Personalized physiologic flow waveforms improve wave reflection estimates compared to triangular flow waveforms in adults

Endothelial dysfunction influences augmented aortic hemodynamic responses to metaboreflex activation in postmenopausal women

PURPOSE

Postmenopausal women experience augmented aortic hemodynamic responses to isometric handgrip (IHG) exercise and metaboreflex activation post-exercise muscle ischemia (PEMI). Relationships between endothelial function brachial artery flow-mediated dilation (FMD) and aortic stiffness carotid-femoral pulse wave velocity (cfPWV) with aortic pulsatile hemodynamics during IHG and PEMI have not been determined. The relationships between aortic hemodynamic responses to PEMI were evaluated.

METHODS

Aortic blood pressure (BP), wave reflection, and pressure of forward (Pf) and backward (Pb) waves were measured using arterial tonometry at rest, IHG at 30% maximal force, and PEMI in 30 (15/group) postmenopausal women with low (≤ 4.5%) and normal (≥ 5.5%) FMD. Hemodynamic responses were analyzed as the change (Δ) from rest to the last minute of IHG and PEMI.

RESULTS

Brachial and aortic systolic BP (SBP) responses to IHG were higher in the low vs normal FMD group (P < 0.05). Aortic SBP (Δ20 ± 8 vs Δ11 ± 7 mmHg), pulse pressure (PP) (Δ12 ± 8 vs Δ6 ± 4 mmHg), augmented pressure (AP) (Δ5 ± 3 vs Δ2 ± 2 mmHg), and Pb (Δ6 ± 4 vs Δ3 ± 2 mmHg) responses to PEMI were greater (P < 0.05) in women with low vs. normal FMD. FMD was negatively correlated with aortic SBP, PP, AP, and Pb (P < 0.05) responses to PEMI. cfPWV was not correlated with responses to PEMI.

CONCLUSION

Endothelial dysfunction relates to augmented aortic pulsatile load during metaboreflex activation, which may increase cardiovascular risk in postmenopausal women.

Superiority of a Representative MRI Flow Waveform over Doppler Ultrasound for Aortic Wave Reflection Assessment in Children and Adolescents With/Without a History of Heart Disease

Wave separation analysis (WSA) reveals the impact of forward- and backward-running waves on the arterial pressure pulse, but the calculations require a flow waveform. This study investigated (1) the variability of the ascending aortic flow waveform in children and adolescents with/without a childhood heart disease history (CHD); (2) the accuracy of WSA obtained with a representative flow waveform (RepFlow), compared with the triangulation method and published ultrasound-derived adult representative flow; (3) the impact of limitations in Doppler ultrasound on WSA; and (4) generalizability of results to adults with a history of CHD. Phase contrast MRI was performed in youth without (n = 45, Group 1, 10-19 years) and with CHD (n = 79, Group 2, 7-18 years), and adults with CHD history (n = 29, Group 3, 19-59 years). Segmented aortic cross-sectional area was used as a surrogate for the central pressure waveform in WSA. A subject-specific virtual Doppler ultrasound was performed on MRI data by extracting velocities from a sample volume. Time/amplitude-normalized ascending aortic flow waveforms were highly consistent amongst all groups. WSA with RepFlow therefore yielded errors < 10% in all groups for reflected wave magnitude and return time. Absolute errors were typically 1.5-3 times greater with other methods, including subject-specific (best-case/virtual) Doppler ultrasound, for which velocity profile skewing introduced waveform errors. Our data suggest that RepFlow is the optimal approach for pressure-only WSA in children and adolescents with/without CHD, as well as adults with CHD history, and may even be more accurate than subject-specific Doppler ultrasound in the ascending aorta.

