Man With Prosthetic Valve Who Needs GI Surgery

A 68-year-old man with a prosthetic mitral valve presents to the emergency department with acute abdominal pain, nausea, vomiting, and constipation. Surgical evaluation is performed; the results suggest a complete bowel obstruction. Urgent celiotomy is recommended.

HISTORY
The patient has hypertension and hypercholesterolemia. He underwent coronary artery bypass grafting and mitral valve replacement 10 years earlier. As a child, he experienced hives and wheezing after receiving an injection of penicillin for pneumonia.

PHYSICAL EXAMINATION
The patient is alert and oriented but in mild distress. Temperature is 36.7°C (98.2°F); heart rate, 110 beats per minute; respiration rate, 18 breaths per minute; blood pressure, 160/88 mm Hg; and weight, 70 kg (154 lb). Mucous membranes are dry, and lungs are clear. Cardiac examination reveals a metallic S₁ murmur. His abdomen is distended but remains soft; bowel sounds are decreased. There is left lower quadrant tenderness without rebound or guarding.

IMAGING RESULTS
A radiographic obstruction series reveals dilated loops of large and small bowel with air-fluid levels. There is no air in the rectum.

Which of the following statements about endocarditis prophylaxis for this patient is most accurate?

A.

Prophylaxis is not indicated because there is no evidence that it is effective.

B.

Options for prophylaxis include clindamycin, 600 mg IV, or erythromycin base, 1 g PO (single dose).

C.

Vancomycin, 1 g IV, and gentamicin, 100 mg IV, should be given before the procedure; no additional doses are needed.

D.

Vancomycin, 1 g IV, and gentamicin, 150 mg IV, should be given before the procedure and again 8 hours later.

CORRECT ANSWER: C

This case highlights several points about endocarditis prophylaxis. The American Heart Association (AHA) guidelines for endocarditis prophylaxis were last revised in 1997.¹ These guidelines emphasize that most cases of endocarditis are not linked to procedures. In fact, some population-based, case-controlled studies have failed to confirm any benefit for endocarditis prophylaxis.² Concerns about control bias³ and the exclusion of important procedures (eg, dental extractions) from the final analysis detract from the impact of these studies. Nonetheless, the AHA writing group was aware of these concerns and took them into account when they finalized the guidelines. A recent systematic review of endocarditis prophylaxis for dental procedures concluded that “there is no evidence about whether prophylaxis is effective or ineffective against bacterial endocarditis.”⁴ While the paucity of convincing evidence should prompt further study and scrutiny of the current guidelines, it is still prudent to heed the expert consensus that they reflect. Thus, choice A (no prophylaxis) would be considered inappropriate by most authorities.

When is prophylaxis indicated? The need for (as well as the type of) prophylaxis varies with risk. As used by the AHA, the term “risk” refers not only to the likelihood of the development of endocarditis but also to the likelihood of morbidity and mortality should endocarditis occur. Risk depends on the patient's history and condition and on the type of procedure.

This patient's prosthetic valve puts him in the high-risk category. The only other high-risk conditions are:

• History of endocarditis.
• Complex cyanotic congenital heart disease.
• Surgically constructed systemic pulmonary shunts.

Once a patient's risk category has been determined, consider the risk of the planned procedure. Emergent surgery for a bowel obstruction, which will probably involve incision through the intestinal mucosa, is a procedure for which prophylaxis is indicated in high-risk patients such as this man.

Which prophylactic regimen? The next step is to determine which antibiotic, route of administration, and dosage are most appropriate. Because this patient has a history of systemic type 1 hypersensitivity to penicillin, β-lactam antibiotics are contraindicated. Erythromycin was formerly recommended as the alternative to amoxicillin in patients who were undergoing dental procedures and who were allergic to penicillin; however, the guidelines no longer recommend erythromycin because of the high incidence of associated GI adverse effects. In addition, the various formulations of this agent were thought to be too confusing. In any event, this patient's bowel obstruction mandates parenteral antibiotics; thus, oral erythromycin (choice B) is incorrect.

Clindamycin (choice B) is the currently recommended alternative to amoxicillin for patients allergic to penicillin who are undergoing dental procedures; however, it is not appropriate for this patient. Gut flora are intrinsically different from oral flora. α-Hemolytic streptococci are the main pathogens targeted by prophylaxis during dental procedures, but activity against Enterococcus faecalis must be the chief criterion for choosing antibiotics to prevent endocarditis during GI tract surgery. Infection with these bacteria, while not especially virulent, is extremely difficult to treat once established, especially in a patient with a prosthetic valve. Clindamycin does not have activity against enterococci. For prevention of endocarditis caused by E faecalis, vancomycin is the agent of choice in patients who cannot take ampicillin.

Although Gram-negative enteric pathogens are much less likely to cause endocarditis than are Gram-positive gut flora, Gram-negative endocarditis does occur-and disproportionately often in patients with prosthetic valves. Moreover, Gram-negative endocarditis can be very difficult to manage. Thus, prophylaxis that targets these organisms is recommended, although only in high-riskpatients. Aminoglycosides have reliable activity against most enteric Gram-negative pathogens and are highly active in the bloodstream. Thus, the addition of gentamicin to the prophylactic regimen is recommended. However, the dose specified in the guidelines is 1.5 mg/kg IV. This means that 150 mg would be too much for a 70-kg man, which makes choice D incorrect.

One dose or two? Endocarditis prophylaxis is justified only when the benefit of preventing infections outweighs the risks and costs associated with the antibiotics used. Thus, a sound prophylactic regimen should minimize the risk of allergic reactions and the induction of antibiotic resistance. This can be accomplished by limiting antibiotic exposure as much as possible-such as by initially administering antibiotics in a manner that ensures the agent is present only for a limited time after exposure to any transient bacteremia. Consequently, second doses of antibiotics after a procedure are no longer recommended-even in high-risk patients-for dental procedures in which clindamycin or amoxicillin is used for prophylaxis or for bowel surgery in which vancomycin and gentamicin are used (choice D).

There are some exceptions to one-time dosing. When ampicillin is used, a second dose is sometimes given 6 hours later because of this agent's short half-life; however, this is done only in high-risk patients. Also, when a procedure is so prolonged that the antibiotics will dissipate before the surgery is over, it is reasonable to give a second dose. However, neither of these conditions exists here. Thus, vancomycin, 1 g IV, and gentamicin, 100 mg IV, in a single dose to be completed 30 minutes before the procedure (choice C), is the most appropriate regimen for this patient.

Outcome of this case.

After receiving prophylactic antibiotics, the patient underwent exploratory surgery. This revealed an obstructing mass in the sigmoid colon; a left-sided colectomy with diverting colostomy was performed. His postoperative course was complicated by anemia that required transfusion; however, there was no evidence of bloodstream infection. He was transferred to another facility for subacute rehabilitation.

References:

REFERENCES:

1.

Dajani AS, Taubert KA, Wilson W, et al. Prevention of bacterial endocarditis. Recommendations of the American Heart Association.

JAMA.

1997;277:1794-1801.

2.

Strom BL, Abrutyn E, Berlin JA, et al. Dental and cardiac risk factors for infective endocarditis. A population-based, case-control study.

Ann Intern Med.

1998;129:761-769.

3.

Seymour RA, Lowry R, Whitworth JM, Martin MV. Infective endocarditis, dentistry and antibiotic prophylaxis; time for a rethink?

Br Dent J.

2000;189:610-616.

4.

Oliver R, Roberts GJ, Hooper L. Penicillins for the prophylaxis of bacterial endocarditis in dentistry.

Cochrane Database Syst Rev.

2004;(2):CD003813.