Augmentation pressure is influenced by ventricular contractility/relaxation dynamics: novel mechanism of reduction of pulse pressure by nitrates

Effect of acute nitrate ingestion on central hemodynamic load in hypoxia

Acute hypoxia results in local vasodilation that may temporarily unload the left ventricle (LV) through nitric oxide (NO)-mediated mechanisms. Whether increasing NO levels augments LV unloading and improves ventricular-vascular coupling in hypoxia remains unknown.

PURPOSE

Investigate the effect of acute nitrate ingestion on central hemodynamic load in hypoxia.

METHODS

20 Healthy men (23 ± 3 yrs, BMI 24.6 ± 2.8 kg m(-2)) consumed 70 mL of either a) 0.45 g nitrate (NIT) or b) an inert placebo (PLA) prior to 105 min of normobaric hypoxia (11.6 ± 0.1%) in this randomized, double-blind, crossover-design study. Wave reflection index (RIX; ratio of forward to reflected wave pressure), augmentation index (AIX75) and pulse wave velocity were calculated as measures of wave reflection magnitude and aortic stiffness, respectively, from the aortic blood pressure (BP) waveform. LV wasted pressure effort (WPE) was calculated as an index of LV work due to wave reflections. Subendocardial viability ratio (SEVR) was assessed a measure of myocardial O2 supply/demand ratio.

RESULTS

Aortic diastolic BP decreased in hypoxia compared to normoxia (p < 0.05). Aortic RIX, AIX75, and LV WPE significantly decreased in hypoxia compared to normoxia (p < 0.05). Aortic systolic BP, SEVR, and PWV were unaffected by hypoxia (p > 0.05). Compared to placebo, nitrate ingestion did not significantly alter central hemodynamics in hypoxia (p > 0.05).

CONCLUSIONS

Acute hypoxic exposure unloads the LV (WPE, AIX75, and RIX) without disturbing myocardial O2 supply-demand ratio (SEVR). Reductions in LV work with hypoxia are likely due to reductions in pressure from wave reflections as hypoxia had negligible effects on aortic stiffness. Nitrate ingestion did not affect the central hemodynamic response to acute systemic hypoxia.

Intrafamilial Aggregation and Heritability of Aortic Reflected (Backward) Waves Derived From Wave Separation Analysis

BACKGROUND

Although aortic wave reflection may be inherited, the extent to which indexes of wave reflection derived from wave separation analysis (reflected (backward) wave index (RI) and pressure (Pb)) show intrafamilial aggregation and heritability is uncertain. We therefore aimed to determine the intrafamilial aggregation and heritability of RI and Pb and compare these with indexes of pressure augmentation.

METHODS

Aortic Pb, RI, augmented pressure (Pa), and augmentation index (AIx) were determined using radial applanation tonometry and SphygmoCor software in 1,152 participants of 315 families (111 father-mother, 705 parent-child, and 301 sibling-sibling pairs) from an urban developing community of black Africans. Heritability estimates were determined from Statistical Analysis for Genetic Epidemiology software.

RESULTS

With appropriate adjustments, significant correlations were noted between parent-child pairs for Pb and Pa (P < 0.05 for all), but not for RI (P = 0.50) or AIx (P = 0.90) and between sib-sib pairs for Pb and Pa (P < 0.05), but not for RI (P = 0.54) or AIx (P = 0.14). No correlations for indexes of wave reflection were noted between fathers and mothers (P > 0.57). After adjustments, Pb (h2 = 0.24±0.07) and Pa (h2 = 0.23±0.07) (P < 0.001 for both) but not RI (h2 = 0.04±0.06, P = 0.27) or AIx (h2 = 0.10±0.07, P = 0.07) showed significant heritability.

CONCLUSIONS

Aortic reflected (backward) waves derived from either wave separation (Pb) or pulse wave analysis (Pa) show a similar degree of intrafamilial aggregation and heritability, but the use of RI or AIx may underestimate reflected wave effects.

Relation of Central Arterial Stiffness to Incident Heart Failure in the Community

BACKGROUND

Arterial stiffness, pressure pulsatility, and wave reflection are associated with cardiovascular disease. Left ventricular function is coupled to proximal aortic properties, but the association of central aortic stiffness and hemodynamics with incident clinical heart failure (HF) is not well described.

