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Eur J Cardiothorac Surg 1999;16:74-80
© 1999 Elsevier Science NL

Reoperations for acute prosthetic thrombosis and pannus: an assessment of rates, relationship and risk

^a Istituto Chirurgia Cardiovascolare, University of Padova Padua, Italy
^b Istituto di Anatomia Patologica, University of Padova Padua, Italy

Corresponding author. Cardiochirurgia Università di Padova, Via Giustiniani 2, 35128 Padova, Italy. Tel.: +39-49-8212408; fax: +39-49-8212409
e-mail: rzzgli{at}ux1.unipd.it

	Abstract

Top Abstract 1. Introduction 2. Materials and methods 3. Results 4. Discussion Appendix A References

 
Objective: Mechanical valvular prostheses have the advantage of longevity but carry a risk of thrombosis which is dependant on valve design, materials and host-related interface. While pannus is common to both biologic and mechanical valves, acute prosthetic thrombosis is mostly a complication of mechanical valves; therefore we investigated to find rates and risk of these obstructive complications. Methods: Between 1/1/70 and 31/12/97, 2680 patients received at least one mechanical prosthesis in the aortic or mitral or tricuspid position and a total of 3014 operations were performed. Follow-up included 18523 years and was 98% complete. Incidence rates, Kaplan–Meier estimates, modeling of the hazard and multivariate analysis in the hazard domain were used in the analysis. Results: Overall survival was 76%, 64%, 51%, 38.5% and 29% at 5, 10, 15, 20 and 25 years, respectively. It was significantly better in aortic than in mitral than in double prosthesis. 290 patients received a single reoperation, 37 a second, six a third and one a fourth reoperation. Two-hundred and fifty-one of these reoperations were exclusively due to malfunction of mechanical prosthesis, nine to malfunction of both mechanic and biologic prostheses. Most frequent reoperative indications was dehiscence (133), pannus (48) and thrombosis (29). The linearized rate of reoperations for pannus was 0.24%/patient per year, for valvular thrombosis 0.15%/patient per year. The shape of the thrombotic hazard was constant (at random) and the relative risk 12 times higher for tricuspid prosthesis, seven times higher for mitral prosthesis. Multivariate analysis controlling for prosthetic position, age, sex and prosthetic size, showed a 67% risk reduction with larger prosthesis (>27 mm), a 69% risk reduction with the Sorin tilting disk prosthesis and an 83% risk reduction with the bileaflet prosthesis. Pannus hazard shows a delayed exponential rise and was two times higher in tricuspid and three times higher in mitral position. Multivariate analysis showed a 50% risk reduction with larger prosthesis, an 11 times higher hazard of old (caged-disk, caged ball) prosthesis and a three times higher hazard of Lillehei–Kaster prosthesis. Reoperation for thrombosis has a 62% perioperative (30 days) survival compared to 92% survival of pannus reoperation. Conclusions: Mechanical valves have a low incidence of reoperation, mostly for prosthetic dehiscence. Pannus development is the next frequent complication increasing with time since implant, therefore in this series it was related to old valvular models and tilting disk prosthesis, with longer follow-up. Acute thrombosis occurs significantly earlier than pannus formation. Despite shorter follow-up we are therefore confident that bileaflet prostheses are less prone to this complication and pannus is a rare early etiologic factor. Thrombosis has very high operative risk as compared to pannus, justifying the present trend to thrombolysate selected cases.

Key Words: Heart-valve-prosthesis • Follow-up • Thrombosis • Pannus • Hazard

	1. Introduction

Top Abstract 1. Introduction 2. Materials and methods 3. Results 4. Discussion Appendix A References

 
Mechanical valvular prostheses have the advantage of longevity but carry a risk of thrombosis which is dependant on valve design, materials and host-related interface [1]. While endocarditis dehiscence and pannus are common to both biologic and mechanical valves, acute prosthetic thrombosis is mostly a complication of mechanical valves [2].

Thrombotic valvular obstruction is most frequently imputed to faulty anticoagulation [3,4] but it has also been related to pannus formation [2]. Our valvular database has been therefore investigated to find rates, risks and relationships of these obstructive complications.

	2. Materials and methods

Top Abstract 1. Introduction 2. Materials and methods 3. Results 4. Discussion Appendix A References

 
Between 1/1/70 and 31/12/97, 2680 patients received at least one mechanical prosthesis in the aortic or mitral or tricuspid position. Three mechanical valves were replaced in two patients, a mechanic mitroaortic operation was performed on 333 patients, an aortic operation on 1492 patients; mitral operations were performed on 850 patients. Three patients had an isolated tricuspid operation. The average age was 52±13 years (range 1–89) and it was similar between aortic and mitral patient. The female/male ratio was 1163:1517.

