O2max, maximal cardiac output ( Q˙max; indicator‐dilution or the Fick method), a-v¯O2 difference (catheters or the Fick equation) and systemic O₂ extraction fraction were identified. For the peripheral responses, group‐mean data (articles: n = 27; subjects: n = 234) on leg blood flow (LBF; thermodilution), a‐v_fO₂ difference and O₂ extraction fraction (arterial and femoral venous catheters) were obtained. Q˙max and two‐LBF increased linearly by 4.9‐6.0 L · min^–1 per 1 L · min^–1 increase in V˙O2max (R² = .73 and R² = .67, respectively; both P < .001). The a-v¯O2 difference increased from 118‐168 mL · L^–1 from a V˙O2max of 2‐4.5 L · min^–1 followed by a reduction (second‐order polynomial: R² = .27). After accounting for a hypoxemia‐induced decrease in arterial O₂ content with increasing V˙O2max (R² = .17; P < .001), systemic O₂ extraction fraction increased up to ~90% ( V˙O2max: 4.5 L · min^–1) with no further change (exponential decay model: R² = .42). Likewise, leg O₂ extraction fraction increased with V˙O2max to approach a maximal value of ~90‐95% (R² = .83). Muscle O₂ diffusing capacity and the equilibration index Y increased linearly with V˙O2max (R² = .77 and R² = .31, respectively; both P < .01), reflecting decreasing O₂ diffusional limitations and accentuating O₂ delivery limitations. In conclusion, although O₂ delivery is the main limiting factor to V˙O2max, enhanced O₂ extraction fraction (≥90%) contributes to the remarkably high V˙O2max in endurance‐trained individuals.

Disentangling the Gordian knot of local metabolic control of coronary blood flow

Recognition that coronary blood flow is tightly coupled with myocardial metabolism has been appreciated for well over half a century. However, exactly how coronary microvascular resistance is tightly coupled with myocardial oxygen consumption (MV̇o₂) remains one of the most highly contested mysteries of the coronary circulation to this day. Understanding the mechanisms responsible for local metabolic control of coronary blood flow has been confounded by continued debate regarding both anticipated experimental outcomes and data interpretation. For a number of years, coronary venous Po₂ has been generally accepted as a measure of myocardial tissue oxygenation and thus the classically proposed error signal for the generation of vasodilator metabolites in the heart. However, interpretation of changes in coronary venous Po₂ relative to MV̇o₂ are quite nuanced, inherently circular in nature, and subject to confounding influences that remain largely unaccounted for. The purpose of this review is to highlight difficulties in interpreting the complex interrelationship between key coronary outcome variables and the arguments that emerge from prior studies performed during exercise, hemodilution, hypoxemia, and alterations in perfusion pressure. Furthermore, potential paths forward are proposed to help to facilitate further dialogue and study to ultimately unravel what has become the Gordian knot of the coronary circulation.

Resting Coronary Flow Varies With Normal Cardiac Catheter Laboratory Stimuli

BACKGROUND

Growing evidence supports physiology-guided revascularization, with Fractional Flow Reserve (FFR) the most commonly used invasive measure of coronary blood flow impairment at the time of diagnostic angiography. Recently, there has been growing interest in stenosis severity indices measured at rest, such as Instantaneous Wave Free Ratio (iFR) and the ratio of distal coronary to aortic pressure at rest (resting Pd/Pa). Their reliability may, theoretically, be more susceptible to changes in microvascular tone and coronary flow. This study aimed to assess variability of resting coronary flow with normal catheter laboratory stimuli.

METHODS

Simultaneous intracoronary pressure (Pd) and Doppler Average Peak Flow Velocity (APV) recordings were made at rest and following the verbal warning preceding an intravenous adenosine infusion.

RESULTS

72 patients undergoing elective angiography were recruited (mean age 62 years, 52.7% male) with a wide range of coronary artery disease severity (FFR 0.86 ± 0.09). Average peak flow velocity varied significantly between measurements at rest and just prior to commencement of adenosine, with a mean variation of 10.2% (17.82 ± 9.41 cm/s vs. 19.63 ± 10.44 cm/s, p < 0.001) with an accompanying significant drop in microvascular resistance (6.27 ± 2.73 mm Hg·cm-1·s-1 vs. 5.8 ± 2.92 mm Hg·cm-1·s-1, p < 0.001). These changes occurred without significant change in systemic hemodynamic measures. Whilst there was a trend for an associated change in the resting indices, Pd/Pa and iFR, this was statistically and clinically not significant (0.92 ± 0.08 vs. 0.92 ± 0.08, p = 0.110; and 0.90 ± 0.11 vs. 0.89 ± 0.12, p = 0.073).

CONCLUSION

Resting coronary flow and microvascular resistance vary significantly with normal catheter laboratory stimuli, such as simple warnings. The clinical impact of these observed changes on indices of stenosis severity, particularly those measured at rest, needs further assessment within larger cohorts.

Cardiovascular Effects and Benefits of Exercise

It is widely accepted that regular physical activity is beneficial for cardiovascular health. Frequent exercise is robustly associated with a decrease in cardiovascular mortality as well as the risk of developing cardiovascular disease. Physically active individuals have lower blood pressure, higher insulin sensitivity, and a more favorable plasma lipoprotein profile. Animal models of exercise show that repeated physical activity suppresses atherogenesis and increases the availability of vasodilatory mediators such as nitric oxide. Exercise has also been found to have beneficial effects on the heart. Acutely, exercise increases cardiac output and blood pressure, but individuals adapted to exercise show lower resting heart rate and cardiac hypertrophy. Both cardiac and vascular changes have been linked to a variety of changes in tissue metabolism and signaling, although our understanding of the contribution of the underlying mechanisms remains incomplete. Even though moderate levels of exercise have been found to be consistently associated with a reduction in cardiovascular disease risk, there is evidence to suggest that continuously high levels of exercise (e.g., marathon running) could have detrimental effects on cardiovascular health. Nevertheless, a specific dose response relationship between the extent and duration of exercise and the reduction in cardiovascular disease risk and mortality remains unclear. Further studies are needed to identify the mechanisms that impart cardiovascular benefits of exercise in order to develop more effective exercise regimens, test the interaction of exercise with diet, and develop pharmacological interventions for those unwilling or unable to exercise.

