Clinical achievements of impedance analysis

Aortic Characteristic Impedance and Suspected Mild Cognitive Impairment in a Community-Based Healthy Population

BACKGROUND

Aging of the proximal aorta can lead to increased pressure and flow pulsatility in the cerebral microcirculation, which may cause cognitive impairment. This study investigated the association between aortic characteristic impedance (Zc), an indicator of regional stiffness of the proximal aorta, and suspected mild cognitive impairment (MCI), compared with carotid-femoral pulse wave velocity (CFPWV).

METHODS AND RESULTS

A total of 1461 healthy community residents (46.4% men; age range, 35-96 years [mean±SD, 59.9±11.8 years]) without a history of cardiovascular events or stroke were included in the study. Zc was estimated using applanation tonometry and echocardiography. Cognitive function was assessed using the Mini-Mental State Examination. Education-adjusted cut points were used to define suspected MCI. Subjects with suspected MCI (n=493 [33.7%]) had significantly higher Zc and CFPWV than those without. In multivariable analysis, both Zc and CFPWV were inversely associated with Mini-Mental State Examination score. Zc (odds ratio per 1 SD, 1.22 [95% CI, 1.09-1.37] and CFPWV (odds ratio per 1 SD, 1.18 [95% CI, 1.01-1.38]) was also significantly associated with suspected MCI, after adjusting for age, sex, education level, mean arterial pressure, hypertension, diabetes, low-density lipoprotein cholesterol, and smoking status. In joint analysis, Zc was significantly associated with suspected MCI, but CFPWV was not. In the age subgroups of <50 years and 50 to 70 years, only Zc was significantly associated with suspected MCI.

CONCLUSIONS

This study found that Zc was significantly associated with Mini-Mental State Examination score and suspected MCI, especially in younger and middle-aged adults. These findings suggest that Zc may be a useful biomarker for identifying individuals at risk for MCI.

Excess pressure but not pulse wave velocity is associated with cognitive function impairment: a community-based study

BACKGROUND

Carotid-femoral pulse wave velocity (cf-PWV), an index of mainly distal aortic stiffness, has been inconsistently associated with cognitive function. Excess pressure, derived from the arterial reservoir-excess pressure analysis, may integrate the pulsatile load of the proximal aorta. The present study examined whether increased excess pressure is associated with cognitive function impairment in community adults.

METHODS

A total of 992 community participants (69.5% females; mean age: 67.3 years; education 13.6 years) without cerebrovascular disease or dementia received the Montreal Cognitive Assessment (MoCA) to evaluate global cognition. Arterial reservoir and excess pressure, arterial stiffness, and wave reflections were assessed, using carotid tonometry and aortic Doppler flowmetry.

RESULTS

Excess pressure integral (XSPI), percentage XSPI, cf-PWV, characteristic impedance (Zc), and forward and backward pressure amplitude (Pf, Pb, respectively) were significantly higher in 197 participants (19.9%) with a low MoCA score (<26 or <25, depending on level of education). In multivariable analyses, XSPI (standardized odds ratio, 95% confidence interval, 1.30, 1.06-1.59), and percentage XSPI (1.27, 1.06-1.52) but not cf-PWV (1.04, 0.85-1.26) were significantly associated with a low MoCA. Further analysis revealed that Pf and Zc were the major determinants of XSPI (partial R2: Pf = 0.656, Zc = 0.467) and percentage XSPI (Pf = 0.459, Zc = 0.371). In contrast, age, instead of Pf and Zc, was the major determinant of cf-PWV (partial R2: age = 0.187).

CONCLUSIONS

Excess pressure (XSPI/percentage XSPI), mainly determined by the pulsatile hemodynamics of the proximal aorta, was significantly associated with cognitive function impairment in middle-aged and elderly community adults.

Transplantation of bone marrow cells from miR150 knockout mice improves senescence-associated humoral immune dysfunction and arterial stiffness

BACKGROUND AND PURPOSE

The senescence-accelerated mouse P1 (SAMP1) suffers from humoral immune deficiency, arterial stiffness and accelerated aging. In contrast, the microRNA-150 knockout (miR-150-KO) mice show enhanced humoral immune function including increased B cell population and elevated serum immunoglobulin levels and enjoy extended lifespan. The purpose of this study was to investigate whether transplantation of bone marrow cells (BMCs) from miR-150-KO mice affects immune deficiency and arterial stiffening in SAMP1 mice.

