Quantitative Doppler measures in coiled vessels: investigation on excised umbilical veins

Increased tissue oxygenation explains the attenuation of hyperemia upon repetitive pneumatic compression of the lower leg

The rapid hyperemia evoked by muscle compression is short lived and was recently shown to undergo a rapid decrease even in spite of continuing mechanical stimulation. The present study aims at investigating the mechanisms underlying this attenuation, which include local metabolic mechanisms, desensitization of mechanosensitive pathways, and reduced efficacy of the muscle pump. In 10 healthy subjects, short sequences of mechanical compressions ( n = 3–6; 150 mmHg) of the lower leg were delivered at different interstimulus intervals (ranging from 20 to 160 s) through a customized pneumatic device. Hemodynamic monitoring included near-infrared spectroscopy, detecting tissue oxygenation and blood volume in calf muscles, and simultaneous echo-Doppler measurement of arterial (superficial femoral artery) and venous (femoral vein) blood flow. The results indicate that 1) a long-lasting (>100 s) increase in local tissue oxygenation follows compression-induced hyperemia, 2) compression-induced hyperemia exhibits different patterns of attenuation depending on the interstimulus interval, 3) the amplitude of the hyperemia is not correlated with the amount of blood volume displaced by the compression, and 4) the extent of attenuation negatively correlates with tissue oxygenation ( r = −0,78, P < 0.05). Increased tissue oxygenation appears to be the key factor for the attenuation of hyperemia upon repetitive compressive stimulation. Tissue oxygenation monitoring is suggested as a useful integration in medical treatments aimed at improving local circulation by repetitive tissue compression.

NEW & NOTEWORTHY This study shows that 1) the hyperemia induced by muscle compression produces a long-lasting increase in tissue oxygenation, 2) the hyperemia produced by subsequent muscle compressions exhibits different patterns of attenuation at different interstimulus intervals, and 3) the extent of attenuation of the compression-induced hyperemia is proportional to the level of oxygenation achieved in the tissue. The results support the concept that tissue oxygenation is a key variable in blood flow regulation.

Doppler indices in the umbilical arteries: influence of vessel curvature induced by bladder filling

Doppler indices are widely used to assess normal versus pathologic haemodynamics. In obstetrics, the assessment of abnormal values in some critical compartments, such as the umbilical arteries (UA), may be crucial in the clinical management of growth-restricted foetuses. It was recently proposed that the UA should be sampled in their perivesical portion (PVC), i.e., where they surround the foetal urinary bladder. However, measurements at this site could be biased by the degree of curvature of the vessel due to bladder filling. We investigated this possibility in vivo and in vitro, i.e., measurements on rubber tubes at different radii of curvature R(c). There was significant dependence of the Doppler indices A/B and PI on the vessel curvature and insonation angle; in fact, we recorded errors of about 25% when R(c) was 10 times larger than the radius of the vessel and about 100% when R(c) was five times larger than the radius of the vessel. Therefore, measurements of the UA at the PVC site should only be performed when the foetal bladder is empty.

Reference ranges for umbilical vein blood flow in the second half of pregnancy based on longitudinal data

OBJECTIVES

To construct new reference ranges for serial measurements of umbilical vein (UV) blood flow.

METHODS

Prospective longitudinal study of blood flow velocities and diameter of the UV measured at four-weekly intervals during 19 to 42 weeks' gestation in 130 low-risk singleton pregnancies. Regression models and multilevel modeling were used to construct the reference ranges.

RESULTS

On the basis of 511 sets of longitudinal observations, we established new reference percentiles of UV diameter, blood flow velocities, volume flow, and blood flow normalized for fetal weight and abdominal circumference. They reflected some of the developmental patterns of previous cross-sectional studies, but with important differences, particularly near term. The UV blood flow showed a continuous increase until term, whereas the flow normalized per unit fetal weight, a corresponding reduction. Calculating the blood flow on the basis of intensity-weighted mean velocity or 0.5 of the maximum velocity gave almost interchangeable results for most fetuses.

