Isosorbide Dinitrate / Isosorbide Mononitrate
Isosorbide dinitrate (ISDN) and isosorbide mononitrate (ISMN) are **organic nitrates** that act as potent **venodilators**, primarily used to reduce cardiac preload. * **Veterinary Use:** While widely used in human medicine for angina and congestive heart failure (CHF), clinical experience and proven efficacy in small animal medicine remain limited. They are occasionally utilized as adjunctive therapy for refractory heart failure or in patients unable to tolerate ACE inhibitors. * **Pharmacological Nuances:** Dogs appear to require significantly higher dosages of isosorbide to achieve therapeutic hemodynamic effects compared to humans. * **Clinical Pearl:** Like other organic nitrates (e.g., nitroglycerin), continuous use of isosorbide nitrates can lead to **nitrate tolerance** (tachyphylaxis), where the vasodilatory effects diminish over time. In human medicine, a "nitrate-free" interval (usually 10-12 hours daily) is required to maintain efficacy, though the exact requirement for this in veterinary patients is unknown.
Mechanism: Organic nitrates act as prodrugs that donate nitric oxide (NO) to vascular smooth muscle, leading to profound vasodilation (predominantly venous, with some arterial effects at higher doses). **Mechanism Pathway:** Organic Nitrate → converted to **free radical Nitric Oxide (NO)** → activates **soluble guanylate cyclase (sGC)** → increases intracellular **cyclic GMP (cGMP)** → activates **Protein Kinase G (PKG)** → decreases intracellular calcium and dephosphorylates **myosin light chain** → **Vascular Smooth Muscle Relaxation**. * **Hemodynamic Effects:** By dilating the venous capacitance vessels, nitrates significantly decrease venous return to the heart (**reduced preload**), which lowers ventricular filling pressures and helps alleviate pulmonary edema in CHF. * **Additional Effects:** Nitrates functionally antagonize acetylcholine, norepinephrine, and histamine. They also relax non-vascular smooth muscle, including biliary, bronchial, gastrointestinal, ureteral, and uterine tissues.
Dosing by species
- Adjunctive treatment of heart failure associated with thyroid storm · Isosorbide dinitrate at 0.5-2 mg/kg PO q8-12h · PO · q8-12h · Start at lowest level and titrate upward.
- Refractory heart failure or in combination with hydralazine or amlodipine in patients unable to tolerate ACE inhibitors · Isosorbide dinitrate at 0.5-2 mg/kg PO twice daily OR isosorbide mononitrate at 0.25-2 mg/kg PO twice daily · PO · twice daily · Efficacy is unknown.
- Refractory heart failure or in combination with hydralazine or amlodipine in patients unable to tolerate ACE inhibitors · Isosorbide dinitrate at 0.5-2 mg/kg PO twice daily OR isosorbide mononitrate at 0.25-2 mg/kg PO twice daily · PO · twice daily · Efficacy is unknown. It is unknown if nitrate-free periods are required to prevent nitrate tolerance.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Cardiogenic or hypovolemic shock
- Severe hypotension
- Hypovolemia
- Use as a sole agent for treating heart failure
- Concurrent use with selective phosphodiesterase inhibitors (e.g., sildenafil)
Adverse effects
- Postural hypotension
- Reflex tachycardia
- Lethargy or weakness (secondary to hypotension)
- Gastrointestinal upset
- Restlessness
- Headache (well-documented in humans, difficult to assess in animals)
- Hypersensitivity reactions (rare)
Drug interactions
- Antihypertensive Drugs (e.g., ACE inhibitors, amlodipine, beta-blockers) · Possible additive hypotensive effects; monitor blood pressure closely.
- Phenothiazines (e.g., acepromazine) · Possible additive hypotensive effects due to alpha-adrenergic blockade.
- Selective Phosphodiesterase Inhibitors (e.g., sildenafil, pimobendan) · Concurrent use with sildenafil can cause profound, life-threatening hypotension and is strictly contraindicated. Use with pimobendan (a PDE3 inhibitor) is common in vet med but requires monitoring for excessive vasodilation.
Monitoring
- Clinical efficacy (improvement in respiratory rate/effort, resolution of pulmonary edema)
- Systemic blood pressure (monitor for hypotension)
- Heart rate (monitor for reflex tachycardia)
- Renal values and electrolytes (if used concurrently with diuretics and ACE inhibitors)
Overdose
**Toxicity Profile:** Isosorbide mononitrate caused significant lethality in rats and mice at massive dosages (2000 mg/kg and 3000 mg/kg, respectively). **Clinical Signs of Overdose:** * Profound venous pooling * Decreased cardiac output * Severe hypotension * Reflex tachycardia * Collapse or syncope **Treatment:** * Treatment is primarily **supportive**. * **Avoid Epinephrine:** Drug therapies with agents such as epinephrine are NOT recommended as they may exacerbate the cardiovascular collapse. * **Fluid Therapy:** Increasing central fluid volume may be useful to counteract venous pooling, but in patients with pre-existing congestive heart failure (CHF), IV fluids must be administered with **extreme caution** to avoid precipitating fulminant pulmonary edema.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.