Aortic pressure-only wave separation analysis in adolescents: accuracy and associations with left ventricular mass index

Early-life exposure to high blood pressure (BP) is associated with cardiovascular target organ damage but not all BP-related risk is attributable to systolic and diastolic BP alone. In adolescence, aortic wave separation (WS) parameters are associated with increased left ventricular mass index (LVMI) but this approach is limited by the requirement for aortic flow measurements. Several methods for estimating the aortic flow waveform from pressure waveforms have emerged, but their accuracy and associations with LVMI have never been tested in adolescents, which was the aim of our study. Carotid pressure waveforms were acquired by tonometry from 58 adolescents (age 16 ± 1.5 years, 59% female). Measured (aortic) flow and LVMI were acquired via 2D echocardiography. Three pressure-only approximations of aortic flow were synthesized, including triangular, excess, and individualized-physiologic flow. A 4th aortic flow (average flow) was approximated from the average of all 58 measured flow waveforms. Forward (Pf) and backward (Pb) pressure and reflection magnitude (Rm) were derived from WS analysis. The individualized-physiologic flow produced the best approximations of Pf (mean difference ± SD, -0.15 ± 2.38 mmHg), Pb (0.14 ± 0.25 mmHg), and Rm (0.01 ± 0.02 mmHg). Pf derived using measured, individualized-physiologic, and average flow, was similarly associated with LVMI adjusting for age, brachial systolic BP, cardiac output, and BMI (P ≤ 0.03 all). Pb derived using all flow waveforms was associated with LVMI and all associations yielded similar effect estimates. Of the estimated flow waveforms, individualized-physiologic flow yielded the best approximation of WS parameters and may provide important physiological and clinical insight among adolescents.

Relation of forward and backward traveling pressure waves with subclinical carotid artery wall remodeling and central pulse pressure

Central pulse pressure (PP) is the sum of forward and backward traveling pressure waves that have been associated with cardiovascular disease (CVD) risk. However, previous studies have reported differential findings regarding the importance of the forward versus the backward wave for CVD risk. Therefore, we sought to determine the degree to which the forward and backward pressure waves are associated with subclinical carotid artery wall remodeling and central PP in healthy adults. Using applanation tonometry, carotid pressure waveforms were acquired in 308 healthy individuals (aged 45 ± 17 years, range 19-80 years, 61% women), from which the time integral of the forward (PfTI) and backward (PbTI) pressure waves were derived via pressure-only wave separation analysis. Common carotid artery intima-media thickness (cIMT), a biomarker of subclinical CVD risk, was derived via B-mode ultrasonography measured ∼2 cm from the carotid bulb. Both PfTI (r = 0.31, P < 0.001) and PbTI (r = 0.40, P < 0.001) were correlated with cIMT. However, further analysis revealed that PbTI mediated the relation between PfTI and cIMT (proportion mediated = 156%, P < 0.001). The association between PbTI and cIMT remained after adjusting for age, sex, body mass index, blood glucose, low-density lipoprotein cholesterol, heart rate, brachial systolic pressure, and aortic stiffness (B = 0.02, 95% confidence interval = 0.01, 2.77, P < 0.001). Both PfTI (r = -0.58, P < 0.001) and PbTI (r = -0.50, P < 0.001) were correlated with central PP, however, PfTI fully mediated the association between PbTI and central PP (proportion mediated = 124%, P < 0.001). Although PfTI is correlated with higher central PP, it is PbTI that is directly associated with carotid artery wall remodeling.NEW & NOTEWORTHY The present study contributes to the growing body of evidence highlighting the physiological and clinical insight provided by the pulsatile hemodynamic components of central artery pulse pressure. The notable findings of this study are: 1) The reflected (backward) pressure wave is associated with carotid intima-media thickness independent of traditional cardiovascular risk factors, including systolic blood pressure and aortic stiffness. 2) The incident (forward) pressure wave, and not the reflected pressure wave, is associated with greater central pulse pressure.

Effect of acute handgrip and aerobic exercise on wasted pressure effort and arterial wave reflections in healthy aging

