V. C. Roberts

Triaxial force transducer for investigating stresses at the stump/socket interface

Although normal pressures at the stump socket interface of the lowerlimb amputee have been investigated, little is known about the shear stresses that also occur. Studies suggest that the combination of both shear and normal stresses significantly exacerbates discomfort and vascular and tissue damage. A means of simultaneously measuring normal and shear stresses will aid in the investigation and improvement of prosthetic fit. A miniature triaxial force transducer (4·9×16 mm diameter) has been developed which can be recessed into the socket wall. The principle of operation, construction, performance and limitations of the device are described. Preliminary measurements of the interface stress variations over the gait cycle in a supra-condylar PTB socket are presented. These show clear differences in the stress patterns present when two different prosthetic feet are used.

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%).

Raynaud's phenomenon and thermal entrainment: an objective test.

A new objective test for diagnosing Raynauds phenomenon was assessed in practice. The test is based on entrainment of the thermal vasomotor control system and entails non-invasive measurement of blood-flow responses in one hand while alternating thermal stimuli are applied to the contralateral hand. A significant (p less than 0.001) abnormality of vasomotor control was found in patients with Raynauds phenomenon compared with normal subjects. When applied clinically this test is diagnostic and indicates the severity of the disease and the effect of treatment.

On-line analysis of the femoral artery flow velocity waveform and its application in the diagnosis of arterial disease

Analysis of the instantaneous peak velocity waveform obtained from the common femoral artery can be used to provide an objective assessment of arterial disease in the leg. The calculation of waveform indices pulsatility index and rise time together with principal component analysis has been found to be the ideal method by which to assess the flow data on-line.

The effect of adjuvant oxygen therapy on transcutaneus p02 and healing in the below-knee amputee

The effects on tissue oxygenation of postoperative adjuvant oxygen have been studied in a group of 20 patients undergoing below-knee (BK) amputation for vascular disease. Ten patients received no therapy, the remainder receiving 28% oxygen for 48 hours following surgery. The results showed that the trancutaneous p02 in the amputation flaps fell significantly by some 20 mmHg (p<0.01) following surgery and that this fall was prevented by the use of adjuvant oxygen. The fall was not observed in the non-amputated limbs. Tcp02 took almost two weeks to reach its pre-operative levels in the amputated limbs. The effect on stump healing of adjuvant oxygen therapy was investigated in a randomized controlled trial in a series of 39 patients undergoing BK amputation. There were 22 patients in the control (untreated) group and 17 in the treated group (adjuvant oxygen for 48 hours). In the treated group 14 patients healed primarily and three amputations failed. In the untreated group 14 limbs healed primarily, one secondarily and there were 7 failures. The pre-operative transcutaneous values in the stumps which failed (26 mmHg±14) was significantly lower (p<0.005) than in those which healed (40 mmHg±9). The mean pre-operative Tcp02 in the patients in whom healing occurred in the treated group (35 mmHg±10) was significantly lower (p<0.001) than the mean pressure observed in the untreated group (44 mmHg±9).

Digital artery calibre measurement—a new technique of assessing Raynaud's phenomenon

This study evaluated the use of a high frequency A-mode ultrasound scanner (CUTECH DM70) for monitoring the digital artery vasospasm of Raynauds Phenomenon (RP). The technique was evaluated on 12 RP patients and 12 age and sex matched controls. The diameter of a single digital artery was measured in these subjects over a range of finger temperatures between 14-35 degrees C. The reproducibility of the measurements was assessed by repeating the procedure in two subjects (a control and an RP patient) on 5 separate days. The results confirmed that the technique is reproducible and monitored vasospasm occurring in all 12 RP patients. In the temperature range tested, none of the controls developed digital artery vasospasm. At finger temperatures greater than 25 degrees C the response to thermal stimuli of the digital arteries in RP patients and controls was similar. However, at temperatures less than 25 degrees C a marked vasoconstriction leading to complete digital artery closure was seen in the RP patients and not in the controls. This technique may prove useful in monitoring RP patients.

Detection of low-grade arterial stenosis using an automatic minimum-flow-velocity tracking system (MVTS) as an adjunct to pulsed ultrasonic Doppler vessel imaging.

A microprocessor-controlled system for detecting minor flow disturbances in blood vessels is described. The system is composed of a multigated pulsed Doppler ultrasonic velocimeter (MAVIS) and an 8085 microprocessor acting as an interface for a signal processing system and/or a storage system. A minimum-particle-velocity method is used to extract. Doppler signals from selected sites as near as possible to the vessel wall. These Doppler signals can be monitored constantly during any vessel scanning procedure revealing flow abnormalities caused by stenoses of as little as 15 per centarea reduction. The performance of the microprocessor-controlled system was assessed using signals generated electronically. For thein vitro assessment of the system as a whole a test rig which generates pulsatile flow through nonsymmetric stenoses varying from 15 to 30 per cent area reduction was used. The study shows the development of disturbed flow not only downstream, but also upstream of the stenosis. The results show the capability of this method in discriminating flow disturbances caused by minor stenoses. Preliminaryin vivo assessment was carried out in patients with suspected carotid artery disease and the results have indicated how this method can be used to localise the exact position where flow disturbance is occurring.

Ultrasound imaging of digital arteries

This study assessed the use of high frequency ultrasound (CUTECH DM70) in the measurement of digital artery diameters. The accuracy of the technique was assessed using a model. Four silicone tubes of different diameters were embedded in opaque agar. Three independent observers measured the diameter of the tubes using the CUTECH DM70 and the measurements obtained were compared with those obtained using a travelling light microscope. The technique was then evaluated on digital arteries. Ten recordings of digital artery diameter were made at a fixed point on the index finger in two subjects at 20 degrees C and at 30 degrees C. There was no statistical difference between the mean diameters obtained by the three observers using the CUTECH DM70 on the model. No statistical difference was noted when the mean diameters measured using the travelling microscope were compared with those obtained by the three observers. The diameter differences between the tubes were all highly significant (P less than 0.001 Mann Witney). The variability in the recordings of digital artery diameter in the two subjects was very low (maximum coefficient of variance less than 4%). A significant difference in the mean measured vessel diameter at the two different temperatures was obtained (P less than 0.001 Mann Witney). We conclude that digital artery diameter can be measured using the CUTECH DM70.

Per-operative haemodynamic assessment of lower limb arterial surgery Part 1: Hydraulic impedance measurement

Hydraulic impedance measurements have been made during surgery on 32 patients undergoing reconstructive arterial surgery. The reconstructive procedures included aorto-iliac, femoro-popliteal and axillo-femoral bypass, and extended deep femoral angioplasty. It was found that in aorto-iliac reconstruction the phase curves provide the means of assessing success, but that in other cases the impedance measurements though showing a similar trend are unable unequivocally to differentiate between success or failure.

Design of a continuous-wave Doppler ultrasonic flowmeter for perivascular application

The paper details the design of the signal processing system of an ultrasonic Doppler flowmeter intended for perivascular application. The system is designed to work in conjunction with a transducer system described in an earlier paper, in a range of vessels varying from 5 mm to 12 mm. The system incorporates conventional quadrature processing for directional discrimination and uses phaselocked loop technology to extract signals corresponding to the instantaneous mean frequency (velocity equivalent). The use of a PLL also permits the extraction of a quasi-spectral analysis signal. The baseline stability of the Doppler system is enhanced by the use of an auto-zeroing system which enables the system to resolve Doppler shifts down to 20Hz. Testing of the complete transducer/signal processor system in a hydraulic rig demonstrates the ability of the flowmeter to accurately measure both steady and pulsatile flow.