Introduction
Heart and lung drugs keep many older dogs and cats comfortable at home for months or years. The veterinary technician fills the prescriptions, counts the doses, draws the drug level at the right time, teaches the owner to count the resting breathing rate and shows the cat owner how to use an inhaler with a spacer. The technician is also often the first person to hear that a dog on digoxin has stopped eating.
This guide groups the drugs by where they act: the heart muscle, the blood vessels, the kidney and the airways. For each class it gives the monitoring the technician performs and the adverse effects the VTNE likes to test.
Key facts
- Pimobendan is an inodilator: it makes the heart muscle more sensitive to calcium and dilates both arteries and veins.2
- Digoxin has a narrow therapeutic window. Vomiting, diarrhea and loss of appetite are often the earliest signs of toxicosis.2
- Draw a serum digoxin sample 8 to 12 hours after the most recent dose. The target range is 0.8 to 1.2 ng/mL.2
- ACE inhibitors (enalapril, benazepril) can cause azotemia, so BUN and creatinine are monitored.3
- Furosemide is a loop diuretic. It acts on the thick ascending loop of Henle, and hypokalemia is a common side effect of loop diuretics.5
- Spironolactone is a potassium-sparing aldosterone antagonist. With an ACE inhibitor it may cause hyperkalemia.35
- Cats are more susceptible than dogs to lidocaine toxicity, so the cat dose is far lower.4
- An albuterol inhaler is a rescue drug for a cat in an asthma crisis. It is given through a spacer and face mask while the cat breathes normally.10
How the VTNE tests this
- Apply anatomy and physiology to drug use, for example, which part of the nephron furosemide acts on (Domain 1 Task A).1
- Recognize drug classes, mechanisms and clinically relevant side effects, such as digoxin toxicosis or ACE inhibitor azotemia (Domain 1 Task F; K8, drug classification).1
- Calculate a dose and tablet count for pimobendan, furosemide or atenolol (Domain 1 Task C).1
- Choose the right client instruction or monitoring step in a short scenario.
Core concepts
Preload, afterload and contractility
Preload is the volume of blood filling the heart before it contracts. Afterload is the resistance the heart pumps against, mostly set by arterial tone. Contractility (inotropy) is the strength of each beat. In congestive heart failure (CHF), the body retains salt and water through the renin-angiotensin-aldosterone system (RAAS). Fluid backs up into the lungs (pulmonary edema) or the body cavities. Every drug below either strengthens the beat, opens the vessels, removes fluid or controls the rhythm.
Positive inotropes: pimobendan and digoxin
Pimobendan increases contractility mainly by sensitizing the heart's contractile proteins to calcium. As a phosphodiesterase-3 (PDE3) inhibitor it also gives balanced vasodilation, which lowers both preload and afterload. That two-part action is why it is called an inodilator.2 The canine label gives a total daily dose of 0.5 mg/kg, divided into 2 portions about 12 hours apart, rounded to the nearest half tablet. It is contraindicated in hypertrophic cardiomyopathy and aortic stenosis, where more forceful pumping is harmful.7 The ACVIM guideline for myxomatous mitral valve disease (MMVD) recommends 0.25 to 0.3 mg/kg by mouth every 12 hours from Stage B2.6
Digoxin is a cardiac glycoside. It blocks the sodium-potassium pump and slows conduction through the atrioventricular (AV) node, so it is used mainly to slow the heart rate in atrial fibrillation, alongside another drug. Its inotropic effect is modest. It is rarely if ever given to cats, and cats are more sensitive to it than dogs.2
Vasodilators: ACE inhibitors and amlodipine
Angiotensin-converting enzyme (ACE) inhibitors such as enalapril and benazepril block the formation of angiotensin II. This prevents vasoconstriction and reduces sodium and water retention. They are balanced vasodilators. Azotemia (raised BUN and creatinine) may develop because angiotensin II helps the kidney hold its own blood flow steady. Giving an ACE inhibitor with an NSAID can cause acute kidney injury.3
Amlodipine is a calcium channel blocker that acts mostly on vascular smooth muscle, with minimal effect on the heart. It is recommended for high blood pressure (systemic hypertension) in cats and dogs.4
Antiarrhythmics and beta-blockers
Lidocaine (class IB) is the drug for acute treatment of ventricular arrhythmias. It has a rapid onset and a half-life of about 1 hour in dogs, so it is given as IV boluses and then a constant rate infusion (CRI). Low potassium seriously reduces its effect.4