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.

Decreased Perfusion in Myocardial Region of Normal Donor Artery Secondary to Collateral Development

Thirty-one patients suffering from single vessel exertional angina with collaterals (Group A) were evaluated by stress 201T1 myocardial emission CT (T1-SPECT) with 16 controls of severely stenotic single vessel exertional angina without collaterals (Group B). Group A included 21 patients (68%) who showed an extensive perfusion defect in double artery myocardial regions, including the normal donor artery myocardial region (DMR). However, there were no such cases in Group B, giving a significant difference between these 2 groups (p < 0.001). Four patients in Group A, having a perfusion defect both in DMR and in the collateral dependent myocardial region (CMR) underwent a successful percutaneous transluminal coronary angioplasty (PTCA) with disappearance of collaterals. T1-SPECT findings after PTCA showed no perfusion defect either in CMR or in DMR. This has been explained on the basis that the coronary collaterals stole blood and produced perfusion defect in DMR.

Regulation of increased blood flow (hyperemia) to muscles during exercise: a hierarchy of competing physiological needs

This review focuses on how blood flow to contracting skeletal muscles is regulated during exercise in humans. The idea is that blood flow to the contracting muscles links oxygen in the atmosphere with the contracting muscles where it is consumed. In this context, we take a top down approach and review the basics of oxygen consumption at rest and during exercise in humans, how these values change with training, and the systemic hemodynamic adaptations that support them. We highlight the very high muscle blood flow responses to exercise discovered in the 1980s. We also discuss the vasodilating factors in the contracting muscles responsible for these very high flows. Finally, the competition between demand for blood flow by contracting muscles and maximum systemic cardiac output is discussed as a potential challenge to blood pressure regulation during heavy large muscle mass or whole body exercise in humans. At this time, no one dominant dilator mechanism accounts for exercise hyperemia. Additionally, complex interactions between the sympathetic nervous system and the microcirculation facilitate high levels of systemic oxygen extraction and permit just enough sympathetic control of blood flow to contracting muscles to regulate blood pressure during large muscle mass exercise in humans.

Neuroprotection by argon ventilation after perinatal asphyxia: a safety study in newborn piglets

Hypothermia is ineffective in 45% of neonates with hypoxic-ischemic encephalopathy. Xenon has additive neuroprotective properties, but is expensive, and its application complicated. Argon gas is cheaper, easier to apply, and also has neuroprotective properties in experimental settings. The aim was to explore the safety of argon ventilation in newborn piglets.

Methods

Eight newborn piglets (weight 1.4–3.0 kg) were used. Heart rate, blood pressure, regional cerebral saturation, and electrocortical brain activity were measured continuously. All experiments had a 30 min. baseline period, followed by three 60 min. periods of argon ventilation alternated with 30 min argon washout periods. Two animals were ventilated with increasing concentrations of argon (1h 30%, 1 h 50%, and 1 h 80%), two were subjected to 60 min. hypoxia (FiO₂ 0.08) before commencing 50% argon ventilation, and two animals received hypothermia following hypoxia as well as 50% argon ventilation. Two animals served as home cage controls and were terminated immediately.

Results

Argon ventilation did not result in a significant change of heart rate (mean ± s.d. −3.5±3.6 bpm), blood pressure (−0.60±1.11 mmHg), cerebral oxygen saturation (0.3±0.9%), electrocortical brain activity (−0.4±0.7 µV), or blood gas values. Argon ventilation resulted in elevated argon concentrations compared to the home cage controls (34.5, 25.4, and 22.4 vs. 7.3 µl/ml).

Conclusion

Ventilation with up to 80% argon during normoxia, and 50% argon after hypoxia did not affect heart rate, blood pressure, cerebral saturation and electrocortical brain activity. Clinical safety studies of argon ventilation in humans seem justified.

Recovery mismatch between myocardial blood flow and cardiac workload after physical exercise: a positron emission tomography study

AIMS

We studied the interrelation between oxygen consumption and myocardial blood flow (MBF) during recovery. MBF is directly dependent on oxygen consumption. The latter is linearly related to the heart rate-blood pressure product (RPP, bpm × mmHg), an index reflecting external cardiac work. In the immediate post-exercise period, cardiac output decreases considerably. This is expected to be paralleled by a rapid fall in oxygen demand, rendering ischaemia unlikely. Thus, the phenomenon of ST-segment depression during recovery remains unexplained.

METHODS AND RESULTS

(15)O-labelled water and positron emission tomography were used to measure MBF in 14 young healthy volunteers (mean age 27 ± 3 years) during the following study conditions: (i) at rest, (ii) during a steady submaximal supine bicycle exercise stress within the scanner, and (iii) during recovery immediately after cessation of exercise. During recovery, RPP decreased by 43% (18 768 ± 1337 vs. 11 652 ± 3224, P < 0.001). In contrast, the associated decrease in MBF (2.52 ± 0.52 vs. 1.93 ± 0.50 mL/min/g, P < 0.001) and perfusion reserve (2.68 ± 0.51 vs. 2.03 ± 0.42, P < 0.001) was significantly less pronounced (-24%, P < 0.01), indicating a relative delay in MBF recovery compared with cardiac work load.

CONCLUSION

The mismatch between a rapid decrease in cardiac workload but preserved hyperaemic response early after cessation of physical exercise suggests an uncoupling of cardiac work and MBF during recovery.

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.

Peripheral circulation

Blood flow (BF) increases with increasing exercise intensity in skeletal, respiratory, and cardiac muscle. In humans during maximal exercise intensities, 85% to 90% of total cardiac output is distributed to skeletal and cardiac muscle. During exercise BF increases modestly and heterogeneously to brain and decreases in gastrointestinal, reproductive, and renal tissues and shows little to no change in skin. If the duration of exercise is sufficient to increase body/core temperature, skin BF is also increased in humans. Because blood pressure changes little during exercise, changes in distribution of BF with incremental exercise result from changes in vascular conductance. These changes in distribution of BF throughout the body contribute to decreases in mixed venous oxygen content, serve to supply adequate oxygen to the active skeletal muscles, and support metabolism of other tissues while maintaining homeostasis. This review discusses the response of the peripheral circulation of humans to acute and chronic dynamic exercise and mechanisms responsible for these responses. This is accomplished in the context of leading the reader on a tour through the peripheral circulation during dynamic exercise. During this tour, we consider what is known about how each vascular bed controls BF during exercise and how these control mechanisms are modified by chronic physical activity/exercise training. The tour ends by comparing responses of the systemic circulation to those of the pulmonary circulation relative to the effects of exercise on the regional distribution of BF and mechanisms responsible for control of resistance/conductance in the systemic and pulmonary circulations.

