Heart Disease in Dogs and Cats: Early Signals and the Diagnostic Workup That Confirms It

Heart disease in dogs and cats is a family of slowly progressing structural problems that, in most cases, give owners months to years of warning before anything dramatic happens. The job of an early-detection plan is simple: catch the warning while it is still a medication conversation rather than an emergency-room conversation. This guide walks through what to look for at home, what your vet will actually do in the clinic, which drugs get used and at what doses, and the specific numbers that should send you straight to the ER.

Why early detection changes the outcome

By the time a small-breed dog with mitral valve disease collapses, the heart has usually been remodeling for two to four years. The same is true for the average cat with hypertrophic cardiomyopathy (HCM). During that pre-clinical window, the heart is changing on imaging while the pet still looks essentially normal at home. Catching the disease then — what cardiologists call Stage B1 or B2 — is the difference between a single daily pill and a multi-drug regimen after a crisis.

The two diseases you are most likely to meet:

• Myxomatous mitral valve disease (MMVD) in dogs, especially small breeds under 20 kg — Cavaliers, Yorkies, Shih Tzus, Mini Poodles, Chihuahuas, and most terriers. Roughly 75% of canine heart disease.

• Dilated cardiomyopathy (DCM) in large-breed dogs — Dobermans, Boxers, Great Danes, Irish Wolfhounds. More aggressive course, often with arrhythmia.

• Hypertrophic cardiomyopathy (HCM) in cats, especially Maine Coons, Ragdolls, British Shorthairs, and middle-aged to senior cats generally.

Early signals you can actually catch at home

Pets hide cardiac disease well; the signs are usually subtle and slow, and owners often only notice in retrospect. Train yourself to look for five things:

1. Cough that is new, soft, and dry, often worse at night or first thing in the morning. In dogs this points to an enlarging left atrium pressing on the left mainstem bronchus. Cats with HCM almost never cough; a coughing cat is more likely to have asthma or bronchitis.

1. Sleeping respiratory rate (SRR) above 30 breaths per minute, counted over a full minute while the pet is sound asleep. Normal is 15–25. Above 30 = call the vet. Above 40 = same-day appointment. Above 50 or with effort = ER.

1. Exercise intolerance that is not "just getting older." Stopping on walks the dog used to finish, refusing the second flight of stairs, breathing hard ten minutes after play.

1. Resting heart rate above 140 bpm in a calm adult dog, or above 200 bpm in a calm adult cat, with a rhythm that is not perfectly regular.

1. Fainting or "wobbly" episodes, especially after excitement or coughing. Fainting in a dog with a heart murmur is a same-day cardiology red flag.

In cats, the most reliable early signs are different: rapid, shallow breathing (not panting), a squatting, hunched posture with elbows out, and hind-leg weakness or sudden lameness from a thrown clot (aortic thromboembolism, ATE) — a cold, painful, dragging rear leg is a true emergency.

Triage: when to call, when to book, when to go now

Not every hint needs a panic run. Use this three-tier framework.

Book a routine appointment (within 1–2 weeks).

• A new soft cough in a small-breed dog older than 6.

• Sleeping respiratory rate 25–30 on a consistent home log.

• A heart murmur found on a wellness exam in an otherwise normal pet.

• New exercise intolerance without distress.

Same-day veterinary visit.

• Sleeping respiratory rate consistently above 30.

• A cat breathing with mouth open, even once.

• A new gallop rhythm (three-beat cadence) on exam.

• A fainting episode within the last 48 hours.

• A non-weight-bearing, cold, painful rear leg in a cat.

Go directly to the ER.

• Open-mouth breathing in a cat, or rapid effort breathing in any pet.

• Pale, blue, or grey gums.

• Collapse that does not resolve within 30 seconds.

• Resting respiratory rate above 50 in a dog, or above 40 in a cat.

• Distended abdomen with sudden weakness.

• Suspected aortic thromboembolism in a cat.

If you can, take a 30-second video of the breathing pattern on your phone. It is the single most useful piece of home data you can hand to the triage nurse.

The diagnostic workup, step by step

A reasonable first-pass workup for a pet with a new murmur, cough, or exercise change includes the following in order:

1. Physical exam — murmur grade (I–VI), timing, point of maximal intensity, and a search for a gallop rhythm. Jugular vein assessment and pulse quality are included.

1. Thoracic radiographs (three views) — look at the vertebral heart score (VHS): normal in dogs is roughly 8.5–10.5, in cats roughly 6.7–8.1. Above 11 in a dog, or with a tall left atrium and pulmonary vein distension, the disease has usually reached Stage B2. Pulmonary edema on the right caudal and right middle lung lobes is the classic CHF pattern in dogs.

1. Blood pressure — measured with Doppler or oscillometric. Hypertension can be either a cause or an effect.

1. NT-proBNP — a biomarker; high values push the probability of cardiac versus respiratory cause for a cough.

1. ECG — for arrhythmias. Atrial fibrillation in a large-breed dog with a fast ventricular rate (often 160–220) is a treatment trigger on its own.

1. Echocardiogram — the gold standard. This is where you get the actual diagnosis: valve leaflet thickening and prolapse, chamber dimensions, fractional shortening, left atrial-to-aortic ratio, and wall thickness.