CONCLUSION

New reference ranges for UV blood flow based on longitudinal observations appear slightly different from cross-sectional studies, and should be more appropriate for serial evaluation of fetal circulation.

Curvature affects Doppler investigation of vessels: implications for clinical practice

In clinical practice, blood velocity estimations from Doppler examination of curved vascular segments are normally different from those of nearby straight segments. The observed "accelerations," sometimes considered as a sort of stochastic disturbances, can actually be related to very specific physical effects due to vessel curvature (i.e., the development of nonaxial velocity [NAV] components) and the spreading of the axial velocity direction in the Doppler sample volume with respect to the insonation axis. The relevant phenomena and their dependence on the radius of curvature of the vessels and on the insonation angle are investigated with a beam-vessel geometry as close as possible to clinical setting, with the simplifying assumptions of steady flow, mild vessel curvature, uniform ultrasonic beam and complete vessel insonation. The insonation angles that minimize the errors are provided on the basis of the study results.

Assessment of the effect of vessel curvature on Doppler measurements in steady flow

Blood vessel curvature is responsible for the appearance of nonaxial velocity components and for minor changes in the pattern of the axial flow. All the velocity components are expected to contribute to the Doppler signal produced by the ultrasound (US) backscattered by the insonated blood cells, the axial velocity, contributing to the actual volumetric blood flow, and the transverse velocity, causing the recirculating vortices. A detailed, separate analysis of the velocity components is, therefore, mandatory to quantify how vessel curvature can affect results and clinical diagnosis. Both experimental in vitro measures and numerical simulations were performed on a curved tube and the Doppler power spectra so obtained were compared. The satisfactorily agreement of the above spectra shows that the nonaxial velocity components are easily detectable with clinical equipment and that their amplitude, as expected, is not negligible and can bias Doppler measurements and resulting clinical diagnosis.

Umbilical venous volume flow in the normally developing and growth-restricted human fetus

OBJECTIVE

To determine the reproducibility of measurement of umbilical venous volume flow components and to calculate umbilical venous volume flow in normal and growth-restricted (small-for-gestational age) fetuses in a cross-sectional study.

METHOD

Using Labview and Imaq-vision software, the cross-sectional inner area of the umbilical vein was traced. Vessel area (mm2) and Doppler-derived time-averaged flow velocity (mm/s) were multiplied to calculate volume flow (mL/min) including flow per kg fetal weight. The coefficient of variation for vessel area and flow velocity scans and tracings were determined (n = 13; 26-35 weeks). Normal charts for components and volume flow were constructed (n = 100; 20-36 weeks) and related to data from growth restricted fetuses (birth weight < 5th centile) (n = 33; 22-36 weeks). In growth-restricted fetuses the umbilical artery pulsatility index was also obtained.

RESULTS

Reproducibility: The coefficient of variation was 5.4% (vessel area) and 7.3% (time-averaged velocity) for scans and 6.6% and 10.5% for measurements, resulting in a coefficient of variation of 8.1% (scans) and 11.9% (measurements) for volume flow. A gestational age-related increase exists for vessel area, time-averaged flow velocity and umbilical venous volume flow from 33.2 (SD, 15.2) mL/min at 20 weeks to 221.0 (SD, 32.8) mL/min at 36 weeks of gestation, but there is a reduction from 117.5 (SD, 33.6) mL/min to 78.3 (SD, 12.4) mL/min for volume flow per kg fetal weight. In small-for-gestational age fetuses, the values were below the normal range in 31 of 33 cases for volume flow and in 21 of 33 cases for volume flow per kg fetal weight. Umbilical artery pulsatility index was significantly different between the subsets with normal and those with reduced volume flow per kg fetal weight.

CONCLUSIONS

Measurements of umbilical venous vessel area and time-averaged velocity resulted in acceptable reproducibility of volume flow calculations, which show a seven-fold increase at 20-36 weeks of gestation. In growth-restricted fetuses, volume flow is significantly reduced. When calculated per kg/fetus, the values were reduced in 21 (63.6%) out of 33 cases.