Molecular basis of physiological heart growth: fundamental concepts and new players

The heart hypertrophies in response to developmental signals as well as increased workload. Although adult-onset hypertrophy can ultimately lead to disease, cardiac hypertrophy is not necessarily maladaptive and can even be beneficial. Progress has been made in our understanding of the structural and molecular characteristics of physiological cardiac hypertrophy, as well as of the endocrine effectors and associated signalling pathways that regulate it. Physiological hypertrophy is initiated by finite signals, which include growth hormones (such as thyroid hormone, insulin, insulin-like growth factor 1 and vascular endothelial growth factor) and mechanical forces that converge on a limited number of intracellular signalling pathways (such as PI3K, AKT, AMP-activated protein kinase and mTOR) to affect gene transcription, protein translation and metabolism. Harnessing adaptive signalling mediators to reinvigorate the diseased heart could have important medical ramifications.

The coronary circulation in exercise training

Exercise training (EX) induces increases in coronary transport capacity through adaptations in the coronary microcirculation including increased arteriolar diameters and/or densities and changes in the vasomotor reactivity of coronary resistance arteries. In large animals, EX increases capillary exchange capacity through angiogenesis of new capillaries at a rate matched to EX-induced cardiac hypertrophy so that capillary density remains normal. However, after EX coronary capillary exchange area is greater (i.e., capillary permeability surface area product is greater) at any given blood flow because of altered coronary vascular resistance and matching of exchange surface area and blood flow distribution. The improved coronary capillary blood flow distribution appears to be the result of structural changes in the coronary tree and alterations in vasoreactivity of coronary resistance arteries. EX also alters vasomotor reactivity of conduit coronary arteries in that after EX, α-adrenergic receptor responsiveness is blunted. Of interest, α- and β-adrenergic tone appears to be maintained in the coronary microcirculation in the presence of lower circulating catecholamine levels because of increased receptor responsiveness to adrenergic stimulation. EX also alters other vasomotor control processes of coronary resistance vessels. For example, coronary arterioles exhibit increased myogenic tone after EX, likely because of a calcium-dependent PKC signaling-mediated alteration in voltage-gated calcium channel activity in response to stretch. Conversely, EX augments endothelium-dependent vasodilation throughout the coronary arteriolar network and in the conduit arteries in coronary artery disease (CAD). The enhanced endothelium-dependent dilation appears to result from increased nitric oxide bioavailability because of changes in nitric oxide synthase expression/activity and decreased oxidant stress. EX also decreases extravascular compressive forces in the myocardium at rest and at comparable levels of exercise, mainly because of decreases in heart rate and duration of systole. EX 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. While there is evidence that EX can decrease the progression of atherosclerotic lesions or even induce the regression of atherosclerotic lesions in humans, the evidence of this is not strong due to the fact that most prospective trials conducted to date have included other lifestyle changes and treatment strategies by necessity. The literature from large animal models of CAD also presents a cloudy picture concerning whether EX can induce the regression of or slow the progression of atherosclerotic lesions. Thus, while evidence from research using humans with CAD and animal models of CAD indicates that EX increases endothelium-dependent dilation throughout the coronary vascular tree, evidence that EX reverses or slows the progression of lesion development in CAD is not conclusive at this time. This suggests that the beneficial effects of EX in CAD may not be the result of direct effects on the coronary artery wall. If this suggestion is true, it is important to determine the mechanisms involved in these beneficial effects.

High intensity interval training alters substrate utilization and reduces oxygen consumption in the heart

AIMS

although exercise training induces hypertrophy with improved contractile function, the effect of exercise on myocardial substrate metabolism and cardiac efficiency is less clear. High intensity training has been shown to produce more profound effects on cardiovascular function and aerobic capacity than isocaloric low and moderate intensity training. The aim of the present study was to explore metabolic and mechanoenergetic changes in the heart following endurance exercise training of both high and moderate intensity.

METHODS AND RESULTS

C57BL/6J mice were subjected to 10 wk treadmill running, either high intensity interval training (HIT) or distance-matched moderate intensity training (MIT), where HIT led to a pronounced increase in maximal oxygen uptake. Although both modes of exercise were associated with a 10% increase in heart weight-to-body weight ratio, only HIT altered cardiac substrate utilization, as revealed by a 36% increase in glucose oxidation and a concomitant reduction in fatty acid oxidation. HIT also improved cardiac efficiency by decreasing work-independent myocardial oxygen consumption. In addition, it increased cardiac maximal mitochondrial respiratory capacity.

CONCLUSION

This study shows that high intensity training is required for induction of changes in cardiac substrate utilization and energetics, which may contribute to the superior effects of high compared with moderate intensity training in terms of increasing aerobic capacity.

The Clinical Utility of Cardiopulmonary Exercise Testing in Suspected or Confirmed Myocardial Ischemia

Heart disease is a major cause of morbidity and mortality in the United States, with coronary artery disease (CAD) representing more than half of all cardiovascular events. Stable patients presenting with symptoms suggestive of CAD are likely to undergo an exercise electrocardiogram (ECG) and/ or imaging study as a first-line diagnostic assessment. A cardiopulmonary exercise test (CPX) is an ECG stress test plus ventilatory gas analysis. Recently, CPX has been used to detect exercise-induced myocardial ischemia (EIMI) suggestive of underlying CAD. Two CPX variables, oxygen pulse (VO2/HR) and the slope of oxygen consumption versus work rate (Δ VO2/ Δ WR), have been identified to be especially indicative of EIMI. Currently, there are a number of diagnostic tests available for the identification of CAD, with the most widely used being stress ECG, stress myocardial perfusion imaging (MPI) and echocardiography, and cardiac catheterization. Exercise ECG, although inexpensive, has a number of well-recognized limitations, including low sensitivity resulting in false-negative results. Stress (exercise or pharmaceutically induced) MPI and catheterization are more accurate but also more invasive and expensive. It appears that CPX may improve the diagnostic accuracy of exercise ECG. This review will address the potential utility of CPX in patients with suspected or confirmed myocardial ischemia.

