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, receives blood into the heart and transfers it to a larger, more muscular ventricle, which is situated at the bottom and pumps blood from the heart into circulation (Marieb 2010).
The right atrium receives low-oxygen blood from vessels throughout the body (systemic circulation), and the right ventricle then pumps it to the lungs to become oxygenated. The left atrium of the heart receives high-oxygen blood from vessels of the lungs (pulmonary circulation), and the left ventricle then pumps it into systemic circulation. Thus, the two sides of the heart work in parallel to collect oxygen-poor blood from peripheral tissues, send it to the lungs for gas exchange (to pick up oxygen and remove carbon dioxide), and redistribute the newly oxygenated blood to tissues and organs to meet metabolic demands (Marieb 2010).
As the heart begins to fail, compensatory mechanisms throughout the body are initiated to ensure that adequate oxygen is delivered to tissues. During compensated heart failure, signals from the brain and kidneys result in fluid retention (to increase blood pressure in an attempt to better distribute oxygenated blood), increased heart rate and contractile force, dilatation (expansion) of the ventricle to hold more blood, and growth of new heart muscle to increase the force of ejection. 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 (Goldman 2013). Since not all patients have fluid accumulation at the time of initial evaluation, the term “heart failure” is preferred over the older term “congestive heart failure” (Hunt 2005; Hunt 2009).
As heart failure progresses, compensatory reactions are increased to keep up with tissue oxygen demands. However, the compensatory mechanisms of the heart are not without their limits; 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 retain so much water before fluid begins to infiltrate other organs and tissues. Once the limits of compensation are reached, the cardiovascular system is no longer able to satisfy tissue oxygen demands. This is called decompensated heart failure and requires aggressive medical intervention or death will occur (Goldman 2013).