Prosthetic heart valve

The risk of embolism associated with mechanical heart valves is 2 to 6% per patient per year despite anticoagulation and is highest with valves in the mitral position. Warfarin therapy (INR 2.5 to 3.5) is recommended in these patients. The addition of enteric-coated aspirin (100 mg/d) to warfarin (INR 3.0 to 4.5) in high-risk patients (preoperative atrial fibrillation, coronary artery disease, history of thromboembolism) with mechanical valves decreases the incidence of systemic embolism and death from vascular causes (1.9 vs. 8.5% per year), but increases the risk of bleeding. 

Thrombo-embolism is a well known complication in patients with prosthetic heart valves. The pathogenesis of thrombo-embolic incidents is unclear but it seems that platelets play a significant role, due to the interaction between these cells and the artificial surfaces of the prosthetic valve. In an attempt to detect possible metabolic abnormalities, heat production rate was measured in platelets from patients with prosthetic cardiac valves [84]. Significantly decreased calorimetric values were observed, indicating that the metabolic activity of platelets in these patients is abnormally low (see Table 16). 

Heat production rate (P) of platelets from patients with valvular heart diseases, operated and not operated with prosthetic heart valves, compared to healthy subjects. Source reference 84. 

These results are in agreement with the observations of other investigators who had previously found decreased platelet ADP and ATP as well as shortened platelet survival, that correlated with thrombo-embolic manifestations in patients with prosthetic heart valves. No conclusive explanation can be offered for the observation of decreased heat production rate in the platelets of patients with prosthetic heart valves. Previous investigators have advanced the hypothesis that mechanical, chemical or immunological factors may cause alterations of blood cell function by interaction of the cells with the prosthetic valves. It is interesting to note that in the calorimetric study, the two patients who had double prosthetic valves were found to have P values below the mean value of the whole patient group. Moreover, the only patient who exibited thrombo-embolic manifestations, had the lowest value of platelet heat production rate. 

These studies seem to indicate that microcalorimetry is a suitable method for the quantitative measurement of overall metabolism in platelets from patients with prosthetic heart valves. It might be of help for identification of patients with high risk to develop thrombo-embolic complications after replacement of heart valves. 

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All patients with a mechanical prosthetic heart valve should receive concomitant unfractionated heparin or a low molecular weight heparin in combination with warfarin pharmacotherapy until the INR is therapeutic and stable for two consecutive days... 

If a patient with a mechanical prosthetic heart valve suffer a systemic embolic event despite having a therapeutic INR... 

FIGURE 6-9. Decision algorithm for stroke prevention in atrial fibrillation.27 Risk factors for stroke prior transient ischemic attack or stroke hypertension heart failure rheumatic heart valve disease prosthetic heart valve. Target International Normalized Ratio = 2.5 (range 2 to 3). 

Additionally, the patient needs to be counseled on the necessity of prophylactic antibiotics prior to any dental or surgical procedure in order to prevent recurrent infections. This is critical in patients with risk factors that predispose them to developing IE, such as prosthetic heart valves, other valvular defects, or previous IE. 

Aortic aneurysm, prosthetic heart valve, vascular or orthopedic prosthesis... 

In 1960, the Starr-Edwards heart valve was invented by Albert Starr and Lowell Edwards. Before the invention of this heart valve, people had only survived for 3 months following the introduction of a prosthetic heart valve. The Starr-Edwards heart valve was a great success as it significantly extended patient survival. This was a particularly impressive invention because the replacement of a heart valve is highly challenging due to the continuous flow of blood. 

Even with this somewhat stricter definition, there is room for discretion. A sportsman who takes an occasional puff of a bronchodilator for exercise-induced asthma but is otherwise asymptomatic may be considered eligible by some. Individuals who have undergone surgery for a congenital condition and are in excellent health may or may not be suitable. Thus, an asymptomatic patient with a hip prosthesis who is taking no medication may be acceptable whereas an equally healthy individual with a prosthetic heart valve should be excluded from a study involving a cannula because of the risk, however remote, of endocarditis. Clearly, whatever definition of a healthy volunteer is used, sensible clinical judgement is still required. 