METHODS AND RESULTS

Pulse wave velocity and blood pressure were increased significantly in SAMP1 mice (10 months), indicating arterial stiffening and hypertension. Interestingly, transplantation of BMCs from miR-150-KO mice significantly attenuated arterial stiffening and hypertension in SAMP1 mice within eight weeks. BMC transplantation from miR-150-KO mice partially rescued the downregulation of B lymphocytes, largely restored serum IgG and IgM levels, decreased inflammatory cytokine and chemokine expression, and attenuated macrophage and T cell infiltration in aortas in SAMP1 mice. BMC transplantation nearly abolished the upregulation of collagen 1, TGFβ1, Scleraxis, MMP-2 and MMP-9 expression and the downregulation of elastin levels in aortas in SAMP1 mice. FISH staining confirmed existence of the transplanted BMCs at end of the experiment. In cultured endothelial cells, IgG-deficient medium invoked upregulation of inflammatory cytokine/chemokine expression which can be rescued by treatment with IgG.

CONCLUSIONS

Accelerated senescence caused arterial stiffening via impairing the humoral immune function in SAMP1 mice. BMC transplantation from miR-150-KO mice attenuated arterial matrix remodeling and stiffening and hypertension in SAMP1 mice partly via improving the humoral immune function which attenuates vascular inflammation.

Harmonic Distortion of Blood Pressure Waveform as a Measure of Arterial Stiffness

Aging and disease alter the composition and elastic properties of the aortic wall resulting in shape changes in blood pressure waveform (BPW). Here, we propose a new index, harmonic distortion (HD), to characterize BPW and its relationship with other in vitro and in vivo measures. Using a Fourier transform of the BPW, HD is calculated as the ratio of energy above the fundamental frequency to that at the fundamental frequency. Male mice fed either a normal diet (ND) or a high fat, high sucrose (HFHS) diet for 2–10 months were used to study BPWs in diet-induced metabolic syndrome. BPWs were recorded for 20 s hourly for 24 h, using radiotelemetry. Pulse wave velocity (PWV), an in vivo measure of arterial stiffness, was measured in the abdominal aorta via ultrasound sonography. Common carotid arteries were excised from a subset of mice to determine the tangent modulus using biaxial tension-inflation test. Over a 24-h period, both HD and systolic blood pressure (SBP) show a large variability, however HD linearly decreases with increasing SBP. HD is also linearly related to tangent modulus and PWV with slopes significantly different between the two diet groups. Overall, our study suggests that HD is sensitive to changes in blood pressure and arterial stiffness and has a potential to be used as a noninvasive measure of arterial stiffness in aging and disease.

Causal Inference of Carnitine on Blood Pressure and potential mediation by uric acid: A mendelian randomization analysis

Background

Dietary change alters blood pressure (BP) but the specific causal dietary elements are unclear. Given previous observational data suggesting serum carnitine or uric acid affect BP, we investigated the role of serum carnitine and serum uric acid concentrations on BP, and considered mediation by lipids and insulin resistance using two-sample bi-directional Mendelian randomization (MR) analysis.

Methods

We performed MR to characterize bi-directional causal relationships of carnitine or uric acid on cardiometabolic traits. We performed two-sample MR using genome-wide association summary data from separate large-scale genomic analyses of carnitine, uric acid, BP, lipids, and glycemic traits. We used inverse variance weighted (IVW) meta-analysis and MR Egger regression to test for causal relations in the absence and presence of pleiotropy, respectively, and performed sensitivity analyses to identify confounders and intermediates.

Results

In our analysis, carnitine was directly, causally associated with systolic BP (IVW effect = 0.2, causal p-value = 0.03) but not diastolic BP (IVW causal p = 0.1). Our findings additionally support direct and indirect relationships of carnitine on TG and on uric acid. No causal associations of carnitine were found with glycemic traits. Uric acid was not associated with BP, nor TG.

Conclusion

Two-sample bi-directional MR demonstrated an unconfounded causal effect of carnitine, but not uric acid, on systolic but not diastolic BP, suggesting a role of carnitine in arterial stiffness. Carnitine, but not uric acid, also has direct and indirect effects on TG but are independent of the causal effect of carnitine on systolic BP.

Biological Pathways in Adolescent Aortic Stiffness

Background

Aortic stiffening begins in youth and antedates future hypertension. In adults, excess weight, systemic inflammation, dyslipidemia, insulin resistance, neurohormonal activation, and altered adipokines are implicated in the pathogenesis of increased aortic stiffness. In adolescents, we assessed the relations of comprehensive measures of aortic stiffness with body mass index (BMI) and related but distinct circulating biomarkers.

Methods and Results

A convenience sample of 246 adolescents (mean age, 16±2 years; 45% female, 24% Black, and 43% Hispanic) attending primary care or preventive cardiology clinics at 2 tertiary hospitals was grouped as normal weight (N=98) or excess weight (N=148, defined as BMI ≥age‐ and sex‐referenced 85th percentile). After an overnight fast, participants underwent anthropometry, noninvasive arterial tonometry, and assays for serum lipids, CRP (C‐reactive protein), glucose, insulin, renin, aldosterone, and leptin. We used multivariable linear regression to relate arterial stiffness markers (including carotid‐femoral pulse wave velocity) to BMI z score and a biomarker panel. Carotid‐femoral pulse wave velocity was higher in excess weight compared with normal weight group (5.0±0.7 versus 4.6±0.6 m/s; P<0.01). After multivariable adjustment, carotid‐femoral pulse wave velocity was associated with BMI z score (0.09 [95% CI, 0.01–0.18]; P=0.04) and with low‐density lipoprotein cholesterol (0.26 [95% CI, 0.03–0.50]; P=0.03).

