Bactrim CHF Case Study – Med Ed 101

One thing I always encourage is critical thinking. Reviewing a diagnosis list will give you clues as to the likely medications that a patient is taking or should be taking if they are following guideline-based medication therapy. In this Bactrim CHF case study, I review some of the important clinical pearls that may come up when you see Bactrim (sulfamethoxazole/trimethoprim) used in a patient with CHF.

KE is a 77-year-old female who lives at an assisted living facility. Her caregivers have encouraged her to see a primary care provider due to increasing lethargy, mild confusion, and mild pain associated with urination. At her appointment, she was diagnosed with a urinary tract infection and prescribed Bactrim DS 1 tablet BID for 7 days, then reassess.

Her past medical history is complex and includes CHF and atrial fibrillation amongst other diagnoses. She is taking numerous medications which includes:

  • Aspirin 81 mg daily
  • Entresto 97/103 mg BID
  • Toprol XL 25 mg QD
  • Spironolactone 50 mg QD
  • Empagliflozin 10 mg daily
  • Omeprazole 20 mg daily
  • Acetaminophen 500 mg PRN
  • Cyclobenzaprine 10 mg HS PRN back pain
  • Atorvastatin 80 mg daily
  • Lidoderm patch as needed
  • Sertraline 25 mg daily
  • Warfarin titrated to an INR of 2-3

As a pharmacist, I’m always looking at the medication list to ensure we don’t run into drug interactions or unnecessary complications from new medications.

The first concern I have with Bactrim is the potential for it to cause hyperkalemia. This patient is on two medications that are commonly used for heart failure that can cause hyperkalemia. Both spironolactone and Entresto (contains valsartan) are going to raise the risk for hyperkalemia. At the very minimum, we need to ensure that potassium levels are monitored if the Bactrim prescription is going to be given to this patient. Here’s one of my all-time favorite donated cases demonstrating how high a potassium level can go due to this interaction!

Obviously, this patient likely has a history of HFrEF with the medications that are being prescribed. If Bactrim is prescribed to a heart failure patient, it is additionally important to ask about fluid restrictions. It is recommended to give Bactrim with 8 ounces of water to reduce the risk of crystalluria. This may be another reason why Bactrim is not advantageous.

In addition to the potential additive drug interaction with potassium-elevating medications, warfarin can also interact with Bactrim. This one recently made my Top 10 list! Bactrim will likely raise the concentrations of warfarin leading to an elevated INR. We need to monitor the INR with this drug interaction. If Bactrim has to be used as the antibiotic of choice, most clinicians will assess INR within 3-5 days of starting the Bactrim. In rare instances, I have seen clinicians proactively reduce the dose of warfarin. It was unclear why this patient is not on apixaban or another DOAC but that remains a consideration to review.

The last issue of note is the UTI itself. Empagliflozin should at least be assessed to ensure that this drug is not contributing to the UTI problem. I’d want to know some past history here and see if recurrent UTIs have been happening and if so, we may have to consider alternative therapy.

Hopefully this Bactrim CHF case study reminds you to look at the diagnosis list in addition to the medications. It may help you identify patients who are at risk for drug interactions and other complications.

Eric Christianson, PharmD, BCPS, BCGP

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