Thorax (1969), 24, 142.

Implantation of homologous and heterologousaortic valves in prosthetic vascular tubes'

An experimental study

C. M. G. DURAN2, R. WHITEHEAD, AND A. J. GUNNINGFrom the Nuffield Departmenit of Suirgery and the Department of Pathology, Radcliffe Infirmary, Oxford

Freeze-dried dog and pig aortic valves sutured within tubes of woven Teflon and knitted Dacronhave been inserted into the descending aorta of 56 dogs with aortic incompetence. All the valvesplaced inside woven Teflon were destroyed, with one exception, where six months after insertionthe valve was competent and in good condition. Most of the valves in knitted Dacron remainedin good condition for up to three months after surgery. The few bad results with knitted Dacronwere related to infection or bad haemodynamic conditions. There was no observable differencein behaviour between the homologous and the heterologous valves during the short follow-up ofthese experiments.

Homologous and heterologous aortic valves havebeen used, with encouraging results, to replace thediseased aortic valve in patients. Although the tech-nique for their insertion is becoming standardized,further technical improvements are still desirable.The insertion of a homologous or heterologousaortic valve in a prosthetic framework wouldtheoretically have technical advantages. A pros-thetic framework would, for instance, simplify thesurgical insertion by providing a rigid ring. Geha(1967) and Weldon, Ameli, Morovati, and Shaker(1966) have reported experimental work using ametal frame covered with Dacron in the sub-coronary position in the calf and in the mitralarea of the dog. Ionescu, Wooler, Smi,th,- andGrimshaw (1967) and Carpentier (1967) haverecently used a Teflon ring to insert heterologousaortic valves into the mitral area.

In this work we have studied the fate of homo-logous and heterologous aortic valves sutured in-side tubes of Teflon or Dacron and inserted intothe descending aorta of 56 dogs in whom aorticincompetence had been produced. We felt that thesurgical simplicity of this method would yielduseful knowledge about the combination of heartvalves and prosthetic materials.

'This work was sponsored by grants from the British Heart Foun-dation and the Medical Research Council2Present address: Cardiovascular Department, Faculty of Medicine,University of Navarra, Pamplona, Spain

TECHNIQUE

The pig aortic valves were obtained unsterile fromthe slaughter-house within 12 hours after the animalwas sacrificed, the hearts having been kept in arefrigerated chamber. The dog aortic valves were col-lected immediately after death in the research labora-tories. All specimens were kept in sterile jars inHanks' solution (Gross, Bill, and Peirce, 1949) in acommercial refrigerator for the final dissection. Thisperiod of storage never exceeded eight days, themajority being stored for only 24 hours. The aorticvalves were then trimmed so that only a few milli-metres of aortic wall remained above, and a smallcuff of myocardium and mitral valve below, the in-sertion of the cusps. The valve was then sutured intoa Teflon3 or Dacron' vascular tube and the wholewas sterilized by immersion in liquid ethylene oxidefor 30 minutes and subsequently freeze dried andstored in a vacuum. At the time of the insertion thevalves were reconstituted by immersion in warmsaline for 20 minutes.The dogs were anaesthetized with intravenous nem-

butal, and the trachea was intubated and ventilatedwith a Starling's respirator. A left thoracotomythrough the fourth intercostal space was performedand a segment of descending thoracic aorta was dis-sected out. A left sub-clavian artery/left femoralartery bypass was used with a siliconized 4 mm.internal diameter Tygon tube. No heparin or trans-

3Teflon-Usci Woven, Catalogue No. 30104Dacron-Usci Knitted, Catalogue No. 6015

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Implantation of homologous and heterologous aortic valves in prosthetic vascular tubes 143

fusion was used in any of the animals. Arterial pres-sure was recorded with polythene catheters intro-duced into the right brachial and right femoralarteries. The aorta was clamped and transected, anda small segment of about 1 cm. in length was excised.The clamps were applied for between 11 and 25minutes. During this time the arterial pressure distalto the clamp averaged a mean of 40 mm. Hg. Theprosthetic tube was then anastomosed end-to-end tothe aorta with an anterior and posterior suture of4/0 Mersilene. The aortic clamps were removed andthe bypass was discontinued. The stump ef the leftsub-clavian artery was then used to introduce a metalhook which was directed retrogradely towards thenon-coronary cusp of the dog's aortic valve, whichwas perforated and split on withdrawal to make itincompetent. The degree of aortic incompetence wasrecorded from the right brachial artery pulsationwhile the competence of the grafted valve was re-corded from the femoral artery pulsation. The chestwas closed in layers. The chest was drained with acatheter which was removed at the end of the pro-cedure. All dogs received penicillin and streptomycinfor one week after surgery.

RESULTS

Fifty-six dogs were operated upon. Teflon wasused in 15 dogs. In 10 of these dogs a pig valvewas inserted, and in the remaining five a dog valvewas used. In 31 dogs a Dacron tube was used, with20 pig and 11 dog valves.The operative mortality was high; 19 died with-

in 24 hours of surgery (33 %). The main causes ofdeath were aortic incompetence, which was re-sponsible for 12 deaths, and haemorrhage, whichwas the cause of three deaths. One died fromfailure of the respirator, and in three cases thecause of death could not be established.