Aging increases arterial stiffness and wave reflections that augment left ventricular wasted pressure effort (WPE). A single bout of exercise may be effective at acutely reducing WPE via reductions in arterial wave reflections. In young adults (YA) acute aerobic exercise decreases, whereas handgrip increases, wave reflections. Whether acute exercise mitigates or exacerbates WPE and arterial wave reflection in healthy aging warrants further examination. The purpose of this study was to determine if there are age-related differences in WPE and wave reflection during acute handgrip and aerobic exercise. When compared with baseline, WPE increased substantially in older adults (OA) during handgrip (5,219 ± 2,396 vs. 7,019 ± 2,888 mmHg·ms, P < 0.001). When compared with baseline, there was a robust reduction in WPE in OA during moderate-intensity aerobic exercise (5,428 ± 2,084 vs. 3,290 ± 1,537 mmHg·ms, P < 0.001), despite absolute WPE remaining higher in OA compared with YA during moderate-intensity aerobic exercise (OA 3,290 ± 1,537 vs. YA 1,188 ± 962 mmHg·ms, P < 0.001). There was no change in wave reflection timing indexed to ejection duration in OA during handgrip (40 ± 6 vs. 38 ± 4%, P = 0.41) or moderate-intensity aerobic exercise (40 ± 5 vs. 42 ± 8%, P = 0.99). Conversely, there was an earlier return of wave reflection in YA during handgrip (60 ± 11 vs. 52 ± 6%, P < 0.001) and moderate-intensity aerobic exercise (59 ± 7 vs. 51 ± 9%, P < 0.001). Changes in stroke volume were not different between groups during handgrip (P = 0.08) or aerobic exercise (P = 0.47). The greater increase in WPE during handgrip and decrease in WPE during aerobic exercise suggest that aortic hemodynamic responses to acute exercise are exaggerated with healthy aging without affecting stroke volume.NEW & NOTEWORTHY We demonstrated that acute aerobic exercise attenuated, whereas handgrip augmented, left ventricular hemodynamic load from wave reflections more in healthy older (OA) compared with young adults (YA) without altering stroke volume. These findings suggest an exaggerated aortic hemodynamic response to acute exercise perturbations with aging. They also highlight the importance of considering exercise modality when examining aortic hemodynamic responses to acute exercise in older adults.

A fast computational model for circulatory dynamics: effects of left ventricle-aorta coupling

The course of diseases such as hypertension, systolic heart failure and heart failure with a preserved ejection fraction is affected by interactions between the left ventricle (LV) and the vasculature. To study these interactions, a computationally efficient, biophysically based mathematical model for the circulatory system is presented. In a four-chamber model of the heart, the LV is represented by a previously described low-order, wall volume-preserving model that includes torsion and base-to-apex and circumferential wall shortening and lengthening, and the other chambers are represented using spherical geometries. Active and passive myocardial mechanics of all four chambers are included. The cardiac model is coupled with a wave propagation model for the aorta and a closed lumped-parameter circulation model. Parameters for the normal heart and aorta are determined by fitting to experimental data. Changes in the timing and magnitude of pulse wave reflections by the aorta are demonstrated with changes in compliance and taper of the aorta as seen in aging (decreased compliance, increased diameter and length), and resulting effects on LV pressure-volume loops and LV fiber stress and sarcomere shortening are predicted. Effects of aging of the aorta combined with reduced LV contractile force (failing heart) are examined. In the failing heart, changes in aortic properties with aging affect stroke volume and sarcomere shortening without appreciable augmentation of aortic pressure, and the reflected pressure wave contributes an increased proportion of aortic pressure.

Wave reflection quantification analysis and personalized flow wave estimation based on the central aortic pressure waveform

Pulse wave reflections reflect cardiac afterload and perfusion, which yield valid indicators for monitoring cardiovascular status. Accurate quantification of pressure wave reflections requires the measurement of aortic flow wave. However, direct flow measurement involves extra equipment and well-trained operator. In this study, the personalized aortic flow waveform was estimated from the individual central aortic pressure waveform (CAPW) based on pressure-flow relations. The separated forward and backward pressure waves were used to calculate wave reflection indices such as reflection index (RI) and reflection magnitude (RM), as well as the central aortic pulse transit time (PTT). The effectiveness and feasibility of the method were validated by a set of clinical data (13 participants) and the Nektar1D Pulse Wave Database (4,374 subjects). The performance of the proposed personalized flow waveform method was compared with the traditional triangular flow waveform method and the recently proposed lognormal flow waveform method by statistical analyses. Results show that the root mean square error calculated by the personalized flow waveform approach is smaller than that of the typical triangular and lognormal flow methods, and the correlation coefficient with the measured flow waveform is higher. The estimated personalized flow waveform based on the characteristics of the CAPW can estimate wave reflection indices more accurately than the other two methods. The proposed personalized flow waveform method can be potentially used as a convenient alternative for the measurement of aortic flow waveform.

