The distribution of whole blood viscosity, its determinants and relationship with arterial blood pressure in the community: cross-sectional analysis from the Gutenberg Health Study.

BACKGROUND: Blood viscosity has a role in modulating cardiovascular homeostasis; changes in this parameter have been associated with cardiovascular mortality and morbidity. However, it remains unclear whether these changes are (1) involved in the pathophysiology of disease, (2) an epiphenomenon, or (3) the expression of counterregulatory mechanisms. We report data on the normal values of blood viscosity and its association with cardiovascular risk factors, prevalent cardiovascular disease, and blood pressure in a large population-based cohort study. METHODS AND RESULTS: Viscosity was calculated using validated formulae and its associations were explored in 15,010 participants (mean 55.0, min-max: 35-74 years old; 49.5% women) from the Gutenberg Health Study as well as in a subgroup of 3223 subjects (61.1% women, mean age 49.2, min-max 35-74 years old) without risk factors or self-reported cardiovascular disease. Age- and gender-adjusted mean values for viscosity were defined. Regression models showed a relationship between classical risk factors and blood viscosity measures; the overall R(2) of the multiple linear regression model was however as low as 0.067 and 0.049 for high and low shear stress viscosity, respectively. After correction for cardiovascular risk factors, there was a very mild association between viscosity and prevalent coronary artery disease and heart failure. Systolic, mean and diastolic blood pressure increased with increasing blood viscosity after correction for age and gender. CONCLUSIONS: We provide reference values for viscosity in a population-based cohort. Blood viscosity decreases in older subjects and shows a very mild association with cardiovascular risk factors and prevalent disease in our cohort. There is a linear positive association between viscosity and blood pressure.

Endothelium and hemorheology.

A number of factors are involved in the regulation and maintenance of vascular homeostasis. The role of the vascular endothelium has been identified almost three decades ago, and a number of lines of evidence provide solid support to the role of this tissue in modulating not only vascular tone, but also phenomena such as platelet, red blood cell aggregation and deformability. In turn, hemorheological characteristics have been proven to impact on the endothelial release of mediators and therefore on vascular tone. Both biochemical and physical stimuli are sensed by the endothelium as stimuli for the release of oxygen free radicals and nitric oxide. In particular, changes in blood viscosity have a direct effect on shear stress, which is believed to be the physiological stimulus for endothelial activation. These considerations have lead us to formulate an alternative hypothesis for the meaning of hyperviscosity in the setting of ischemic syndromes. While this hypothesis is supported by animal data, the evidence of cross-sectional human studies is controversial. This evidence is discussed in the present review.

Evidence of a weak correlation between peripheral endothelial function measures and carotid intima-media thickness.

Previous studies from our and other laboratories have demonstrated the existence of a clear relationship between different measures of endothelial function and the extent of coronary atherosclerosis. The relationship between endothelial function and carotid intima-media thickness has not been extensively investigated. Endothelial function using radial artery flow-mediated constriction (L-FMC) and dilation (FMD) was assessed in 513 consecutive patients undergoing diagnostic coronary angiography. Intima-media thickness of both carotid arteries was also measured. IMT was greater in patients with diabetes, males, those with body mass index >30, and in those older than 65 years (all p < 0.05). There was a strong correlation between age and IMT (p < 0.0001). Hypercholesterolemia and a family history for cardiovascular disease had no impact on IMT. In contrast, the relationship between either L-FMC or FMD and IMT was weak at best (p = 0.008 for the relationship between L-FMC and IMT, p = 0.13 for the relationship between FMD and IMT). There was a positive correlation between IMT and resting radial artery diameter (p = 0.008). IMT increased with the extent of coronary artery disease, but this trend did not reach statistical significance (p = 0.07). Resting (L-FMC), but not recruitable (FMD) endothelial function correlates with the extent of subclinical carotid atherosclerosis. This correlation is however weaker in comparison to that with age.

Parameters of blood viscosity do not correlate with the extent of coronary and carotid atherosclerosis and with endothelial function in patients undergoing coronary angiography.