Atenolol is a selective beta-1 blocker and the most commonly used beta-blocker in veterinary medicine. Because it spares beta-2 receptors, it is less likely than propranolol to cause bronchospasm. Adverse effects include bradycardia, AV block, low blood pressure and weaker contractions. Doses are increased gradually, and abrupt discontinuation should be avoided.4 Sotalol (class III, with beta-blocking activity) is the most commonly used long-term drug for serious ventricular arrhythmias in dogs and cats. Diltiazem slows the heart and should not be given with a beta-blocker.4
Diuretics
Furosemide is the most commonly used diuretic for CHF in dogs, cats, cattle and horses. It blocks sodium, potassium and chloride reabsorption in the thick ascending loop of Henle.5 Thiazides act on the distal convoluted tubule. Spironolactone competes with aldosterone in the distal tubule. It is a mild diuretic and is used more for its antifibrotic, heart-protective effect.5
Loop and thiazide diuretics share the same dose-dependent risks: dehydration, prerenal and renal azotemia, hypokalemia, hyponatremia, hypomagnesemia and metabolic alkalosis. Adverse effects are typically more severe in cats than in dogs. Diuretics may increase the risk of digoxin toxicosis.5 Very high IV doses of furosemide (over 20 mg/kg) can cause hearing loss in dogs, and furosemide worsens aminoglycoside ototoxicity and nephrotoxicity.5
Respiratory drugs
Antitussives (cough suppressants) such as codeine, hydrocodone and butorphanol act on the cough center in the medulla. Dextromethorphan suppresses cough in some species but is not analgesic or addictive.8 Some opioid antitussives are controlled substances. For example, the DEA moved hydrocodone combination products from Schedule III to Schedule II in 2014.12 Check the schedule on the label and record every controlled dose in the log.
Bronchodilators open narrowed airways. Beta-2 agonists include albuterol and terbutaline. Theophylline is a methylxanthine, a group classed by some as PDE inhibitors (a debated point).2 For feline asthma, oral or inhaled corticosteroids (fluticasone, budesonide) are the treatment. Bronchodilators may be added but are not used alone.9 Inhaled steroids take effect more slowly than systemic steroids.10 Note the trap: propranolol, a non-selective beta-blocker, should be avoided in animals with asthma.4
| Drug | Class | Main use | Technician monitors | Key adverse effect |
|---|---|---|---|---|
| Pimobendan | Inodilator (PDE3 inhibitor, calcium sensitizer) | MMVD Stage B2, CHF in dogs | Doses 12 hours apart, resting breathing rate | Contraindicated in HCM and aortic stenosis |
| Digoxin | Cardiac glycoside | Heart rate control in atrial fibrillation | Serum level 8 to 12 hours after dose; appetite; potassium | Vomiting, anorexia, any arrhythmia |
| Enalapril, benazepril | ACE inhibitor | CHF, hypertension | BUN, creatinine | Azotemia, low blood pressure |
| Amlodipine | Calcium channel blocker (vascular) | Hypertension in cats and dogs | Blood pressure | Acts on vessels, not heart rate |
| Atenolol | Beta-1 blocker | Rate and rhythm control | Heart rate | Bradycardia, AV block; do not stop abruptly |
| Lidocaine | Class IB antiarrhythmic | Acute ventricular arrhythmias | ECG, potassium, CNS signs | Twitching, seizures; cats more sensitive |
| Furosemide | Loop diuretic | Pulmonary edema in CHF | Hydration, electrolytes, kidney values | Hypokalemia, dehydration, azotemia |
| Spironolactone | Potassium-sparing diuretic | Add-on in CHF | Potassium | Hyperkalemia with ACE inhibitors |
Sources: Merck Veterinary Manual cardiovascular pharmacology chapters; Vetmedin label; ACVIM MMVD guideline.23457
| Drug | Class | Use | Technician note |
|---|---|---|---|
| Butorphanol, hydrocodone, codeine | Opioid antitussives | Cough suppression | Check the schedule; log controlled doses |
| Dextromethorphan | Non-opioid-receptor antitussive | Cough | Not analgesic or addictive |
| Albuterol, terbutaline | Beta-2 agonist bronchodilators | Bronchospasm; feline asthma add-on | Albuterol inhaler for rescue, not daily use |
| Theophylline | Methylxanthine bronchodilator | Airway disease | Fluoroquinolones raise its blood level |
| Fluticasone, budesonide | Inhaled corticosteroids | Feline asthma maintenance | Slower onset than oral steroids |
Sources: Merck Veterinary Manual; Barchilon and Reinero, J Feline Med Surg 2023.89101112
Procedure or technique
Drawing a serum digoxin level
- Check the order: the first level is usually taken 2 to 7 days after treatment starts.2
- Confirm with the owner the exact time of the last dose.