Follow-up data was collected from outpatient visits by telephone or mail during 1998. Fifty-four patients have not yet been traced or were lost to follow-up at various time intervals; therefore goodness of follow-up is presently 98%. Patients reoperated elsewhere were included as well as patients who had their primary operation elsewhere, assuming random migration of some patients among centers.

Follow-up ranges from a few hours to 27 years: the overall time at risk was 18523 years, with a mean of 6.9 and a median of 5.5 years.

Prosthetic implant and heart preservation techniques have varied widely in the course of previous experiences. Cardioplegia has been used consistently since 1978 and single pledjetted sutures since 1980. Orientation of the mitral prosthesis was usually anatomic. The tilting disk prosthesis had a posterior opening of the larger orifice. Bileaflet and tilting disk aortic prostheses were implanted respectively with their axis or largest opening facing the left sinus.

Database coding of the cause of prosthetic obstruction from thrombosis or pannus was done from the operating surgeon at operation. Events were classified as thrombosis if not obviously due to pannus as in Horstkotte paper [5], they were classified as pannus elsewhere.

All patients received oral anticoagulation to reduce prothrombin time to 20–35% of normal and more recently to maintain INR between 2.5 and 3.5.

2.1. Statistics
Contingency tables, incidence rates and Kaplan–Meier estimates of survival were used in the exploratory analysis.

The data set was structured per patient, each having the possibility of one or more prosthetic replacements. The hazard ratios express the likelihood that, given the occurrence of an event, the patient will have the stated characteristics.

In this analysis survival time of patients with repeated operations was calculated by segmenting the individual's longitudinal record into inter-operation observations, marked by a beginning time (left censoring) and an event or final censoring time (modulated renewal method). Pannus and thrombosis were considered terminal events

The shaping and scaling of the two events, thrombosis and pannus, were calculated with the hazard procedure [6]. Multivariate analysis of specific prosthetic models was subsequently performed controlling for prosthetic position, age, sex, and prosthetic size (large vs. small, cutpoints >21 for aortic position, >27 for mitral position) by means of the Weibull and exponential models. The results were expressed as hazard ratios.

	3. Results

Top Abstract 1. Introduction 2. Materials and methods 3. Results 4. Discussion Appendix A References

 
We have observed 962 deaths: 235 patients died within 30 days; late deaths occurred in 727 patients. The linearized death incidence was therefore 5.2%/patient/year, with an expected median survival time of 15.6 years. The Kaplan–Meier survival was 64% (95% CL 62–66) at 10 years, 38% (95% CL 35–42) at 20 years, 29% (95% CL 24–34%) at 25 years. The curve shows an early risk phase followed by a constant attrition.

3.1. Reoperations
Valvular operations (3014) were performed on the patients of this series. The pool of mechanical valves that were at risk of reoperation are shown in Table 1. Prosthetic size was <21 mm in 71 aortic prosthesis, <27 mm in 184 mitral prosthesis and one tricuspid.

3.2. Pannus
Forty operative notes were reviewed. Annular concentric growth was described in 14% of cases; secondary thrombus was described in 25% of cases. Seven patients were reoperated elsewhere. Reoperations for pannus occurred on the prosthesis shown in Table 2.

A single patient was classified in both categories because of uncertainty.

Thrombosis occurred on the prosthetic devices shown in Table 2. Two patients had recurrence of the thrombosis on the new prosthesis.

The median interval to prosthetic thrombosis was 6.2 years; the 25th/75th percentiles were 2–10 years. Reoperations for pannus occurred significantly later (P<0.0001) at a median of 13 years (interquartile range 10–16), the earliest occurrence was after 1.8 years. The cumulative distribution of the events is shown in Fig. 1.

View larger version (20K): [in this window] [in a new window]   Fig. 1. Cumulative distribution of the events pannus and thrombosis.

View larger version (22K): [in this window] [in a new window]   Fig. 2. Squares, Kaplan–Meier failure function; triangles, 70% confidence limits of the Kaplan–Meier estimates; continuous line, parametric failure function; dashed line, 70% confidence limits of the parametric failure; Hazard ratio, relative risk of the event in the stated position.

View larger version (19K): [in this window] [in a new window]   Fig. 3. Squares, Kaplan–Meier failure function; triangles, 70% confidence limits of the Kaplan–Meier estimates; continuous line, parametric failure function; dashed line, 70% confidence limits of the parametric failure; hazard ratio, relative risk of the event in the stated position.

Prosthetic thrombosis had a severe perioperative risk. The 30 days survival was 49% (95% CL 26–69%) and 17 years survival was 18% (95% CL 4–39%). The reoperative risk was significantly higher (P<0.001) than the risk of reoperation for pannus which was 0.94% (95% CL 77–98%) at 30 days and 74% (95% CL 49–88%) at 17 years.

