In a patient with acute deep vein thrombosis on therapeutic heparin who now has hyperkalemia and mildly elevated creatinine, which medication should be discontinued: heparin, furosemide, lisinopril, or metformin?

Lisinopril Should Be Stopped

In this patient with hyperkalemia (K 6.0 mEq/L) and acute kidney injury on heparin for DVT, lisinopril must be discontinued immediately as it is the primary cause of the hyperkalemia and is contraindicated in this clinical scenario.

Rationale for Stopping Lisinopril

The patient presents with two critical findings that mandate stopping the ACE inhibitor:

• Hyperkalemia (K 6.0 mEq/L): The ACC/AHA guidelines explicitly state that aldosterone receptor antagonists and RAAS blockers should not be used when potassium is ≥5.0 mEq/L, and that inappropriate use when potassium >5.0 mEq/L is potentially harmful due to life-threatening hyperkalemia 1

• Acute kidney injury with intercurrent illness: The Canadian Society of Nephrology recommends temporary discontinuation of RAAS blockers (including ACE inhibitors) in patients with serious intercurrent illness that increases the risk of AKI, particularly when GFR <60 mL/min/1.73 m² 1

• Triple threat combination: This patient is likely on the dangerous combination of ACE inhibitor + diuretic (furosemide) + acute illness, which creates a "perfect storm" for acute kidney injury and hyperkalemia 2

Why Other Medications Should Continue

Heparin (Continue)

• Essential for DVT treatment: Heparin is the cornerstone of acute DVT management and must be continued with dose adjustments to maintain therapeutic aPTT 3, 4
• The prolonged PT/aPTT is expected with therapeutic heparin and does not indicate toxicity requiring discontinuation 1
• Stopping heparin would expose the patient to high risk of thrombosis propagation and pulmonary embolism 3

Furosemide (Continue with caution)

• Loop diuretics do not cause hyperkalemia; they actually promote potassium excretion 1
• While the "sick-day rules" suggest temporary discontinuation of diuretics during acute illness 1, furosemide may be needed to manage volume status
• The diuretic should be held only if the patient is volume depleted, but can be restarted once lisinopril is stopped

Metformin (Continue for now)

• The patient's creatinine is 120 μmol/L (approximately 1.4 mg/dL), which corresponds to an eGFR likely >45 mL/min/1.73 m² 1
• Canadian guidelines recommend metformin can be continued with GFR ≥45 mL/min/1.73 m² and should only be reviewed (not necessarily stopped) when GFR is 30-44 mL/min/1.73 m² 1
• However, metformin should be temporarily held during this acute illness per "sick-day rules" and restarted once the patient is stable 1

Immediate Management Algorithm

1. Stop lisinopril immediately - do not wait for potassium to normalize 1

2. Treat hyperkalemia urgently with standard protocols (calcium gluconate if ECG changes, insulin/glucose, sodium polystyrene sulfonate)

3. Continue heparin with aPTT monitoring to maintain therapeutic range of 1.5-2.5 times control 1

4. Temporarily hold metformin during acute illness, restart when stable and eating normally 1

5. Reassess furosemide - hold if volume depleted, continue if volume overloaded

6. Monitor closely: Check potassium and creatinine daily until stable, then every 2-3 days 1

Common Pitfalls to Avoid

• Do not restart lisinopril until potassium is <5.0 mEq/L and creatinine has returned to baseline 1
• Do not stop heparin - the prolonged coagulation studies are expected and therapeutic 1
• Do not assume the ACE inhibitor is "needed" for heart failure during acute illness - patient safety takes priority 1
• Do not continue the triple combination of ACE inhibitor + diuretic + acute illness, as this dramatically increases AKI risk 2

The answer is C - Lisinopril must be stopped.