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Biomedical subjects

M Horrocks

Publications and source records attributed to M Horrocks.

122 records · Page 7Linked to original sources
PubMed1987

An analogue mean frequency estimator for the quantitative measurement of blood flow by Doppler ultrasound.

Ultrasonic Doppler-shifted signals from blood can be displayed in various forms using either analogue or digital processing techniques. One of the most commonly extracted parameters from the Doppler spectrum is the mean frequency envelope. This waveform can be used for the interpretation of physiological changes within man and can also be used for the estimation of volume flow rate within a vessel. This paper describes a practical implementation of an analogue, instantaneous mean frequency estimator and its use in the measurement of blood flow volume. The circuit is linear to within two per cent and operates over a frequency range of 200 Hz to 10 kHz. It can extract a velocity-time envelope from a Doppler signal within noise and performs well in a clinical environment. It is relatively inexpensive compared with digital signal processing techniques and requires minimal setting up. Simple connection to a Doppler velocimeter using phase quadrature demodulation will produce a directional output.

Blood Flow Velocity↗
PubMed1986

Haemodynamics of the interposition vein cuff.

It has been suggested that the use of an interposition vein cuff for anastomosing PTFE grafts to small calibre arteries confers a haemodynamic advantage. Pulsatile blood flow through a standard anastomosis was compared, in vitro, to that incorporating a vein cuff. 6 mm PTFE was anastomosed end-to-side to 10 pairs of cadaver internal mammary arteries matched for length and internal diameter (median 1.8 mm, range 1.6-2.4). Blood flow was significantly higher through the vein cuff anastomosis than the standard anastomosis (comparison of regression analysis, P less than 0.001) when the arterial diameter was less than 2.0 mm (five pairs). These results suggest that the vein cuff does confer a significant haemodynamic advantage when PTFE is anastomosed to arteries less than 2.0 mm diameter and this may be because the compliant vein cuff allows more distension of the anastomosis thus reducing anastomotic resistance.

Bioprosthesis↗
PubMed1986

A Doppler flowmeter for use in theatre.

We have developed a Doppler flowmeter based on a 10 MHz pencil probe and mean frequency estimator which overcomes many of the limitations of existing electromagnetic and ultrasonic flowmeters. The output of the flowmeter, which is proportional to the first moment of the Doppler power spectrum and hence mean blood velocity is linear from 1.3 to over 50 cm s-1 for pulsatile flow. Variation in vessel diameter and angle of insonation, which are the common sources of error in Doppler flowmetry, are minimised by constraining the vessel in a plastic cuff which fixes the probe angle at 50 degrees. A simple gauge is used to compress the vessel flat, before the cuff is applied, to measure the wall thickness to within 0.25 mm. The vessel internal diameter and hence blood flow can then be calculated using an experimentally determined calibration factor to compensate for non-even insonation. A range of sterilizable cuffs from 3-12 mm diameter have been built and the flowmeter is now being used routinely during all arterial reconstructive surgery. The accuracy and reproducibility of the system was tested for range of different sized silastic tubes on a hydraulic model and found to be less than 12% for vessels greater than 2 mm internal diameter. Satisfactory signals were easily obtained from all prosthetic materials with the exception of PTFE. The instantaneous output was compared to an electromagnetic flowmeter using a fast Fourier transform algorithm; the moduli of the harmonics were virtually identical but the Doppler system produced a smaller phase shift with increasing harmonics.(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗
PubMed1986

A comparison of the costs of vascular surgery and balloon dilatation in lower limb ischaemic disease.

The length of stay in hospital was recorded for 24 patients admitted for investigation and treatment of vascular disease of the lower limb and a detailed analysis of the costs incurred was made for 10 patients. It was found that the mean cost of angiography and surgery was pounds 1577 and of angiography and dilatation, pounds 301. The major factor in the cost difference was the shorter hospital stay for dilatation, with lesser factors being the use of fewer staff and the avoidance of general anaesthesia.

Angiography↗
PubMed1986

The effects of introducing balloon dilatation into vascular surgical practice.

The numbers of angiograms, balloon dilatations, reconstructive operations and amputations carried out annually over a 5-year period following the introduction of balloon dilatation have been counted and the change in estimated costs calculated. Over the 5-year period the numbers of angiograms and reconstructive operations carried out have doubled and the number of amputations has fallen from 52 in the first year to 39 in the fifth year. The number of dilatations has increased from six in five patients in the first year to 97 in 75 patients in the fifth year. The cost of investigation and treatment has doubled but since more patients are being treated effectively, the mean cost per patient treated has fallen from pounds 1855 to pounds 1277. The introduction of balloon dilatation into vascular practice causes patients to be investigated earlier in the progress of their disease and enables more patients to be treated effectively with greater cost-efficiency, but does not reduce the overall cost of investigation and treatment of ischaemic disease of the lower limb.

Angiography↗
PubMed1986

Does the in situ technique for autologous vein femoropopliteal bypass offer any hemodynamic advantage?