Conclusions

Higher BMI and low‐density lipoprotein cholesterol were associated with greater aortic stiffness in adolescents. Maintaining optimal BMI and lipid levels may mitigate aortic stiffness.

Serum Albumin Is a Marker of Myocardial Fibrosis, Adverse Pulsatile Aortic Hemodynamics, and Prognosis in Heart Failure With Preserved Ejection Fraction

Background

Data regarding the phenotypic correlates and prognostic value of albumin in heart failure with preserved ejection fraction (HFpEF) are scarce. The goal of the current study is to determine phenotypic correlates (myocardial hypertrophy, myocardial fibrosis, detailed pulsatile hemodynamics, and skeletal muscle mass) and prognostic implications of serum albumin in HFpEF.

Methods and Results

We studied 118 adults with HFpEF. All‐cause death or heart‐failure–related hospitalization was ascertained over a median follow‐up of 57.6 months. We measured left ventricular mass, myocardial extracellular volume, and axial muscle areas using magnetic resonance imaging. Pulsatile arterial hemodynamics were assessed with a combination of arterial tonometry and phase‐contrast magnetic resonance imaging. Subjects with lower serum albumin exhibited a higher body mass index, and a greater proportion of black ethnicity and diabetes mellitus. A low serum albumin was associated with higher myocardial extracellular volume (52.3 versus 57.4 versus 39.3 mL in lowest to highest albumin tertile, respectively; P=0.0023) and greater N‐terminal pro B‐type natriuretic peptide levels, but not with a higher myocardial cellular volume (123 versus 114 versus 102 mL; P=0.13). Lower serum albumin was also associated with an increased forward wave amplitude and markedly increased pulsatile power in the aorta. Serum albumin was a strong predictor of death or heart failure hospitalization even after adjustment for N‐terminal pro B‐type natriuretic peptide levels and the Meta‐Analysis Global Group in Chronic Heart Failure (MAGGIC) risk score (adjusted standardized hazard ratio=0.56; 95% CI=0.37–0.83; P<0.0001).

Conclusions

Serum albumin is associated with myocardial fibrosis, adverse pulsatile aortic hemodynamics, and prognosis in HFpEF. This readily available clinical biomarker can enhance risk stratification in HFpEF and identifies a subgroup with specific pathophysiological abnormalities.

Proximal pressure reducing effect of wave reflection in the pulmonary circulation disappear in obstructive disease: insight from a rabbit model

Locating the site of increased resistance within the vascular tree in pulmonary arterial hypertension could assist in both patient diagnosis and tailoring treatment. Wave intensity analysis (WIA) is a wave analysis method that may be capable of localizing the major site of reflection within a vascular system. We investigated the contribution of WIA to the analysis of the pulmonary circulation in a rabbit model with animals subjected to variable occlusive pulmonary disease. Animals were embolized with different sized microspheres for 6 wk ( n = 10) or underwent pulmonary artery (PA) ligation for 6 wk ( n = 3). These animals were compared with a control group ( n = 6) and acutely embolized animals ( n = 4). WIA was performed and compared with impedance-based methods to analyze wave reflections. The control group showed a relatively high extent of reflected waves (15.7 ± 10.6%); reflections had a net effect of pressure reduction during systole, suggesting an open-end reflector. The pattern of wave reflection was not different in the group with partial PA ligation (12.4 ± 4.1%). In the chronically embolized group, wave reflection was not observed (3.6 ± 1.5%). In the acute embolization group, wave reflection was more prominent (37.3 ± 12.6%), with the appearance of a novel wave increasing pressure, suggesting the appearance of a closed-end reflector. Wave reflections of an open-end type are present in the normal rabbit pulmonary circulation. However, the pattern and nature of reflections vary according to the extent of pulmonary vascular occlusion.

NEW & NOTEWORTHY The study proposes an original framework of a complementary analysis of wave reflections in the time domain and in the frequency domain. The methodology was used in the pulmonary circulation with different forms of chronic obstructions. The results suggest that the pulmonary vascular tree generates a reflection pattern that could actually assist the heart during ejection, and chronic obstruction significantly modifies the pattern.

