Maximum Velocity Waveforms Research Articles

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

Objective: Local arterial wave speed, a surrogate of vessel stiffness, can be estimated via the pressure-velocity (PU) and diameter-velocity (ln(D)U) loop methods. These assume negligible early-systolic reflected waves (RWes) and require measurement of cross-sectionally averaged velocity (Umean), which is related to volumetric blood flow. However, RWes may not always be negligible and Doppler ultrasound typically provides maximum velocity waveforms or estimates of mean velocity subject to various errors (Uraw). This study investigates how these issues affect wave speed estimation and explores more robust methods for obtaining local wave speed and Umean. Approach: Using aortic phase-contrast MRI (PCMRI, n = 34) and a simulated virtual cohort (n = 3325), we assessed errors in calculated wave speed caused by RWes and use of Uraw rather than true Umean. By combining PUraw and ln(D)Uraw loop wave speed values, (i) a corrected wave speed (ln(D)P), insensitive to RWes and velocity errors, was derived; and (ii) a novel method for estimating Umean from Uraw was proposed (where Uraw can be any scaled version of Umean). Main results: Proof-of-principle was established via PCMRI data and in the ascending aorta, carotid, brachial and femoral arteries of the virtual cohort, with acceptably low wave speed and Umean errors obtained even when local pressure was estimated from diameter and mean/diastolic brachial pressures. Significance: Given a locally measured diameter waveform and brachial cuff pressures, (i) the velocity- and RWes-independent ln(D)P method can be applied non-invasively and is likely more robust than ln(D)U and PU loop methods; and (ii) Umean can be estimated from routinely-acquired Uraw.

Investigation of Ultrasound-Measured Flow Velocity, Flow Rate and Wall Shear Rate in Radial and Ulnar Arteries Using Simulation

Parameters of blood flow measured by ultrasound in radial and ulnar arteries, such as flow velocity, flow rate and wall shear rate, are widely used in clinical practice and clinical research. Investigation of these measurements is useful for evaluating accuracy and providing knowledge of error sources. A method for simulating the spectral Doppler ultrasound measurement process was developed with computational fluid dynamics providing flow-field data. Specific scanning factors were adjusted to investigate their influence on estimation of the maximum velocity waveform, and flow rate and wall shear rate were derived using the Womersley equation. The overestimation in maximum velocity increases greatly (peak systolic from about 10% to 30%, time-averaged from about 30% to 50%) when the beam–vessel angle is changed from 30° to 70°. The Womersley equation was able to estimate flow rate in both arteries with less than 3% error, but performed better in the radial artery (2.3% overestimation) than the ulnar artery (15.4% underestimation) in estimating wall shear rate. It is concluded that measurements of flow parameters in the radial and ulnar arteries with clinical ultrasound scanners are prone to clinically significant errors.

Compensatory fuzzy neural networks-based intelligent detection of abnormal neonatal cerebral Doppler ultrasound waveforms.

Compensatory fuzzy neural networks (CFNN) without normalization, which can be trained with a backpropagation learning algorithm, is proposed as a pattern recognition technique for intelligent detection of Doppler ultrasound waveforms of abnormal neonatal cerebral hemodynamics. Doppler ultrasound signals were recorded from the anterior cerebral arteries of 40 normal full-term babies and 14 mature babies with intracranial pathology. The features of normal and abnormal groups as inputs to pattern recognition algorithms were extracted from the maximum velocity waveforms by using principal component analysis. The proposed technique is compared with the CFNN with normalization and other pattern recognition techniques applied to Doppler ultrasound signals from various arteries. The results show that the proposed method is superior to the others, and can be a powerful technique to be used in analyzing Doppler ultrasound signals from various arteries.

Nonlinear transcranial Doppler analysis demonstrates age-related changes of cerebral hemodynamics

We studied the age- and gender-related changes in complexity of arterial blood flow by analyzing the maximum velocity waveforms. Data were collected by insonation of the middle cerebral artery blood flow by a 2-MHz transcranial pulsed Doppler system. Following the paradigm of nonlinear dynamical systems or “chaos” theory, complexity is best evaluated by estimating the correlation dimension ( D 2) and the largest Lyapunov exponent (λ 1). Forty healthy persons (male/female ratio: 1 1 ; mean age 48.6 y; range 19–86 y) were studied. No gender-related differences were observed. The age-effect showed a diminishing λ 1 [ df(1, 36) = 5.687; p < 0.022] and an increasing D 2 at higher age [ df(1, 36) = 4.997; p < 0.032]. The age-related decline of the λ 1 implies a more prominent periodicity, explained by reduced fluctuations in R-R intervals and an altered gain of the baroreceptor reflex. The increased D 2 might be related to more prominent vessel wall oscillations due to the increased vessel wall stiffness at higher age.

