pericardial heart valves pericardial heart valves the pericardial heart valve was invented by marian ionescu, a british surgeon working at the general infirmary in leeds, england. he created this artificial bioprosthetic heart valve as a three-cusp structure made of chemically treated bovine pericardium attached to a dacron cloth-covered titanium frame. the realisation of a concept the experimental and in vitro testing of this novel device took place in 1970, and in march 1971, ionescu began, for the first time, the implantation of the pericardial valve in all three cardiac positions in humans. between 1971 and 1976 the valves had been made in ionescu's own hospital laboratory. throughout these five years of usage in 212 patients the performance of the pericardial valve was thoroughly evaluated. the results showed that this original valve exhibited the best haemodynamic performance, at rest and during exercise, when compared with the reported results of all other artificial valves in existence. it demonstrated a very low risk of embolisation even in the absence of long term anticoagulation treatment of the patients. there were no cases of valve thrombosis, intra-vascular haemolysis or sudden, unexpected valve failure. the durability of the valve was good at 5 years of follow-up. based on these results, the shiley laboratory in irvine, california, began to manufacture this valve and to distribute it worldwide under the name of the 'ionescu - shiley pericardial xenograft.' from 1976 onwards a series of modifications were made in order to improve the qualities and the performance of the pericardial xenograft. the selection and preparation of the bovine pericardium were standardised and rigorously controlled. for tissue fixation at zero pressure a solution of 0.5% purified glutaraldehyde was used. it contained an optimal proportion of monomers and polymers and an ideal cross-link density was obtained by controlling the concentration and the ph of the solution as well as its temperature and exposure time of the tissue to its action. the thickness and pliability of the pericardium were standardized and the direction of macroscopically visible fibres matched for each three cusps of a particular valve. the supporting stent was changed. the titanium was replaced with machined delrin which is an acetyl homopolymer with low 'creep' properties due to a stable molecular memory. it is flexible and shock absorbent, essential qualities for a tissue heart valve support. this new stent contained a radio-opaque marker at its base for easy identification. the contour of the scalloped posts was modified and the height of the stent reduced. the entire delrin structure was covered with seamless dacron velour and at a later stage, the margins of the scalloped edges were covered with a thin layer of pericardium in an attempt to prevent or reduce the abrasion of the leaflets when in contact with this margin during valve closure. the sewing rim was bolstered for better and safer attachment to the heart annuli and its shape was anatomically contoured into two different configurations to better fit in the aortic and the atrio-ventricular positions. two other additions were made: an integral valve holder which prevents the touching of the valve's cusps, and a 'freeze-watch' indicator as a safe—guard against exposing the valves during transportation or storage at temperatures below 4 degrees celsius. the geometry of the valve was slightly modified due to changes in the shape of the stent and by removing the outside pledgets around the posts. this gave a more streamlined shape of the whole structure. these modifications had been progressively introduced and all of them were incorporated in the 'ionescu - shiley low profile pericardial xenograft' valve, which became available in 1983. approximately 200,000 pericardial valves manufactured by shiley laboratories were distributed around the world between 1976 and 1987 and it is presumed that most of them were implanted in patients. the use of this valve generated a lot of interest expressed in several specialist symposia, academic meetings, and numerous scientific articles published over the years. the appropriation and organisation of this enormous material and the classification and interpretation of data has been a very difficult and complex task, especially because - contrary to what it is claimed - there remains a great deal of variation in standards of reporting in all essential chapters of a scientific work. in some cases it is quite impossible to follow such standards as it will be described later. despite all these difficulties and impediments, a general view of the performance of the pericardial valve as close to reality as possible could be obtained. one should however keep in mind that any single investigator should resist the temptation to write a review of such a complex matter as tissue heart valves, and to cover the subject completely and fairly. one should also remember that if we study complex and variable conditions, averages must be rejected because they confuse while aiming to unify, and distort while aiming to simplify. from the material available it is evident that the reported hospital mortality and, up to a certain point, late mortality are similar among the various publications of different authors, and do not directly reflect on the quality of the valve used. haemodynamic investigations and in-vitro testing of valves the ionescu pericardial valve had a large central opening almost equal with the inner surface area of the supporting stent. this, plus the pliability of the pericardial tissue, confer this valve exceptional hydraulic qualities. haemodynamic studies by several authors investigating patients with pericardial valves demonstrated that in all respects the haemodynamic function of this valve is superior to that reported for the porcine valves and, generally speaking, equal to that of the best mechanical prostheses. the haemodynamic results reported by other investigators are very similar to those by tandon's group. some authors stressed the advantage of very low pressure gradients across small pericardial valves (viz: 17, 19 and 21mm diameter) for implantation in small aortic roots without the need of complex surgical techniques for root enlargement. tandon and associates performed pre- and postoperative haemodynamic investigations at rest and during exercise in 110 patients. there were 51 with aortic valve replacement, 44 with mitral replacement, 3 with tricuspid and 12 who received multiple valve replacement. following a technique and protocol developed at leeds general infirmary, from the group of 110 patients