Furosemide
Furosemide is a potent, rapidly acting **loop diuretic** widely used in veterinary medicine to manage fluid overload conditions. It is the cornerstone of therapy for **congestive heart failure (CHF)** and **pulmonary edema** across multiple species. By promoting profound diuresis, it rapidly reduces cardiac preload and relieves venous congestion. Beyond its primary cardiac applications, furosemide is utilized for: * **Oliguric renal failure:** To convert oliguria to polyuria and promote urine flow. * **Hypercalcemia:** To promote renal calcium excretion (requires prior volume expansion). * **Exercise-Induced Pulmonary Hemorrhage (EIPH):** Used prophylactically in racehorses to reduce epistaxis. * **Udder edema:** FDA-approved for post-parturient use in cattle. Because of its potency, furosemide can lead to significant fluid and electrolyte shifts, necessitating careful monitoring of hydration status and serum electrolytes (especially potassium).
Mechanism: Furosemide acts primarily on the **thick ascending limb of the loop of Henle** in the nephron. * **Inhibition of NKCC2:** It reversibly binds to and inhibits the **Naโบ/Kโบ/2Clโป cotransporter** on the luminal membrane โ prevents the reabsorption of sodium, potassium, and chloride. * **Loss of Medullary Hypertonicity:** This disruption abolishes the hypertonic medullary interstitium โ impairs the kidney's ability to concentrate urine โ results in profound, dose-dependent diuresis. * **Electrolyte Excretion:** It significantly increases the renal excretion of water, Naโบ, Kโบ, Clโป, Caยฒโบ, Mgยฒโบ, Hโบ, NHโโบ, and bicarbonate. * **Hemodynamic Effects:** Furosemide induces mild **renal venodilation** and transiently increases glomerular filtration rate (GFR) via prostaglandin release. This venodilation provides rapid relief in acute cardiogenic pulmonary edema, often before the onset of significant diuresis.
Dosing by species
- As a general diuretic ยท 2.5-5 mg/kg (lower dose suggested for cats) ยท PO, IV or IM ยท once or twice daily at 6-8 hour intervals
- For severe acute pulmonary edema (parenteral dosing) ยท Initially 2- 4 mg/kg IV or IM. The dose may be repeated within 1-2 hours. ยท IV, IM ยท Repeat within 1-2h ยท Cats that can tolerate an IV injection may benefit from a faster onset of action. Reduce dosing sharply once respiratory rate starts to decrease.
- For heart failure (oral dosing) ยท Maintenance dose usually ranges from 6.25 mg once a day to 12.5 mg PO q8h (higher doses, up to 37.5 mg PO q12h have been used) ยท PO ยท SID to q8h ยท Doses must be titrated carefully. Instruct owner to count sleeping respiration rates.
- For chronic therapy (CHF) ยท initially, try 6.25 mg/cat/day. Some can be maintained on 6.25 mg every other day. ยท PO ยท SID or EOD ยท Goal is to give the 'lowest possible dose'.
- In combination with rest, vasodilators and rate-control ยท 1-4 mg/kg ยท IV, IM or PO ยท q1-12 hours
- For adjunctive treatment of moderate to severe hypercalcemia ยท 2-4 mg/kg ยท IV, SC or PO ยท two to three times daily ยท Volume expansion is necessary prior to use.
- For acute oliguric renal failure ยท Initially 2 mg/kg IV; if no substantial diuresis develops in one hour, double to 4 mg/kg. If successful, repeat at 8 hour intervals ยท IV ยท q8h
- For acute oliguric renal failure (fluid replete patients) ยท bolus at 1-6 mg/kg IV. If diuresis ensues, can repeat dose every 6-8 hours or a CRI at 0.1-1 mg/kg/hr ยท IV ยท q6-8h or CRI
Routes of administration
Contraindications
- Anuria
- Hypersensitivity to furosemide
- Progressive renal disease if increasing azotemia and oliguria occur during therapy
- Seriously depleted electrolytes
- Severe dehydration
- Severe electrolyte depletion
- Hepatic coma
- Dehydration
- Pericardial effusion where cardiac tamponade is confirmed
Adverse effects
- Fluid and electrolyte abnormalities (hyponatremia, hypokalemia, hypocalcemia, hypomagnesemia)
- Prerenal azotemia (secondary to dehydration)
- Ototoxicity (especially in cats with high-dose IV therapy)
- Gastrointestinal distress
- Hematologic effects (anemia, leukopenia)
- Weakness and restlessness
- Hypokalaemia
- Hypochloraemia
- Hypocalcaemia
- Hypomagnesaemia
- Hyponatraemia
- Dehydration
- Polyuria/polydipsia (PU/PD)
- Prerenal azotaemia
- Ototoxicity (especially in cats)
- Gastrointestinal disturbances
Drug interactions
- ACE INHIBITORS (e.g., enalapril, benazepril) ยท Increased risks for hypotension, particularly in patients who are volume or sodium depleted secondary to diuretics
- AMINOGLYCOSIDES (gentamicin, amikacin, etc.) ยท Increased risk for ototoxicity
- AMPHOTERICIN B ยท Loop diuretics may increase the risk for nephrotoxicity development; hypokalemia
- CORTICOSTEROIDS ยท Increased risk for GI ulceration; hypokalemia ยท moderate
- DIGOXIN ยท Furosemide-induced hypokalemia may increase the potential for digoxin toxicity ยท major
- INSULIN ยท Furosemide may alter insulin requirements
- MUSCLE RELAXANTS, NON-DEPOLARIZING (e.g., atracurium, tubocurarine) ยท Furosemide may prolong neuromuscular blockade
- PROBENECID ยท Furosemide can reduce uricosuric effects
- SALICYLATES ยท Loop diuretics can reduce the excretion of salicylates
- SUCCINYLCHOLINE ยท Furosemide may potentiate effects
- THEOPHYLLINE ยท Pharmacologic effects of theophylline may be enhanced when used with furosemide ยท moderate
- Aminoglycosides ยท Potentiates nephrotoxicity and ototoxicity ยท major
Monitoring
- Serum electrolytes (especially potassium, sodium, calcium, magnesium)
- BUN, creatinine
- Blood glucose
- Hydration status
- Blood pressure, if indicated
- Clinical signs of edema, patient weight, if indicated
- Evaluation of ototoxicity, particularly with prolonged therapy or in cats
- Resting respiratory rate (at home by owner)
- Serum electrolytes (Potassium, Sodium, Chloride, Calcium, Magnesium)
- Renal parameters (BUN, Creatinine)
- Blood pressure
- Clinical signs of ototoxicity (especially in cats)
- Serum electrolytes (especially Potassium, Sodium, Chloride)
- Renal function (BUN, Creatinine)
- Clinical signs of edema/heart failure resolution
Overdose
The LD50 in dogs after oral administration is >1000 mg/kg; after IV injection >300 mg/kg. Chronic overdosing at 10 mg/kg for six months in dogs led to development of calcification and scarring of the renal parenchyma. **Acute overdosage** may cause severe electrolyte and water balance problems, CNS effects (ranging from lethargy to coma and seizures), and cardiovascular collapse. **Treatment:** * Empty the gut after recent oral ingestion using standard protocols. * **Avoid concomitant cathartics** as they may exacerbate fluid and electrolyte imbalances. * Aggressively monitor and treat electrolyte and water balance abnormalities supportively. * Monitor respiratory, CNS, and cardiovascular status.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.