Comparison of noninvasive pulse transit time determined from Doppler aortic flow and multichannel bioimpedance plethysmography

The main goal of this study was to make a comparison of aortic flow timing obtained by PW Doppler in four aortic sections with timing of - dZ/dt max obtained by bioimpedance measurement in nine locations on the thorax and neck. This knowledge is essential for determination of which bioimpedance channel could be used as a proximal for evaluation of pulse wave velocity (PWV) from the beginning of the ascending aorta or another aortic section. Time intervals between the Doppler flow and bioimpedance information (- dZ/dt max) were compared. It was found that the channel located on the left part of the neck is the most suitable as a proximal bioimpedance channel which corresponds to the aortic arch. This match is obtained with regard to the value of the time difference as well as inter-subject stability. This channel can be used as a proximal for evaluation of pulse wave velocity from the aortic arch to the desired distal target place in the body when distance between measured parts is known. The data from 35 volunteers with adequate signal quality were analyzed. Graphical abstract ᅟ.

Deep Phenotyping of Systemic Arterial Hemodynamics in HFpEF (Part 2): Clinical and Therapeutic Considerations

Multiple phase III trials over the last few decades have failed to demonstrate a clear benefit of various pharmacologic interventions in heart failure with a preserved left ventricular (LV) ejection fraction (HFpEF). Therefore, a better understanding of its pathophysiology is important. An accompanying review describes key technical and physiologic aspects regarding the deep phenotyping of arterial hemodynamics in HFpEF. This review deals with the potential of this approach to enhance our clinical, translational, and therapeutic approach to HFpEF. Specifically, the role of arterial hemodynamics is discussed in relation to (1) the pathophysiology of left ventricular diastolic dysfunction, remodeling, and fibrosis, (2) impaired oxygen delivery to peripheral skeletal muscle, which affects peripheral oxygen extraction, (3) the frequent presence of comorbidities, such as renal failure and dementia in this population, and (4) the potential to enhance precision medicine approaches. A therapeutic approach to target arterial hemodynamic abnormalities that are prevalent in this population (particularly, with inorganic nitrate/nitrite) is also discussed.

Deep Phenotyping of Systemic Arterial Hemodynamics in HFpEF (Part 1): Physiologic and Technical Considerations

A better understanding of the pathophysiology of heart failure with a preserved left ventricular ejection fraction (HFpEF) is important. Detailed phenotyping of pulsatile hemodynamics has provided important insights into the pathophysiology of left ventricular remodeling and fibrosis, diastolic dysfunction, microvascular disease, and impaired oxygen delivery to peripheral skeletal muscle, all of which contribute to exercise intolerance, the cardinal feature of HFpEF. Furthermore, arterial pulsatile hemodynamic mechanisms likely contribute to the frequent presence of comorbidities, such as renal failure and dementia, in this population. Our therapeutic approach to HFpEF can be enhanced by clinical phenotyping tools with the potential to "segment" this population into relevant pathophysiologic categories or to identify individuals exhibiting prominent specific abnormalities that can be targeted by pharmacologic interventions. This review describes relevant technical and physiologic aspects regarding the deep phenotyping of arterial hemodynamics in HFpEF. In an accompanying review, the potential of this approach to enhance our clinical and therapeutic approach to HFpEF is discussed.

Ventricular-Arterial Coupling in Chronic Heart Failure

Measures of interaction between the left ventricle (LV) and arterial system (ventricular-arterial coupling) are important but under-recognised cardiovascular phenotypes in heart failure. Ventriculo-arterial coupling is commonly assessed in the pressure-volume plane, using the ratio of effective arterial elastance (E_A) to LV end-systolic elastance (E_ES) to provide information on ventricular-arterial system mechanical efficiency and performance when LV ejection fraction is abnormal. These analyses have significant limitations, such as neglecting systolic loading sequence, and are less informative in heart failure with preserved ejection fraction (HFpEF). E_A is almost entirely dependent on vascular resistance and heart rate. Assessment of pulsatile arterial haemodynamics and time-resolved myocardial wall stress provide critical incremental physiological information and should be more widely utilised. Pulsatile arterial load represents a promising therapeutic target in HFpEF. Here, we review various approaches to assess ventricular-arterial interactions, and their pathophysiological and clinical implications in heart failure.

Associations of Alterations in Pulsatile Arterial Load With Left Ventricular Longitudinal Strain

BACKGROUND

Increased arterial stiffness leads to increased pulsatile load on the heart. We investigated associations of components of pulsatile load with a measure of left ventricular (LV) systolic function-global longitudinal strain (GLS), in a community-based cohort ascertained based on family history of hypertension.

METHODS

Arterial tonometry and echocardiography with speckle tracking were performed in 520 adults with normal LV ejection fraction (EF) (age 67±9 years, 70% hypertensive) to quantify measures of pulsatile load (characteristic aortic impedance (Zc), total arterial compliance (TAC), and augmentation index (AI)) and GLS. The associations of log-Zc, log-TAC, and AI with GLS were assessed using sex-specific z-scores for each measure of arterial load.