While the role of physical forces on the control of atherogenesis and the modulation of endothelial function is well known, studies investigating the impact of shear stress on the extent of central atherosclerosis and flow-mediated dilation in humans produced controversial results. We investigated the relationship between viscosity, coronary atherosclerosis, carotid intima-media thickness and flow-mediated dilation in patients undergoing coronary angiography. 451 patients (306 males, mean age 66 ± 10) were enrolled. Viscosity, which was calculated using a validated formula, showed a positive association with platelet activation (P = 0.01), leukocyte counts (P = 0.006) and C-reactive protein (P = 0.03), a marker of inflammation; surprisingly, visocsity showed a negative association with FMD (FMD decreased 0.14 ± 0.05% per each cPoise increase in viscosity) but only in patients without coronary artery disease. Viscosity showed no association with the extent of coronary or carotid artery disease. We provide cross-sectional data on the relationship between whole blood viscosity and parameters of vascular structure and function. While viscosity correlated with parameters of vascular inflammation, it showed no relationship with the presence and severity of central atherosclerosis.

Endothelial function and hemorheological parameters modulate coronary blood flow in patients without significant coronary artery disease.

BACKGROUND: Coronary (micro)vascular resistance is regulated by the complex interplay of several factors. Two potentially important determinants include endothelial function and the rheological properties of blood. However, their impact on the control of the coronary resistance vasculature is poorly understood. METHODS: The corrected Thrombolysis In Myocardial Infarction frame count (TIMIfc, an index of coronary flow velocity), conduit artery endothelial function, intima-media thickness of the common carotid artery and complete blood counts were measured in 145 patients undergoing elective coronary angiography. Patients with obstructive coronary artery disease or systemic conditions thought to be associated with microvascular disease were excluded from the analysis. RESULTS: There was a strong correlation between the TIMIfc measured in the three main coronary artery distributions (R values between 0.71 and 0.85, P < 0.00001). The TIMIfc was higher in males (P < 0.05), but there was no association with traditional risk factors for coronary artery disease (all P > 0.1). There was a correlation between TIMIfc and L-FMC, a parameter of resting endothelial function (R = 0.33, P < 0.0005). TIMIfc also correlated with mean platelet volume (a marker of platelet activation, R = 0.33, P < 0.001), and hematocrit (R = 0.33, P = 0.0002). There was no correlation between TIMIfc and carotid intima-media thickness and the degree of coronary atherosclerosis. Logistic regression analysis showed that L-FMC and hemorheological variables may explain as much as 19% of the variability in TIMIfc. CONCLUSIONS: Resting peripheral endothelial function, as well as parameters of platelet function, correlate with coronary TIMIfc. These data emphasize the existence of an association between endothelial function, hemorheological variables and coronary blood flow velocity.

Peripheral hemorheological and vascular correlates of coronary blood flow.

The slow coronary flow phenomenon (SCF), a condition described by the presence of inappropriate delay in the progression of intracoronary contrast during angiography in the absence of stenoses, has been shown in some patients presenting with chest pain. While several conditions leading to "secondary" slow flow are known, there are no definitive conclusions regarding the exact pathogenesis of "primary" SCF. The present paper outlines the mechanisms that may lead to SCF, emphasizing the role of hemorheological and vascular factors in the pathogenesis of this phenomenon. Small vessel dysfunction has been proposed in the pathogenesis of SCF since the first description of this syndrome in 1972. Abnormalities in coronary microvascular function result from increased microvascular resistances and impaired endothelial release of vasoactive substances, especially in production and bioavailability of endothelium derived NO. Inflammatory conditions (increased levels of C-reactive protein, interleukin-6 and adhesion molecules) and metabolic abnormalities such as impaired glycemic control, hyperuricemia and elevated serum gamma-glutamyltransferase were also found to contribute to microvascular dysfunction in patients with SCF. New studies have also indicated that increased blood viscosity and one of its major determinants, erythrocyte aggregation, is associated with the SCF. Rheological variables play a role in the control of shear stress and contribute to blood flow velocity changes. Although platelets do not have a significant influence on blood viscosity, it has been demonstrated that they are involved in the development of SCF. Increased mean platelet volume (MPV), an indicator of platelet activation and platelet aggregability is also significantly higher in patients with SCF compared with patients with normal coronary flow.

Chronic pharmacological preconditioning against ischemia.