Myocardial blood flow and adenosine A2A receptor density in endurance athletes and untrained men

Previous human studies have shown divergent results concerning the effects of exercise training on myocardial blood flow (MBF) at rest or during adenosine-induced hyperaemia in humans. We studied whether these responses are related to alterations in adenosine A2A receptor (A2AR) density in the left-ventricular (LV) myocardium, size and work output of the athlete's heart, or to fitness level. MBF at baseline and during intravenous adenosine infusion, and A2AR density at baseline were measured using positron emission tomography, and by a novel A(2A)R tracer in 10 healthy male endurance athletes (ET) and 10 healthy untrained (UT) men. Structural LV parameters were measured with echocardiography. LV mass index was 71% higher in ET than UT (193 +/- 18 g m(-2) versus 114 +/- 13 g m(-2), respectively). MBF per gram of tissue was significantly lower in the ET than UT at baseline, but this was only partly explained by reduced LV work load since MBF corrected for LV work was higher in ET than UT, as well as total MBF. The MBF during adenosine-induced hyperaemia was reduced in ET compared to UT, and the fitter the athlete was, the lower was adenosine-induced MBF. A2AR density was not different between the groups and was not coupled to resting or adenosine-mediated MBF. The novel findings of the present study show that the adaptations in the heart of highly trained endurance athletes lead to relative myocardial 'overperfusion' at rest. On the other hand hyperaemic perfusion is reduced, but is not explained by A2AR density.

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.

Exercise hyperaemia in the heart: the search for the dilator mechanism

Coronary blood flow is tightly coupled to myocardial oxygen consumption to maintain a consistently high level of myocardial oxygen extraction over a wide range of physical activity. This tight coupling has been proposed to depend on periarteriolar oxygen tension, signals released from cardiomyocytes (adenosine acting on K(ATP) channels) and the endothelium (prostanoids(,) nitric oxide, endothelin) as well as neurohumoral influences (catecholamines, endothelin), but the contribution of each of these regulatory pathways, and their interactions, to exercise hyperaemia in the human heart are still incompletely understood. Thus, in the human heart, nitric oxide, prostanoids, adenosine and K(ATP) channels each contribute to resting tone, but evidence for a critical contribution to exercise hyperaemia is lacking. In dogs K(ATP) channel activation together with adenosine and nitric oxide contribute to exercise hyperaemia in a non-linear redundant fashion. In contrast, in swine nitric oxide, adenosine and K(ATP) channels contribute to resting coronary resistance vessel tone control in a linear additive manner, but are not mandatory for exercise hyperaemia in the heart. Rather, exercise hyperaemia in swine appears to involve K(Ca) channel opening that is mediated, at least in part, by exercise-induced beta-adrenergic activation, possibly in conjunction with exercise-induced blunting of an endothelin-mediated vasoconstrictor influence. In view of these remarkable species differences in coronary vasomotor control during exercise, future studies are required to determine whether exercise hyperaemia in humans follows a canine or porcine control design.

Myocardial perfusion during exercise in endurance-trained and untrained humans

Because of technical challenges very little is known about absolute myocardial perfusion in humans in vivo during physical exercise. In the present study we applied positron emission tomography (PET) in order to 1) investigate the effects of dynamic bicycle exercise on myocardial perfusion and 2) clarify the possible effects of endurance training on myocardial perfusion during exercise. Myocardial perfusion was measured in endurance-trained and healthy untrained subjects at rest and during absolutely the same (150 W) and relatively similar [70% maximal power output (W(max))] bicycle exercise intensities. On average, the absolute myocardial perfusion was 3.4-fold higher during 150 W (P < 0.001) and 4.9-fold higher during 70% W(max) (P < 0.001) than at rest. At 150 W myocardial perfusion was 46% lower in endurance-trained than in untrained subjects (1.67 +/- 0.45 vs. 3.00 +/- 0.75 ml x g(-1) x min(-1); P < 0.05), whereas during 70% W(max) perfusion was not significantly different between groups (P = not significant). When myocardial perfusion was normalized with rate-pressure product, the results were similar. Thus, according to the present results, myocardial perfusion increases in parallel with the increase in working intensity and in myocardial work rate. Endurance training seems to affect myocardial blood flow pattern during submaximal exercise and leads to more efficient myocardial pump function.

Increased physical activity decreases hepatic free fatty acid uptake: a study in human monozygotic twins

Exercise is considered to be beneficial for free fatty acid (FFA) metabolism, although reports of the effects of increased physical activity on FFA uptake and oxidation in different tissues in vivo in humans have been inconsistent. To investigate the heredity-independent effects of physical activity and fitness on FFA uptake in skeletal muscle, the myocardium, and liver we used positron emission tomography (PET) in nine healthy young male monozygotic twin pairs discordant for physical activity and fitness. The cotwins with higher physical activity constituting the more active group had a similar body mass index but less body fat and 18 +/- 10% higher (P < 0.001) compared to the less active brothers with lower physical activity. Low-intensity knee-extension exercise increased skeletal muscle FFA and oxygen uptake six to 10 times compared to resting values but no differences were observed between the groups at rest or during exercise. At rest the more active group had lower hepatic FFA uptake compared to the less active group (5.5 +/- 4.3 versus 9.0 +/- 6.1 micromol (100 ml)(-1) min(-1), P = 0.04). Hepatic FFA uptake associated significantly with body fat percentage (P = 0.05). Myocardial FFA uptake was similar between the groups. In conclusion, in the absence of the confounding effects of genetic factors, moderately increased physical activity and aerobic fitness decrease body adiposity even in normal-weighted healthy young adult men. Further, increased physical activity together with decreased intra-abdominal adiposity seems to decrease hepatic FFA uptake but has no effects on skeletal muscle or myocardial FFA uptake.