-Fifteen dogs with a Teflon graft survived theoperation (2 dog valves and 13 pig valves). Alldied or were sacrificed from a few days up to sixmonths after surgery. The specimens from thesedogs showed, at the end of six days, a normalvalve, but from then onwards all but one speci-men showed a destroyed aortic valve. Both theaortic wall and the valve cusps were torn andhad partially disappeared. In one dog (with a pigvalve), sacrificed three months after surgery, thebase of the destroyed valve had become calcified.The exception was a dog sacrificed six monthsafter operation. Arterial pressures taken aboveand below the grafted pig valve prior to sacrificeshowed a perfectly competent valve. The speci-men of this dog showed the Teflon graft and ana-stomoses covered with neointima which bridgedthe pig aortic valve anastomosis, and the threecusps were in perfect condlition (Fig. 1).

FIG. 1. Specimen from a dog with a Teflon graft and apig aortic valve s..x months after insertion.

Twenty-two dogs with Dacron grafts survivedthe operation (8 dogs valves and 14 pig valves).Ail, except three which have been kept for long-term studies, have died or been sacrificed at one,two or three monthly intervals (11 dogs). Thevalve cusps were thin and present in all but threespecimens (sacrificed 41, 72, and 82 days post-operatively). In two of these, the aortic wall wasdestroyed and specks of calcium deposits werepresent. In the third animal, one cusp had sepa-rated from the aortic wall. Two of them had in-fected thoracotomy wounds, and the other wassuffering from distemper as well.

In five cases, all between 33 and 64 days aftersurgery, one or more cusps, although thin andpresent, were stuck to the aortic wall, making thevalve incompetent (Fig. 2). These bad results were

FIG. 2. Dacron graft and a pig aortic valve 64 days afterimplantation. Note that the valve cusps, although present,are adherent to the sinus aortic wall.

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144 C. M. G. Duran, R. Whitehead, and A. J. Gunning

possibly related to mechanical factors, because inone dog the valve grafted was incompetent from A

the moment of insertion. In another, not enoughaortic incompetence had been created, and in athird, good incompetence at the time of surgeryhad become compensated one month later.The other 11 dogs had valves which were thin

and competent (Fig. 3). No difference in behaviourcould be detected between the homologous andheterologous valves in the short follow-up of theseexperiments. In fact the two longest survivors withperfect valves had received pig valves (6 monthswith Teflon and 3 months with Dacron).

HISTOLOGY

The earliest change seen in the aortic wall, myo-cardial cuff, and valve is a loss of nuclei from ithe cells. This nuclear loss tends to be complete > Y ,in the myocardial and aortic tissue, but smallpyknotic aggregates remain in the valve cusp cells.These are found near the base or in relation tosurface thrombus. In this respect no differencebetween the dog and pig valves could be seen.

In the first few days there is an acute inflam- FIG. 3. Dacron graft with a pig aorticmatory reaction at the outer surface of the pros- valve three months after implantation.

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FIG. 4. Dacron transplant at one month. Inflammatory infiltrate and fibrous cuff on outer aspect ofDacron (below). (H. & E. x 85.)

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Implantation of homologous and heterologous aortic valves in prosthetic vascular tubes 145N w F .^ X , ; * : . S . iA.; tO 4 .'Ss,..',' >

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FIG. 5. Dacron transplant at three months. Fibrous outer layer continuous via wedge-shaped connexions withinner layer on luminal surface (above). (H. & E. x 45.)

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FIG. 6. Teflon graft at six months. Fibrous layer inside (above) and outside (below) Teflon is present hutthere are fewer and smaller wedge-shaped bridges as compared to Figure 5. (H. & E. x 40.)

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C. M. G. Duran, R. Whitehead, and A. J. Gunning

FIG. 7. Teflon transplant at six months. The valve is acellular with only patches of persistentnuclei. Valve base is fibrous and continuous with luminal layer. Note acellular areas representing remainsofaortic cuff. (H. & E. x 40.)

thetic tubes. This reaction penetrates the graft butdoes not involve the valve base tissues, seeming toend more or less abruptly at the outer layer ofaortic tissue or myocardium. In the ensuing weeksthis infiltrate assumes a more chronic form andis attended by a proliferation of fibroblasts andfibrous tissue (Fig. 4). Later, fibrous tissue growsthrough the interstices of the prosthesis to becomecon,tinuous with a fibrous layer on the inside whosenuclei lie parallel to the wall (Fig. 5). At this stagethere is also some fibrous replacement of the outerpart of the cardiac muscle cuff but the aortic tissueappears to remain separate.The inner fibrous layer extends usually to the