METHODS AND RESULTS

Framingham Study participants without clinical HF (n=2539, mean age 64 years, 56% women) underwent applanation tonometry to measure carotid-femoral pulse wave velocity (CFPWV), central pulse pressure, forward wave amplitude, and augmentation index. CFPWV was inverse-transformed to reduce heteroscedasticity and multiplied by -1 to restore effect direction (iCFPWV). Over 10.1 (range 0.04-12.9) years, 170 HF events developed. In multivariable-adjusted analyses, iCFPWV was associated with incident HF in a continuous, graded fashion (hazards ratio [HR] per SD unit [SDU] 1.29, 95% confidence interval [CI] 1.02-1.64, P=0.037). iCFPWV was associated with HF with reduced ejection fraction (HR=1.69/SDU, 95% CI 1.19-2.42, P=0.0037) in age- and sex-adjusted models, which was attenuated in multivariable-adjusted models (P=0.065). Central pulse pressure and forward wave amplitude were associated with HF in age- and sex-adjusted models (per SDU, HR=1.20, 95% CI 1.06-1.37, P=0.006, and HR=1.15, 95% CI 1.01-1.31, P=0.036, respectively), but not in multivariable-adjusted models (both P≥0.28). Augmentation index was not associated with HF risk (P≥0.19 in all models).

CONCLUSIONS

In our prospective investigation of a large community-based sample of middle-aged to elderly individuals, greater aortic stiffness (reflected by higher iCFPWV) was associated with increased risk of HF. Future studies may investigate the impact of modifying aortic stiffness in reducing the community burden of HF.

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.

Associations and clinical relevance of aortic-brachial artery stiffness mismatch, aortic reservoir function, and central pressure augmentation

Central augmentation pressure (AP) and index (AIx) predict cardiovascular events and mortality, but underlying physiological mechanisms remain disputed. While traditionally believed to relate to wave reflections arising from proximal arterial impedance (and stiffness) mismatching, recent evidence suggests aortic reservoir function may be a more dominant contributor to AP and AIx. Our aim was therefore to determine relationships among aortic-brachial stiffness mismatching, AP, AIx, aortic reservoir function, and end-organ disease. Aortic (aPWV) and brachial (bPWV) pulse wave velocity were measured in 359 individuals (aged 61 ± 9, 49% male). Central AP, AIx, and aortic reservoir indexes were derived from radial tonometry. Participants were stratified by positive (bPWV > aPWV), negligible (bPWV ≈ aPWV), or negative stiffness mismatch (bPWV < aPWV). Left-ventricular mass index (LVMI) was measured by two-dimensional-echocardiography. Central AP and AIx were higher with negative stiffness mismatch vs. negligible or positive stiffness mismatch (11 ± 6 vs. 10 ± 6 vs. 8 ± 6 mmHg, P < 0.001 and 24 ± 10 vs. 24 ± 11 vs. 21 ± 13%, P = 0.042). Stiffness mismatch (bPWV-aPWV) was negatively associated with AP (r = -0.18, P = 0.001) but not AIx (r = -0.06, P = 0.27). Aortic reservoir pressure strongly correlated to AP (r = 0.81, P < 0.001) and AIx (r = 0.62, P < 0.001) independent of age, sex, heart rate, mean arterial pressure, and height (standardized β = 0.61 and 0.12, P ≤ 0.001). Aortic reservoir pressure independently predicted abnormal LVMI (β = 0.13, P = 0.024). Positive aortic-brachial stiffness mismatch does not result in higher AP or AIx. Aortic reservoir function, rather than discrete wave reflection from proximal arterial stiffness mismatching, provides a better model description of AP and AIx and also has clinical relevance as evidenced by an independent association of aortic reservoir pressure with LVMI.

Arterial stiffness: insights from Framingham and Iceland

PURPOSE OF REVIEW

To examine the putative measures of arterial stiffness and the mechanisms of adverse effects of stiffness on blood pressure and target organ damage using data from comprehensive hemodynamic profiles obtained in the Framingham Heart Study and the Age, Gene/Environment Susceptibility-Reykjavik Study.

RECENT FINDINGS

Once thought to be a consequence of longstanding hypertension, recent evidence suggests that aortic stiffness antedates and contributes to the pathogenesis of hypertension and target organ damage in the heart, brain, and kidneys. Carotid-femoral pulse-wave velocity (CFPWV) has emerged as the reference standard measure of aortic stiffness and a powerful predictor of cardiovascular disease risk. Augmentation index, a putative measure of arterial stiffness and wave reflection, has complex relations with stiffness and risk. Recent evidence suggests that wave reflection, which is a normal consequence of impedance mismatch between compliant aorta and stiff muscular arteries, is protective and limits the exposure of target organs to potentially harmful pulsatile energy. Aortic stiffening produces impedance matching that reduces wave reflection and exposes the microcirculation to excessive pulsatile stress, resulting in microvascular target organ damage and dysfunction.