Myocardial and peripheral vascular functional adaptation to exercise training

Exercise training seems to restore impaired vascular function in both peripheral and myocardial vessels in patients with coronary artery and peripheral vascular disease or in patients with risk factors for these diseases. However, the results on the effects of exercise training on vascular function in apparently healthy subjects are controversial. We studied the effects of long-term volitionally increased physical activity on peripheral and myocardial vascular function in nine young healthy male monozygotic twin pairs discordant for physical activity and fitness. The brothers were divided into more (MAG) and less active groups according to physical activity and fitness. The difference between groups in VO(2max) was 18+/-10% (P<0.001). Myocardial perfusion at rest, during adenosine-induced vasodilatation and during cold-pressor test and myocardial oxygen consumption were measured with positron emission tomography. In addition, endothelial function was measured using ultrasound in brachial and left anterior descending coronary arteries, and standard echocardiographic measures were taken. No differences were observed in myocardial perfusion measurements between groups. MAG tended to have a lower oxygen extraction fraction (P=0.06), but oxygen consumption was similar between the groups. No differences were found in coronary artery, myocardial resistance vessel or peripheral endothelial function between groups. These results suggest that when the effects of heredity are controlled, myocardial perfusion reserve and endothelial function, both in peripheral arteries and myocardial vessels, are not enhanced by increased physical activity and fitness in young healthy adult men.

Regulation of myocardial substrate metabolism during increased energy expenditure: insights from computational studies

In response to exercise, the heart increases its metabolic rate severalfold while maintaining energy species (e.g., ATP, ADP, and Pi) concentrations constant; however, the mechanisms that regulate this response are unclear. Limited experimental studies show that the classic regulatory species NADH and NAD+ are also maintained nearly constant with increased cardiac power generation, but current measurements lump the cytosol and mitochondria and do not provide dynamic information during the early phase of the transition from low to high work states. In the present study, we modified our previously published computational model of cardiac metabolism by incorporating parallel activation of ATP hydrolysis, glycolysis, mitochondrial dehydrogenases, the electron transport chain, and oxidative phosphorylation, and simulated the metabolic responses of the heart to an abrupt increase in energy expenditure. Model simulations showed that myocardial oxygen consumption, pyruvate oxidation, fatty acids oxidation, and ATP generation were all increased with increased energy expenditure, whereas ATP and ADP remained constant. Both cytosolic and mitochondrial NADH/NAD+ increased during the first minutes (by 40% and 20%, respectively) and returned to the resting values by 10-15 min. Furthermore, model simulations showed that an altered substrate selection, induced by either elevated arterial lactate or diabetic conditions, affected cytosolic NADH/NAD+ but had minimal effects on the mitochondrial NADH/NAD+, myocardial oxygen consumption, or ATP production. In conclusion, these results support the concept of parallel activation of metabolic processes generating reducing equivalents during an abrupt increase in cardiac energy expenditure and suggest there is a transient increase in the mitochondrial NADH/NAD+ ratio that is independent of substrate supply.

Coronary microcirculation into different models of left ventricular hypertrophy-hypertensive and athlete's heart: a contrast echocardiographic study

The study was carried out in two different models of left ventricular hypertrophy: athlete's heart and essential arterial hypertension. Three groups of strictly age-matched males were studied: one group of 10 young adult untreated essential hypertensive patients (H), a second group of 10 athletes (A), and a group of 10 healthy individuals as controls (C). A Sonos 5500 echograph with S4 harmonic transducer was used with Levovist (ultrasonic tracer) before and after dipyridamole injection; digitised images of quantitative myocardial contrast echocardiography were collected with Power Harmonic Doppler. Angio images were analysed using dedicated PC software by placing a region-of-interest on the septum. Peak intensity, half-time (HT), the area under the curve of appearance and disappearance of microbubbles at 2/3 of PI, both in absolute and indexed values (/LVMi), were sampled. The per cent increase of PI after dipyridamole was significantly higher in C (+73%, P < 0.01) than in H (+31%) and in A (+33%) (P < 0.05). The area of appearance was significantly lower in H in comparison with C and A, both at rest and after vasodilatation. The disappearance area after dipyridamole was significantly higher in C and in A (+124%) than in H (+104%) (P < 0.05). Some hypothesis could be made: an impairment in the coronary microcirculatory function in hypertensive patients could be because of an in-crease in the arteriolar resistance. Angiogenesis and several different functional adaptations are the mechanisms that allow an optimal distribution of oxygen and of substrates to the hypertrophied myocardium of the athletes.

The role of quantitative myocardial contrast echocardiography in the study of coronary microcirculation in athlete's heart

Quantitative myocardial contrast echocardiography was performed with harmonic power Doppler analysis using the background subtraction and Levovist (Schering AG, Berlin, Germany) as contrast agent in a triggered modality. Quantitative analysis of echocontrast was performed offline with PC software, obtaining the transit curves of microbubbles through the coronary capillary system. Coronary microcirculation in athletes showed a behavior substantially comparable with control participants, although at a higher level. Training determines a physiologic left ventricular hypertrophy that counterbalances the dilatation in the left ventricular chambers because of the higher blood volume in athletes compared with control participants. Angiogenesis and several functional adaptations (relaxation of small coronary arteries, increased production of nitric oxide by the coronary endothelium, or both), represent the potential mechanisms that allow an optimal distribution of oxygen and of substrates to the hypertrophied myocardium of the athletes.

Myocardial perfusion and perfusion reserve in endurance-trained men

PURPOSE

This study was undertaken to determine whether endurance training is associated with changes in myocardial perfusion in humans.

METHODS

Myocardial perfusion was measured in eleven trained and nine sedentary men at rest and during adenosine-stimulated hyperemia using positron emission tomography (PET). Left ventricular (LV) dimensions and mass were measured using echocardiography. Myocardial work per gram of tissue was calculated as (cardiac output. mean arterial blood pressure)/LV mass.