investigated, 13 patients with aortic and 6 with mitral valve replacement were subjected to multiple, sequential haemodynamic studies at rest and during exercise at the following intervals: aortic: preoperatively and at 9.9, 42.2 and 68.3 months postoperatively; mitral: preoperatively and at 11.2, 42.3 and 68.7 months postoperatively. the results obtained showed that the considerable improvement recorded at the first postoperative investigation was maintained up to 68 months following valve replacement. in order to demonstrate visually the reasons for the great haemodynamic difference between the pericardial and porcine valves, ionescu recorded in a 'pulse duplicator' the opening characteristics of two porcine valves (hancock modified orifice and the recently modified edwards valve) and two pericardial valves ( the standard and the low-profile shiley valves). all four valves were manufactured for clinical use and all had an implantation diameter of 25mm. the valves were tested under identical conditions in the mitral compartment of the pulse duplicator and photographs were taken at the peak of diastole. the flow rates were for each frame, from left to right: 0, 100, 200, 300 and 400 ml per second. the opening of the cusps of both types of pericardial valves is synchronous and regular, without three-dimensional flexure and the low-profile pericardial valve shows an even larger opening when compared with the standard pericardial valve. there are no crevices or dead spaces behind the open cusps of the pericardial valves. the difference between the porcine and the pericardial valves is flagrant in all respects. (fig ) many authors had studied, in-vitro, the hydrodynamic performance of the pericardial valve and found that it possesses better functional characteristics than the porcine valves and similar to those of the best prostheses. in summary, the excellent haemodynamic function of the pericardial valve is one of its great advantages and sets it aside from all other tissue valves. valve-related complications thrombosis, embolism and anticoagulation-related haemorrhage while dealing with a very large number of reports from different hospitals with various numbers of patients who received pericardial valves and were followed up for differing durations of time, from five to ten years, and especially because the reporting did not follow an 'established' albeit loose 'standard' of identification, description and grading of the events, it is better to enumerate the results from some of the more representative series reported and draw only general conclusions. the following data show the results as given in actuarial percentages of freedom from embolism. table i. freedom from embolism main author no. of patients duration follow-up actuarial freedom from embolisation da cooley 2701 5 years 93.2% for all patients, 96.1% for aortic, 89.9% for mitral and 94% for mitral and aortic replacement a pavie 675 5 years 93.8% for all patients m holden 290 6 years 5 emboli (1 certain, 4 doubtful) 0.70% per patient year j m reveulta 90 8 years 93.6% for all patients l gonzalez-lavin 224 8 years 95.3% for aortic, 97.4% for mitral replacement jb garcia-bengochea 248 8 years 97.5% for all patients n p silverton 492 6–10 years 96.8% for mitral, 97.2% for multiple replacements x d zhu 520 9 years 95.8% for all patients m i ionescu 1171 10 years 96.4% for aortic, 96.8% for mitral, 97.2% for multiple replacements from perusing innumerable publications on the results of heart valve replacement with pericardial valves concerning the rate of embolic complications, one may formulate several conclusions. a clear picture concerning the exactitude of thrombotic and embolic complications of artificial heart valves, and especially of pericardial valves, seems to be very difficult. the knowledge at present is superficial and incomplete concerning the real causes and the risk and contributing factors to this complex phenomenon. consequently, it has never been practical to try to standardise definitions, and even more complicated to establish lines of treatment. everyone talks of 'causes' and 'risk factors' but no-one possesses any scientific evidence to this effect. the so-called 'risk factors' for embolisation, with the exception of atrial fibrillation, can be called, at best, 'scientific illusions'. consequently, any scientific, logical way of establishing a therapeutic means for preventing such phenomena due to unknown or incompletely understood causes is doomed to remain empirical, and the end results uncertain. there are myriads of reports for and against anticoagulant treatment for patients with tissue heart valve replacement. in addition, heart valve replacement patients are followed-up by a 'committee' made up successively by the surgeon, the cardiologist, the general practitioner in this or another town, etc. the impression of knowledge or our acceptance of ignorance compound this matter further. the only solution, for patients and doctors alike, would be an artificial heart valve which carries a very low risk of clotting, and therefore would not require, in the majority of cases, anticoagulant treatment. one main drawback in the recent 'scientific' literature on pericardial valves is that the essential data for arriving at an intelligent interpretation of results is missing. there is no description of the pre-operative condition of the patients concerning cardiac rhythm, various arrythmias, atrial fibrillation, anticoagulant treatment, previous systemic emboli, etc., and scant information about the post-operative condition: cardiac rhythm, the nature and duration of anticoagulation, the time of occurrence of embolic phenomena and the magnitude and sequelae, if any. all this is already in the past, now, for practical purposes, one can conclude that the pericardial valve carries a very small risk of embolisation, much smaller than that of the porcine valves even in the absence of anticoagulant treatment. the risk of pericardial valve thrombosis is exceedingly remote. the very few cases reported have not been thoroughly investigated as far as the cause or the contributing factors, related or not to the valve, are concerned. anticoagulant related haemorrhage was extremely rarely reported because few patients received prothrombin depressants for long periods of time (sublata causa tollitur effectus). there are several reports on tissue valves stating that the incidence of emboli was higher among patients receiving anticoagulation. in addition, it was observed that the rate of embolisation appears to be decreasing with the passage of time with the pericardial valves, unlike the experience with porcine valves