RESULTS

In univariable analyses, higher Zc was associated with worse GLS (less negative) and higher TAC and AI were associated with better GLS (all P < 0.001). In a multivariable model including age, sex, heart rate (HR), LVEF, mean arterial load (systemic vascular resistance), and measures of pulsatile load, Zc remained associated with GLS (β = 0.28, P < 0.001), while the associations of TAC and AI were no longer significant (P > 0.5). Additional adjustment for cardiovascular risk factors and history of coronary heart disease and stroke did not attenuate the association of Zc with GLS; Zc, sex, HR, LVEF remained associated with GLS after stepwise elimination (all P < 0.001).

CONCLUSIONS

Greater proximal aortic stiffness, as manifested by a higher Zc, is independently associated with worse LV longitudinal function.

Effective arterial elastance is insensitive to pulsatile arterial load

Effective arterial elastance (E(A)) was proposed as a lumped parameter that incorporates pulsatile and resistive afterload and is increasingly being used in clinical studies. Theoretical modeling studies suggest that E(A) is minimally affected by pulsatile load, but little human data are available. We assessed the relationship between E(A) and arterial load determined noninvasively from central pressure-flow analyses among middle-aged adults in the general population (n=2367) and a diverse clinical population of older adults (n=193). In a separate study, we investigated the sensitivity of E(A) to changes in pulsatile load induced by isometric exercise (n=73). The combination of systemic vascular resistance and heart rate predicted 95.6% and 97.8% of the variability in E(A) among middle-aged and older adults, respectively. E(A) demonstrated a quasi-perfect linear relationship with the ratio of systemic vascular resistance/heart period (middle-aged adults, R=0.972; older adults, R=0.99; P<0.0001). Aortic characteristic impedance, total arterial compliance, reflection magnitude, and timing accounted together for <1% of the variability in E(A) in either middle-aged or older adults. Despite pronounced changes in pulsatile load induced by isometric exercise, changes in E(A) were not independently associated with changes pulsatile load but were rather a nearly perfect linear function of the ratio of systemic vascular resistance/heart period (R=0.99; P<0.0001). Our findings demonstrate that E(A) is simply a function of systemic vascular resistance and heart rate and is negligibly influenced by (and insensitive to) changes in pulsatile afterload in humans. Its current interpretation as a lumped parameter of pulsatile and resistive afterload should thus be reassessed.

The contribution of vascular smooth muscle to aortic stiffness across length scales

The operation of the cardiovascular system in health and disease is inherently mechanical. Clinically, aortic stiffness has proven to be of critical importance as an early biomarker for subsequent cardiovascular disease; however, the mechanisms involved in aortic stiffening are still unclear. The etiology of aortic stiffening with age has been thought to primarily involve changes in extracellular matrix protein composition and quantity, but recent studies suggest a significant involvement of the differentiated contractile vascular smooth muscle cells in the vessel wall. Here, we provide an overview of vascular physiology and biomechanics at different spatial scales. The processes involved in aortic stiffening are examined with particular attention given to recent discoveries regarding the role of vascular smooth muscle.

Young little mice express a premature cardiovascular aging phenotype

To investigate the effect of growth hormone and insulin-like growth factor 1 deficiency on the aging mouse arterial system, we compared the hemodynamics in young (4 months) and old (30 months) growth hormone-releasing hormone receptor null dwarf (Little) mice and their wild-type littermates. Young Little mice had significantly lower peak and mean aortic velocity and significantly higher aortic impedance than young wild-type mice. However, unlike the wild-type mice, there were no significant changes in arterial function with age in the Little mice. Aortic pulse wave velocity estimated using characteristic impedance increased with age in the wild-type mice, but it changed minimally in the Little mouse. We therefore conclude that arterial function in Little mice expresses a premature aging phenotype at young age and may neither enhance nor reduce their longevity.

Ventricular-arterial coupling: Invasive and non-invasive assessment

Interactions between the left ventricle (LV) and the arterial system, (ventricular-arterial coupling) are key determinants of cardiovascular function. Ventricularearterial coupling is most frequently assessed in the pressure-volume plane using the ratio of effective arterial elastance (E_A) to LV end-systolic elastance (E_ES). E_A (usually interpreted as a lumped index of arterial load) can be computed as end-systolic pressure/stroke volume, whereas E_ES (a load-independent measure of LV chamber systolic stiffness and contractility) is ideally assessed invasively using data from a family of pressure-volume loops obtained during an acute preload alteration. Single-beat methods have also been proposed, allowing for non-invasive estimations of E_ES using simple echocardiographic measurements. The E_A/E_ES ratio is useful because it provides information regarding the operating mechanical efficiency and performance of the ventricular-arterial system. However, it should be recognized that analyses in the pressure-volume plane have several limitations and that "ventricular-arterial coupling" encompasses multiple physiologic aspects, many of which are not captured in the pressure-volume plane. Therefore, additional assessments provide important incremental physiologic information about the cardiovascular system and should be more widely used. In particular, it should be recognized that: (1) comprehensive analyses of arterial load are important because E_A poorly characterizes pulsatile LV load and does not depend exclusively on arterial properties; (2) The systolic loading sequence, an important aspect of ventricular-arterial coupling, is neglected by pressure-volume analyses, and can profoundly impact LV function, remodeling and progression to heart failure. This brief review summarizes methods for the assessment of ventricular-arterial interactions, as discussed at the Artery 12 meeting (October 2012).