Sex differences in microvascular function and arterial hemodynamics in nondialysis chronic kidney disease

Cardiovascular disease (CVD) is the leading cause of death in chronic kidney disease (CKD). Abnormal arterial hemodynamics contribute to CVD, a relationship that can be mediated by microvascular dysfunction. The purpose of this study was to investigate potential sex differences in arterial hemodynamics and microvascular dysfunction in patients with stages 3 to 4 CKD. Vascular function was assessed in 22 male (mean ± SD; age, 56 ± 13 yr) and 10 female (age, 63 ± 9 yr) patients. Arterial hemodynamics were acquired with combined tonometry and oscillometry. Skin blood flow was used as a model of microvascular function. Participants were instrumented with three microdialysis fibers for the delivery of 1) Ringer's solution; 2) superoxide dismutase mimetic, Tempol; and 3) nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor, apocynin. Blood flow was measured via laser-Doppler flowmetry during standardized local heating (42°C). Central pulse pressure (mean ± SE; 62 ± 9 vs. 46 ± 3 mmHg; P = 0.01) and augmentation index (36 ± 3 vs. 26 ± 3%; P = 0.03) were higher in females. There was a trend for higher central systolic pressures in females (146 ± 9 vs. 131 ± 3 mmHg; P = 0.06). Females reported higher forward (39 ± 4 vs. 29 ± 2 mmHg; P = 0.004) and reflected (27 ± 3 vs. 19 ± 1 mmHg; P < 0.001) wave amplitudes. Cutaneous vascular function was impaired in females compared with males (77 ± 3 vs. 89 ± 1%, P = 0.001). Microvascular function was improved following the delivery of Tempol and apocynin in females but not in males. Female patients with CKD had poorer central hemodynamics and reduced microvascular function compared with their male counterparts. Oxidative stress may contribute to lower microvascular function observed in females.NEW & NOTEWORTHY There are limited data regarding the physiological mechanisms of potential sex differences in central hemodynamics and vascular function in chronic kidney disease (CKD). We report that older female patients with nondialysis CKD have higher central pulse pressures compared with male patients with CKD. In addition, older females with CKD have lower microvascular function compared with their male counterparts, and oxidative stress contributes to the lower microvascular function in older female patients with CKD.

Arterial pressure pulse wave separation analysis using a multi-gaussian decomposition model

OBJECTIVE

Methods for separating the forward-backward components from blood pulse waves rely on simultaneously measured pressure and flow velocity from a target artery site. Modelling approaches for flow velocity simplify the wave separation analysis (WSA), providing a methodological and instrumentational advantage over the former; however, current methods are limited to the aortic site. In this work, a multi-Gaussian decomposition (MGD) modelled WSA (MGDWSA) is developed for a non-aortic site asuch as the carotid artery. While the model is an adaptation of the existing wave separation theory, it does not rely on the information of measured or modelled flow velocity.

APPROACH

The proposed model decomposes the arterial pressure waveform using weighted and shifted multi-Gaussians, which are then uniquely combined to yield the forward (PF(t)) and backward (PB(t)) pressure wave. A study using the database of healthy (virtual) subjects was used to evaluate the performance of MGDWSA at the carotid artery and was compared against reference flow-based WSA methods.

MAIN RESULTS

The MGD modelled pressure waveform yielded a root-mean-square error (RMSE) < 0.35 mmHg. Reliable forward-backward components with a group average RMSE < 2.5 mmHg for PF(t) and PB(t) were obtained. When compared with the reference counterparts, the pulse pressures (ΔPF and ΔPB), as well as reflection quantification indices, showed a statistically significant strong correlation (r > 0.96, p < 0.0001) and (r > 0.83, p < 0.0001) respectively, with an insignificant (p > 0.05) bias.

SIGNIFICANCE

This study reports WSA for carotid pressure waveforms without assumptions on flow conditions. The proposed method has the potential to adapt and widen the vascular health assessment techniques incorporating pulse wave dynamics.