SUMMARY

CFPWV provides a powerful new tool for risk stratification and elucidation of the pathogenesis of target organ damage in hypertension.

Determinants and covariates of central pressures and wave reflections in systolic heart failure

BACKGROUND

In general, higher blood pressure levels and increased central pulsatility are indicators for increased cardiovascular risk. However, in systolic heart failure (SHF), this relationship is reversed. Therefore, the aim of this work is to compare pulsatile hemodynamics between patients with SHF and controls and to clarify the relationships between measures of cardiac and arterial function in the two groups.

METHODS

We used parameters derived from angiography, echocardiography, as well as from pulse wave analysis (PWA) and wave separation analysis (WSA) based on non-invasively assessed pressure and flow waves to quantify cardiac function, aortic stiffness and arterial wave reflection in 61 patients with highly reduced (rEF) and 122 matched control-patients with normal ejection fraction (nEF).

RESULTS

Invasively measured pulse wave velocity was comparable between the groups (8.6/8.05 m/s rEF/nEF, P = 0.24), whereas all measures derived by PWA and WSA were significantly decreased (augmentation index: 18.1/24.8 rEF/nEF, P < 0.01; reflection magnitude: 56.3/62.1 rEF/nEF, P < 0.01). However, these differences could be explained by the shortened ejection duration (ED) in rEF (ED: 269/308 ms rEF/nEF, P < 0.01; AIx: 22.2/22.8 rEF/nEF, P = 0.7; RM: 59.3/60.6 rEF/nEF, P = 0.47 after adjustment for ED). Ventricular function was positively associated with central pulse pressures in SHF in contrast to no or even a slightly negative association in controls.

CONCLUSIONS

The results suggest that the decreased measures of pulsatile function may be caused by impaired systolic function and altered interplay of left ventricle and vascular system rather than by a real reduction of wave reflections or aortic stiffness in SHF.

Acute effect of high-intensity cycling exercise on carotid artery hemodynamic pulsatility

PURPOSE

Investigate the effects of acute high-intensity exercise on common carotid artery (CCA) dimensions, stiffness, and wave intensity.

METHODS

Fifty-five healthy men and women (22 ± 5 year; 24.5 ± 2.7 kg m(-2)) underwent 30 s of high-intensity cycling (HIC; Wingate anaerobic test). CCA diameter, stiffness [β-stiffness, Elastic Modulus (E p)], pulsatility index (PI), forward wave intensities [due to LV contraction (W 1) and LV suction (W 2)], and reflected wave intensity [negative area (NA)] were assessed using a combination of Doppler ultrasound, wave intensity analysis, and applanation tonometry at baseline and immediately post-HIC.

RESULTS

CCA β-stiffness, E p, PI and pulse pressure increased significantly immediately post-HIC (p < 0.05). CCA diameter decreased acutely post-HIC (p < 0.05). There were also significant increases in W 1 and NA and a significant decrease in W 2 (p < 0.05). A significant correlation was found between change in W 1 and PI (r = 0.438, p < 0.05), from rest to recovery as well as a significant inverse correlation between W 2 and PI (r = -0.378, p < 0.05). Change in PI was not associated with change in CCA stiffness or NA (p > 0.05).

CONCLUSIONS

Acute HIC results in CCA constriction and increases in CCA stiffness along with increases in hemodynamic pulsatility. The increase in pulsatility may be due to a combination of increased forward wave intensity from increased LV contractility into a smaller vessel (i.e. impaired matching of diameter and flow) coupled with reduced LV suction.

Indexes of aortic pressure augmentation markedly underestimate the contribution of reflected waves toward variations in aortic pressure and left ventricular mass