Evaluation of cerebral blood flow in rabbits with transcranial doppler sonography: First results

A transcranial Doppler sonographic system with a special 2-MHz probe for prolonged experimentation has been developed for Doppler waveform recordings from the basilar artery on rabbits. Measurements were made from the maximum velocity wave form, and Pourcelot's resistance index was used to express the results. The diastolic flow, similar to that observed in humans in physiological conditions, decreases with increased intracranial pressure during chronic intracranial hypertension. This Doppler model may be valuable for assessing rapid changes of cerebral blood flow in conscious animals during prolonged or acute experimental procedures.

Cerebral Blood Flow During Labor In The Human Fetus

A method is described for recording blood flow velocity waveforms from fetal cerebral vessels during labor, using a 2 MHz pulsed Doppler ultrasound technique. Fifteen healthy women with uncomplicated pregnancies and labor without signs of fetal distress participated in the study. With membranes ruptured and cervix orifice open 4-9 cm, the Doppler transducer was placed transvaginally on the fetal skull and the Doppler shift signals from the middle cerebral artery were located. The maximum velocity waveforms were recorded before, during and after uterine contractions and analysed off-line for pulsatility index (PI). No differences in the PI were found with regard to the uterine contractions. The results suggest an unchanged peripheral resistance in the fetal cerebral vascular bed during the first stage of normal labor.

Doppler assessment of the fetal and uteroplacental circulation during nifedipine therapy for preterm labor

To investigate the effects of nifedipine on the human fetal circulation, 11 fetuses whose mothers were treated with nifedipine for treatment of preterm labor were studied. Maximum velocity waveforms were obtained in the middle cerebral artery (n = 8), renal artery (n = 6), ductus arteriosus (n = 8), and umbilical artery (n = 10). Transvalvular maximal velocity waveforms were obtained across the aortic (n = 11) and pulmonary (n = 7) valves. Maternal uterine arteries also were studied (n = 7). Doppler data were collected before and 5 hours after nifedipine therapy. Patients received an oral loading dose of 30 mg of nifedipine followed by a second oral dose of 20 mg 4 hours later. No significant difference in the flow velocity waveforms was found in the vessels studied 5 hours after the initial dose. These results suggest that short-term nifedipine therapy does not influence either fetal or uteroplacental circulation as evaluated with the Doppler technique.

Doppler assessment of the pulsatility index of the middle cerebral artery during constriction of the fetal ductus arteriosus after indomethacin therapy

To investigate the effects of constriction of the ductus arteriosus on the pulsatility index of the middle cerebral artery, maximum velocity waveforms were obtained in 13 fetuses (one set of twins) whose mothers were treated with indomethacin for preterm labor (n = 9) or polyhydramnios (n = 3). Eleven of the fetuses manifested ductal constriction within 48 hours of therapy, whereas two fetuses had constriction after 1 week of therapy. Six of the 13 fetuses also manifested tricuspid insufficiency in association with constriction of the ductus arteriosus. All abnormal cardiac changes resolved in utero after discontinuation of indomethacin. No difference in the pulsatility index values of the middle cerebral artery was observed in the fetuses with ductal constriction but without tricuspid regurgitation (n = 7) when the values were compared with those obtained in absence of ductal constriction (1.87 +/- 0.37 vs. 1.88 +/- 0.33). In the fetuses that manifested both ductal constriction and tricuspid insufficiency (n = 6), the pulsatility index values in the middle cerebral artery were significantly lower in the presence of ductal constriction when compared with the values obtained in the absence of ductal constriction (2.22 +/- 0.26 vs. 1.57 +/- 0.34). These results indicate that a response to indomethacin sufficient to cause both ductal constriction and tricuspid insufficiency decreases the pulsatility index of the middle cerebral artery.

Ultrasound Doppler evaluation of uteroplacental and fetoplacental circulation in pre-eclampsia.

