Between B1 and B2 — what the left atrium is saying

The words heard most often in an HCM consult room are not actually the disease name — they are a letter and a digit. "Still B1." Or: "We should call this B2 now." Both cats look equally fine. Why is one B1 and the other B2 — and when that single character changes, what is supposed to change with it?

This article covers the middle of the HCM staging system: the fork between two asymptomatic stages that look identical from the outside. The short answer: what stands at that fork is not the heart wall. It is the left atrium.

1. Where B sits on the map

The ACVIM stages (2020) in one line: A (predisposed only) → B1·B2 (disease without signs) → C (has experienced failure or a clot) → D (refractory). Most HCM cats spend most of their lives in B. That is why position within B is nearly the whole of managing this disease — and why ACVIM split B in two: the same "asymptomatic" can carry entirely different imminent risk.

2. Why the atrium — the reservoir where risk collects

One metaphor carries the whole article. The thickened ventricle is a stiffened pump. What the pump cannot accept backs up into the chamber before it — the left atrium. The atrium is this disease's reservoir.

• Reservoir still normal size → upstream is coping despite the stiff pump → B1
• Reservoir starting to swell → pressure is climbing upstream → B2
• Reservoir overflows → fluid into the lungs (heart failure) → C
• Standing water congeals → the clot (ATE)

Atrial size is the cumulative report card of the load the heart has carried. Wall thickness names the disease's existence; the atrium tracks its progress. Which is why, on any recheck report, the direction of LA/Ao is bigger news than the wall (reading the echo report).

3. B1 — a surveillance grade, not a sentence

B1's numbers are the most comforting in this bible:

• Median survival of cats diagnosed while asymptomatic: 10.9 years — many live to old age and die of something else (Payne 2013)
• In a natural-history study following rehoming-centre cats for a median 5.6 years (CatScan II), 79% of HCM cats had no cardiovascular event during follow-up
• No drug has been shown to change outcomes in B1 — "watch without medication" is the standard of care (ACVIM 2020)

B1 care fits in three lines: echo recheck every 6–12 months, resting breaths 2–3 times a week, announce the heart before any anaesthesia or fluids. Everything else — play, food, routines — stays. B1 is less a diagnosis than a surveillance grade.

4. B2 — the atrium has started talking

B2 means moderate or worse left atrial enlargement (roughly LA/Ao 1.9 and above on 2D). Beyond size alone, ACVIM weighs auxiliary findings that raise the risk:

The size of the risk, in research numbers: in the large follow-up of preclinical HCM cats (REVEAL, 1,730 cats), about one in four to five experienced heart failure or a clot within five years, and left atrial size was the leading predictor. Split B in two, and most of that risk piles onto the B2 side.

5. The three things that change at B2

• ① The clopidogrel discussion — an antiplatelet that keeps stagnant blood from congealing. Proven superior to aspirin for preventing recurrence after a clot (the FAT CAT trial), and ACVIM recommends preventive use from B2, when clot risk rises. A quarter to half tablet daily, usually for life; its bitter taste calls for capsules or pill pockets
• ② Shorter rechecks — from 6–12 months to 3–6 months, so the atrium's direction is never missed. Blood-side markers like NLR and PNR also earn their place from here
• ③ Breathing counts go daily — from 2–3 times weekly to once a day, because the earliest sign of crossing from B2 into C is a rise in the resting respiratory rate

6. The cat on the border — between 1.6 and 1.9

• No single number fixes a stage — measurements sway by 0.1–0.2 with angle and operator. Read a trend only when the direction repeats twice
• Was the method the same? — 2D and M-mode values are not interchangeable (chapter 02)
• The auxiliary findings referee — the same 1.7 leans B2 with a gallop, arrhythmia or weak atrial function, and B1 with none. That weighing is the spirit of the ACVIM system
• The usual prescription in the grey zone is "again in 3–4 months" — a sign of care, not confusion

7. From B to C — what all this surveillance is for

The goal of B2 management in one sentence: push the crossing into C (failure or clot) as far away as possible — and if it comes, catch it in a clinic visit, not an emergency room. So memorise the crossing signals: a stepwise rise in the resting breathing rate (earliest and most common), sudden hind-leg paralysis with crying (the clot), open-mouth breathing or collapse. The first is caught by one minute of daily counting; the latter two mean the hospital, now.

And if C does come — many cats stabilise on treatment and go home to live on. That story (heart failure and the clot, in practice) continues in chapters 06 and 07.

8. What to ask your vet

• "What exactly was the LA/Ao — and do you read this cat as B1 or B2?"
• "Beyond atrial size, did anything stand out — gallop, arrhythmia, atrial function, smoke?"
• "(If B2) do we start clopidogrel? What dose, and any tricks for giving it?"
• "(If borderline) when do we look again — and what should I watch at home meanwhile?"
• "Of the signals of crossing into C, which should this particular cat's family watch hardest?"

9. Related

• Reading the echo report — finding LA/Ao on the page, and its traps
• Resting respiratory rate — B2's one minute a day
• NLR and PNR — blood markers that matter more from B2
• The day of diagnosis — the full stage map