Gentamincin Therapy for Endocarditis

To summarize - the addition of gentamicin to nafcillin for MSSA endocarditis is often talked about, but in reality has relatively weak evidence.  In a widely quoted study, which was a multicenter randomized trial of ~78 patients (published in 1982 in Annals of Internal Medicine), gentamicin was added for the initial 2 weeks to nafcillin (vs nafcillin alone) in pts with MSSA endocarditis - duration of bacteremia was decreased by 1 day, but no change in mortality, and there was also an increase in nephrotoxicity (not surprisingly) in the gentamicin group.  

There is even less evidence to support the role of gentamicin caused in MRSA endocarditis (no similar randomized control trial has been done), and the use of gentamicin in these cases has been largely extrapolated from the MSSA data.  

Thus, the IDSA guidelines (published in 2005 Circulation) recommend that addition of gentamicin is optional, and if done, only for the first 3-5 days - both for MSSA and MRSA endocarditis (to nafcillin for MSSA, and to vancomycin for MRSA).  Many clinicians would add gentamicin in cases of severe or fulminant endocarditis - in fact, for the patient discussed in report, when it was discovered that he had new severe aortic regurgitation, ID did in fact recommend adding gentamicin.  An additional concern for MRSA cases is the potential for synergistic nephrotoxocity and ototoxicity between vancomycin and gentamicin.  

To reiterate an important point discussed this morning, the role of gentamicin is much stronger and has much more evidence in Strep and Enterococcal endocarditis.  The exact regimens can be somewhat complex and depend on the MICs to penicillin, but basically most regimens for Strep endocarditis contain gentamicin for 2 weeks in addition to beta-lactam therapy, and for enterococcus gentamicin is used for even longer - 4-6 weeks.

(Chanu Rhee MD, 5/11/2010)