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Biomedical subjects

J D Beard

Publications and source records attributed to J D Beard.

At least 109 records · Page 6Linked 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

Compliance changes in in-situ femoropopliteal bypass vein grafts.

Changes in wall structure, including neo-intimal proliferation and medial fibrosis, have been implicated as a cause of late occlusion in reversed femoropopliteal vein grafts. These changes can be measured indirectly as a fall in compliance. It has been suggested that long-term patency might be improved by the in situ technique because the nutrient vasa vasorum are left intact and therefore wall structure preserved. We have measured the compliance of 62 in situ vein grafts, with times after operation ranging from 2 days to 6 years, and also compared the compliance changes, in the first 3 months after operation, of 15 undisturbed in situ vein grafts with 15 fully mobilized in situ vein grafts. Compliance was derived non-invasively from the pulse wave velocity using Doppler ultrasound. There was a significant fall in compliance after operation (P less than 0.001) and no difference could be found between the undisturbed and mobilized in situ vein grafts (P greater than 0.1). Histological examination of 6 grafts suggested that the fall in compliance was due to neo-intimal proliferation which still occurred although medial fibrosis was reduced. Any potential improvement in long-term patency rates using the in situ technique must be due to other factors.

Aged↗
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

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↗
PubMed1984

Plasma lipoprotein changes during abstinence in chronic alcoholics.

Plasma lipoprotein changes during a 2-week period of abstinence were followed in 6 male, chronic alcoholics without evidence of severe liver disease and with initial HDL cholesterol (HDL-CH) greater than 60 mg/dl. Fasting blood samples were obtained on days 1, 3, 7 and 14 of the study. Reference data were obtained from healthy non-alcoholic normolipidemic men who had abstained from alcohol for 10 days. At day 1, plasma- and HDL-CH (by the heparin-Mn2+ technique) were 17 and 69% higher, respectively, than those of controls. During abstinence, VLDL-CH increased 52% whereas HDL-CH decreased 30% compared to day 1 values. (IDL + LDL)-CH increased during abstinence to levels 33% higher than that of controls. Plasma, SF greater than 400, VLDL- and d greater than 1.006 g/ml-triglycerides (TG) were not significantly different from those of the control group. Only the d greater than 1.006 g/ml-TG showed a significant effect of abstinence, increasing by 37%. Initial plasma and d greater than 1.006 g/ml-phospholipid (PL) concentrations were 17 and 31% higher, respectively, than those of controls; and the latter was the only fraction to change significantly with abstinence, decreasing by 13%. Density gradient ultracentrifugation was employed to further resolve the d greater than 1.006 g/ml fraction into IDL, LDL, HDL and VHDL subfractions. Initial levels of IDL- and LDL-CH and -PL in the alcoholic group did not differ from those of controls. IDL-CH and -PL were invariant during abstinence, whereas LDL-CH and -PL levels increased 38 and 28%, during this period. Apo A-I, CH and PL contained in the 'lighter' density HDL region of the gradient (d = 1.063-1.125 g/ml) were 70-108% increased over the corresponding parameters for controls; and with abstinence decreased 30-40% between days 1 and 14. CH and PL in the 'heavier' density HDL region (d = 1.125-1.21 g/ml) were 30% increased over those of controls, while apo A-I levels were similar to controls. During abstinence, 'heavy' HDL-CH, -PL and -apo A-I decreased 25% compared to day 1 values. Inspection of 'difference' density gradient plots indicated that the dominant decrease in HDL occurred in the 'light' HDL subspecies in only one-half of the subjects. Other investigators have reported a preferential decrease in 'light' HDL subspecies during abstinence in chronic alcoholics, and have related the decrease to decreased TG turnover. We conclude that additional mechanisms related to the synthesis and/or catabolism of HDL may need to be considered in explaining decreases in HDL subspecies in alcoholics during abstinence.

Adult↗
PubMed1980

Twenty-four-hour fluid intake and renal handling of electrolytes after various doses of ethanol.

The effects of single ethanol doses on fluid and electrolyte metabolism were studied in 31 male mongrel dogs. The animals were given either 0.75 g/kg 1.50 g/kg, or 2.25 g/kg of a 25% (v/v) ethanol solution or isovolumetric quantities of water. Fluid intake, urine output, and electrolyte (Na, K, Cl, Mg) excretions were measured at 0--3, 3--8, and 8--24 hr. During the ascending portion of the plasma ethanol curve (0--3 hr) there was a diuresis and renal magnesium loss in the two highest dosage ethanol groups. During the initial portion of the descending plasma ethanol curve (3--8 hr), each ethanol group had a significant elevation in voluntary intake. At 8--24 hr, renal retention of sodium, potassium, and chloride was found in the 1.5 and 2.25 g/kg groups, and magnesium excretion was also reduced in the 2.25 g/kg group. Over the 0--24 hr, none of the ethanol groups showed fluid loss, while the 2.25 g/kg ethanol group had significant retention of water. The administration of the 2.25 g/kg ethanol dose also resulted in 24-hr retention of sodium and potassium.

Animals↗
PubMed1975

Renal hemodynamics and electrolyte excretions after reserpine and ethanol.

The participation of the peripheral nervous system in the renal response to an acute ethanol administration was investigated in dogs. The animals received water or ethanol (3 g/kg), with and without 2 days pretreatment with reserpine. Each group was examined 18 hours after the water or ethanol administration. In comparison to water controls, both reserpine and ethanol produced similar increases with respect to renal hemodynamics. The combined treatment produced a further significant increase in effective renal plasma flow. Reserpine alone produced increases in the rates of filtration and reabsorption of sodium and chloride; ethanol and reserpine-ethanol treatments produced further increases but did not alter the excretory rates. Although all three treatments produced increases in the rate of filtration of potassium, only ethanol and reserpine-ethanol treatments increased the excretory rate. Magnesium excretion was decreased by reserpine and reserpine-ethanol in comparison to water and ethanol, respectively; but calcium excretion was elevated in all the treatment groups. Zinc excretion was increased in the ethanol group. The results suggest that the secondary action of ethanol to produce release and possible depletion of peripheral catecholamine stores is not primarily involved with its renal effects, except in the case of magnesium and zinc metabolism.

Animals↗