Despite decades of research and thousands of experimental publications, acute preconditioning strategies have yet to be implemented in clinical practice. While some have attributed this to a failure of the experimental studies to mimic the clinical environment, others have suggested that acute preconditioning strategies themselves may possess physiological limitations. In particular, there is evidence to suggest a reduced efficacy of acute preconditioning in the aged heart and in disease states, such as diabetes, hypertension, hyperlipidemia, and atherosclerosis. In addition, pharmacologic agent commonly used in clinical practice, such as sulfonylureas and non-steroidal anti-inflammatory agents may interfere with acute preconditioning signaling pathways. Such considerations may preclude the translation of acute preconditioning strategies to the clinical setting. This has led some to shift attention to alternate strategies of cardioprotection, one such strategy being the possibility of generating a prolonged state of cardioprotection. Although preliminary, studies to date have suggested that sustained preconditioning strategies may not be associated with the same drawbacks as acute preconditioning. Further, cardioprotective signaling pathways that elicit the sustained preconditioning response may be distinct from acute signaling pathways, which permit pharmacologic targeting of these pathways in the future. Additionally, sustained preconditioning strategies may be clinically applicable in the setting of acute myocardial infarction, a setting where acute preconditioning strategies are inherently limited. This review will briefly discuss the current data regarding sustained preconditioning strategies, including those in humans, and discuss the goal of future studies in this setting.

Endothelium and hyperviscosity.

The role of viscosity, and of interindividual variations in this parameter, in the pathophysiology of cardiovascular disease remain incompletely understood. Any speculation regarding the possible impact of "hemorheological" therapies is therefore even more complex. In the last years, the debate regarding the relationship between increased viscosity and atherogenesis has been opened again. While the traditional view postulates that an increased blood viscosity has invariably a negative impact on tissue perfusion and therefore should be considered as a risk factor (when not as a true disease), a more recent hypothesis has been formulated based on the observation that small increases in viscosity actually have vasodilatory effects, potentially improving tissue perfusion.

The mechanism of nitrate-induced preconditioning.

Nitroglycerin (GTN) has been shown, in both human and animal studies, to induce a protective phenotype that limits tissue damage after ischemia and reperfusion. This phenomenon is similar to ischemic preconditioning, and several reports suggest that also the molecular pathways involved in this protective effect of nitrates are the same that determine ischemic preconditioning. Our group conducted a series of studies aimed at investigating, using a human model of endothelial IR injury, the mechanism of nitrate-induced preconditioning and particularly the role of reactive oxygen species formation and of the opening of mitochondrial permeability transition pores. Our data demonstrate that GTN protects the endothelium against postischemic endothelial dysfunction in a mechanism that is mediated by oxygen free radical release and opening of mitochondrial permeability transition pores. In contrast, the protective effect of pentaerithrityl tetranitrate appears to be independent of these mechanisms, and it seems to be mediated by induction of antioxidant genes. Finally, isosorbide mononitrate seems to be devoid of a significant protective effect. These data are summarized and discussed in the present paper.

Comparison of blood viscosity using a torsional oscillation viscometer and a rheometer.

The absence of a simple and clinically practical method to determine whole blood viscosity can partly justify why the medical community has been slow in realizing the significance of whole blood viscosity. For this reason, the availability of a technique able to evaluate blood viscosity in a rapid and direct manner is welcome. To evaluate the feasibility in hemorheological laboratory of a new torsional oscillation viscometer, it was compared with a conventional cone-plate system. The viscosity comparison has been related to hematocrit value both on whole blood and suspended blood in a saline solution. The results showed a good repeatability and reproducibility of the new equipment, with a best-fitting data of the hematocrit 0-100% range characterized by coefficient of determinations, r2>0.95. Furthermore, a comparison of whole blood viscosity as measured by the two instruments was done on blood samples collected from hospitalized patients. Reasonable agreement for the viscosity values was found between the two methods with linear determination coefficients between the two measurement methods comprised between r2=0.7329 and 0.9263, depending on shear stress phase and the corresponding shear rate.

Nitroglycerine causes mitochondrial reactive oxygen species production: in vitro mechanistic insights.

