Martin Morris Black

Artificial heart valves

A bioprosthetic mitral valve replacement comprises a flexible frame (1) having a ring-shaped base (2) and at least a pair of upstanding posts (3), which divide the base into at least two portions (2A, 2B) of varying flexibility, together with animal tissue leaflets (4) each having a periphery consisting of a free portion (4A) extending between the tips (3X) of posts (3) and a fixed portion (4B) secured and sealed to corresponding sides of the posts and the adjacent portion of the base. A bicuspid mitral valve replacement has a pair of posts (3) disposed at opposite ends of a diameter of the undistorted base (2), or displaced therefrom towards the portion (2A) of lesser flexibility to accommodate leaflets (4) of unequal size. The frame (1) is formed of Delrin covered with Dacron cloth, the Delrin having differing thicknesses to either side of the posts (3) which merge into the base (2) by way of a continuous curve (6) on each side. The leaflets (4) are cut from flat sheet in a special way to avoid stress-fixing.

Validation of the orifice formula for estimating effective heart valve opening area

Interest in the Gorlin formula for estimating heart valve effective orifice area (EOA) has recently been rekindled and the formula itself has been challenged. In this validation study, explanted native heart valves, unimplanted mechanical prostheses, unimplanted bioprostheses and explanted bioprostheses have been tested in vitro in a pulsatile flow simulator. Pressures have been measured 30 mm upstream and 100 mm downstream from the plane of the valve sewing ring (to give pressure drop, pd in kPa). Flow (Q in 1 min-1) has been measured directly by electromagnetic flowmeter and orifice areas have either been taken from manufacturer supplied data (mechanical valves) or have been digitised from video images at maximum orifice (biological valves). The formula EOA = Q/(6.96 x pd 1/2) - 0.7 fitted the data with good correlation, r = 0.96 (n = 179). The orifice assumption on which this formula is based (cf. Gorlin formula) is confirmed though it is recommended that the formula should be modified to account for (i) the pressure recovery phenomenon and (ii) the fact that forward flow through a valve only occurs over a portion of the cycle in pulsatile flow. Heart rates used in the study ranged from 40 to 140 min-1, stroke volumes ranged from 20 to 114.3 ml, cardiac outputs from 2.0 to 8.0 1 min-1 and peripheral resistance from 0.1 to 1.6 kPa 1-1 min (1 - 12 mmHg l-1 min). Application of the formula was independent of the flow conditions.

Understanding valve/host interactions through explanted valve analysis

It is essential that the factors leading to bioprosthetic valve dysfunction are fully understood if a more durable bioprosthesis is to be developed and the incidence of premature failure reduced. Studies of explanted bioprostheses yield important information on both the aetiology of valve failure and influence of valve/host interactions. Details are given of the Sheffield Explant Study which holds data on 570 explants and 34 different models. Advantages and disadvantages of this type of study are discussed.

A Hydrodynamic Model For The Left Side Action Of The Human Heart

paper gives details of a hydrodynamic model which simulates the action of the left side of the heart and the systemic circulation. The control of the system, its calibration and the recording and analysis of measured results are all computer controlled.

Valve data collection: problems and pitfalls

Since 1981, the Department of Medical Physics and Clinical Engineering at the University of Sheffield has been responsible for the organization, management and data collection associated with the largest multicentre heart valve implant patient follow-up study in the Western world. At the present time, the database comprises information on over 16,000 valve implants, which have been provided by 57 surgeons working at 22 centres in the UK. All this data is available for in-depth statistical analysis. Over 30 individual valve models presently are included in the Study and these can be categorized into five main types: ball, disc, porcine, pericardial and homograft. Analysis includes descriptive statistics as well as valuable information on the various performances of the different valves. Survival and event-free survival graphs are obtained by actuarial methods and individual valve types can be studied in depth in terms of freedom from thromboembolic complications and valve dysfunction. Whilst this approach provides interesting and valuable survival data, it does not take account of the wide variation in prognostic factors which occur within large groups of patients. This latter problem can be addressed by the use of proportional hazards analysis and this paper provides details of this approach and typical results obtained from the use of this method. These include the comparative performances of the major types of valves currently in use in terms of the event-free survival of the patients.

The Long-Term Clinical Assessment of Heart Valve Substitutes

This paper describes the organization and routine management of the UK Multicentre Valve Trial. Details are given of the present data bank comprising information on 9859 artificial valve implants collected from 44 surgeons in 22 centres. The paper discusses various analyses of the long term performances of different valves. The validity of these analyses and this approach to valve performance assessment is discussed with reference to: (i) the limitations of the present statistical techniques, (ii) the inherent variability of data arising from differences in patient/prosthesis selection and surgical techniques amongst the various participating centres, and (iii) a new approach for the estimation of the failure rates of valve implants taking such inherent variability of the data into consideration.

Elective removal of convexe-concave Björk-Shiley valves

Replacement has been an accepted method for treating advanced cardiac valvular disease for more than 25 years. However, the perfect prosthesis has yet to be developed, judging by the number of devices available. A prosthesis that initially appears promising may cause problems in due course, and indeed some devices have been modified or withdrawn from clinical use. A notable example of a prosthetic valve that has give problems is the Björk-Shiley convexo-concave prosthesis, some models of which have undergone mechanical failure due to strut fracture. We report the elective removal of such a valve and the subsequent examination of the prosthesis. The results of this examination suggest that a policy of elective removal is justified.

Biological Tissue For Heart Valve Substitutes

There are currently two major categories of commercially available heart value substitutes, namely, mechanical or bioprosthetic. In the case of the latter there are again two principal types: chemically treated and frame mounted natural porcine valves; frame mounted valves in which the leaflets are made from chemically treated biological tissue, usually bovine pericardium.