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Cardiology · Clinical background

Furosemide CRI

Loop diuretic. Inhibits the Na-K-2Cl symporter at the thick ascending limb of the loop of Henle, reducing sodium and chloride reabsorption and producing potent diuresis with kaliuresis, magnesuria, and modest calciuresis. Onset 5 min IV; peak diuresis 30 min; duration 2 hr (single bolus). Also produces mild venodilation that contributes to preload reduction independent of diuresis, which is useful in acute pulmonary edema. Hepatic metabolism with renal excretion. CRI delivery produces a smoother diuresis curve, reduced ototoxicity, and better natriuretic efficiency per total dose than intermittent bolus.

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Clinical background

Furosemide is the cornerstone diuretic in vet cardiology and is the most commonly used loop diuretic in vet ICU practice. It is prescribed routinely as an intermittent IV or oral bolus for congestive heart failure; the CRI formulation matters for the smaller set of patients in whom intermittent dosing is no longer producing adequate diuresis. The clinical pivot is the natriuretic-efficiency evidence: at equivalent total daily doses, continuous infusion produces more sodium and water excretion than intermittent bolus delivery, with less ototoxicity and a smoother diuresis curve. This was demonstrated in Greyhounds by Adin in 2003 and has since been the foundational vet reference supporting CRI use in refractory cases.

Pharmacology

Loop diuretic. The molecular action is inhibition of the Na-K-2Cl symporter on the apical membrane of the thick ascending limb of the loop of Henle:

Onset is within 5 minutes IV; peak diuresis at 30 minutes; duration of action 2 hours after a single bolus. The pharmacokinetic implication of this short duration is that intermittent dosing produces alternating peaks and troughs of natriuretic effect, with periods of compensatory sodium retention between doses. CRI delivery eliminates the troughs and produces sustained natriuresis at a lower peak plasma concentration.

Hepatic metabolism contributes about 30% of clearance; renal excretion of unchanged drug accounts for the rest. Renal dysfunction prolongs the elimination half-life but does not eliminate the diuretic response unless GFR has collapsed.

Indications

Primary use cases:

Furosemide is not appropriate as a first-line diuretic in patients who are not yet receiving any diuretic for a new diagnosis of heart failure; oral diuresis with conventional intermittent dosing is the starting point. The CRI is reserved for refractory cases.

Dosing

Cat dosing runs lower than dog dosing for the same indication. Cats have less margin between effective diuresis and dehydration; the natriuretic effect rises with dose, but so does the rate at which cats decompensate from intravascular depletion. Most feline CHF patients respond at 0.25–0.5 mg/kg/hr.

Reassess fluid status, electrolytes, and BUN/creatinine every 4–6 hours during the CRI. The most common error in furosemide CRI use is failing to recognize that the patient has crossed from “volume overloaded and underperfused” to “volume contracted and underperfused.” Both look like worsening clinical signs, and both look bad on the monitor, but the treatment is exactly the opposite.

Administration

Stock is 50 mg/mL injectable, 50 mL multi-dose vial (2500 mg per vial). The InfusionFox calculator preselects three weight-banded preparations (5, 2, or 1 mg/mL) to keep the pump rate in the precision range. Furosemide CRIs run at relatively low mg/kg/hr doses, so the working concentrations need to be more dilute than catecholamines to avoid sub-2-mL/hr pump rates at typical patient weights.

Diluent: 0.9% sodium chloride. Lactated Ringer’s and other acidic fluids cause precipitation; 5% dextrose is borderline (some references list compatibility, others suggest avoidance) and saline is the preferred choice.

Compatibility constraints:

Light protection is not strictly required for short-term IV infusions, but the prepared bag should be discarded if any color change to yellow or amber appears. Furosemide degrades in ambient light over hours, particularly at elevated temperature.

Companion measures during the CRI: pair with electrolyte monitoring at 4–6 hour intervals, and consider proactive potassium and magnesium supplementation rather than waiting for severe derangement to develop. Hypokalemia almost always develops within the first 12 hours of a furosemide CRI; treating it preemptively with potassium-rich IV fluid or potassium chloride supplementation is better practice than reactive treatment after the patient has dropped into the 2 mEq/L range.

Drug interactions

Adverse effects

Monitoring

Weaning

For most patients, the CRI is a 12–48 hour bridge between presentation in crisis and stabilization on oral therapy. Wean by 25–50% every 6–12 hours once urine output is established and clinical signs are improving. Transition to oral furosemide at a dose adjusted for the patient’s home regimen, with concurrent introduction of any planned chronic therapy (pimobendan, ACE inhibitor, spironolactone, additional thiazide if sequential nephron blockade is the strategy).

In patients with refractory CHF where the CRI is being used as part of a sequential nephron blockade strategy, the duration may extend longer; the dose adjustments are driven by the response of body weight and electrolyte trends rather than a fixed taper schedule.

Abrupt cessation is usually well tolerated for short-duration CRIs and is occasionally necessary if the patient becomes volume contracted; tapering is preferred where time allows.

Sources