Understanding the Heart and Heart Failure
The human heart consists of left and right halves, which behave as two parallel “pumps” with distinct roles in circulation. Both the left and right side of the heart contain two chambers: a smaller atrium, at the top, which receives blood and transfers it to a larger, more muscular ventricle. The ventricles, situated at the bottom of the heart, pump blood from the heart into circulation.17
The right atrium receives low-oxygen blood from systemic circulation, and the right ventricle then pumps it to the lungs to become re-oxygenated. The left atrium of the heart receives high-oxygen blood from the lungs (pulmonary circulation), and the left ventricle then pumps it into systemic circulation. Thus, the two sides of the heart work in conjunction to collect oxygen-poor blood from peripheral tissues, send it to the lungs to pick up oxygen and deposit carbon dioxide, and redistribute the newly oxygenated blood to tissues and organs.17
Heart failure can occur when the heart becomes weakened or damaged (see “Heart Failure: Causes and Risk Factors”). The ventricles may become too stiff to fill properly or stretch too much to pump blood efficiently. Ejection fraction (EF) is a measure of the percentage of blood ejected from the left ventricle with each heartbeat, reflecting the efficiency of the heart’s pumping action. A normal EF is 55‒70%. In other words, a normally functioning heart will eject 55‒70% of the total blood in the left ventricle with each heartbeat. Heart failure can occur with reduced EF or preserved EF. Whether EF is reduced or preserved may influence treatment decisions and the course of the condition.
As the heart begins to fail, the body tries to compensate to ensure that adequate oxygen is delivered to tissues. Signals from the nervous system, kidneys, and blood vessels result in fluid retention (to increase blood pressure in an attempt to better distribute oxygenated blood), increased heart rate and contractile force, and dilatation of the ventricle (to hold more blood) to increase ejection force.
Increases in blood volume and ventricular filling pressures cause blood to “back up” in systemic or pulmonary circulation and leak fluid into peripheral tissues, causing edema (swelling) in the lungs, abdomen, and extremities. This is termed “congestive” heart failure.18 Fluid can sometimes collect in the lungs, hindering breathing. This is known as pulmonary edema, which can cause respiratory distress if untreated.19
As heart failure progresses, the body tries to keep up with tissue oxygen demands. However, the heart is restricted in how much it can expand to hold more blood or increase its contractile force and rate, and the kidneys can only process so much water before fluid infiltrates other organs and tissues. Once compensation limits have been reached, the cardiovascular system can no longer satisfy tissue oxygen demands. This is called decompensated heart failure, which requires immediate medical intervention.18,20
Types of Heart Failure
Distinction is sometimes made between “left-sided” and “right-sided” heart failure. In left-sided heart failure, the left ventricle is primarily affected. Right-sided heart failure usually arises after left-sided heart failure progresses, and typically does not occur independently. In less common conditions, such as cor pulmonale (a lung problem), the right side of the heart may be primarily affected.
Left-sided heart failure: reduced and preserved ejection fraction. The left ventricle, the largest and most muscular of the four heart chambers, must generate a substantial amount of force to pump blood into the systemic circulation. Generally, heart failure begins with the left ventricle.1 In left-sided heart failure, the ability of the left ventricle to push oxygenated blood into circulation is compromised, meaning the heart must work harder to pump the same amount of blood.
There are two types of left-sided heart failure: heart failure with preserved ejection fraction (HFpEF) and heart failure with reduced ejection fraction (HFrEF).21 As previously noted, ejection fraction is a measure of the amount of blood that leaves the left ventricle and enters systemic circulation with each heartbeat. It represents how efficiently the left ventricle empties itself. In HFrEF, the ventricle cannot contract normally and lacks the force to adequately eject blood. In HFpEF, the ventricle is unable to relax and fill properly. Patients with HFrEF typically respond well to standard treatments and have a more favorable prognosis than those with HFpEF.22
Ejection fractions between 40‒55% may indicate cardiac damage, and an ejection fraction < 40%, termed reduced ejection fraction, indicates heart failure or significant heart muscle damage.23 An ejection fraction of > 75% may indicate hypertrophic cardiomyopathy, in which abnormally thick heart muscle makes it difficult for the heart to pump out enough blood.
The most common causes of heart failure are ischemic heart disease and coronary artery disease. Other causes include high blood pressure, valvular heart disease, congenital heart disease, and a variety of cardiomyopathies.24,25
Right-sided heart failure. The right side of the heart pumps oxygen-poor blood to the lungs so it can be oxygenated. Right-sided heart failure usually occurs as a result of left-sided failure. When the left ventricle fails, increased fluid pressure backs up through the pulmonary circulation and increases the resistance against which the right ventricle must pump. As the right side of the heart fails, blood backs up in the body's veins. This may cause swelling in the legs, ankles, and abdomen.26