BACKGROUND: Nitroglycerine (GTN) is an organic nitrate that has been used for more than 100 years. Despite its widespread clinical use, several aspects of the pharmacology of GTN remain elusive. In a recent study, the authors of the present study showed that GTN causes opening of the mitochondrial permeability transition pore (mPTP) and mitochondrial production of reactive oxygen species (ROS). OBJECTIVE: In the present study, it was tested whether GTN-induced ROS production depends on mitochondrial potassium ATP-dependent channel or mPTP opening, and/or GTN biotransformation. METHODS AND RESULTS: Isolated rat heart mitochondria were incubated with succinate (a substrate for complex II) and GTN, causing immediate ROS production, as manifested by chemiluminescence. This ROS production was prevented by concomitant vitamin C incubation. Conversely, inhibitors of potassium ATP-dependent channels, mPTP opening or of GTN biotransformation did not modify ROS production. CONCLUSIONS: GTN triggers mitochondrial ROS production independently of the opening of mitochondrial channels and/or of GTN biotransformation. The present data, coupled with previous evidence published by the same authors that GTN causes opening of mPTPs, provide further evidence on the pharmacology of GTN. It is proposed that GTN causes direct uncoupling of the respiratory chain, which determines ROS production and subsequent mPTP opening. The clinical implications of these findings are also discussed.

Endothelium and haemorheology.

The vascular endothelium has been recognized to have a central importance in maintaining vascular homeostasis and in preventing cardiovascular disease. The mechanisms underlying the regulation of its function are extremely complex, and are principally determined by physical forces imposed on the endothelium by the flowing blood. In the present paper, we describe the interactions between the rheological properties of blood and the vascular endothelium. The role of shear stress, viscosity, cell-cell interactions, as well as the molecular mechanisms that are important for the transduction of these signals are discussed both in physiology and in pathology, with a particular attention to the role of reactive oxygen species. In the final conclusions, we propose an hypothesis regarding the implications of changes in blood viscosity, and particularly on the significance of secondary hyperviscosity syndromes.

Nitroglycerin protects the endothelium from ischaemia and reperfusion: human mechanistic insight.

AIMS: Nitroglycerin (GTN) modulates tissue damage induced by ischaemia and reperfusion (IR) in a mechanism that is similar to ischaemic preconditioning. We set out to study, using a human model of endothelial IR injury, whether GTN-induced endothelial preconditioning is mediated by reactive oxygen species (ROS) formation and/or opening of mitochondrial permeability transition pores (mPTP). METHODS: In two double-blind, randomized, parallel studies, a total of 66 volunteers underwent measurement of radial artery endothelium-dependent, flow-mediated dilation (FMD) before and after local IR. Subjects were treated, 24 h before IR, with different drugs in order to test the mechanism of GTN-induced endothelial protection. RESULTS: Transdermal GTN (0.6 mg h(-1) for 2 h, administered 24 h before IR) significantly reduced the impairment of FMD caused by IR (placebo group: FMD after IR, 1.3 +/- 0.8%; GTN group: FMD after IR, 5.3 +/- 0.9%, P < 0.01 compared with placebo). This protective effect was lost when vitamin C (2 g i.v. at the time of GTN administration) or ciclosporin (an inhibitor of mPTP, 100 mg 2 h prior to GTN administration) were coadministered (FMD after IR: vit C + GTN group, 2.1 +/- 1.0%; ciclosporin + GTN group, 1.7 +/- 0.8%; both P < 0.05 compared with GTN alone). CONCLUSIONS: We demonstrate that GTN protects the endothelium against IR-induced endothelial dysfunction, in an effect similar to delayed ischaemic preconditioning. Using a human model, we provide evidence supporting the concept that this protective effect is mediated by ROS release and mPTP opening upon GTN administration.

Correlation analysis between different parameters of conduit artery and microvascular vasodilation.