Although indexes of wave reflection enhance risk prediction, the extent to which measures of aortic systolic pressure augmentation (augmented pressures [Pa] or augmentation index) underestimate the effects of reflected waves on cardiovascular risk is uncertain. In participants from a community sample (age >16), we compared the relative contribution of reflected (backward wave pressures and the reflected wave index [RI]) versus augmented (Pa and augmentation index) pressure wave indexes to variations in central aortic pulse pressure (PPc; n=1185), and left ventricular mass index (LVMI; n=793). Aortic hemodynamics and LVMI were determined using radial applanation tonometry (SphygmoCor) and echocardiography. Independent of confounders, RI and backward wave pressures contributed more than forward wave pressures, whereas Pa and augmentation index contributed less than incident wave pressure to variations in PPc (P<0.0001 for comparison of partial r values). In those <50 years of age, while backward wave pressures (partial r=0.28, P<0.0001) contributed more than forward wave pressures (partial r=0.15, P<0.001; P<0.05 for comparison of r values), Pa (partial r=0.13, P<0.005) contributed to a similar extent as incident wave pressure (partial r=0.22, P<0.0001) to variations in LVMI. Furthermore, in those ≥50 years of age, backward wave pressures (partial r=0.21, P<0.0001), but not forward wave pressures (P=0.98), while incident wave pressure (partial r=0.23, P<0.0001), but not Pa (P=0.80) were associated with LVMI. Pa and augmentation index underestimated the effect of wave reflection on PPc and LVMI in both men and women. Thus, as compared with relations between indexes of aortic pressure augmentation and PPc or LVMI, strikingly better relations are noted between aortic wave reflection and PPc or LVMI.

Components of hemodynamic load and cardiovascular events: the Framingham Heart Study

BACKGROUND

Elevated blood pressure is the leading modifiable risk factor for cardiovascular disease (CVD) and premature death. The blood pressure waveform consists of discrete hemodynamic components, derived from measured central pressure and flow, which may contribute separately to risk for an adverse outcome. However, pressure-flow measures have not been studied in a large, community-based sample.

METHODS AND RESULTS

We used proportional hazards models to examine the association of incident CVD with forward pressure wave amplitude, mean arterial pressure, and global reflection coefficient derived from wave separation analysis and echocardiography in 2492 participants (mean age 66±9 years, 56% women) in the Framingham Heart Study. During follow-up (0.04-6.8 years), 149 participants (6%) had a CVD event. In multivariable models adjusting for age, sex, antihypertensive therapy, body mass index, heart rate, total and high-density lipoprotein cholesterol concentrations, smoking, and the presence of diabetes mellitus, forward pressure wave amplitude (hazard ratio, 1.40; 95% confidence interval, 1.16-1.67; P=0.0003) was associated with incident CVD, whereas mean arterial pressure (hazard ratio, 1.10; 95% confidence interval, 0.94-1.29; P=0.25) and global wave reflection (hazard ratio, 0.93; 95% confidence interval, 0.78-1.12; P=0.58) were not. After adding systolic blood pressure and carotid-femoral pulse wave velocity to the model, forward pressure wave amplitude persisted as a correlate of events (hazard ratio, 1.33; 95% confidence interval, 1.05-1.68; P=0.02).

CONCLUSIONS

Higher forward pressure wave amplitude (a measure of proximal aortic geometry and stiffness) was associated with increased risk for incident CVD, whereas mean arterial pressure and relative wave reflection (correlates of resistance vessel structure and function) were not associated with increased risk for incident CVD.

Forward and backward wave morphology and central pressure augmentation in men and women in the Framingham Heart Study

Central pressure augmentation is associated with greater backward wave amplitude and shorter transit time and is higher in women for reasons only partially elucidated. Augmentation also is affected by left ventricular function and shapes of the forward and backward waves. The goal of this study was to examine the relative contributions of forward and backward wave morphology to central pressure augmentation in men and women. From noninvasive measurements of central pressure and flow in 7437 participants (4036 women) aged from 19 to 90 years (mean age, 51 years), we calculated several variables: augmentation index, backward wave arrival time, reflection factor, forward wave amplitude, forward wave peak width, and slope of the backward wave upstroke. Linear regression models for augmentation index, adjusted for height and heart rate, demonstrated nonlinear relations with age (age: B=4.6±0.1%; P<0.001; age2: B=−4.2±0.1%; P<0.001) and higher augmentation in women (B=4.5±0.4%; P<0.001; model R2=0.35). Addition of reflection factor and backward wave arrival time improved model fit (R2=0.62) and reduced the age coefficients: age (B=2.3±0.1%; P<0.001) and age2 (B=−2.2±0.1%; P<0.001). Addition of width of forward wave peak, slope of backward wave upstroke, and forward wave amplitude further improved model fit (R2=0.75) and attenuated the sex coefficient (B=1.9±0.2%; P<0.001). Thus, shape and amplitude of the forward wave may be important correlates of augmentation index, and part of the sex difference in augmentation index may be explained by forward and backward wave morphology.