Heart Failure
Mechanisms of Heart Failure
Heart failure develops through several interconnected processes that progressively weaken the heart's ability to pump blood. Understanding these mechanisms matters because nearly every drug used to treat heart failure targets one or more of them.
- Impaired contractility leads to decreased cardiac output, a reduced ejection fraction (the percentage of blood pumped out of the ventricle with each contraction), and increased end-systolic volume (blood left in the ventricle after it contracts).
- Increased afterload results from elevated systemic vascular resistance (the resistance blood encounters in peripheral vessels). This increases ventricular wall tension and decreases stroke volume (the volume of blood ejected per contraction). Think of it as the heart having to push against a higher pressure with every beat.
- Ventricular remodeling involves myocardial hypertrophy (thickening of the heart muscle), ventricular dilation (enlargement of the chambers), and altered ventricular geometry (the ventricle changes shape, becoming more spherical and less efficient). Over time, remodeling causes further deterioration of heart function, creating a vicious cycle.
- Neurohormonal activation is the body's attempt to compensate for falling cardiac output, but it ultimately makes things worse:
- RAAS activation (renin-angiotensin-aldosterone system) causes vasoconstriction and sodium/water retention, increasing both preload and afterload.
- Sympathetic nervous system activation increases heart rate, contractility, and vasoconstriction. While these responses temporarily maintain perfusion, they increase myocardial oxygen demand and accelerate remodeling.
Cardiac Function and Adaptation
A few foundational concepts are essential for understanding how heart failure disrupts normal cardiac physiology.
- Preload is the volume of blood in the ventricles at the end of diastole. It determines how much the cardiac muscle fibers are stretched before contraction.
- The Frank-Starling mechanism describes how increased preload stretches cardiac fibers, which increases the force of contraction and stroke volume. In a healthy heart, this allows the heart to adapt to changes in blood volume. In heart failure, the curve flattens: adding more volume no longer improves output and instead causes congestion.
- Cardiac output = stroke volume × heart rate. It represents the total volume of blood the heart pumps per minute.
- Heart failure can manifest as two types of dysfunction:
- Systolic dysfunction (HFrEF): reduced contractility and a low ejection fraction (typically below 40%). The heart can't squeeze forcefully enough.
- Diastolic dysfunction (HFpEF): the ventricles are stiff and have difficulty relaxing and filling properly, even though ejection fraction may be preserved.
- Natriuretic peptides (BNP, ANP) are hormones released by the heart in response to increased wall stress. They promote vasodilation and sodium/water excretion, counteracting RAAS. Their blood levels are used diagnostically to confirm heart failure.
Signs and Symptoms of Heart Failure
The symptoms of heart failure reflect two core problems: inadequate forward blood flow (low output) and fluid backing up behind the failing ventricle (congestion).
- Dyspnea (shortness of breath) is the hallmark symptom. It presents in several patterns:
- Exertional dyspnea: occurs during physical activity
- Orthopnea: worsens when lying flat (patients often sleep propped up on pillows)
- Paroxysmal nocturnal dyspnea (PND): sudden episodes of breathlessness that wake the patient at night
- Fatigue and weakness result from reduced cardiac output and decreased oxygen delivery to tissues.
- Edema (fluid accumulation) presents as peripheral edema (swelling in the legs and ankles, often worse at the end of the day) and pulmonary edema (fluid in the lungs, causing crackles on auscultation).
- Jugular venous distension (JVD) is visible bulging of the jugular veins in the neck, indicating elevated venous pressure from right-sided congestion.
- Cardiac enlargement can be detected on physical exam (displaced point of maximal impulse) or imaging.
- S3 gallop is an extra heart sound heard during early diastole, caused by blood rushing into an already volume-overloaded ventricle. It's a classic finding in systolic heart failure.
- Pulmonary crackles are abnormal lung sounds caused by fluid in the alveoli.
- Tachycardia is a compensatory increase in heart rate as the body tries to maintain cardiac output.
- Decreased exercise tolerance occurs because the heart cannot meet the body's increased oxygen demands during activity.
Causes and Diagnosis of Heart Failure
Primary causes:
- Coronary artery disease is the most common cause, leading to myocardial infarction and ischemic cardiomyopathy (weakened heart muscle from inadequate blood supply).
- Hypertension forces the heart to work harder over time, eventually causing hypertrophy and failure.
- Valvular heart disease, such as aortic stenosis (narrowing) or mitral regurgitation (leaking), places chronic volume or pressure overload on the heart.
- Cardiomyopathies are diseases of the heart muscle itself, including dilated (enlarged and weakened), hypertrophic (abnormally thickened), and restrictive (stiff and less compliant) types.
- Arrhythmias such as chronic atrial fibrillation or sustained tachycardia can impair cardiac function over time.
- Congenital heart defects are structural abnormalities present at birth that can lead to heart failure if uncorrected.
Diagnostic methods:
- Physical examination includes auscultation for S3 gallop and pulmonary crackles, assessment of JVD, and evaluation of peripheral edema.
- Chest X-ray can reveal cardiomegaly (enlarged heart silhouette) and pulmonary congestion.
- Electrocardiogram (ECG) can detect arrhythmias, evidence of prior myocardial infarction, or ventricular hypertrophy.
- Echocardiography is the single most useful diagnostic test. It assesses ventricular function, measures ejection fraction, evaluates valve function, and identifies structural abnormalities.
- Cardiac biomarkers: Elevated BNP or NT-proBNP levels help confirm heart failure and correlate with disease severity. Troponins may be elevated in acute settings (e.g., myocardial infarction triggering decompensation).