Class 111, for example, prosthetic heart valves, rechargeable non-active drug delivery systems, absorbable sutures, spinal stents, neurological catheters, temporary pacemaker leads. 

Mechanical prosthetic heart valves The use of enoxaparin injection has not been adequately studied for thromboprophylaxis or long-term use in patients with mechanical prosthetic heart valves. Isolated cases of prosthetic heart valve thrombosis have been reported in patients with mechanical prosthetic heart valves who have received enoxaparin for thromboprophylaxis. Some of these cases were pregnant women in whom thrombosis led to maternal and fetal deaths. Frequent monitoring of peak and trough anti-Factor Xa levels and dose adjustment may be needed. 

Adverse effects include nausea, dizziness, skin rash and headache. Though it is not useful as an antianginal drug, but it has been employed for prophylaxis of coronary and cerebral thrombosis in post MI and post stroke patients, as well as to prevent thrombosis in patients with prosthetic heart valves. 

It is used in venous thromboembolism, pulmonary embolism, atrial fibrillation and for prophylaxis after insertion of prosthetic heart valves. 

Dipyridamole is a vasodilator that inhibits platelet function by inhibiting adenosine uptake and cGMP phosphodiesterase activity. Dipyridamole by itself has little or no beneficial effect. Therefore, therapeutic use of this agent is primarily in combination with aspirin to prevent cerebrovascular ischemia. It may also be used in combination with warfarin for primary prophylaxis of thromboemboli in patients with prosthetic heart valves. A combination of dipyridamole complexed with 25 mg of aspirin is now available for secondary prophylaxis of cerebrovascular disease. 

Prophylaxis should be targeted to those with the following risk factors prosthetic heart valves, previous bacterial endocarditis, congenital cardiac malformations, cardiac transplantation patients who develop cardiac valvulopathy. 

Dipyridamole (Persantine) is a vasodilator that, in combination with warfarin, inhibits embolization from prosthetic heart valves and, in combination with aspirin, reduces thrombosis in patients with thrombotic diseases. Dipyridamole by itself has little or no benefit in fact, in trials where a regimen of dipyridamole plus aspirin was compared with aspirin alone, dipyridamole provided no additional beneficial effect. Dipyridamole interferes with platelet function by increasing the cellular concentration of adenosine 3, 5 -monophosphate (cyclic AMP). This effect is mediated by inhibition of cyclic nucleotide phosphodiesterase and by blockade of uptake of adenosine, which acts at A2 receptors for adenosine to stimulate platelet adenylyl cyclase. The only current recommended use of dipyridamole is for primary prophylaxis of thromboemboli in patients with prosthetic heart valves the drug is given in combination with warfarin. 

Apart from atrial fibrillation, there are many other causes of cardioembolic stroke including prosthetic heart valves and patent foramen ovale (see Ch. 6). 

Thorough cardiac examination should look for possible cardiac source of embolism, including atrial fibrillation, mitral stenosis and prosthetic heart valves. Left ventricular hypertrophy suggests hypertension or aortic stenosis, and a displaced apex from a dilated left ventricle indicates underlying cardiac or valvular pathology. 

Mitral and aortic valve vegetations Prosthetic heart valve malfunction Aortic arch atherothrombosis/dissection Mitral valve prolapse... 

INR 3.0-4.5 Recurrent deep vein thrombosis and pulmonary embolism arterial disease including myocardial infarction mechanical prosthetic heart valves. 

Prevention of thrombosis. Postoperatively or after myocardial infarction 5000 units of unfractionated heparin should be given s.c. every 8 or 12 h without monitoring (this dose does not prolong the APPT), or in pregnancy 5000-10 000 units s.c. every 12 h with monitoring (except for pregnant women with prosthetic heart valves for whom specialist monitoring is needed). 

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