Expect the full workup to take 45–90 minutes, and expect a referral for the echo unless your general clinic has cardiology time on-site.

Medications and doses: what actually gets prescribed

I am giving you the doses pet owners most often ask me to write down, so you can double-check your label. Do not start or change any of these without a prescription and a recheck. The numbers below are for general reference in stable patients and are not a substitute for a specific dosing plan from your vet.

Pre-clinical mitral valve disease (Stage B2, dogs with VHS >10.5 or LA/Ao >1.6):

• Pimobendan (Vetmedin) — 0.25–0.3 mg/kg orally every 12 hours, given one hour before food. The EPIC trial showed a meaningful delay to the onset of CHF when started at this stage in appropriate candidates.

• Optional add-on: Benazepril 0.25–0.5 mg/kg orally every 24 hours, if systemic hypertension is also present.

Congestive heart failure (Stage C, dogs):

• Pimobendan 0.25–0.3 mg/kg PO q12h.

• Furosemide 1–4 mg/kg PO q8–12h, titrated to breathing rate and weight. In an acute crisis the loading dose is 2–4 mg/kg IV or IM, repeatable.

• ACE inhibitor — Enalapril 0.5 mg/kg PO q12–24h, or Benazepril 0.25–0.5 mg/kg PO q24h.

• Spironolactone 1–2 mg/kg PO q12h, for the potassium-sparing and antifibrotic effect.

• Diltiazem 0.5–1.5 mg/kg PO q8h or sustained-release at 3–5 mg/kg PO q12h, for rate control if atrial fibrillation is present. Some cardiologists prefer a beta-blocker instead.

• For refractory cases, sildenafil 0.5–2 mg/kg PO q8–12h if pulmonary hypertension is documented, and torsemide 0.1–0.3 mg/kg PO q12–24h as a more potent alternative to furosemide.

Pre-clinical DCM (large-breed dogs, especially Dobermans):

• Pimobendan 0.25–0.3 mg/kg PO q12h, started in the pre-clinical occult phase based on echo or 24-hour Holter showing ventricular ectopy. Evidence supports earlier intervention in this group.

Feline HCM:

• Most cats are not started on medication in the pre-clinical phase. Treatment is triggered by clinical signs, gallop rhythm, very high heart rate, evidence of CHF on imaging, or ATE.

• Atenolol 6.25–12.5 mg per cat PO q12–24h for rate control in obstructive HCM, titrated up to effect. Avoid in decompensated cats.

• Diltiazem sustained-release 10 mg/kg PO q24h (≈ one 30 mg tablet per average cat) as an alternative, especially for non-obstructive HCM.

• Clopidogrel 18.75 mg per cat PO q24h for cats with a history of ATE or at high clot risk; often combined with aspirin at 0.5–1 mg/kg q72h, though aspirin alone is considered insufficient.

• Pimobendan 0.25–0.3 mg/kg PO q12h is increasingly used in feline CHF, though the evidence base is thinner than in dogs; the decision is typically cardiology-driven.

Keep a one-page medication card for the pet: drug, dose, frequency, with-food or empty-stomach, and which side effects to watch for (loss of appetite, vomiting, lethargy, sudden change in thirst).

Monitoring at home after a diagnosis

Three numbers, tracked weekly, tell you 80% of what you need to know:

• Sleeping respiratory rate. Log it in a phone note. Trend matters more than any single reading.

• Resting body weight. A gain of 0.5 kg overnight in a small dog, or 0.3–0.5 kg in a cat, often means fluid retention. A drop over weeks without diet change can mean cardiac cachexia.

• Appetite and attitude. Any 24-hour downturn is worth a call.

Bring the log to every recheck. Rechecks are usually every 3–6 months for stable Stage B, and every 1–3 months once on heart failure medication, with a chest X-ray and a blood kidney panel (furosemide and ACE inhibitors both affect renal values).

When to escalate: the hard-stop numbers

Use this as your personal go-now list, on top of the ER criteria above:

• SRR above 40 in a dog, or above 35 in a cat, on two separate counts an hour apart, while the pet is at rest.

• Open-mouth breathing in a cat at any time, even briefly.

• A fainting episode, even a single one.

• Sudden abdominal distension, sudden weight gain, or new ascites.

• A cold, painful, non-weight-bearing rear leg in a cat.

• A resting heart rate above 160 in a dog not currently on a beta-blocker, or a sudden new irregular rhythm.

• Gums that look pale, blue, or grey instead of bubble-gum pink.

If any of those happen, do not email. Call the ER, say "I think my pet is in heart failure," and drive.

A short note on prevention and screening

There is no lifestyle that reliably prevents MMVD or HCM. What helps is screening at the right age: auscultation at every annual visit, a baseline radiograph in small-breed dogs around age 7, a one-time echo in any Doberman or Boxer over age 4, and an annual echo in any predisposed cat breed. Early detection is the single most powerful thing modern cardiology can offer your pet.

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Reviewed for general information. Specific doses and escalation thresholds are typical ranges from current ACVIM consensus guidelines; your pet's plan should be set by the prescribing veterinarian. If you found this useful, save the home-log template and the go-now list — these are the two pages that actually move the needle in a cardiac case.