Anticoagulation

All these patients received anticoagulation. In a single case oral anticoagulation was interrupted for hysterectomy. Faulty anticoagulation control was possibly present in six cases (35%) because of out of target preoperative values (4) or because of relaxed control times. Three patients had prior embolic episodes.

	4. Discussion

Top Abstract 1. Introduction 2. Materials and methods 3. Results 4. Discussion Appendix A References

 
Mechanical valvular prostheses are selected in younger recipients because of their durability. The favorable longterm outcome after prosthetic valve replacement is mostly related to reversibility of myocardial and organic failures and is therefore a question of operation timing [7,8], nonetheless complications of prosthetic function and/or prosthetic tissue interface have an equally important role on survival. Thrombogenicity is a persistent risk requiring anticoagulation: therefore severe bleeding complications as high as 18.6% vs. 7.1% (P<0.01) have been found in a prospective trial of Bjork–Shiley versus porcine valves [9].

In our study we focus on obstructive complication, pannus and thrombosis that are most common in mechanic valves. Complications were classified as mutually exclusive events by the operating surgeon, based on the operative analysis of the mechanism of occlusion [5], although thrombus was described in 25% of pannus cases and pannus in two thrombus cases.

We do not have the ambition to define the true incidence of these complications, which would require an investigation of all death causes and autopsies. We think that reoperation rates can give a fair idea of the true incidence because as Horstkotte [5] pointed out, the clinical symptoms progress slowly over weeks and months, and should permit diagnosis and treatment in countries with a reasonable health system. In fact a fresh thrombosis was seen in only 17% of our patients and in the majority of operative notes (42%) a variable amount of organized thrombosis with successive thrombotic stratification and apposition was described.

Although thrombosis has been directly related to anticoagulation [10], a direct relationship with the intensity of anticoagulation has not been demonstrated [11]. In this experience we did not find major deviations from the standard anticoagulant regimen but for a single patient; minor deviations were probably present in 14% of patients.

Apart from anticoagulation failure, recurrent embolism, size and design of the prosthesis, atrial rhythm, cardiac failure and individual propensity are concomitant risk factors. Furthermore, to fully appreciate the hazard of faulty anticoagulation control on 2680 patients will require collection and averaging of huge amount of data, because the thrombotic process is most often gradual or occurs with random thrombotic apposition. In fact it has been shown by a perspective study of a small cohort of patients that periods of high and low variability of PT ratios define high and low risk of thromboembolism, respectively [12].

Our study extends for 18 523 patient years. The survival rates of our patients series compares favorably with the survival rates reported at 5, 10, 15 and 20 years (76.6%, 59.6%, 44.9% and 31.2%, respectively) by Orszulak [13] for the Starr–Edwards, as benchmark for subsequent improvement and comparison of current prostheses. This signifies that our series is representative of the average surgical results.

The thrombosis rates we found are similar to those reported by Murphy [14] for the Bjork–Shiley prosthesis and to many others summarized by Horstkotte [5] in his report.

On the contrary, the shape of the hazard function differs from the one found in 1084 Bjork–Shiley standard valves by Blackstone [15]. In our surgical experience, with the use of a mix of different prosthetic devices, thrombosis seems to occur at random, with a constant rate and without time relationships, as can be seen from the Kaplan–Meier failure function (Fig. 2).

The peculiar thrombotic hazard for prosthesis in the atrio-ventricular position [5] is confirmed in our study with crude rates seven to 12 times larger than in the aortic position. This finding is modulated by concomitant variables. A novel finding is the protective effect of larger size prosthesis (>27 mm diameter) which could be explained by a more regular transvalvular flow and accounts for a 67% reduction of the mitral risk. Also Sorin tilting disk prostheses and especially bileaflet prostheses seem to reduce the thrombotic hazard, pointing to advantages from different design and materials. An 83% hazard reduction with the bileaflet prosthesis results in a thrombosis rate of 0.017%/patient per year. Therefore the probability of reoperations for mitral prosthetic thrombosis in large bileaflet prosthesis versus a small caged disk prosthesis, can be simulated in Fig. 4.

View larger version (27K): [in this window] [in a new window]   Fig. 4. Predicted probability of reoperation for thrombosis if mitral prosthesis: yes, bileaflet large prosthesis vs. old prosthetic model. Large=size >27 mm. Old models: includes caged ball-disk or tilting disk (Sorin tilting disk excluded) prosthesis. Continuous lines, predictions; dashed lines, 70% confidence limits.

The crude hazard is two times higher in tricuspid and 3.2 times higher in mitral position than in aortic position. Multivariate analysis shows a 50% risk reduction with the large mitral prosthesis. Furthermore the Lillehei–Kaster prosthesis has a significant three times higher hazard and the old prosthesis (caged ball and caged disk) has a significant 11 times higher hazard. Therefore the probability of mitral reoperation for pannus with a more recent prosthesis versus an old prosthesis, can be predicted from Fig. 5.

