Design considerations for a small bore vascular prosthesis.

No synthetic prosthesis has yet been found which can satisfactorily be used as a substitute for saphenous veins in vascular surgery. One explanation for the lack of success is that their properties and behaviour do not mimic those of the arteries they replace. The essential features for a successful vascular prosthesis, over and above biocompatibility, are a low friction non-thrombogenic flow surface, mechanical (elastic) behaviour that matches natural artery and dimensions that approximate the host vessels. This will allow a high velocity of flow with low impedance. A prosthesis with this specification would avoid the problems of flow disturbances and reflection at the anastomoses, pulse wave damping and large pressure gradients, all of which reduce pulsatile energy. We have designed a vascular prosthesis based upon the above criteria. It has an internal diameter of 4 mm and a wall thickness of 1 mm. Its geometry makes it kink resistant and a good match for tibial and coronary arteries. The prosthesis is made from polyurethane and its mechanical properties, which approximate to those of peripheral arteries, are achieved via a mechanism of internal wall compression rather than external circumferential expansion. To assess the in-vivo potential of this prosthesis, 21 grafts were implanted in 11 sheep, replacing a section of carotid artery with a 6 cm long prosthesis. All 21 grafts were patent after 3 months. One graft was removed for histological examination. 16 of the remaining 20 grafts were patent after 7 months. Problems associated with thrombosis were not encountered and we conclude that this prosthesis shows good potential as a small bore vascular substitute.

Early results of femoropopliteal bypass using a five millimeter "thin wall" polytetrafluoroethylene (Gore-Tex) prosthesis.

We report the early results of a series of 86 femoropopliteal bypass operations in which a 5 mm diameter thin wall polytetrafluoroethylene (Gore-Tex) prosthesis was used. Sixty-five bypasses were implanted in men and 21 in women. Thirty bypasses were done in conjunction with an aortic bifurcation graft and 18 patients had a bilateral procedure. The indication for operation was severe claudication in 74 cases and critical ischaemia in 12 cases. The angiographic run-off was good (three patent vessels) in 22 limbs and poor (one or two patent vessels) in 64 limbs. The cumulative patency rate for the whole group was 62% after 18 months. Angiographic run-off and the indication for operation were both found to influence cumulative patency rate significantly (p = 0.035 and p = 0.055, respectively). We also compared the results obtained when run-off was poor with equivalent data from our own previously published series in which a standard 6 mm diameter Gore-Tex prosthesis was used. This shows a difference in patency rate, for example 57% against 37% after 18 months, in favour of the smaller bore thin wall graft.

Femoropopliteal bypass with a Goretex prosthesis: a long-term follow-up.

This report describes a series of 101 femoropopliteal bypasses performed on 85 patients (16 bilateral procedures), in whom a 6 mm diameter polytetrafluoroethylene prosthesis (Goretex) was used. The indication for operation was severe claudication in 58 cases and pregangrene in 43 cases. After one year there was a statistically significant difference between the cumulative patency rates for these two subgroups at 66 and 29 per cent respectively (P less than 0.001). In patients with good 'run-off, i.e. three patent vessels in the lower leg, there was a tendency for the bypasses to remain patent for longer than in patients in whom there were only one or two vessels present (e.g. 58 and 33 per cent respectively at 2 years), although this trend was not statistically significant. At 3 years the cumulative patency rate for the whole group was 36 per cent and for claudicants alone was 48 per cent.

The uses and abuses of life-table methods in vascular surgery.

This paper details the calculations involved in a life-table analysis and appraises the current use of this technique in vascular research. We suggest that the presentation of patency curves should be standardized and recommend that: curves are presented as step rather than continuous graphs; the number of patients remaining in a study are shown at intervals along patency curves; the length of time for which a curve is claimed to hold is more rigorously defined; an attempt is made to increase, where possible, the frequency of patient review, particularly in the immediate postoperative period; ways of increasing the total sample size (e.g. multicentre studies) are investigated. We also recommend that a set of definitive guidelines on the presentation of graft patency data acceptable to all the major vascular journals be sought.

A comparison between two types of waveform analysis in patients with multisegmental arterial disease.

The results of an experiment to compare two different methods of analysing ultrasound signals are presented. Normal volunteers, both young and old, and patients with arteriosclerosis were examined with Doppler shifted ultrasound and the results of two forms of waveform analysis retrospectively compared with angiograms. The angiograms were used to divide the patients into groups with similar patterns of disease. The ability of the two methods of analysis, 'pulsatility Index' (PI) and 'Laplace transform damping factor' (LTD), to identify patients with haemodynamically significant disease in the aorta or iliac arteries was tested statistically (Mann-Whitney U test) and compared in receiver operator curves. The results showed that both tests were equally good in the identification of aortoiliac disease, even when the superficial femoral artery was occluded, and that, contrary to theoretical arguments, LTD was influenced by distal impedance.

An investigation into how the geometry of an end-to-side arterial anastomosis affects its function.

The effects of altering the geometry of an end-to-side junction were investigated in a hydraulic model. The dimensions of the tubes, pressures and flow rate and the frequency of pulsation were calculated by dimensional analysis, to produce a model which simulated pulsatile flow in small arteries. The ratio between the impedance of the 'bypass' and the impedance downstream from the junction was greater than unity when the two tubes forming the junction were of unequal diameter. No increase in downstream impedance was measured when the angle of junction was increased from 45 degrees to 135 degrees. Cine photographs of streamlines at the junction showed that flow passed smoothly through the junction irrespective of angle and that perturbations occurred only during that part of the cycle when flow was reversed. The results indicate that the angle at which a junction is made affects flow. The effect is small and is only seen when the ratio of diameters of the tubes is unity. The ratio of the diameters of the two tubes which make up the junction has the greatest effect on flow through the junction.