Robust and practical non-invasive estimation of local arterial wave speed and mean blood velocity waveforms

Carotid wave analysis in young adults with a history of adolescent anorexia nervosa: a case control study

BACKGROUND

Anorexia nervosa (AN) is associated with abnormalities that may increase the risk of future cardiovascular disease. This study assessed the cardiovascular health of individuals who recovered from AN during adolescence by conducting wave power analysis.

METHODS

Former AN patients discharged from the Royal Children's and Monash Children's Hospitals (N = 17) in Melbourne, Australia underwent ultrasound imaging of the right carotid artery. Wave power analysis was conducted to assess biomechanical interactions of the cardiovascular system. Patient measures were compared to healthy controls (N = 51).

RESULTS

Eighty-eight percent of the former AN patients and controls were female, aged approximately 25 years, with a healthy body mass index. Mean carotid flow and pulsatility index were not different between groups. Carotid arterial strain and distensibility were lower, and the wave speed and beta stiffness index higher in the former AN patients. Characteristic impedance was not different nor were the forward and backward wave amplitudes. However, wave reflection indices (ratios of backward-to-forward compression wave area, and wave-related effect on pressure and hydraulic power) were 12-18% lower in the former AN patients (p < 0.05).

CONCLUSIONS

Increased carotid artery stiffness and reduced wave reflection are evident in young adults who recovered from adolescent AN. This may relate to an adaptive process that helps to maintain or restore flow and characteristic impedance despite increased vessel stiffness, with this warranting future investigation.

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.

Non-invasive Assessment by B-Mode Ultrasound of Arterial Pulse Wave Intensity and Its Reduction During Ventricular Dysfunction

Arterial pulse waves contain clinically useful information about cardiac performance, arterial stiffness and vessel tone. Here we describe a novel method for non-invasively assessing wave properties, based on measuring changes in blood flow velocity and arterial wall diameter during the cardiac cycle. Velocity and diameter were determined by tracking speckles in successive B-mode images acquired with an ultrafast scanner and plane-wave transmission. Blood speckle was separated from tissue by singular value decomposition and processed to correct biases in ultrasound imaging velocimetry. Results obtained in the rabbit aorta were compared with a conventional analysis based on blood velocity and pressure, employing measurements obtained with a clinical intra-arterial catheter system. This system had a poorer frequency response and greater lags but the pattern of net forward-traveling and backward-traveling waves was consistent between the two methods. Errors in wave speed were also similar in magnitude, and comparable reductions in wave intensity and delays in wave arrival were detected during ventricular dysfunction. The non-invasive method was applied to the carotid artery of a healthy human participant and gave a wave speed and patterns of wave intensity consistent with earlier measurements. The new system may have clinical utility in screening for heart failure.

Measurement, Analysis and Interpretation of Pressure/Flow Waves in Blood Vessels

The optimal performance of the cardiovascular system, as well as the break-down of this performance with disease, both involve complex biomechanical interactions between the heart, conduit vascular networks and microvascular beds. ‘Wave analysis’ refers to a group of techniques that provide valuable insight into these interactions by scrutinizing the shape of blood pressure and flow/velocity waveforms. The aim of this review paper is to provide a comprehensive introduction to wave analysis, with a focus on key concepts and practical application rather than mathematical derivations. We begin with an overview of invasive and non-invasive measurement techniques that can be used to obtain the signals required for wave analysis. We then review the most widely used wave analysis techniques—pulse wave analysis, wave separation and wave intensity analysis—and associated methods for estimating local wave speed or characteristic impedance that are required for decomposing waveforms into forward and backward wave components. This is followed by a discussion of the biomechanical phenomena that generate waves and the processes that modulate wave amplitude, both of which are critical for interpreting measured wave patterns. Finally, we provide a brief update on several emerging techniques/concepts in the wave analysis field, namely wave potential and the reservoir-excess pressure approach.

How to Measure Arterial Stiffness in Humans

Despite the wide recognition of larger artery stiffness as a highly clinically relevant and independent prognostic biomarker, it has yet be incorporated into routine clinical practice and to take a more prominent position in clinical guidelines. An important reason may be the plethora of methods and devices claiming to measure arterial stiffness in humans. This brief review provides a concise overview of methods in use, indicating strengths and weaknesses. We classified and graded methods, highly weighing their scrutiny and purity in quantifying arterial stiffness, rather than focusing on their ease of application or the level at which methods have demonstrated their prognostic and diagnostic potential.