RESULTS

LV mass was significantly higher and myocardial work per gram of tissue lower in the trained than in the untrained subjects. Basal (0.78 +/- 0.10 and 0.76 +/- 0.15 mL. min-1. g-1, P = NS) and adenosine-stimulated perfusion (3.46 +/- 0.91 and 3.14 +/- 0.70 mL. min-1. g-1, P = NS) were similar between trained and untrained men, respectively. Consequently, myocardial perfusion reserve was similar in both groups (4.4 +/- 1.2 and 4.1 +/- 0.7, P = NS). In addition, coronary resistance at baseline (115 +/- 17 vs 119 +/- 22, mm Hg. mL. min-1. g-1, P = NS) and during adenosine infusion (28 +/- 8 vs 30 +/- 8, mm Hg. mL. min-1. g-1, P = NS) were similar in both groups. Resting myocardial work correlated with resting myocardial perfusion in both groups, but the relationship between perfusion and work was different between the groups so that perfusion for a given myocardial work was significantly higher in trained subjects (0.56 +/- 0.04 and 0.34 +/- 0.05 mL. (mm Hg. L)-1, P < 0.001).

CONCLUSIONS

These findings suggest that endurance trained subjects do not have different resting or adenosine-stimulated myocardial perfusion. However, the relationship between myocardial perfusion and work appears altered in the athletes.

Coronary flow reserve is supranormal in endurance athletes: an adenosine transthoracic echocardiographic study

OBJECTIVE

To compare coronary flow reserve in endurance athletes and healthy sedentary controls, using adenosine transthoracic echocardiography.

METHODS

29 male endurance athletes (mean (SD) age 27.3 (6.6) years, body mass index (BMI) 22.1 (1.9) kg/m(2)) and 23 male controls (age 27.2 (6.1) years, BMI 23.9 (2.6) kg/m(2)) with no coronary risk factors underwent transthoracic echocardiographic assessment of distal left anterior descending coronary artery (LAD) diameter and flow, both at rest and during intravenous adenosine infusion (140 microg/kg/min).

RESULTS

Distal LAD diameter and flow were adequately assessed in 19 controls (83%) and 26 athletes (90%). Distal LAD diameter in athletes (2.04 (0.25) mm) was not significantly greater than in sedentary controls (1.97 (0.27) mm). Per cent increase in LAD diameter following 400 microg sublingual nitrate was greater in the athletes than in the controls, at 14.1 (7. 2)% v 8.8 (5.7)% (p < 0.01). Left ventricular mass index in athletes exceeded that of controls, at 130 (19) v 98 (14) g/m(2) (p < 0.01). Resting flow among the athletes (10.6 (3.1) ml/min; 4.4 (1.2) ml/min/100 g left ventricular mass) was less than in the controls (14.3 (3.6) ml/min; 8.2 (2.2) ml/min/100 g left ventricular mass) (both p < 0.01). Hyperaemic flow among the athletes (61.9 (17.8) ml/min) exceeded that of the controls (51.1 (14.6) ml/min; p = 0.02), but not when corrected for left ventricular mass (25.9 (5.6) v 28.5 (7.4) ml/min/100 g left ventricular mass; NS). Coronary flow reserve was therefore substantially greater in the athletes than in the controls, at 5.9 (1.0) v 3.7 (0.7) (p < 0.01).

CONCLUSIONS

Coronary flow reserve in endurance athletes is supranormal and endothelium independent vasodilatation is enhanced. Myocardial hypertrophy per se does not necessarily impair coronary flow reserve. Adenosine transthoracic echocardiography is a promising technique for the investigation of coronary flow reserve.

Development of normative values for resting and exercise rate pressure product

PURPOSE

The purpose of this study was to develop multivariate models to quantify resting, submaximal, and maximal rate pressure products (RPP).

METHODS

A validation sample (N = 1623) was randomly selected from a clinically healthy population, and four cross-validation samples were randomly selected from a clinical cohort. The cross-validation samples were patients who had a negative exercise ECG with (Neg-Med, N = 179) and without cardiovascular drug (Neg-NoMed, N = 350), and patients who had a positive exercise ECG with (Pos-Med, N = 60) and without cardiovascular drug (Pos-NoMed, N = 75). Men made up 83% of the validation sample (mean age = 44.2+/-8.7) and women 17% (mean age = 39.7+/-10.1). The validation sample was used to develop multiple regression equations to quantify resting, submaximal, and maximal RPP.

RESULTS

Results indicated that gender, body mass index (BMI), and physical activity level (Ex-code) were significantly related with resting RPP. Gender, age, BMI, and Ex-code were significantly related with maximal RPP. Gender, age, BMI, Ex-code, and percent of maximal heart rate at submaximal exercise (%HRmax) were significantly related with submaximal RPP. The multiple correlations for the resting, submaximal, and maximal models were 0.29 (SE = 16.75 beats x min(-1) x mm Hg), 0.87 (SE = 29.04 beats x min(-1) x mm Hg), and 0.31 (SE = 42.41 beats x min(-1) x mm Hg), respectively. The accuracy of the models was confirmed when applied to the Neg-NoMed and Pos-NoMed samples but not the Neg-Med and Pos-Med samples. This result suggest that the regression models developed from this study can be generalized to other populations where patients were not taking cardiovascular medication. Microcomputer programs were suggested to evaluate RPP at rest, maximal exercise, and submaximal exercise.

CONCLUSION

Normative RPP for resting and exercise relies on multiple fitness parameters. Practical regression models are developed and can be applied to patients without cardiovascular medication.

Myocardial blood flow, oxygen consumption, and fatty acid uptake in endurance athletes during insulin stimulation

We have previously demonstrated reduced myocardial glucose uptake rates in hearts of endurance athletes, which could be due to increased use of alternative fuels or reduced energy demands. In the present study myocardial blood flow, oxygen consumption, and free fatty acid uptake were measured with [(15)O]H(2)O, [(15)O]O(2), [(18)F]FTHA, and positron emission tomography (PET) in 9 endurance athletes and 11 sedentary men during euglycemic hyperinsulinemia. Compared with sedentary men, athletes had 33% lower myocardial blood flow, 27% lower oxygen consumption, and 20% lower estimated myocardial work per gram of tissue. Myocardial fatty acid uptake rates were not significantly different in endurance athletes (0.83 +/- 0.29) and sedentary men (1.0 +/- 0.31 micromol. 100 g(-1). min(-1), P = 0.232). In conclusion, myocardial blood flow and oxygen consumption per unit mass of myocardium are reduced at rest in endurance athletes. This can be explained by reduced energy requirements per gram of tissue due to anatomic and physiological changes of the athlete's heart.