L-Citrulline supplementation attenuates aortic pulse pressure and wave reflection responses to cold stress in older adults

BACKGROUND AND AIMS

Augmented aortic systolic blood pressure (SBP) and wave reflection via sympathetic-mediated vasoconstriction elevates the risk for adverse cardiovascular events in older adults. L-citrulline (L-CIT) supplementation has shown to reduce aortic SBP and pulse pressure (PP) responses to cold pressor test (CPT) induced sympathoactivation in young men. The aim of this study was to elucidate the efficacy of L-CIT supplementation to attenuate aortic hemodynamic responses to CPT in older adults.

METHODS AND RESULTS

Sixteen older adults were randomly assigned to placebo or L-CIT (6 g/day) for 14-days in a crossover, double-blind, placebo-controlled design. Brachial SBP and aortic SBP, PP, augmented pressure (AP), augmentation index standardized at 75 bpm (AIx@75), and pressure of the forward (Pf) and reflected (Pb) waves were evaluated at rest and during CPT pre- and post-intervention. Although no hemodynamic changes at rest, brachial SBP (Δ-12 ± 18 vs. Δ4 ± 14 mmHg; P = 0.008) and aortic SBP (Δ-10 ± 14 vs. Δ4 ± 12 mmHg; P = 0.005), PP (Δ-10 ± 12 vs. Δ4 ± 11 mmHg; P = 0.002), AP (Δ-4 ± 4 vs. Δ2 ± 7 mmHg; P = 0.004), AIx@75 (Δ-3.2 ± 7.2 vs. Δ2.2 ± 6.9%; P = 0.038), Pf (Δ-6 ± 10 vs. Δ3 ± 9 mmHg; P = 0.019), and Pb (Δ-4 ± 6 vs. Δ2 ± 6 mmHg; P = 0.008) responses to the CPT were significantly attenuated following L-CIT supplementation vs. placebo.

CONCLUSIONS

L-CIT supplementation attenuated aortic pulsatile pressure and pressure wave reflection responses to CPT in older adults, providing possible cardioprotection during cold-induced sympathoactivation in older adults.

Lower-body dynamic exercise reduces wave reflection in healthy young adults

NEW FINDINGS

What is the central question of this study? There is a paradoxical reduction in augmentation index during lower-body dynamic (LBD) exercise in the face of an increase in central pressure. To determine causality, the amplitudes of forward and backward pressure waves were assessed separately using wave separation analysis. What is the main finding and its importance? Reflection magnitude decreased during LBD exercise in healthy young adults and was attributable to an increased forward pressure wave amplitude and decreased backward pressure wave amplitude. This vasoactive response might limit the adverse effects of wave reflection during LBD exercise, optimizing ventricular-arterial interactions.

ABSTRACT

Acute lower-body dynamic (LBD) exercise decreases surrogate measures of wave reflection, such as the augmentation index. However, the augmentation index is influenced by the combined effects of wave reflection timing, magnitude and other confounding factors external to wave reflection, which make it difficult to discern the origin of changes in surrogate measures. The relative contributions of forward (Pf) and backward (Pb) pressure wave amplitudes to central pressure can be determined by wave separation analysis. Reflection magnitude (RM = Pb/Pf) and the timing of apparent wave reflection return can also be determined. We tested the hypothesis that acute LBD exercise decreases RM and reflected wave transit time (RWTT). Applanation tonometry was used to record radial artery pressure waveforms in 25 adults (24 ± 4 years of age) at baseline and during light-, moderate- and vigorous-intensity exercise. Wave separation analysis was conducted offline using a personalized physiological flow wave to determine Pf, Pb, RM and RWTT. The RM decreased during all intensities of exercise compared with baseline (all P < 0.001; baseline, 43 ± 5%; light, 33 ± 6%; moderate, 23 ± 7%; vigorous, 17 ± 5%). The reduction in RM was attributable to the combined effect of increased Pf and decreased Pb during exercise. The RWTT decreased during all intensities of exercise compared with baseline (all P < 0.04; baseline, 156 ± 17 ms; light, 144 ± 15 ms; moderate, 129 ± 16 ms; vigorous, 121 ± 17 ms). Lastly, in a stepwise multilinear regression, Pf, but not Pb and RWTT, contributed to increased central pulse pressure during LBD exercise. These data show that wave reflection decreased and that central pulse pressure is most influenced by Pf during LBD exercise.