- Draw the sample 8 to 12 hours after that dose (a trough-type sample).2
- Record the time of the last dose and the draw time on the submission form.
- Report vomiting, diarrhea, anorexia or lethargy to the veterinarian, even if the last level was normal. Toxic effects can occur within the therapeutic range.2
Teaching an owner to use a feline inhaler with a spacer
- Acclimate the cat first: let it sniff the mask and eat treats placed in it over several days.10
- Attach the metered-dose inhaler to the valved holding chamber (spacer), with the face mask on the other end.
- Place the mask snugly over the nose and mouth.
- Actuate one puff into the chamber. The one-way valve lets the cat breathe the drug in during normal (tidal) breathing. A cat cannot time a breath or hold its breath.10
- Hold the mask in place for several breaths, as the veterinarian directs, then remove it.
- Remind the owner that a steroid inhaler is the daily controller and the albuterol inhaler is for rescue only.
Flowchart
The ACVIM guideline encourages home monitoring of breathing and heart rate. A rise in resting respiratory rate above the pet's normal baseline is the best single predictor of impending decompensation.6
Species differences
- Lidocaine: dog boluses are 2 mg/kg IV to effect, up to 8 mg/kg over about 30 minutes. Cats receive 0.1 to 0.4 mg/kg, because cats are more susceptible to toxic effects and may show cardiac suppression and CNS excitation.4
- Digoxin: cats are more sensitive than dogs, and digoxin is rarely if ever used in cats.2
- Diuretics: adverse effects are typically more severe in cats than in dogs.5
- Quinidine should not be given to cats.4
- Horses: lidocaine given too fast IV can cause excitement, sweating and seizures. Furosemide is also used for CHF in horses and cattle.45
- Inhalers: cats need a spacer with a face mask because they cannot coordinate inhalation with actuation.10
Calculation
Order: pimobendan at the label total daily dose of 0.5 mg/kg, divided twice daily, for a 30 kg dog. Tablets are 5 mg, scored.
Total daily dose: 30 kg x 0.5 mg/kg = 15 mg per day (kg cancels).
Per dose: 15 mg per day / 2 doses per day = 7.5 mg per dose.
Tablets per dose: 7.5 mg / 5 mg per tablet = 1.5 tablets (mg cancels).
Answer: 1 and a half 5 mg tablets every 12 hours. The label allows rounding to the nearest half tablet.7
Safety, legal and client education
- Teach owners to count sleeping or resting breaths for one minute, keep a log and call if the rate rises above the pet's baseline.6
- Pimobendan doses go about 12 hours apart. Keep chewable tablets secured: the label warns to store them out of reach of dogs, cats and children.7
- Owners should not give over-the-counter NSAIDs to a pet on an ACE inhibitor, because the combination can injure the kidneys.3
- A pet on furosemide will urinate more. Once diuresis starts, the patient should have free access to water.6
- Do not stop atenolol suddenly unless the veterinarian says so.4
- Do not run lidocaine through the same IV line as other medications.4
- Check opioid antitussive labels for a controlled-substance schedule, and record controlled doses in the log.
Common mistakes and exam traps
- Reading vomiting in a dog on digoxin as a stomach upset. GI signs are often the first sign of toxicosis.2
- Thinking furosemide spares potassium. It wastes potassium, and low potassium makes digoxin toxicity more likely and lidocaine less effective.24
- Using the dog lidocaine dose in a cat.
- Choosing a bronchodilator alone for feline asthma. Corticosteroids are the core treatment.9
- Choosing pimobendan for a patient with hypertrophic cardiomyopathy or aortic stenosis: the label contraindicates both.7
- Calling amlodipine a heart-rate drug. It works on vessels; diltiazem is the calcium channel blocker that acts on the heart.4
Quick review
- Pimobendan: inodilator, calcium sensitizer plus balanced vasodilator.
- Label dose: 0.5 mg/kg per day split into 2 doses 12 hours apart.
- Digoxin: narrow window; GI signs first; cats more sensitive.
- Digoxin level: 8 to 12 hours post dose; target 0.8 to 1.2 ng/mL.
- ACE inhibitors: watch BUN and creatinine; avoid NSAIDs.
- Amlodipine: vascular calcium channel blocker for hypertension.
- Atenolol: beta-1 selective; bradycardia; taper, never stop abruptly.
- Lidocaine: acute ventricular arrhythmias; much lower dose in cats.
- Furosemide: thick ascending loop of Henle; hypokalemia, dehydration.
- Spironolactone: aldosterone antagonist; hyperkalemia risk with ACE inhibitors.
- Feline asthma: steroids first; albuterol by spacer for rescue.
- Rising resting breathing rate at home: see the veterinarian today.