Maximal oxygen uptake: "classical" versus "contemporary" viewpoints: a rebuttal

Bassett and Howley contend that the 1996 J. B. Wolffe lecture is erroneous because: 1) A. V. Hill did establish the existence of the "plateau phenomenon," 2) the maximum oxygen consumption (VO2max) is limited by the development of anaerobiosis in the active muscle, and 3) endurance performance is also determined by skeletal muscle anaerobiosis because the VO2max is the best predictor of athletic ability. As a result, 4) cardiovascular and not skeletal muscle factors determine endurance performance. They further contend that Hill's "scientific hunches were correct," requiring "only relatively minor refinements" in the past 70 yr. But the evidence presented in this rebuttal shows that Hill neither sought nor believed in either the "plateau phenomenon" or the concept of the individual maximum oxygen consumption. These twin concepts were created by Taylor et al. (97) in 1955 and erroneously attributed to Hill. Rather Hill believed that there was a universal human VO2max of 4 L x min(-1). His error resulted from his incorrect belief that the real VO2 unmeasurable because it includes a large "anaerobic component," rose exponentially at running speeds greater than 13.2 km x h(-1). But Hill and his colleagues were indeed the first to realize the danger that a plateau in cardiac output (CO) and hence in VO2 would pose for the heart itself. For unlike skeletal muscle, the pumping capacity of the heart is both dependent on, but also the determinant of, its own blood supply. Thus, if the CO reaches a peak causing the "plateau phenomenon," the immediate cause of that peak will have been a plateau in myocardial oxygen delivery, causing a developing myocardial ischemia. The ischemia must worsen as exercise continues beyond the supposed VO2 "plateau." To accommodate this dilemma, Hill and his colleagues proposed a governor "either in the heart muscle or in the nervous system" necessary to prevent myocardial ischemia developing during maximal exercise. This governor would cause maximal exercise to terminate before the development of a plateau in either coronary flow, CO, or VO2, or the onset of skeletal muscle anaerobiosis. Accordingly, a new physiological model is proposed in which skeletal muscle recruitment is regulated by a central "governor" specifically to prevent the development of a progressive myocardial ischemia that would precede the development of skeletal muscle anaerobiosis during maximum exercise. As a result cardiovascular function "limits" maximum exercise capacity, probably as a result of a limiting myocardial oxygen delivery. The model is compatible with all the published findings of cardiovascular function during exercise in hypobaric hypoxia, in which there is a greater likelihood that myocardial hypoxia will develop.

Autonomic control of vasomotion in the porcine coronary circulation during treadmill exercise: evidence for feed-forward beta-adrenergic control

To date, no studies have investigated coronary vasomotor control of myocardial O2 delivery (MDO2) and its modulation by the autonomic nervous system in the porcine heart during treadmill exercise. We studied 8 chronically instrumented swine under resting conditions and during graded treadmill exercise. Exercise up to 85% to 90% of maximum heart rate produced an increase in myocardial O2 consumption (MVO2) from 163+/-16 micromol/min (mean+/-SE) at rest to 423+/-75 micromol/min (P< or =0.05), which was paralleled by an increase in MDO2, so that myocardial O2 extraction (79+/-1% at rest) and coronary venous O2 tension (cvPO2, 23.7+/-1.0 mm Hg at rest) were maintained. Beta-adrenoceptor blockade blunted the exercise-induced increase of MDO2 out of proportion compared with the attenuation of the exercise-induced increase in MVO2, so that O2 extraction rose from 78+/-1% at rest to 83+/-1% during exercise and cvPO2 fell from 23.5+/-0.9 to 19.6+/-1.1 mm Hg (both P< or =0.05). In contrast, alpha-adrenoceptor blockade, either in the absence or presence of beta-adrenoceptor blockade, had no effect on myocardial O2 extraction or cvPO2 at rest or during exercise. Muscarinic receptor blockade resulted in a decreased O2 extraction and an increase in cvPO2 at rest, an effect that waned during exercise. The vasodilation produced by muscarinic receptor blockade was likely due to an increased beta-adrenoceptor activity, since combined muscarinic and beta-adrenoceptor blockade produced similar changes in O2 extraction and cvPO2, as did beta-adrenoceptor blockade alone. In conclusion, in swine myocardium, MVO2 and MDO2 are matched during exercise, which is the result of feed-forward beta-adrenergic vasodilation in conjunction with minimal a-adrenergic vasoconstriction. Beta-adrenergic vasodilation is due to an increase in sympathetic activity but may also be supported by withdrawal of muscarinic receptor-mediated inhibition of beta-adrenergic coronary vasodilation. The observation that cvPO2 levels are maintained even during heavy exercise suggests that a decrease in cvPO2 is not essential for coronary vasodilation during exercise.

Left ventricle haemodynamics and vaso-active hormones during graded supine exercise in healthy male subjects

Left ventricle systolic and diastolic functional parameters were measured by gated equilibrium radionuclide cardiography in 12 healthy men (age 33-51 years) at rest and during graded supine exercise. The leftventricle end-diastolic volume showed an initial small (11%) increase during low submaximal exercise [from mean 163 (SD 40) at rest to mean 181 (SD 48) ml], while left ventricle end-systolic volume decreased successively [from mean 59 (SD 19) to mean 39 (SD 21) ml] with increasing exercise. Stroke volume was therefore elevated at all exercise levels compared with rest [mean 104 (SD 23) ml], and the peak value [mean 128 (SD 33) ml] was found at the lowest exercise level, contributing 40% to the initial increase in cardiac output. Cardiac output increased from mean 6.2 (SD 1.4) at rest to mean 20.2 (SD 5.0) l.min-1 at maximum. Left ventricle peak ejection and peak filling rates increased from mean 449 (SD 89) and mean 442 (SD 85) ml.s-1 at rest to mean 996 (SD 227) and mean 1255 (SD 333) ml.s-1, respectively, at maximum. The myocardium oxygen consumption, assumed to be proportional to the sum of the stroke work and the potential energy, increased fourfold, but absolute values were twice as high as expected, indicating that extrapolation from data obtained in dog hearts (as we have done) cannot be directly applied to humans. Selected vaso-active hormones were measured at all exercise intensities. Noradrenaline (NA), adrenaline (A) and angiotensin II (AII) concentrations showed a very pronounced increase at maximal exercise compared with the preceding lower intensites, while atrial natriuretic factor (ANF) and cyclic guanosinemonophosphate (cGMP) concentrations showed a more continuous increase, and dopamine (DA) remained almost unchanged. This speaks in favour of a crucial role for NA, A and AII in preserving blood pressure at maximum exercise, while DA probably has no importance for the cardiovascular homeostasis during exercise. Increases in concentrations of ANF and cGMP were highly correlated (r = 0.86). Our data supported the opinion that there is a cardiac limitation to maximal performance connected to the cardiac pumping capacity.