Mapping the site-specific accuracy of loop-based local pulse wave velocity estimation and reflection magnitude: a 1D arterial network model analysis

OBJECTIVE

Local pulse wave velocity (PWV) can be estimated from the waterhammer equation and is an essential component of wave separation analysis. However, previous studies have demonstrated inaccuracies in the estimations of local PWV due to the presence of reflections. In this study we compared the estimates of local PWV from the PU-loop, ln(D)U-loop, QA-loop and ln(D)P-loop methods along the complete human arterial tree, and analyzed the impact of the estimations on subsequent wave separation analysis.

APPROACH

Estimated values were derived from the numerical outputs (pressure, flow, flow velocity, area and diameter waveforms) of a 1D model of the human circulation, and compared against a reference PWV obtained from the Bramwell-Hill equation in a reference configuration, and in a configuration with lower distensibility representing ageing.

MAIN RESULTS

When including all nodes, the overall performance of the methods was poor (correlations and mean differences of R ²  <  0.4 and 3.0  ±  4.1 m s^-1 for the PU-loop, R ²  <  0.07 and  -0.7  ±  2.3 m s^-1 for the ln(D)U-loop, and R ²  <  0.06 and  -0.4  ±  2.3 m s^-1 for the QA-loop). Focusing on specific sites, the ln(D)U- and QA-loop methods yielded acceptable results in the thoracic aorta and iliac arteries, while the PU-loop method was acceptable at the aortic arch. The reflection-insensitive ln(D)P-loop method performed well over the complete network (R ²  =  0.9 and 0.3  ±  0.3 m s^-1), as did a previously proposed reflection-correction method for most vascular sites. Large errors in PWV estimation are attenuated in subsequent wave separation analysis, but the errors are site-dependent.

SIGNIFICANCE

We conclude that the performances of the PU-loop, ln(D)U-loop and QA-loop methods are highly site-specific. The results should be interpreted with caution at all times.

Increased aortic wave reflection contributes to higher systolic blood pressure in adolescents born preterm

OBJECTIVE

To evaluate the wave reflection characteristics in the aortic arch and common carotid artery of ex-preterm adolescents and assess their relationship to central blood pressure in a cohort followed prospectively since birth.

METHODS

Central blood pressures, pulse wave velocity, augmentation index, microvascular reactive hyperemia, arterial distensibility, compliance and stiffness index, and also aortic and carotid wave intensity were measured in 18-year-olds born extremely preterm at below 28 weeks' gestation (n = 76) and term-born controls (n = 42).

RESULTS

Compared with controls, ex-preterm adolescents had higher central systolic (111 ± 11 vs. 105 ± 10 mmHg; P < 0.001) and diastolic blood pressures (73 ± 7 vs. 67 ± 7 mmHg; P < 0.001). Although conventional measures of arterial function and biomechanics such as pulse wave velocity and augmentation index were no different between groups, wave intensity analysis revealed elevated backward compression wave area (-0.39 ± 0.21 vs. -0.29 ± 0.17 W/m/s × 10; P = 0.03), backward compression wave pressure change (9.0 ± 3.5 vs. 6.6 ± 2.5 mmHg; P = 0.001) and reflection index (0.44 ± 0.15 vs. 0.32 ± 0.08; P < 0.001) in the aorta of ex-preterm adolescents compared with controls. These changes were less pronounced in the carotid artery. On multivariable analysis, forward and backward compression wave areas were the only biomechanical variables associated with central systolic pressure.

CONCLUSIONS

Ex-preterm adolescents demonstrate elevated wave reflection indices in the aortic arch, which correlate with central systolic pressure. Wave intensity analysis may provide a sensitive novel marker of evolving vascular dysfunction in ex-preterm survivors.

Reduced Aortic Distensibility is Associated With Higher Aorto-Carotid Wave Transmission and Central Aortic Systolic Pressure in Young Adults After Coarctation Repair

Background The long-term prognosis of patients with repaired aortic coarctation is characterized by high rates of cardiovascular and cerebrovascular disease related to hypertension, the basis of which remains unclear. To define potential underlying mechanisms, we investigated aortic and carotid arterial biomechanics and wave dynamics, and determinants of aortic systolic blood pressure, in young adults after coarctation repair. Methods and Results Aortic arch and carotid biomechanics, wave intensity and wave power, and central aortic blood pressure, were derived from echocardiography and brachial blood pressure in 43 young adults after coarctation repair and 42 controls. Coarctation subjects had higher brachial and central systolic blood pressure ( P=0.04), while aortic compliance was lower and characteristic impedance (Zc) higher. Although carotid intima-media thickness was higher ( P<0.001), carotid biomechanics were no different. Carotid forward compression wave power was higher and was negatively correlated with aortic compliance ( R2=0.42, P<0.001) and distensibility ( R2=0.37, P=0.001) in coarctation subjects. Aortic wave power and wave reflection indices were no different in control and coarctation patients, but coarctation patients with elevated aortic Zc had greater aorto-carotid transmission of forward compression wave power ( P=0.006). Aortic distensibility was the only independent predictor of central aortic systolic blood pressure on multivariable analysis. Conclusions Young adults following coarctation repair had a less compliant aorta, but no change in carotid biomechanics. Reduced aortic distensibility was related to greater transmission of aortic forward wave energy into the carotid artery and higher central aortic systolic blood pressure. These findings suggest that reduced aortic distensibility may contribute to later cardiovascular and cerebrovascular disease after coarctation repair.