Impaired endothelial responsiveness to specific vasodilator stimuli has been used as a surrogate marker of cardiovascular risk. Multiple methods allow testing endothelial responses in both microvessels and conduit arteries, but it is still unclear whether there is a relationship in endothelial function between these two different vascular beds. Twenty-five healthy young non smoking male volunteers (age range 24-45) were enrolled. Radial (conduit) artery (endothelium-dependent) flow-mediated dilation (FMD), microvascular cutaneous reactive hyperemia (using laser Doppler) and acetylcholine-induced microvascular vasodilation (laser Doppler iontophoresis) were measured. Data were analyzed in a randomized fashion in order to test the existence of a correlation among these measures of endothelium (in)dependent vasodilation. Conduit artery FMD showed a negative correlation with resting radial artery diameter (R=0.44, P<0.05). There was a correlation between peak responsiveness to acetylcholine and peak reactive hyperemia (R=0.41, P<0.05). Conversely, absolutely no correlation was shown between FMD and measures of microvascular vasomotion, including reactive hyperemia (P=ns) and acetylcholine-induced vasodilation (P=ns). Using three different human in vivo models, we test conduit artery and microvascular endothelial vasodilation. While microvascular flow reserve measurements induced by endothelium-dependent and independent stimuli appear to be linearly correlated, we show no correlation in endothelium-dependent vasomotion between the micro- and macrocirculation.

Ischemia and reperfusion: the endothelial perspective. A radical view.

Ischemia and reperfusion (IR) injury causes a variety of changes in tissue homeostasis that lead to necrosis and/or programmed cell death. Due to its strategic location at the luminal surface of vessels, the vascular endothelium is particularly sensitive to IR. In particular, endothelial biosynthetic activities (and their protective effects) appear to be impaired by the oxidative burst induced by a sudden increase in oxygen free radical species upon reperfusion. Importantly, this endothelial damage can be easily assessed in vivo in humans by measuring endothelium-dependent vasorelaxation. Paradoxically, recent studies have emphasized the central role of free radicals (including oxygen free radicals and nitric oxide) also in a protective process, denominated ischemic preconditioning, i.e. a condition whereby a given stimulus can increase the tolerance of a tissue to IR damage. We discuss the role of the endothelium in determining the mechanism of IR injury, and on the other side, the effect of IR injury on endothelial function. In particular, we focus on the role of reactive free radicals in endothelial IR injury and in the development of ischemic preconditioning.

The effect of ischemia and reperfusion on microvascular function: a human in vivo comparative study with conduit arteries.

Although microvascular dysfunction is of critical importance in the pathophysiology of myocardial ischemic syndromes, no study has investigated whether there are differences in the sensitivity to ischemia and reperfusion injury between microvessels and conduit arteries. Ten healthy young nonsmoking male volunteers (age range 24-45) were enrolled. Parameters measured included radial (conduit) artery (endothelium-dependent) flow-mediated dilation, microvascular cutaneous reactive hyperemia (using laser Doppler) and acetylcholine-induced microvascular vasodilation (laser Doppler iontophoresis). Data were acquired before and after ischemic injury (15 minutes of ischemia of the brachial artery followed by 15 minutes reperfusion) and analyzed in a randomized, blinded fashion. Conduit artery FMD was significantly blunted after ischemia (before: 7.5 +/- 1.1%; after: 2.9 +/- 1.0%, P < 0.05). Conversely, ischemia had no effect on microvascular reactive hyperemia (P = ns) and acetylcholine-induced vasodilation (P = ns). Using a human in vivo model, we demonstrate that microvessels are more resistant to ischemic injury as compared to conduit arteries.

Evaluation of a torsional-vibrating technique for the hemorheological characterization.

Clinical measurement of blood viscosity is an important parameter in the diagnosis of different diseases (e.g., diabetes, hypertension, cardiovascular diseases). The significance of blood viscosity in the microcirculatory flow is also of great importance. Thus, a simple and accurate evaluation of hemorheological properties could be an important challenge in clinical practice. Nowadays, validated measurements of plasma viscosity are commonly carried out with rotational viscometers by means of the various geometric configurations. However, red blood cells deform under mechanical force and this aspect could lead to an artificial variation in the apparent viscosity. In this work, an evaluation of a new technique for the viscosity determination is focused. In particular, a torsional oscillation viscometer was adopted (VM10AL, CBC Europe) in the presence and in the absence of stirring conditions at thermostated conditions. The profile of the rheological behaviour as a function of time was recorded and compared with that obtained using a cone-plate rotational viscometer (AR300, TA Instrument).