Alpha 1-adrenergic tone does not influence the transmural distribution of myocardial blood flow during exercise in dogs with pressure overload left ventricular hypertrophy

This study was carried out to test the hypothesis that alpha 1-adrenergic activation during exercise causes preferential vasoconstriction of subepicardial coronary resistance vessels, thereby augmenting blood flow to the subendocardium. Studies were performed in 7 dogs in which left ventricular hypertrophy was produced by banding the ascending aorta at 6-9 weeks of age. Animals were studied at approximately 1 year of age when the left ventricular/body weight ratio was 7.7 +/- 0.3 g/kg (mean +/- SE). Left anterior descending (LAD) coronary artery flow was measured with a Doppler velocity flow probe at rest and during a three-stage graded treadmill exercise protocol. The transmural distribution of myocardial blood flow was assessed with radioactive microspheres. Coronary blood flow increased progressively as a function of heart rate and rate-pressure product in response to exercise. In contrast to normal dogs which maintain preferential blood flow to the subendocardium (ENDO) relative to the subepicardium (EPI) during exercise, the ENDO/EPI flow ratio in the hypertrophied left ventricles was 0.88 +/- 0.10 during exercise. Selective alpha 1-adrenergic blockade by infusion of prazosin (10 micrograms/kg) into the LAD decreased mean aortic pressure during exercise from 86 +/- 6 to 76 +/- 4 mmHg (p < 0.05), but did not change coronary pressure, heart rate, left ventricular systolic or enddiastolic pressures, or LVdP/dtmax. Coronary blood flow was not significantly altered by prazosin at rest, but was progressively increased during increasing levels of exercise levels. During the heaviest level of exercise prazosin caused a 22 +/- 3% increase in mean myocardial blood flow which was similar in all transmural layers, with no change in the transmural distribution of perfusion (ENDO/EPI = 0.85 +/- 0.09). These findings demonstrate that alpha 1-adrenergic vasoconstrictor tone limits blood flow during exercise in the hypertrophied left ventricle, but do not support the concept that alpha 1-adrenergic activation augments perfusion of the subendocardium during exercise.

Equivalence between adenosine and exercise thallium-201 myocardial tomography: a multicenter, prospective, crossover trial

OBJECTIVES

The study was designed to compare pharmacologic and exercise stress during thallium-201 single-photon tomography in a multicenter prospective crossover trial.

BACKGROUND

Both exercise and adenosine myocardial perfusion imaging have high sensitivity and specificity for detection of coronary artery disease. However, few data are available comparing these two stress tests in the same patients.

METHODS

The study group consisted of 175 subjects: 55 healthy volunteers and 120 patients with suspected coronary artery disease. All subjects underwent two thallium tomographic tests performed 30 days apart, one during intravenous administration of adenosine (140 micrograms/kg per min for 6 min) and one during exercise stress. All images were computer quantified and interpreted without knowledge of the stress test performed. Interpretation agreement was assessed by kappa and Z statistics.

RESULTS

Agreement on the presence of normal or abnormal tomograms by adenosine and exercise scintigraphy was 82.8% by visual analysis with kappa and Z statistics of 0.65 (p less than 0.0001) and 11.1 (p less than 0.00001), respectively. The agreement by computer quantification was 86% with kappa and Z statistics of 0.709 (p less than 0.0001) and 12.2 (p less than 0.00001), respectively. Agreement on localization of the perfusion defect to a specific coronary vascular territory varied from 82.7% to 91.4% with highly significant kappa and Z statistics (p less than 0.0001). There was a good correlation between quantified perfusion defect size by adenosine and exercise (r = 0.80, p less than 0.0001), but the values for defect size were significantly greater by adenosine scintigraphy (p = 0.0073). Adenosine side effects were frequent but transient and ceased spontaneously in most subjects within 1 to 2 min after the infusion was discontinued.

CONCLUSIONS

Adenosine thallium-201 scintigraphy provides diagnostic information similar to that of exercise scintigraphy, although values for defect sizes are greater with adenosine.

Myocardial energetics in dilated cardiomyopathy

To assess hemodynamic and energetic effects of different drug interventions on idiopathic dilated cardiomyopathy (IDCM), we determined hemodynamic variables of myocardial oxygen consumption (MVO2) in 37 patients with IDCM. Hemodynamics were measured during routine left and right heart catheterization. MVO2 was analyzed from myocardial blood flow (measured by the argon method) and aortocoronary sinus blood oxygen difference. The hemodynamic variable which correlated best with MVO2 was shown to be the systolic stress time integral (STI). Four different representative compounds were tested with respect to their acute effects on myocardial energetics (MVO2/STI) in patients with IDCM who were in compensated heart failure (NYHA class II-III). The drug interventions were performed at rest. Intravenous injection of the vasodilator nitroprusside yielded a 35% reduction in STI and a 30% reduction in MVO2; in other words, the ratio MVO2/STI was not altered. Injection of the calcium sensitizer and phosphodiesterase inhibitor pimobendan also did not alter this ratio, as both STI (36%) and MVO2 (33%) were lowered. The profound reduction in STI (60%) seen with the phosphodiesterase inhibitor enoximone was accompanied by a much smaller decrease in MVO2 (19%); therefore, the ratio of MVO2/STI increased significantly. An increase of this ratio was also seen with the partial beta-1 receptor agonist xamoterol. However, in this case STI did not change, whereas MVO2 increased by 26%. In summary, vasodilation has energy-saving effects, whereas positive inotropism is an energy-consuming process. We conclude that the overall effect on myocardial energetics of a drug which possesses both positive inotropic and vasodilating properties depends on the balance of the two properties.