Resistance exercise training reduces arterial reservoir pressure in older adults with prehypertension and hypertension

We examined changes in central blood pressure (BP) following resistance exercise training (RET) in men and women with prehypertension and never-treated hypertension. Both Windkessel theory and wave theory were used to provide a comprehensive examination of hemodynamic modulation with RET. Twenty-one participants (age 61±1 years, n=6 male; average systolic blood pressure (SBP)/diastolic blood pressure (DBP)=138/84 mm Hg) were randomized to either 12 weeks of RET (n=11) or an inactive control group. Central BP and augmentation index (AIx) were derived from radial pressure waveforms using tonometry and a generalized transfer function. A novel reservoir-wave separation technique was used to derive excess wave pressure (related to forward and backward traveling waves) and reservoir pressure (related to the capacitance/Windkessel properties of the arterial tree). Wave separation using traditional impedance analysis and aortic flow triangulation was also applied to derive forward wave pressure (Pf) and backward wave pressure (Pb). There was a group-by-time interaction (P<0.05) for central BP as there was a significant ~6 mm Hg reduction in SBP and ~7 mm Hg reduction in DBP following RET with no change in the control condition. There were also group-by-time interactions (P<0.05) for Pf, excess wave pressure and reservoir pressure attributable to reductions in these parameters in the RET group concomitant with slight increases in the control group. There was no change in AIx or Pb (P>0.05). RET may reduce central BP in older adults with hypertension and prehypertension by lowering Pf and reservoir pressure without affecting pressure from wave reflections.

Mechanics and Function of the Pulmonary Vasculature: Implications for Pulmonary Vascular Disease and Right Ventricular Function

The relationship between cardiac function and the afterload against which the heart muscle must work to circulate blood throughout the pulmonary circulation is defined by a complex interaction between many coupled system parameters. These parameters range broadly and incorporate system effects originating primarily from three distinct locations: input power from the heart, hydraulic impedance from the large conduit pulmonary arteries, and hydraulic resistance from the more distal microcirculation. These organ systems are not independent, but rather, form a coupled system in which a change to any individual parameter affects all other system parameters. The result is a highly nonlinear system which requires not only detailed study of each specific component and the effect of disease on their specific function, but also requires study of the interconnected relationship between the microcirculation, the conduit arteries, and the heart in response to age and disease. Here, we investigate systems-level changes associated with pulmonary hypertensive disease progression in an effort to better understand this coupled relationship.

Pulmonary vascular wall stiffness: An important contributor to the increased right ventricular afterload with pulmonary hypertension

Pulmonary hypertension (PH) is associated with structural and mechanical changes in the pulmonary vascular bed that increase right ventricular (RV) afterload. These changes, characterized by narrowing and stiffening, occur in both proximal and distal pulmonary arteries (PAs). An important consequence of arterial narrowing is increased pulmonary vascular resistance (PVR). Arterial stiffening, which can occur in both the proximal and distal pulmonary arteries, is an important index of disease progression and is a significant contributor to increased RV afterload in PH. In particular, arterial narrowing and stiffening increase the RV afterload by increasing steady and oscillatory RV work, respectively. Here we review the current state of knowledge of the causes and consequences of pulmonary arterial stiffening in PH and its impact on RV function. We review direct and indirect techniques for measuring proximal and distal pulmonary arterial stiffness, measures of arterial stiffness including elastic modulus, incremental elastic modulus, stiffness coefficient β and others, the changes in cellular function and the extracellular matrix proteins that contribute to pulmonary arterial stiffening, the consequences of PA stiffening for RV function and the clinical implications of pulmonary vascular stiffening for PH progression. Future investigation of the relationship between PA stiffening and RV dysfunction may facilitate new therapies aimed at improving RV function and thus ultimately reducing mortality in PH.

Fractal scaling of laser Doppler flowmetry time series in patients with essential hypertension

The full diagnostic potential of the fractal complexity measure, α, of detrended fluctuation analysis (DFA) has not been realized yet. To reveal the impaired mechanisms in the blood flow regulation in patients with essential hypertension (EHT), we studied the laser Doppler flowmetry (LDF) time series by applying DFA. Forearm microvascular blood flow was measured by LDF during supine rest. After a 15 min baseline recording, microvascular response to thermal hyperemia was measured over 30 min. We found three distinct scaling regions; corresponding to the integration of local mechanisms, cardiac effect on local blood flow, and the coupling of extrinsic factors (cardiac and respiratory) to local blood flow by myogenic mechanism. In the control group, local scaling exponent, α(L)=0.96 ± 0.08, did not change but cardiac scaling exponent, α(C)=1.53 ± 0.05, for baseline signal was increased to α(CT)=1.73 ± 0.10 and cardio-respiratory scaling exponent, α(CR)=0.73 ± 0.19, was decreased to α(CRT)=0.24 ± 0.06 during vasodilatation in response to local heating. However, we found significantly different scaling exponents, α(LT)<1, α(CT) ≥ α(C)<1.5 and α(CR) ≈ α(CRT)>0.5 in patients with EHT. Our findings suggest that the local regulatory and the cushioning peripheral vascular functions are impaired in patients with EHT, and vascular/microvascular pathology can be evaluated by applying DFA to LDF signal.

