How Heart Failure Develops
Heart failure does not mean the heart has stopped — it means the pump can no longer keep up with the body’s demand. Watch a healthy left ventricle squeeze out about 60% of its blood each beat, then injure it: the chamber dilates, the ejection fraction collapses toward 30%, too little blood reaches the body, and the rest dams up behind the heart — flooding the lungs and swelling the legs. Then meet the cruel twist: the body’s own rescue systems, the sympathetic nerves and RAAS, whip the failing heart harder and drown it in fluid — which is exactly why the drugs that help are the ones that block them.
Try this: start on Healthy pump and watch the squeeze, then click Compensation and let the vicious cycle run for ten seconds — when the lungs are flooding, press 💊 Guideline therapy and watch the wedge pressure and swelling fall.
Live pump readout
What’s happening
The ejection fraction, output, heart rate and pressures shown are realistic clinical values (EF ∼60% healthy vs ≤40% in HFrEF; wedge pressure ∼8–12 mmHg normally, congestion above ∼18 mmHg; output ∼5 L/min at rest). The exact numbers ticking here are an illustrative model driving the picture, not a measurement of any real patient.
The Science in Plain Language
Heart failure does not mean the heart has stopped
The word “failure” sounds like the lights went out. They didn’t. In heart failure the heart is still beating — often for many years — but the pump can no longer move enough blood to meet what the body is asking of it, or it can only do so at the cost of dangerously high filling pressures. Think of an ageing water pump that still runs but can’t keep the tank topped up when everyone turns their taps on at once. Everything on this page follows from that one idea: a pump that can’t quite keep up.
Ejection fraction: reduced vs preserved
Each beat, the left ventricle fills to its end-diastolic volume, then contracts and ejects a portion. That portion is the ejection fraction (EF) — stroke volume divided by the volume it started with. A healthy heart ejects about 55–70%. When EF falls to 40% or below, that is heart failure with reduced ejection fraction (HFrEF) — the weak, dilated pump you see when you click Weak pump. But EF is only half the story: many people have heart failure with a normal EF because the ventricle has become stiff and can’t fill properly — heart failure with preserved ejection fraction (HFpEF), defined as EF ≥ 50%. Between them (41–49%) is a mildly-reduced band. Cardiac output is just stroke volume × heart rate; at rest a normal heart pushes roughly 5 litres a minute.
What injures the pump in the first place
Heart failure is a destination reached by several roads. A heart attack (myocardial infarction) kills a patch of muscle and replaces it with scar that cannot contract. Decades of high blood pressure (hypertension) force the ventricle to eject against high resistance every single beat, until the overworked muscle thickens, stiffens and eventually weakens. A leaking or narrowed heart valve makes the chamber pump the same blood twice or push against an obstruction. Add diabetes, alcohol, viral inflammation of the muscle (myocarditis), certain chemotherapies, or a chronically fast rhythm, and the common ending is the same: a chamber that can’t keep up.
Forward failure: too little reaches the body
When the pump is weak, the first problem is simple arithmetic — not enough blood goes forward. Muscles, brain and kidneys run short of oxygen and fuel. That is why the classic everyday symptoms of a failing heart are fatigue, weakness, breathlessness on exertion and light-headedness — the body is being under-supplied. On the diagram, watch the Body & organs block dim as cardiac output falls: that dimming is forward failure.
Backward failure: the traffic jam behind the pump
The blood the ventricle cannot move doesn’t vanish — it dams up behind the failing chamber, and pressure climbs upstream. Behind the left heart, that pressure backs into the lungs. Once the pulmonary wedge pressure climbs above roughly 18–20 mmHg (normal is ~8–12), fluid is squeezed out of the lung capillaries into the air sacs — pulmonary congestion. It feels like breathlessness that is worse lying flat (orthopnoea, so people prop themselves on extra pillows) and can jolt you awake gasping at night (paroxysmal nocturnal dyspnoea). Behind the right heart, the backup is into the body’s veins: swollen ankles and legs (dependent oedema), a congested, tender liver, and sometimes a swollen belly. Turn on Congestion and watch the lungs fill and the ankles balloon.
The cruel twist: compensation that turns toxic
Here is the part that surprises people. Sensing low output, the body reaches for the very reflexes it evolved to survive blood loss. The sympathetic nervous system floods the heart with adrenaline-like signals (noradrenaline), speeding it up and squeezing it harder. The kidneys, feeling under-perfused, switch on the renin–angiotensin–aldosterone system (RAAS): renin → angiotensin II (which clamps down blood vessels) → aldosterone (which orders the kidney to hoard salt and water). For a day, this props up the blood pressure. But chronically it is a disaster: the extra fluid raises the pressure the heart must fill against and floods the lungs and legs, while relentless adrenaline is directly toxic to heart muscle. The chamber stretches, thickens and remodels into a bigger, rounder, weaker pump — which drops output further, which fires the compensation harder. That is the vicious cycle the Compensation button lets you watch tighten.
Why the drugs that help are the ones that BLOCK the rescue
This flips the intuition. For a failing pump you might expect a drug that whips it harder — and for a century that was tried. It doesn’t work long-term, because more stimulation feeds the cycle. The medicines that actually keep people alive and out of hospital are the ones that switch the over-compensation off. ACE inhibitors (“-prils”) and ARBs (“-sartans”) block RAAS. Beta-blockers (carvedilol, bisoprolol, metoprolol succinate) shield the heart from the sympathetic whip — started low and slow, they let it recover. Mineralocorticoid-receptor antagonists (spironolactone, eplerenone) block aldosterone. Newer pillars add an ARNI (sacubitril/valsartan) and the SGLT2 inhibitors (dapagliflozin, empagliflozin), which help even in people without diabetes and in HFpEF. And diuretics (furosemide) drain the backed-up fluid to relieve breathlessness — they make you feel far better fast, though it is the cycle-blockers that change the long game. Press Guideline therapy to see the wedge pressure and swelling fall and the ventricle begin to reverse-remodel.
The myth worth correcting
A common and damaging belief is that “a normal ejection fraction means your heart is fine, so this can’t be heart failure.” Not true. Roughly half of all heart failure is HFpEF, where the EF is 50% or higher but the stiff ventricle can’t fill — the breathlessness, fatigue and swelling are just as real, and an echocardiogram reading “EF 60%” does not rule it out. A blood test for BNP or NT-proBNP (peptides the stretched ventricle releases) helps confirm it. The other myth — that heart failure means imminent death and calls for bed rest — is also wrong: with the modern combination above, plus a low-salt diet, daily weights (a sudden 1–2 kg gain warns of fluid buildup before you feel it), and supervised exercise, many people live active lives for years. The failing heart is not a stopped heart; it is a pump we now know how to unburden.
How doctors measure it — and what you can track yourself
Two everyday scales turn all of this into numbers a clinic can follow. NYHA class grades how much the failing pump limits you: Class I (no limitation), II (breathless on ordinary activity like stairs), III (breathless on light activity), and IV (breathless at rest). The ACC/AHA stages run in parallel from Stage A (at risk — hypertension or diabetes, heart still normal) through B (structural change, no symptoms yet), C (symptomatic), to D (advanced). An echocardiogram measures the ejection fraction and chamber size directly; the 6-minute walk test tracks functional capacity; and NT-proBNP in a blood tube rises with ventricular stretch. At home the single most useful instrument is a bathroom scale: weigh yourself the same time each morning, because a rapid 1–2 kg gain over a day or two is fluid piling up — a warning to act (often an extra diuretic dose, on a plan agreed with your team) before the breathlessness arrives. Cutting dietary salt keeps that fluid from accumulating in the first place.