The improved patency rates that have been claimed for in situ femoropopliteal vein bypass have been attributed to improved hemodynamics because of the natural taper of the vein and maintenance of wall compliance because of preservation of the nutrient vasa vasorum. Blood flow in 31 in situ and 21 reversed vein grafts was measured noninvasively with a duplex ultrasound scanner at a median of 6 months after operation. There was no significant difference in the resting or hyperemic flows between the two groups (p much greater than 0.1). The compliance of 15 undisturbed and 15 fully mobilized in situ vein grafts was measured noninvasively by Doppler ultrasonography immediately after operation and at 3 months. There was a significant fall in compliance in both groups (p less than 0.001) but no significant difference between them (p much greater than 0.1). We have been unable to demonstrate any significant hemodynamic advantage with the in situ technique but believe that its use is still justified because of technically easier anastomoses and an improved utilization rate. Preservation of the vasa vasorum does not maintain wall compliance and complete mobilization of the vein relieves any uneven tension and ensures that all venous tributaries are ligated, removing the risk of arteriovenous fistulas.

Compliance↗
PubMed1986

The surgical prophylaxis of stroke.

Operations for the prevention of stroke have an uncertain place despite more than 30 years experience. The precise indications, contraindications, risk factors, techniques and long-term results remain controversial with widely differing opinions. Despite the controversy the number of operations have gradually increased yearly.

Anticoagulants↗
PubMed1979

Assessment of carotid artery stenosis using pulse wave transit time.

A prospective study has been undertaken on 24 patients to establish the relationship between the degree of internal carotid artery stenosis, as estimated by arteriography, and the pulse wave transit time in the carotid artery. The technique involves the simultaneous use of two continuous wave Doppler velocimeters, one on the carotid and one on the supraorbital artery, to measure the flow waves and hence deduce the pulse wave transit times. The measured transit times varied from 17 to 124 ms with a beat-to-beat variation of 11 per cent. The absolute transit time in any segment correlated moderately well with the estimated degree of stenosis (r = 0.68, P less than 0.001). However, normalization of the transit times, to correct for the effects of blood pressure and age, produced on improved correlation between the variables (r = 0.8, P less than 0.001). Normalization was achieved by dividing the common carotid to supraorbital transit time by the shortest transit time found between the common carotid and either the superficial temporal or brachial arteries. Significant differences in the normalized transit times were found between arteries that were diametrically stenosed by more than 50 per cent, by between 21 and 50 per cent and by 20 per cent or below. It is concluded that pulse wave transit time measurement can provide a simple and reproducible non-invasive technique for the diagnosis of varying degrees of carotid artery stenosis.

Adult↗
PubMed1978

Plasmapheresis in Raynaud's disease.

Understanding of the pathogenesis, clinical course, and treatment of Raynaud's disease is confused and unsatisfactory, and no success in treatment can be promised. Five patients with severe Raynaud's disease, who had not responded to other forms of treatment, were treated by plasmapheresis, which produced a striking improvement. Ultrasonic velocimetry showed that segments of digital arteries which had been thought to be permanently occluded became patent and remained patent after plasmapheresis.

Adult↗
PubMed1978

Involvement of the carotid artery in tumours of the head and neck.

Carotid artery involvement in neck tumours causes great concern to surgeons because of the unpredictable effects of carotid artery clamping or excision. Various methods of pre- and peroperative assessment of crossover cerebral circulation are reviewed. The management of 3 cases with extensive tumour involving the carotid artery is described. This comprises a combined operation involving a head and neck surgeon and vascular surgeon using a temporary carotid shunt followed by permanent autogenous vein graft to the internal carotid artery. It is suggested that thorough preoperative investigation combined with a more radical surgical approach can give a more satisfactory outcome in cases of head and neck tumours involving the carotid artery.

Adult↗
PubMedDate not supplied

An objective comparison of four stress tests in the assessment of "at risk" femoro-distal grafts.

Femoro-distal vein grafts for limb salvage have a 30% failure rate at 6 months. Graft surveillance may enable the "at risk" grafts to be recognised and corrected, but there remains the need for a simple test to identify these patients. The ankle pressure response to exercise and reactive hyperaemia has been investigated prospectively in 40 "at risk" femoro-distal non-reversed grafts (median age 73 years, range 51-87 years, M/F = 33:7), defined as those with a resting ankle brachial index less than 0.9 or a drop of greater than 0.2 following a stress test. Four different stress tests have been assessed; active ankle plantar-dorsiflexion for 2 minutes (I), occlusive calf cuff 50 mmHg above systolic pressure for 2 minutes (II), treadmill exercise test for 1 minute, slope = 10%, at 3 km/hr (III) and 4 km/hr (IV). Ankle brachial indices (ABI's) were recorded before and immediately following each test and expressed as mean % drop +/- standard error of mean. Test I was only tolerated by 45% of patients whereas 55% and 50% could complete tests III and IV respectively. By contrast, 85% of patients could tolerate occlusive cuff hyperaemia (test II). Test I produced a significantly lower mean percentage drop in ABI when compared with each of the others (p less than 0.02, Mann U Whitney). There was an excellent correlation between test II and both the 3 km/hr (r = 0.77, p less than 0.001) and 4 km/hr (r = 0.84, p less than 0.001) exercise tests.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