Shear rate estimation using a clinical ultrasound scanner.

Wall shear stress is a factor in the development of atherogenesis, thrombus formation, and embolization, but its existence is very difficult to determine with ultrasonography. Instead, we estimated shear rates using a clinical ultrasound scanner and compared results from vessels with and without stenoses. Velocity profiles were obtained from color M-mode images on a P700 scanner. Maximum shear rates were calculated off-line as the maximum velocity gradients. In vitro, studies were performed on a flow phantom with a vessel containing a 50% stenosis. Shear rates within the stenosis were significantly higher than those obtained outside the stenosis (P < 0.00001) and varied more than the peak velocities. In vivo, the internal carotid artery of 10 volunteers and 13 patients (with stenoses) was studied. The mean shear rate was 414 s(-1) +/- 154.5 s(-1) in normal vessels and 687 s(-1) +/- 263.5 s(-1) in stenotic vessels (P = 0.00017). In conclusion, shear rate estimates can be obtained with a clinical ultrasound scanner from color M-mode images. Shear rates estimated in vessels with stenoses are significantly higher than those obtained in normal vessels in vitro as well as in vivo.

Colour velocity flow measurement: in vitro validation and application to human carotid arteries.

Ultrasound measurement of volume blood flow is potentially useful for many clinical situations, yet practical implementation and use are restricted by the many instrumentation and blood flow limitations that can arise. Colour velocity imaging offers a number of theoretical advantages over methods based on duplex imaging. We evaluated a colour velocity flow measurement system (CVI-Q, Philips) both in a flow phantom and in vivo in the extracranial carotid arteries of normal volunteers. Over a range of constant (50-1200 ml/min) and pulsatile (92-366 ml/min) flows and using both steered and unsteered beams with beam angles of 30 degrees and 40 degrees, errors usually within 5% were obtained for constant flow and within 10% for pulsatile flow. However, with a beam angle of 70 degrees, higher errors of 20% were obtained for pulsatile flow. The reproducibility of flow measurements made using both anterior and posterior-lateral scanning approaches was determined in the common (CCA), internal (ICA) and external carotid (ECA) arteries of 18 volunteers. A greater reproducibility was found using the posterior-lateral approach (CCA 6.27%; ICA 9.8%), and mean (SD) flow values were 376 ml/min in the CCA and 255 ml/min in the ICA. The ratio of (ICA + ECA)/CCA flow calculated for each subject individually was mean (SD) 0.95 (0.11). Insonation from an anterior approach resulted in lower reproducibility and lower flow values. In conclusion, colour flow velocity imaging allows repeatable reproducible measurements of CCA and ICA flow, but results are optimal if a posterior-lateral scanning approach is used.

Effect of L-arginine on renal blood flow in normal subjects and patients with hypoxic chronic obstructive pulmonary disease.

BACKGROUND: L-arginine is the precursor of endothelium derived nitric oxide (NO) and increasing the available substrate may increase the production of NO. This has been shown by local infusion in peripheral vascular beds but there are few studies of the effects during systemic infusion. Renal vasoconstriction is known to be important in the pathogenesis of cor pulmonale in patients with hypoxic chronic obstructive pulmonary disease (COPD). The effects of a systemic infusion of L-arginine on renal and aortic haemodynamics were therefore investigated in normal subjects and in patients with hypoxic COPD. METHODS: Ten normal volunteers were recruited from the research staff of King's College Hospital Six patients with COPD and hypoxia (arterial oxygen tension (PaO2) < 8.5 kPa) were recruited from the thoracic medicine outpatient clinic at King's College Hospital and five age and sex matched normal subjects were recruited from a group of normal subjects recruited from the database of the Department of Health Care for the Elderly as volunteers for medical research. There was no history of renal, cardiac, or hepatic disease. Baseline values of time averaged mean of the maximum instantaneous velocity (Tamx) and maximum velocity (Vmax) of blood flow in intrarenal arteries were obtained using colour flow Doppler ultrasound. Using the same technique, Vmax was obtained from the abdominal aorta just distal to the xiphisternum before and after infusion of L-arginine via a large peripheral vein (20 g in 100 ml sterile water over 30 minutes). RESULTS: In normal subjects L-arginine increased blood velocity in the intrarenal vessels from a mean of 0.22 m/s to 0.26 m/s, an increase of 19.8%. There was no effect on arterial blood pressure, heart rate, or aortic blood velocity. L-arginine had no effect on intrarenal or aortic blood velocity in patients with hypoxic COPD. In age matched controls L-arginine increased blood velocity in the intrarenal vessels from a mean of 0.20 m/s to 0.26 m/s, an increase of 36.8%. There was no effect on arterial blood pressure, heart rate, or aortic blood velocity. CONCLUSIONS: L-arginine, at the doses administered, increased renal blood flow, as assessed by renal arterial velocity. This effect was not seen in patients with hypoxic COPD but was present in age matched controls. This suggests that the abnormal renal vascular control seen in hypoxic patients with COPD may reflect a disturbance of the L-arginine/nitric oxide pathway.

Normal renal artery spectral Doppler waveform: a closer look.