Increases in cerebrovascular impedance in older adults

This study explored a novel method for measuring cerebrovascular impedance to quantify the relationship between pulsatile changes in cerebral blood flow (CBF) and arterial pressure. Arterial pressure in the internal or common carotid artery (applanation tonometry), CBF velocity in the middle cerebral artery (transcranial Doppler), and end-tidal CO(2) (capnography) were measured in six young (28 ± 4 yr) and nine elderly subjects (70 ± 6 yr). Transfer function method was used to estimate cerebrovascular impedance. Under supine resting conditions, CBF velocity was reduced in the elderly despite the fact that they had higher arterial pressure than young subjects. As expected, cerebrovascular resistance index was increased in the elderly. In both young and elderly subjects, impedance modulus was reduced gradually in the frequency range of 0.78-8 Hz. Phase was negative in the range of 0.78-4.3 Hz and fluctuated at high frequencies. Compared with the young, impedance modulus increased by 38% in the elderly in the range of 0.78-2 Hz and by 39% in the range of 2-4 Hz (P < 0.05). Moreover, increases in impedance were correlated with reductions in CBF velocity. Collectively, these findings demonstrate the feasibility of assessing cerebrovascular impedance using the noninvasive method developed in this study. The estimated impedance modulus and phase are similar to those observed in the systemic circulation and other vascular beds. Moreover, increases in impedance in the elderly suggest that arterial stiffening, besides changes in cerebrovascular resistance, contributes to reduction in CBF with age.

Noninvasive Evaluation of Left Ventricular Afterload

The mechanical load imposed by the systemic circulation to the left ventricle is an important determinant of normal and abnormal cardiovascular function. Left ventricular afterload is determined by complex time-varying phenomena, which affect pressure and flow patterns generated by the pumping ventricle. Left ventricular afterload is best described in terms of pressure-flow relations, allowing for quantification of various components of load using simplified biomechanical models of the circulation, with great potential for mechanistic understanding of the role of central hemodynamics in cardiovascular disease and the effects of therapeutic interventions. In the second part of this tutorial, we review analytic methods used to characterize left ventricular afterload, including analyses of central arterial pressure-flow relations and windkessel modeling (pressure-volume relations). Conceptual descriptions of various models and methods are emphasized over mathematical ones. Our review is aimed at helping researchers and clinicians obtain and interpret results from analyses of left ventricular afterload in clinical and epidemiological settings.

Noninvasive Evaluation of Left Ventricular Afterload

The mechanical load imposed by the systemic circulation to the left ventricle is an important determinant of normal and abnormal cardiovascular function. Left ventricular afterload is determined by complex time-varying phenomena, which affect pressure and flow patterns generated by the pumping ventricle and cannot be expressed as a single numeric measure or described in terms of pressure alone. Left ventricular afterload is best described in terms of pressure-flow relations. High-fidelity arterial applanation tonometry can be used to record time-resolved central pressure noninvasively, whereas contemporary noninvasive imaging techniques, such as Doppler echocardiography and phase-contrast MRI, allow for accurate assessments of aortic flow. Central pressure and flow can be analyzed using simplified biomechanical models to characterize various components of afterload, with great potential for mechanistic understanding of the role of central hemodynamics in cardiovascular disease and the effects of therapeutic interventions. In the first part of this tutorial, we review noninvasive techniques for central pressure and flow measurements and basic concepts of wave conduction and reflection as they relate to the interpretation of central pressure-flow relations. Conceptual descriptions of various models and methods are emphasized over mathematical ones. Our review is aimed at helping researchers and clinicians apply and interpret results obtained from analyses of left ventricular afterload in clinical and epidemiological settings.

Decreased lung function is associated with increased arterial stiffness as measured by peripheral pulse pressure: data from NHANES III

BACKGROUND

Individuals with impaired lung function have an elevated risk of cardiovascular events. Improved understanding of the factors associated with increased cardiovascular risk in the setting of lung function impairment is needed. We set out to determine in a nationwide population-based cohort whether impaired lung function as measured by forced expiratory volume in 1 s (FEV1) is associated with evidence of increased arterial stiffness measured by peripheral pulse pressure, a risk factor for cardiovascular events.

