left-sided heart failure
The left side of the heart receives oxygen-rich blood from the lungs and pumps it out to the body. When that side falters, blood backs up where it came from — the lungs — like a clogged drain causing the sink above it to overflow. The result is wet, congested lungs and the breathlessness that defines left-sided heart failure.
The left ventricle (and behind it the left atrium) cannot keep pace with the blood arriving from the pulmonary veins, so pressure rises in the lung capillaries and fluid seeps into the air spaces. Symptoms are dominated by the lungs: breathlessness on exertion, difficulty breathing when lying flat (orthopnea), and waking at night gasping for air (paroxysmal nocturnal dyspnea). Because the pulmonary circulation lies just upstream, these are the earliest warning signs in most cases.
Left-sided failure is the most common form and the usual starting point. Importantly, it is the leading cause of right-sided heart failure: chronically high lung pressures eventually strain the right ventricle. When both sides fail, the picture is called biventricular heart failure.
A man with a prior heart attack notices that he can no longer lie flat to sleep — within minutes he feels he is suffocating and must sit up. This orthopnea reflects fluid shifting into his lungs when horizontal, a hallmark of left-sided failure.
Orthopnea points to congestion in the pulmonary circulation.