base of the valve only, but when the sinus containsorganizing thrombus, as sometimes occurs, thenit becomes continuous with this tissue. Much ofthe thrombus seen on the valve surface and on theinside of the prosthetic tubes, however, is of recentorigin and shows no evidence of organization. Itprobably forms preterminally as the circulationslows, or it may be formed and lysed as a con-tinuous process during life.The reaction to the Dacron is considerably

more pronounced than to the Teflon, and the ulti-mate fibrous matting produced in the interstices

of the tubes is less dense in the Teflon (Fig. 6).After several months the valve appears largely un-changed from earlier appearances. It is more orless acellular with persisting patches of pyknoticnuclei (Fig. 7). Thrombus when present is usuallyfresh, but occasionally, as earlier, in the sinusesorganizing thrombus is seen. Even at six monthsthe aortic wall cuff can still be recognized and isquite acellular. Other valve base tissues appear tobe incorporated in the surrounding fibrous re-action. In two cases tiny foci of calcificationoccurred both in the new fibrous cuff and in theaortic wall remains. In long survivors there is stillno appreciable difference between dog and piggrafts. Essentially the changes seen correspondclosely to those described by us in detail withfreeze-dried homografts (Duran and Whitehead,1966) and heterografted aortic valves (Duran,Gunning, and Whitehead, 1967).

DISCUSSION

It is known that a freeze-dried and therefore deadaortic valve remains acellular but will functionfor long periods of time, both in the human(Barratt-Boyes, 1965; Ross, 1967) and in theanimal (Duran, Manley, and Gunning, 1965). We

146

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Implantation of homologous and heterologous aortic valves in prosthetic vascular tubes 147

have recently studied four dogs with freeze-driedhomologous valves inserted into the descendingaorta between two and two-and-a-half years aftertheir implantation in the dog. In one dog thetransplanted aortic wall had become enormouslydilated and the cusps could not be identified, butin the other three the cusps were in perfect condi-tion, although in one of them the aortic wallshowed areas of calcification. It is thereforereasonable to emphasize the biological differencebetween a fresh or freeze-dried aortic wall andan aortic valve cusp. The latter does not seem toneed any intrinsic vascularization which, as wehave shown elsewhere, is not normally present(Duran and Gunning, 1968). This fact might ex-plain the difference between the bad results ob-served with arterial grafts in peripheral vascularsurgery and the good results to date with valvegrafts. If a valve inserted within a Dacron tube isshown to become incorporated into the animal fora few months, it is reasonable to assume that itwill remain functioning for long periods of time.A longer follow-up will be required to establishthe eventual fate of this type of experiment.The difference found in these experiments be-

tween valves placed inside tubes of woven Teflonand those placed in knitted Dacron shows theimportance of the porosity of the prostheticmaterial, allowing for the penetration of smallblood vessels and the strong fixation of the baseof the valve cusps. (The calculated porosity ofknitted Teflon or Dacron is about 10 times higherthan their woven equivalents.)

The lack of difference between the fate offreeze-dried homologous and heterologous valvesin this situation is encouraging when heterologousva!ves are being considered for use in the human.However, the question of the long-term behaviourof these transplants still remains unanswered.The incompetence of the inserted valve present

in five dogs was due to one or more cusps becom-ing attached to the aortic wall. The fact that un-favourable haemodynamic conditions were presentin three dogs emphasizes the importance of purelymechanical factors in the eventual fate of thesetransplants.

REFERENCES

Barratt-Boyes, B. G. (1965). A method for preparing and inserting ahomograft aortic valve. Brit. J. Surg., 52, 847.

Carpentier, A. (1967). Communication to British Research Society,Aberdeen. Or personal communication, Paris.

Duran, C. M. G., and Gunning, A. J. (1968). The vascularization ofthe heart valves: a comparative study. Cardiovasc. Res., 2, 290.

and Whitehead, R. (1967). Experimental aortic valveheterotransplantation. Thorax, 22, 510.Manley, G., and Gunning, A. J. (1965). The behaviour of homo-transplanted aortic valves in the dog. Brit. J. Surg., 52, 549.and Whitehead, R. (1966). Experimental homologous aorticvalve transplantation: a comparison between fresh and freeze-dried grafts. Brit. J. Surg., 53, 1049.

Geha (1967). Communication to American Association for ThoracicSurgery, New York. Or personal communication, Rochester(Minn.).

Gross, R. E., Bill, A. H., and Peirce, E. C. (1949). Methods forpreservation and transplantation of arterial grafts. Surg. Gynec.Obstet., 88, 689.

lonescu, M. I., Wooler, G. H., Smith, D. R., and Grimshaw, V. A.(1967). Mitral valve replacement with aortic heterografts inhumans. Thorax, 22, 305.

Ross, D. N. (1967). Homograft replacement of the aortic valve. Brit.J. Surg., 54, 842.

Weldon, C. S., Ameli, M. M., Morovati, S. S., and Shaker, I. J. (1966).A prosthetic stunted aortic homograft for mitral valve replace-ment. J. surg. Res., 6, 548.

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