METHODS

Cross-sectional study of adults > or = 20 years of age in the Third National Health and Nutrition Examination Survey (NHANES III) who had valid and reproducible FEV1 data and serial blood pressure measurements allowing for the calculation of a mean pulse pressure, a measure of central arterial stiffness (n = 13,090).

RESULTS

There was a significant negative correlation between FEV1 and mean pulse pressure (r = -0.37). After controlling for demographic variables and confounders including cardiovascular risk factors, smoking history, and lung disease, the negative relationship between pulse pressure and FEV1 varies with age and becomes apparent only after age 40. In individuals aged 40-59 and > or = 60 years, there was respectively a 2 and 3 mm Hg increment in pulse pressure for every one standard deviation decrement in FEV1.

CONCLUSIONS

FEV1 is significantly related to pulse pressure, a clinically important measure of arterial stiffness, among those aged > or = 40 years. This relationship may help to explain the increased risk of cardiovascular events found in individuals with impaired lung function.

Model-based correction of the influence of body position on continuous segmental and hand-to-foot bioimpedance measurements

Bioimpedance spectroscopy (BIS) is suitable for continuous monitoring of body water content. The combination of body posture and time is a well-known source of error, which limits the accuracy and therapeutic validity of BIS measurements. This study evaluates a model-based correction as a possible solution. For this purpose, an 11-cylinder model representing body impedance distribution is used. Each cylinder contains a nonlinear two-pool model to describe fluid redistribution due to changing body position and its influence on segmental and hand-to-foot (HF) bioimpedance measurements. A model-based correction of segmental (thigh) and HF measurements (Xitron Hydra 4200) in nine healthy human subjects (following a sequence of 7 min supine, 20 min standing, 40 min supine) has been evaluated. The model-based compensation algorithm represents a compromise between accuracy and simplicity, and reduces the influence of changes in body position on the measured extracellular resistance and extracellular fluid by up to 75 and 70%, respectively.

Wave reflection and arterial stiffness in the prediction of 15-year all-cause and cardiovascular mortalities: a community-based study

The value of increased arterial wave reflection, usually assessed by the transit time-dependent augmentation index and augmented pressure (Pa), in the prediction of cardiovascular events may have been underestimated. We investigated whether the transit time-independent measures of reflected wave magnitude predict cardiovascular outcomes independent of arterial stiffness indexed by carotid-femoral pulse wave velocity. A total of 1272 participants (47% women; mean age: 52+/-13 years; range: 30 to 79 years) from a community-based survey were studied. Carotid pressure waveforms derived by tonometry were decomposed into their forward wave amplitudes, backward wave amplitudes (Pb), and a reflection index (=[Pb/(forward wave amplitude+Pb)]), in addition to augmentation index, Pa, and reflected wave transit time. During a median follow-up of 15 years, 225 deaths occurred (17.6%), including 64 cardiovascular origins (5%). In univariate Cox proportional hazard regression analysis, pulse wave velocity, Pa, and Pb predicted all-cause and cardiovascular mortality in both men and women, whereas augmentation index, reflected wave transit time, and reflection index were predictive only in men. In multivariate analysis accounting for age, height, and heart rate, Pb predicted cardiovascular mortality in both men and women, whereas Pa was predictive only in men. Per 1-SD increment (6 mm Hg), Pb predicted 15-year cardiovascular mortality independent of brachial but not central pressure, pulse wave velocity, augmentation index, Pa, and conventional cardiovascular risk factors with hazard ratios of approximately 1.60 (all P<0.05). In conclusion, Pb, a transit time-independent measure of reflected wave magnitude, predicted long-term cardiovascular mortality in men and women independent of arterial stiffness.

On the reliability of frequency components in systolic arterial pressure in patients with atrial fibrillation

Atrial fibrillation (AF) is characterized by desynchronization of atrial electrical activity causing a consequent irregular ventricular response. In AF, the beat-to-beat variation of blood pressure is increased because of variations in filling time and contractility. However, only a few studies have analyzed short-term blood pressure variations in AF, and we have recently observed a harmonic low-frequency (LF) component in systolic arterial pressure (SAP) during AF. Aim of the present study is to propose a method to verify the reliability of the spectral component found in SAP series, based on the position of the poles of the autoregressive spectral decomposition in the z-plane. In particular, 1,000 random permutations of the series allowed the definition of an area in the z-plane where poles from random process are likely to occur. Poles lying outside this area are considered as reliable oscillations. We tested the method on 53 recordings obtained at rest from patients with persistent AF. LF component was found in, respectively, 51 and 43 recordings in SAP and RR series. High-frequency (HF) component was found in all the recordings for both SAP and RR series. Using the proposed test, the percentage of reliable components in LF and HF bands was 80 and 38 in SAP series, and 20 and 18 in RR series. We concluded that, at variance with RR ones, SAP LF components are likely to represent true physiological oscillations.