Blood flow velocity waveforms (FVW) were recorded weekly from the umbilical and arcuate arteries in 58 hospitalised women with a pregnancy complicated by pre-eclampsia. The maximum velocity waveform was analysed for pulsatility index (PI) and the results from the final antenatal examination were related to the outcome of pregnancy. The umbilical artery FVW was abnormal in 36% of the pre-eclamptic pregnancies, as was the arcuate artery FVW in 42%. No difference in FVW was found between mild and severe pre-eclampsia. Abnormal FVW in the umbilical artery was associated significantly both with intra-uterine growth retardation (IUGR) (P less than 0.001) and with signs of fetal distress (FD) (P less than 0.05). Abnormal arcuate artery FVW was associated with FD (P less than 0.05), but not with IUGR. The outcome of pregnancy was related to Placenta Waveform Class, which was derived from the blood velocity on both maternal and fetal sides of placenta. The results suggest that ultrasound Doppler examination of the umbilical artery is a useful aid in monitoring pregnancies complicated by pre-eclampsia, but that arcuate artery examination needs further evaluation.

A comparison of Doppler ultrasound waveform indices in the umbilical artery—I. Indices derived from the maximum velocity waveform

Various Doppler waveform indices have been used for assessment of the fetal circulation. Comparisons were made to show what relations exist between the indices, and to identify any differences or difficulties which might arise from using one as opposed to another in clinical practice. Both normal pregnancy and cases of fetal growth failure were studied. Indices were obtained from the maximum velocity envelope of the umbilical artery waveform using a curve fitting technique. The values were very reproducible for all indices. The FHR, which varied over the entire normal range, did not significantly affect the values of any index. The downstream impedance indices calculated included the AB ratio, pulsatility index (PI) and Pourcelot ratio. These all gave very closely correlated results for normals but discrepancies occurred in the at risk group, where values were elevated. This could be attributed to differences in the underlying distributions. The indices suggested for cardiac contractility were not as closely related to each other, and moreover the differences between them showed no clear pattern. None of the indices varied independently of the others. The rising slope, which is by definition related to the PI, was more highly correlated with the downstream indices than the relative flow rate index.

The detection of abnormal neonatal cerebral haemodynamics using principal component analysis of the doppler ultrasound waveform

Doppler ultrasound recordings were made from the anterior cerebral arteries of 40 normal full-term babies and 14 mature babies with intracranial pathology. The maximum velocity waveforms were extracted using spectral analysis and further analyzed using the Pourcelot resistance index method and the principal component method. Principal component analysis was found to be superior to the currently widely used resistance index method and may significantly enhance the value of Doppler ultrasound for detecting abnormal cerebral haemodynamics in neonates.

Comparative study of four methods for quantifying doppler ultrasound waveforms from the femoral artery

Currently, there is no agreement as to the best method for quantifying Doppler ultrasound recordings from peripheral arteries in order to detect occlusive disease. The four methods assessed in this study are: the pulsatility index, height-width index, path length index, and a Laplace transform function index. Recordings of the Doppler ultrasound spectral waveforms from the common femoral artery of 232 limbs were digitized to obtain the maximum velocity waveforms. This data was analyzed and the various indices were computed and then compared with the arteriographic grades. The effect of distal disease was also examined. The diagnostic accuracy of each index was determined from receiver operating characteristic curves. We concluded that all four indices were capable of detecting significant aortoiliac disease with approximately equal diagnostic accuracy of 90–95% but that pulsatility index had the advantages of simplicity and ease of calculation.

Relationship between Doppler pulsatility index and direct femoral pressure measurements in the diagnosis of aortoiliac occlusive disease.

One hundred and seventy-five aortofemoral segments were studied in order to determine the role of CW Doppler ultrasound combined with real-time spectral analysis in the assessment of aortoiliac disease. The pulsatility index (PI), determined from the maximum velocity waveform, was compared to the systolic pressure difference between the aorta and common-femoral artery as measured at the time of angiography. Receiver operating characteristic curves were constructed and the optimum sensitivities and specificities measured. Results show that if a PI(max) of 5.5 is taken as the threshold level, then the PI is 95% sensitive and specific in detecting hemodynamically significant aortoiliac disease that corresponds to a resting aortofemoral pressure difference of 10 mm Hg or greater. We also carefully examine those technical aspects of Doppler recording and analysis that influence the accuracy of PI measurements.

Doppler methods for quantitative measurement and localization of peripheral arterial occlusive disease by analysis of the blood flow velocity waveform

Errors concerned with the use of continuous wave Doppler ultrasound for the quantitative assessment of peripheral arterial occlusive disease by analysis of the blood flow velocity waveform are briefly examined. It is shown that, while some of the simpler signal processing techniques are inadequate, techniques such as real-time frequency analysis of the Doppler signal can yield information of quantitative value. A new multifilter system is described that yields the instantaneous maximum velocity waveform. From the results of preliminary patient studies using this system, it is concluded that clinically significant peripheral arterial disease can be quantified and regionally localized.