Comparison of invasive and non-invasive aortic wave intensity and wave power analyses in sheep

OBJECTIVE

Wave intensity (WI) and wave power (WP) analyses are powerful approaches for assessing ventricular-vascular interactions and arterial dynamics using invasive and non-invasive methods. However, in vivo comparison of these methods for large arteries is lacking. This study assessed agreement, correlation and relative changes in wave size in invasive and non-invasive aortic WI/WP analyses, and associated sources of error.

APPROACH

The proximal descending thoracic aorta (DTA) of nine wethers was instrumented with a micromanometer and perivascular transit-time flow probe to measure high-fidelity blood pressure (P) and flow (Q) for invasive WI/WP analyses at baseline and during haemodynamic perturbations produced by cardiac pacing, distal DTA constriction and dobutamine-induced inotropic stimulation. In 212 experimental runs, concurrent echocardiographic DTA diameter and velocity (U) data were acquired for non-invasive WI/WP analyses, with measurement of forward compression wave (FCW), backward compression wave (BCW) and forward decompression wave (FDW) cumulative intensity (CI), cumulative power (CP) and wave-related pressure changes (ΔP).

MAIN RESULTS

Although agreement between invasive and non-invasive FCW, BCW and FDW CI/CP measures was variable (bias  -84% to  +7%), correlation was good (R  =  0.66-0.84), with lower bias and higher correlation for ΔP variables and similar relative changes in FCW and BCW CI/ΔP during haemodynamic perturbations. Main error sources were overestimation of invasive U due to assumed fixed vessel diameter, inaccuracies in non-invasive Q, and non-invasive underestimation of peak P/U and Q rates of change.

SIGNIFICANCE

Despite variable agreement, non-invasive CI/CP indices correlate well with invasive measurements, and detect relative changes in major waves induced by haemodynamic perturbations.

Effects of carotid pressure waveforms on the results of wave separation, wave intensity and reservoir pressure analysis

OBJECTIVE

Recently great attention has been paid to innovative cardiovascular biomarkers obtained from wave separation (WS), wave intensity (WI) and reservoir-wave (RW) theories. All these approaches share a requirement for pressure information. The aim of this study was to evaluate differences in WS-, WI- and RW-derived parameters obtained achieving pressure waveforms in different ways.

APPROACH

Twenty-two individuals (49  ±  17 years, 59% males) were examined. Common carotid blood flow waveforms were obtained from pulsed-wave Doppler images. Carotid pressure waveforms were achieved in four different ways: (1) with applanation tonometry, used as a reference method; (2) linear scaling from an ultrasound (US)-derived diameter curve; (3) exponential scaling from a US-derived diameter curve; and (4) linear scaling from an accelerometric-derived diameter signal. For each case, the reflection magnitude (RM) and index (RI) were obtained from the WS. The amplitude of the first positive peak (W ₁), of the second positive peak (W ₂) and of the negative peak (W _b) were calculated from the WI, while the maximum of the reservoir (maxP_r) and the excess (maxP_ex) pressure were achieved from the RW.

MAIN RESULTS

According to the intra-class coefficient values, the agreement between the standard method and all the others was excellent for the RM (linear: 0.82; exponential: 0.83; accelerometric: 0.86), RI (linear: 0.84; exponential: 0.85; accelerometric: 0.87), maxP_r (linear: 0.97; exponential: 0.96; accelerometric: 0.97) and maxP_ex (linear: 0.85; exponential: 0.87; accelerometric: 0.89), while only a fair/good level was reached for W ₁ (linear: 0.67; exponential: 0.77; accelerometric: 0.52), W ₂ (linear: 0.52; exponential: 0.69; accelerometric: 0.83) and W _b (linear: 0.60; exponential: 0.44; accelerometric: 0.50).

SIGNIFICANCE

Measuring carotid pressure waveforms with different approaches does not influence the cardiovascular parameters obtained by WS and RW; those derived by WI are affected by the carotid pressure curve employed.