PURPOSE: To characterize the spectral Doppler tracing of the normal renal artery. MATERIALS AND METHODS: Doppler tracings were obtained from a phantom of a vessel with variable compliance and from the kidneys of 15 healthy volunteers. RESULTS: In the phantom, vascular compliance had the following effects on systolic flow velocity patterns: low compliance, little change in the sharp appearance of the systolic component of the transmitted pulse; mildly increased compliance, down-stream dampening of early systolic acceleration (ESA) with the appearance of a sharp early systolic transmitted peak and of a more rounded late systolic compliance peak; high compliance, delayed and diminished transmitted peak that eventually disappeared within the enlarging down-stream compliance peak. Healthy subjects with compliant vessels had greater ESA and more frequently visualized early systolic peaks in the renal hilum than in the renal sinus. CONCLUSION: Absence of a discrete early systolic peak is a normal finding in young patients, especially in the more distal interlobar arteries. Loss of the early systolic peak may be explained on the basis of vascular compliance.

The effects of oxygen and dopamine on renal and aortic blood flow in chronic obstructive pulmonary disease with hypoxemia and hypercapnia.

Renal blood flow is reduced in patients with chronic respiratory failure caused by chronic obstructive pulmonary disease (COPD), and changes in renal hemodynamics are likely to be important in the pathogenesis of the edematous state of cor pulmonale. We therefore examined the hypothesis that this renal vasoconstriction is reversible by comparing the effects of oxygen therapy and the renal vasodilator dopamine on renal hemodynamics in both hypoxemic patients with COPD and those who were also hypercapnic. We assessed renal hemodynamics noninvasively with color-flow Doppler ultrasound. In order to validate the technique we recorded renal hemodynamics in a group of healthy volunteers before and during a dopamine infusion, and in a subgroup we simultaneously measured effective renal plasma flow (ERPF) with para-aminohippurate clearance. In the healthy volunteers there was a 22.5% rise in time-adjusted mean arterial velocity (Tamx) measured by Doppler compared with a 22% rise in ERPF with dopamine. This rise was significant (p < 0.05). In hypoxemic, normocapnic subjects Tamx rose by 25% with oxygen (p < 0.005), 20% with dopamine (p < 0.005), and 24% with both therapies. There was no significant change in aortic flow whether the subjects received air, oxygen, or dopamine (p = 0.77). In the hypercapnic patients there was no significant change in aortic or renal velocities while receiving oxygen or dopamine (p = 0.85 and 0.86). We conclude that color-flow Doppler velocity measurements can accurately detect changes in renal blood flow. Oxygen and dopamine are equipotent renal vasodilators in hypoxic COPD, but the effect is not additive.(ABSTRACT TRUNCATED AT 250 WORDS)

Strain gauge plethysmography and Doppler ultrasound in the measurement of limb blood flow.

The maintenance of adequate oxygen delivery (DO2) and tissue uptake (VO2) has become central dogma in the management of the critically ill. However, these parameters are derived using gas tensions measured in mixed venous blood and may not reflect changes in regional blood flow. Therefore, it has become necessary to provide estimates of blood flow to specific organs and to evaluate the most adequate techniques available. In order to define the best means of assessing blood flow to the lower limb noninvasively in normal subjects, measurements of superficial femoral arterial blood flow using Doppler ultrasound (DU) and strain gauge plethysmography (SGP) were compared in 10 normal volunteers at rest and during exercise. To evaluate the effect of strain gauge positioning, results of measurements made under four different combinations of cuff/strain gauge placement were compared in 15 other volunteers. The correlation of the limb blood flow obtained using the two methods at rest and exercise was 0.57 and 0.62 and the limits of agreement (d +/- 2SD) were 0.40 +/- 2.49 and -0.86 +/- 5.22 ml 100 ml-1 tissue min-1 at rest and on exercise, respectively. Results obtained using SGP were more reproducible (Coef. repeat. 0.45 vs. 0.94 ml 100 ml-1 tissue min-1, for SGP and DU, respectively). The various combinations of cuff/strain gauge positioning showed a tendency to over-read when the latter was placed on the thigh, but were not significantly different (P > 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Accuracy of colour Doppler ultrasound velocity measurements in small vessels.

Colour Doppler ultrasound offers the possibility of imaging small vessels not visible by B-mode alone. The colour Doppler image of velocities allows the course of small vessels to be imaged in the X-Y plane of the scan provided the Doppler frequency shift is of sufficient magnitude. This permits alignments of the Doppler cursor, allowing angle correction to provide true velocity measurements from the Doppler shift obtained. Before attempting to make velocity measurements, however, it is essential to be aware of the possible error in the Z plane caused by the thickness of the Doppler sample volume. To quantify this source of error, hydrophone and flow-rig measurements were performed on an Acuson 128 colour Doppler scanner with both 5 MHz linear-array and 3.5 MHz phased-array transducers. Measurements of the transmitted pulses using a point hydrophone showed that both probes employ approximately 3.5 MHz Doppler pulses (in both colour and pulsed Doppler modes). The two transducers have the same axial resolution. In colour Doppler mode the axial length of the sample volume increases automatically with depth by up to 0.5 mm. Measurements of colour and pulsed Doppler signal strength were obtained in a controlled flow rig. Both transducers produced accurate colour flow images of the phantom at their optimum depths; flow velocity errors due to Z-plane thickness are less than 5%. There was, however, substantial error outside these optimum conditions (up to 20%).