View larger version (26K): [in this window] [in a new window]   Fig. 5. Predicted probability of reoperation for pannus if mitral prosthesis: yes. Recent prosthetic model=bileaflets or tilting disk model (Lillehei–Kaster excluded). Large=prosthetic size >27 mm. Continuous lines, predictions; dashed lines, 70% confidence limits.

We conclude that obstruction of a mechanical valve is a rare event. Thrombosis occurs at random whereas pannus formation requires a larger time interval and has a delayed exponential increase. This is a crucial observation. If the hazard function increases with time our expectations of better results for newer prosthetic models will deserve the test of time and the finding of a significant association of pannus with older prosthetic models (especially caged disk and Lillehei–Kaster valves) could be simply due to the longer follow-up time.

On the contrary, acute thrombosis occurs significantly earlier than pannus formation and in a random fashion. Despite shorter follow-up we are therefore confident that bileaflet prostheses are less prone to this complication. A further inference is that pannus formation is an unlikely etiologic factor of early thrombosis. In face of the generally reported high operative risk of prosthetic thrombosis, this criterion can be added to the criteria of Renzulli [19] to select patients for thrombolysis.

	Acknowledgments

 
We thank Dr Francesco Veronese for his helpful assistance in patient follow-up. This study was supported by Regione del Veneto ricerca finalizzata 714/02/96 and Istituto Superiore di Sanita (Roma).

	Footnotes

 
Presented at the 12th Annual Meeting of the European Association for Cardio-thoracic Surgery, Brussels, Belgium, September 20–23, 1998.

¹ Fellow of the European Thoracic Cardiovascular Surgery Association.

	Appendix A

Top Abstract 1. Introduction 2. Materials and methods 3. Results 4. Discussion Appendix A References

 
Conference discussion
Dr A. Renzulli (Naples, Italy): In a larger series of 100 cases of prosthetic thrombosis we found an incidence of pannus in 80% of cases. I should like to ask to you, did you find any difference in pannus formation between the leaflet and tilting disk, in a sense that did you find an eccentric formation in the tilting disk and a concentric formation in the leaflet? And second question, did you find any recurrence of a pannus formation in patients reoperated for pannus formation, because we found a subset of patients undergoing MVR redo procedures for pannus formation for thrombosis that had the second and the third reoperation for recurrence of a pannus, and, independently, which prosthesis has been used?

Dr Rizzoli: Well, to know exactly which is the cause of an event like thrombosis or pannus, it is rather difficult. This is a retrospective study and the patients were classified as acute thrombosis only if the surgeon felt the cause was not pannus. In our experience there is always some thrombosis in patients with severe pannus growth. Anyway, a large amount of pannus growth was found only after 1 year and 7 months in our experience. So all the occurrences of thrombosis before that time are not related in any sense with pannus. I think the main cause of thrombosis is related to anticoagulation. Pannus growth is of course one of the complications of any mechanical prosthesis, and with time, as you have seen, there is an increased occurrence of this complication. Because of the retrospective analysis, based on hospital charts, I can't say exactly how much and if the pannus is really the cause of thrombosis. It is unlikely from our numbers and timing of the event.

Dr I. Virdi (New Delhi, India): At reoperation did you replace those valves?

Dr Rizzoli: Yes.

Dr Virdi: Or did you ever attempt to surgically remove the thrombus from the valve and leave the valve in situ or excise the pannus?

Dr Rizzoli: No. We routinely change the valve. I think there have been only one or two cases in all the series where this has not been done, but this was done because the amount of pannus was very small. Usually we remove the valve if thrombosed or if it has pannus. I must say that there are at least two occasions in our series where a new thrombosis was seen after changing the valve with a different model. This points to some anticoagulation problem more than to pannus formation.

Dr D. Saksena (Bombay, India): Our experience has just not been the same, it has been a little bit different, particularly in the mitral position with the bileaflet and the monoleaflet valves. Of course I must admit that we have a very hostile population in the sense anticoagulation is less than ideal and sometimes is very erratic. In those circumstances we find that the monoleaflet valve has lesser thrombosis than the bileaflet valve in the mitral position.

Dr Rizzoli: Well, as it is shown from our data, it is always difficult to say something on these problems because certain models of prostheses, like, for instance, the bileaflet prosthesis, has of course a shorter follow-up than patients with an older model prosthesis. So it is a suggestion that we make that these prostheses should have a lesser complication rate. We do not actually have the number until we have a follow-up for this prosthesis similar to the other prostheses. As shown in our paper the tilting disk model has in fact a decreased hazard of thrombosis.

	References

Top Abstract 1. Introduction 2. Materials and methods 3. Results 4. Discussion Appendix A References

 

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