Heart Failure: MRA Prescribing

A mineralocorticoid receptor antagonist (MRA) blocks the receptor that aldosterone uses. Spironolactone is described in its Canadian monograph as a specific pharmacologic antagonist of aldosterone that acts by competitive binding at the aldosterone-dependent sodium-potassium exchange site in the distal convoluted renal tubule, so more sodium and water are excreted while potassium loss is minimized. That last clause is the whole prescribing problem in one line: the same action that helps in heart failure keeps potassium in the body. Two ideas organize everything that follows. First, an MRA is not chosen as a diuretic. The Canadian Cardiovascular Society (CCS) 2021 heart failure guideline places it as one of four standard therapies for heart failure with reduced ejection fraction (HFrEF), each with a different mechanism, and recommends combination therapy with one evidence-based medication from each category. Second, the dangerous event is hyperkalemia, so the prescriber's job is to pick a patient whose potassium and kidneys can carry the drug, check the numbers on a fixed schedule, and know in advance what each result changes. The Inspra monograph states the principle plainly: the...